Prof. Dr. Martin R. Hofmann

Photonics and Terahertz Technology
Ruhr-Universität Bochum

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  • Applying machine learning to optical coherence tomography images for automated tissue classification in brain metastases
    Möller, J. and Bartsch, A. and Lenz, M. and Tischoff, I. and Krug, R. and Welp, H. and Hofmann, M.R. and Schmieder, K. and Miller, D.
    International Journal of Computer Assisted Radiology and Surgery (2021)
    Purpose: A precise resection of the entire tumor tissue during surgery for brain metastases is essential to reduce local recurrence. Conventional intraoperative imaging techniques all have limitations in detecting tumor remnants. Therefore, there is a need for innovative new imaging methods such as optical coherence tomography (OCT). The purpose of this study is to discriminate brain metastases from healthy brain tissue in an ex vivo setting by applying texture analysis and machine learning algorithms for tissue classification to OCT images. Methods: Tumor and healthy tissue samples were collected during resection of brain metastases. Samples were imaged using OCT. Texture features were extracted from B-scans. Then, a machine learning algorithm using principal component analysis (PCA) and support vector machines (SVM) was applied to the OCT scans for classification. As a gold standard, an experienced pathologist examined the tissue samples histologically and determined the percentage of vital tumor, necrosis and healthy tissue of each sample. A total of 14.336 B-scans from 14 tissue samples were included in the classification analysis. Results: We were able to discriminate vital tumor from healthy brain tissue with an accuracy of 95.75%. By comparing necrotic tissue and healthy tissue, a classification accuracy of 99.10% was obtained. A generalized classification between brain metastases (vital tumor and necrosis) and healthy tissue was achieved with an accuracy of 96.83%. Conclusions: An automated classification of brain metastases and healthy brain tissue is feasible using OCT imaging, extracted texture features and machine learning with PCA and SVM. The established approach can prospectively provide the surgeon with additional information about the tissue, thus optimizing the extent of tumor resection and minimizing the risk of local recurrences. © 2021, The Author(s).
    view abstract10.1007/s11548-021-02412-2
  • Comparison between optical coherence tomography imaging and histological sections of peripheral nerves
    Carolus, A.E. and Möller, J. and Hofmann, M.R. and van de Nes, J.A.P. and Welp, H. and Schmieder, K. and Brenke, C.
    Journal of Neurosurgery 134 (2021)
    OBJECTIVE Optical coherence tomography (OCT) is an imaging technique that uses the light-backscattering properties of different tissue types to generate an image. In an earlier feasibility study the authors showed that it can be applied to visualize human peripheral nerves. As a follow-up, this paper focuses on the interpretation of the images obtained. METHODS Ten different short peripheral nerve specimens were retained following surgery. In a first step they were examined by OCT during, or directly after, surgery. In a second step the nerve specimens were subjected to histological examination. Various steps of image processing were applied to the OCT raw data acquired. The improved OCT images were compared with the sections stained by H & E. The authors assigned the structures in the images to the various nerve components including perineurium, fascicles, and intrafascicular microstructures. RESULTS The results show that OCT is able to resolve the myelinated axons. A weighted averaging filter helps in identifying the borders of structural features and reduces artifacts at the same time. Tissue-remodeling processes due to injury (perineural fibrosis or neuroma) led to more homogeneous light backscattering. Anterograde axonal degeneration due to sharp injury led to a loss of visible axons and to an increase of light-backscattering tissue as well. However, the depth of light penetration is too small to allow generation of a complete picture of the nerve. CONCLUSIONS OCT is the first in vivo imaging technique that is able to resolve a nerve’s structures down to the level of myelinated axons. It can yield information about focal and segmental pathologies. © AANS 2021, except where prohibited by US copyright law
    view abstract10.3171/2019.8.JNS191278
  • Confocal laser scanning holographic microscopy of buried structures
    Schnitzler, L. and Neutsch, K. and Schellenberg, F. and Hofmann, M.R. and Gerhardt, N.C.
    Applied Optics 60 (2021)
    In this paper, we present a confocal laser scanning holographic microscope for the investigation of buried structures. The multimodal system combines high diffraction limited resolution and high signal-to-noise-ratio with the ability of phase acquisition. The amplitude and phase imaging capabilities of the system are shown on a test target. For the investigation of buried integrated semiconductor structures, we expand our system with an optical beam induced current modality that provides additional structure-sensitive contrast. We demonstrate the performance of the multimodal system by imaging the buried structures of a microcontroller through the silicon backside of its housing in reflection geometry. © 2020 Optical Society of America
    view abstract10.1364/AO.403687
  • Digital holography for spatially resolved analysis of the semiconductor optical response
    Besaga, V.R. and Gerhardt, N.C. and Hofmann, M.R.
    Applied Optics 60 (2021)
    We present spatially resolved measurements of the below-band-gap carrier-induced absorption and concurrent phase change in a semiconductor with the help of transmission digital holography. The application is demonstrated for a bulk GaAs sample, while the holograms are recorded with a conventional CMOS sensor. We show that the phase information enables spatially resolved monitoring of excess carrier distributions. Based on that, we discuss a phase-based approach for separation of carrier and heat related effects in the semiconductor optical response. © 2020 Optical Society of America.
    view abstract10.1364/AO.402488
  • Femtosecond pulse generation from external cavity diode laser based on self-mode-locking
    Alloush, M.A. and Brenner, C. and Calò, C. and Hofmann, M.R.
    Optics letters 46 (2021)
    In this Letter, we report optical pulse generation from a single-section diode gain chip, employed in an external cavity geometry based on the self-mode-locking regime. The gain chip emits light at 1550 nm wavelength range. The external cavity is operated at various repetition rates, ranging from 1 to 2.5 GHz. An optical pulse width of approximately 650 fs is obtained by fitting a Lorentzian distribution. A low RF spectral width of 78.875 kHz is measured corresponding to a low pulse-to-pulse RMS timing jitter of 1.273 ps. This system paves the way towards ultra-compact, cost-effective, and chirp-compensated femtosecond laser pulse sources with adjustable repetition rates.
    view abstract10.1364/OL.415336
  • Investigations on polarization dynamics of birefringent spin-VCSELs
    Jung, N. and Lindemann, M. and Pusch, T. and Hashib, M.F. and Hoti, P. and Debernardi, P. and Michalzik, R. and Hofmann, M.R. and Gerhardt, N.C.
    Proceedings of SPIE - The International Society for Optical Engineering 11704 (2021)
    Vertical-cavity surface-emitting lasers (VCSELs) are widely used in optical data communication mainly in data centers for short-haul transmissions. However, their intensity modulation resonance frequency does not exceed 40 GHz which also limits the achievable modulation bandwidth and data rate. In contrast, spin-VCSELs can overcome these bandwidth limitations by modulating spin and polarization instead of current and intensity. In spin-VCSELs, the birefringence determines the resonance frequency of the polarization dynamics as well as the modulation bandwidth. We control the birefringence and thus the polarization dynamics via the elasto-optic effect by mechanically or thermally induced strain providing polarization oscillation frequencies up to more than 200 GHz. Detailed analysis shows that spin-VCSELs offer polarization dynamics with good signal strength even when operating close to threshold and at high temperatures. Here, we analyze devices with integrated surface gratings. VCSELs with different grating periods as well as mesa diameters and resulting different oxide apertures were investigated. © 2021 SPIE. © 2021 SPIE. All rights reserved.
    view abstract10.1117/12.2577488
  • Lensless digital holographic microscopy as an efficient method to monitor enzymatic plastic degradation
    Schnitzler, L. and Zarzycki, J. and Gerhard, M. and Konde, S. and Rexer, K.-H. and Erb, T.J. and Maier, U.G. and Koch, M. and Hofmann, M.R. and Moog, D.
    Marine Pollution Bulletin 163 (2021)
    A big challenge of the 21st century is to cope with the huge amounts of plastic waste on Earth. Especially the oceans are heavily polluted with plastics. To counteract this issue, biological (enzymatic) plastic decomposition is increasingly gaining attention. Recently it was shown that polyethylene terephthalate (PET) can be degraded in a saltwater-based environment using bacterial PETase produced by a marine diatom. At moderate temperatures, plastic biodegradation is slow and requires sensitive methods for detection, at least at initial stages. However, conventional methods for verifying the plastic degradation are either complex, expensive, time-consuming or they interfere with the degradation process. Here, we adapt lensless digital holographic microscopy (LDHM) as a new application for efficiently monitoring enzymatic degradation of a PET glycol copolymer (PETG). LDHM is a cost-effective, compact and sensitive optical method. We demonstrate enzymatic PETG degradation over a time course of 43 days employing numerical analysis of LDHM images. © 2020 Elsevier Ltd
    view abstract10.1016/j.marpolbul.2020.111950
  • Reduction of surface morphology influence on THz reflection time domain spectroscopy for material classification by using multiple observation angles
    Becke, L. and Gerling, A. and Hofmann, M.R. and Brenner, C.
    Proceedings of SPIE - The International Society for Optical Engineering 11685 (2021)
    Material classification with THz radiation is typically done in transmission geometry.1 However, in many situations a reflection based classification is highly desirable. For a reflection based classification scheme, it is necessary to compensate the impact of the surface morphology on the reflection signal.2 As the surface morphology will mainly change the frequency dependent reflection pattern of the beam, we use different observation angles to improve classification based on THz reflection data. We use a THz TDS reflection setup measuring at several input and output angles. While the sample can be rotated, the transmitter can be moved on a semi-arc (see Fig. 1). We measure the reflection spectrum at different input-/output- A ngle configurations, which can be retrieved by an Euler transform of transmitter angle and sample angle. A measurement of a grating structure can be seen in Fig. 1. To reduce measuring time while maintaining a sufficient signal to noise ratio, we measure small angle variations around the main specular reflection. For classification we use a supervised machine learning approach based on principal component analysis for feature reduction and a support vector machine for classification.3 In this paper we present the impact of different observation angles on the classification accuracy in contrast to single-observation-angle classification, to check on the hypothesis that an increase in observation angle helps to classify a set of known materials by THz TDS reflection spectroscopy. In consequence we can estimate requirements on the observation angle and identify surface structures which will prevent classification. © COPYRIGHT SPIE.
    view abstract10.1117/12.2577607
  • RF Analysis of a Sub-GHz InP-Based 1550 nm Monolithic Mode-Locked Laser Chip
    Ali Alloush, M. and Van Delden, M. and Bassal, A. and Kleemann, N. and Brenner, C. and Lo, M. and Augustin, L. and Guzman, R. and Musch, T. and Carpintero, G. and Hofmann, M.R.
    IEEE Photonics Technology Letters (2021)
    We report a monolithic sub-GHz repetition rate mode-locked laser with record low pulse-to-pulse RMS timing jitter of 3.65 ps in the passive mode locking regime. We analyse the optical pulse generation in passive and hybrid mode-locking operating regimes, finding narrower RF tone linewidth in the passive regime, attributed to the improved contact structure of the gain sections. The noise performance is also characterized in passive and hybrid regimes, showing RMS integrated timing jitter of approximately 600 fs. For hybrid modelocking, the repetition rate can be varied over a large range from 880 to 990 MHz. We observe broad pulse widths of few hundred picoseconds attributed to the (long folded) waveguide architecture and on-chip multimode interference mirrors. This device subjects a stand-alone, ultra-compact, mode-locking based clock source to realize frequency synthesizers operating over a frequency range from sub-GHz up to approximately 15 GHz. IEEE
    view abstract10.1109/LPT.2021.3083096
  • Amplitude noise and RF response analysis of 1 GHz mode-locked pulses from an InP-based laser chip at 1550 nm
    Alloush, A. and Van Delden, M. and Bassal, A. and Brenner, C. and Musch, T. and Lo, M.C. and Augustin, L. and Guzman, R. and Carpintero, G. and Hofmann, M.R.
    2020 IEEE Photonics Conference, IPC 2020 - Proceedings (2020)
    In this work, we investigate a 1 GHz InP-based hybrid mode-locked laser chip and find an amplitude noise of 0.036 percent. An RF response simulation of its custom-designed mounting PCB is performed providing power transmission between 86 and 92 percent. © 2020 IEEE.
    view abstract10.1109/IPC47351.2020.9252313
  • Bias current and temperature dependence of polarization dynamics in spin-lasers with electrically tunable birefringence
    Lindemann, M. and Jung, N. and Stadler, P. and Pusch, T. and Michalzik, R. and Hofmann, M.R. and Gerhardt, N.C.
    AIP Advances 10 (2020)
    We investigate and compare the intensity and polarization dynamics in a vertical-cavity surface-emitting laser (VCSEL) with a monolithically integrated, electrically controlled birefringence tuning mechanism. The influence of the bias current on the polarization dynamics is investigated over a large range of birefringence values. Bias current tuning toward low values and simultaneous maximization of the resonance frequency is an important strategy to optimize the spin-VCSEL toward energy-efficient operation. A polarization dynamics resonance tuning range from a few GHz up to the maximum frequency of 36 GHz was achieved, and polarization dynamics at maximum frequency are demonstrated at minimum bias current and at high temperatures of approximately 70 °C. We propose a strategy for data communication with low energy consumption and low cooling effort. © 2020 Author(s).
    view abstract10.1063/1.5139199
  • Comparison of self-mode-locking in monolithic and external cavity diode laser at 1550 nm
    Alloush, M.A. and Bassal, A. and Brenner, C. and Fortin, C. and Mekhazni, K. and Gamarra, P. and Calo, C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 11301 (2020)
    In this work, a comparison of self-mode-locking of a 100 GHz repetition-rate monolithic diode as a stand-alone laser source and whilst employed in an external cavity arrangement at 1550 nm is reported. We operated our chip at a forward current slightly above the monolithic chip's lasing threshold and compensated the chirp by a single mode fiber. Ultrashort pulses with 1 ps pulse-width were generated. Changes in the dispersion compensation parameters due to the changed cavity dispersion were analyzed. © 2020 SPIE.
    view abstract10.1117/12.2545780
  • Continuous Wave THz System Based on an Electrically Tunable Monolithic Dual Wavelength Y-Branch DBR Diode Laser
    Gwaro, J.O. and Brenner, C. and Theurer, L.S. and Maiwald, M. and Sumpf, B. and Hofmann, M.R.
    Journal of Infrared, Millimeter, and Terahertz Waves 41 (2020)
    We analyse the use of a tunable dual wavelength Y-branch DBR laser diode for THz applications. The laser generates electrically tunable THz difference frequencies in the range between 100 and 300 GHz. The optical beats are tuned via current injection into a micro-resistor heater integrated on top of one of the distributed Bragg reflector (DBR) section of the diode. The laser is integrated in a homodyne THz system employing fiber coupled ion-implanted LT-GaAs log spiral antennas. The applicability of the developed system in THz spectroscopy is demonstrated by evaluating the spectral resonances of a THz filter as well as in THz metrology in thickness determination of a polyethylene sample. © 2020, The Author(s).
    view abstract10.1007/s10762-020-00676-4
  • Continuous wave THz system based on dual wavelength monolithic Y-branch laser diode
    Surkamp, N. and Gerling, A. and O'Gorman, J. and Honsberg, M. and Schmidtmann, S. and Nandi, U. and Preu, S. and Sacher, J. and Brenner, C. and Hofmann, M.R.
    International Conference on Transparent Optical Networks 2020-July (2020)
    In this work, a monolithic Y-branch laser diode is used for continuous wave Terahertz generation and detection. The output of the laser diode is fiber coupled, amplified and fed into a photoconductor based cw Terahertz setup. While this system can be used for time domain measurements using a fiber coupled delay stage, we'll present sample thickness determination based on laser current induced frequency sweeping without delay stage. © 2020 IEEE.
    view abstract10.1109/ICTON51198.2020.9203061
  • Digital holography for evaluation of the refractive index distribution externally induced in semiconductors
    Besaga, V.R. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 11306 (2020)
    In this paper, we analyse the capabilities of the digital holographic approach for evaluation of the refractive index distribution appearing in semiconductor materials due to external optical excitation. The study is based on a modified transmission Mach-Zehnder holographic microscope operating in the near-infrared spectral range. Practical considerations for holographic characterization of semiconductor samples are discussed. Experimentally measured data are compared with simulations as well as approaches to interpretation of the retrieved data are covered. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2544160
  • Femtosecond RMS timing jitter from 1 GHz InP on-chip mode-locked laser at 1550 nm
    Alloush, A. and Bassal, A. and Brenner, C. and Lo, M.C. and Guzman, R. and Augustin, L. and Carpintero, G. and Hofmann, M.R.
    2020 Conference on Lasers and Electro-Optics Pacific Rim, CLEO-PR 2020 - Proceedings (2020)
    In this work, we analyze the timing stability of a 1 GHz InP on-chip monolithic mode-locked laser at 1550 nm. 504 fs RMS timing jitter is achieved by a hybrid mode-locking operation. © 2020 IEEE.
    view abstract10.1364/CLEOPR.2020.C4C_2
  • High-resolution in vivo imaging of peripheral nerves using optical coherence tomography: A feasibility study
    Carolus, A.E. and Lenz, M. and Hofmann, M. and Welp, H. and Schmieder, K. and Brenke, C.
    Journal of Neurosurgery 132 (2020)
    Objective: Because of their complex topography, long courses, and small diameters, peripheral nerves are challenging structures for radiological diagnostics. However, imaging techniques in the area of peripheral nerve diseases have undergone unexpected development in recent decades. They include MRI and high-resolution sonography (HRS). Yet none of those imaging techniques reaches a resolution comparable to that of histological sections. Fascicles are the smallest discernable structure. Optical coherence tomography (OCT) is the first imaging technique that is able to depict a nerve's ultrastructure at micrometer resolution. In the current study, the authors present an in vivo assessment of human peripheral nerves using OCT. Methods: OCT measurement was performed in 34 patients with different peripheral nerve pathologies, i.e., nerve compression syndromes. The nerves were examined during surgery after their exposure. Only the sural nerve was twice examined ex vivo. The Thorlabs OCT systems Callisto and Ganymede were used. For intraoperative use, a hand probe was covered with a sterile foil. Different postprocessing imaging techniques were applied and evaluated. In order to highlight certain structures, five texture parameters based on gray-level co-occurrence matrices were calculated according to Haralick. Results: The intraoperative use of OCT is easy and intuitive. Image artifacts are mainly caused by motion and the sterile foil. If the artifacts are kept at a low level, the hyporeflecting bundles of nerve fascicles and their inner parts can be displayed. In the Haralick evaluation, the second angular moment is most suitable to depict the connective tissue. Conclusions: OCT is a new imaging technique that has shown promise in peripheral nerve surgery for particular questions. Its resolution exceeds that provided by recent radiological possibilities such as MRI and HRS. Since its field of view is relatively small, faster acquisition times would be highly desirable and have already been demonstrated by other groups. Currently, the method resembles an optical biopsy and can be a supplement to intraoperative sonography, giving high-resolution insight into a suspect area that has been located by sonography in advance. © AANS 2020, except where prohibited by US copyright law.
    view abstract10.3171/2019.2.JNS183542
  • Holographic imaging of particles with Lloyd’s mirror interferometer
    Schnitzler, L. and Kleemann, N. and Neutsch, K. and Hofmann, M.R. and Gerhardt, N.C.
    Optics InfoBase Conference Papers (2020)
    We present a holographic microscope for particle detection based on Lloyd’s mirror configuration. It provides a simple and compact setup with high phase sensitivity and a high stability due to its common-path and self-referencing system. © 2020 The Author(s) © OSA 2020
    view abstract10.1364/DH.2020.HTh4H.5
  • In vivo imaging of human peripheral nerves using optical coherence tomography compared to histopathology slices
    Möller, J. and Carolus, A.E. and Van De Nes, J.A.P. and Lenz, M. and Brenke, C. and Schmieder, K. and Welp, H. and Gerhardt, N.C. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 11228 (2020)
    In this work we demonstrate the ability of in vivo optical coherence tomography (OCT) images to resolve all relevant structures of human peripheral nerves. Measurements have been acquired in more than 30 peripheral nerve surgeries using a commercial OCT system (Thorlabs Ganymede) with a hand probe, which can be directly placed on the nerve, covered by a sterile foil. The resulting 3D OCT images were processed using texture analysis to highlight structural tissue features of the nerve. A comparison of OCT images and corresponding histopathology slices was performed in order to confirm the visualization of the nerve's structures by OCT. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2544801
  • In-depth particle localization with common-path digital holographic microscopy
    Neutsch, K. and Schnitzler, L. and Sun, J. and Tranelis, M.J. and Hofmann, M.R. and Gerhardt, N.C.
    Proceedings of SPIE - The International Society for Optical Engineering 11306 (2020)
    Three-dimensional particle tracking and localization has various applications in biology and medicine, where it may be used to analyze contrast agents, or in flow analysis, e.g. for localizing dust particles in a gas stream or to analyze turbulence in a flow. Moreover, particle localization finds applications in IT-security, where a random arrangement of particles in a transparent environment may represent a Physically Unclonable Function (PUF), which is interesting for individual labeling of high value goods. In conventional systems, such as bright field microscopy, a three-dimensional representation of particles is rather difficult, as it is challenging to acquire depth information about the sample. Quantitative phase imaging techniques provide phase and amplitude and thus in-depth information. Furthermore, they offer single shot measurements while providing images from multiple focal planes. Concerning the stability, which is an important aspect in localizing particles of diffraction limited size, common-path digital holographic microscopy is a reliable tool in particular in combination with a self-referencing system. In this article, we show a common-path digital holographic microscope for particle localization. Firstly, the setup is characterized with a test chart in order to evaluate lateral and axial resolution properties. Afterwards a sample with particles distributed in a three-dimensional medium is analyzed. For reconstruction of the holograms, we use the angular spectrum method, numerical phase unwrapping as well as Zernike polynomials for aberration correction. All in all, the system is able to achieve stable particle localization in 3D with lateral resolution in the sub-micrometer range and an axial sensitivity of at least 100 nm. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2545925
  • Inspection of semiconductor-based planar wave-guiding structures with a near-infrared transmission digital holographic microscopy
    Besaga, V.R. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 11369 (2020)
    In this paper we report on practical investigations aimed at failure detection of the integrated optical circuits (IOC) on Silicon substrate during the control measurements of the items in use. Experiments are performed with a near-infrared (1064 nm) digital holographic microscope (≈90×magnification) in transmission mode. The instrument provides non-destructive and fast (<380 ms reconstruction time for 4112×3008 pixels images) data analysis at the diffraction-limited accuracy (lateral resolution of 760 nm). High quality of the instrument performance is shown on example of topography reconstruction of a standard glass-substrate test target. Practical applicability of the approach was proven on example of diffractive input elements of the IOCs designed for sensing purposes. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2553911
  • Integrated surface gratings in VCSELs for high birefringence splitting
    Pusch, T. and Debernardi, P. and Lindemann, M. and Jung, N. and Gerhardt, N.C. and Hofmann, M.R. and Michalzik, R.
    Proceedings of SPIE - The International Society for Optical Engineering 11356 (2020)
    In single-mode vertical-cavity surface-emitting lasers (VCSELs) the frequency difference between the two orthogonal modes, which is defined by the birefringence present in the cavity, is the key factor to enable ultrafast polarization dynamics in spin-lasers. This could be a promising alternative to overcome the bandwidth limitations in short-haul data transmission. Therefore, controlling the birefringence is indispensable to utilize the full potential of the polarization dynamics. Splittings of around 100GHz were realized with an on-chip approach by integration of a surface grating in an oxide-confined AlGaAs-based VCSEL. In this paper we present further details of the parameter search process using a three-dimensional vectorial optical VCSEL electro-magnetics (VELM) model. We also show the geometrical properties of the processed grating structure. © 2020 SPIE.
    view abstract10.1117/12.2554703
  • Intensity and polarization dynamics in ultrafast birefringent spin-VCSELs
    Lindemann, M. and Jung, N. and Burghard, M. and Pusch, T. and Xu, G. and Žutić, I. and Birkedal, D. and Michalzik, R. and Hofmann, M.R. and Gerhardt, N.C.
    Proceedings of SPIE - The International Society for Optical Engineering 11470 (2020)
    Since todays Internet traffic is more and more concentrated in hyperscale datacenters,1 new concepts for shortrange optical communication systems with high modulation bandwidth, high temperature stability, and low energy consumption are urgently needed. Birefringent spin-lasers, in particular spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs), are a novel type of ultrafast laser devices which promise to serve as ultrafast transmitters for the next generation of optical communication systems. While current-driven intensitymodulated VCSELs are state-of-the-art laser devices for short-range communication, their modulation bandwidth is limited to values below 40 GHz.2, 3 Recently, we were able to demonstrate that modulating carrier spin and light polarization in spin-VCSELs instead of carrier density and light intensity in conventional devices enables ultrafast polarization dynamics and a modulation bandwidth of more than 200 GHz.4 This high modulation bandwidth was achieved by increasing the resonance frequency of the coupled carrier spin-photon system by implementing high values of birefringence to the cavity of 850 nm GaAs/GaAlAs VCSELs. Here, we show experimental results for the intensity and polarization dynamics in highly birefringent spin-VCSELs as a function of bias current, birefringence, and temperature and demonstrate the capability of spin-VCSELs for ultralow energy consumption and high temperature stability. Furthermore, we present first results on polarization dynamics in 1.3 μm VCSELs for potential long-range communication systems and discuss novel concepts for future integrated and electrically pumped devices. © 2020 SPIE.
    view abstract10.1117/12.2567628
  • Investigation of the polarization state in spin-VCSELs with thermally tuned birefringence
    Jung, N. and Lindemann, M. and Stadler, P. and Pusch, T. and Michalzik, R. and Hofmann, M.R. and Gerhardt, N.C.
    Proceedings of SPIE - The International Society for Optical Engineering 11356 (2020)
    Vertical-cavity surface-emitting lasers (VCSELs) are commonly used in optical data communication mainly for short-haul transmissions in data centers. Spin-VCSELs can be a promising solution in order to overcome the bandwidth limitations of conventional VCSELs by utilizing the spin and polarization instead of current and intensity. Recently, their polarization dynamics have been enhanced to resonance frequencies of more than 200 GHz by implementing a large amount of birefringence into the laser cavity. For future applications onchip solutions to control the birefringence are preferred. For this purpose, a keyhole-shaped mesa-structure on standard wafer material for an 850nm oxide-confined AlGaAs-VCSEL is used. A variable heating current is driven into the semiconductor ridge connected to the mesa at a constant pump current. This creates an asymmetrical heat gradient. Here we investigate the polarization behaviour in a spin-VCSEL with thermally induced birefringence. We analyze the hysteresis in the heating and pump current of the sample to identify optimized working points near the polarization switching points. © 2020 SPIE.
    view abstract10.1117/12.2555395
  • Manipulation of birefringence in spin-VCSELs
    Pusch, T. and Lindemann, M. and Jung, N. and Gerhardt, N.C. and Hofmann, M.R. and Michalzik, R.
    Proceedings of SPIE - The International Society for Optical Engineering 11470 (2020)
    Data centers play an important role in the ongoing demand for higher data rates. Here intensity-modulated vertical-cavity surface-emitting lasers (VCSELs) are the emitters of choice. The intensity dynamics resonance is currently limited to around 30 GHz. Using the much faster polarization dynamics in VCSELs can be a promising alternative. The polarization dynamics resonance is mainly determined by the frequency difference of the two orthogonal linearly polarized modes, which is defined by the birefringence. We have experimentally investigated mechanisms for birefringence manipulation from large-scale down to on-chip solutions. Polarization oscillations with frequencies in excess of 200 GHz have been observed. © 2020 SPIE.
    view abstract10.1117/12.2567336
  • Mode-locked diode laser-based two-photon polymerisation
    Surkamp, N. and Zyla, G. and Gurevich, E.L. and Klehr, A. and Knigge, A. and Ostendorf, A. and Hofmann, M.R.
    Electronics Letters 56 (2020)
    In this Letter, the authors present the construction of three-dimensional microstructures by two-photon polymerisation induced by ultrashort pulses of a mode-locked diode laser. The ultrafast light source is based on a diode laser with segmented metallisation to realise a waveguide integrated saturable absorber. It is subsequently amplified and compressed resulting in ultrashort laser pulses of 440 fs length and average output power of 160 mW at a fundamental repetition rate of 383.1 MHz. These pulses are coupled into a customised two-photon polymerisation setup. A series of suspended lines were fabricated between support cuboids for testing the process behaviour. A 3D structure with complex features was polymerised to demonstrate the high potential for mode-locked diode lasers in the field of direct laser writing. © The Institution of Engineering and Technology 2020.
    view abstract10.1049/el.2019.2385
  • Mode-locked diode lasers as sources for two-photon polymerization
    Surkamp, N. and Zyla, G. and Gurevich, E.L. and Esen, C. and Klehr, A. and Knigge, A. and Ostendorf, A. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 11349 (2020)
    In this study, we use a hybrid mode-locked external cavity diode laser with subsequent amplification and pulse compression. The system provides laser pulses of 440 fs width (assuming a sech pulse shape) and 160 mW average output power at a repetition rate of 383.1 MHz. The laser oscillator consists of a double quantum well laser diode with a gain segment of 1080 μm length and an absorber element of 80 μm lengths. The chip's back facet is covered with a high reflective coating, the front facet with an anti-reflective coating. The resonator itself is operated in a collimated geometry and folded by two dielectric mirrors. The used output coupler provides a transmission of 20 percent, which is coupled into a tapered amplifier. Two Faraday isolators are used to decouple the laser and the amplifier from any back reflections. Subsequently, the pulses are compressed using a single pass Martinez type pulse compressor. Experiments on Two-Photon Polymerization were conducted on a conventional setup consisting of a 2D galvo-scanner system with an attached microscope objective. The oil immersion objective (NA =1.4) focusses the light pulses through a cover glass into a droplet of the photosensitive material. Process monitoring can be achieved by observing the image on a camera placed behind a semi-transparent mirror in front of the galvo-scanner. Using this experimental setup, test structures that consist of free-hanging lines supported by cuboids were produced. In addition, a procedure for automated linewidth measurements is outlined and used for analyzation of the generated structures. This work shows that mode-locked diode lasers can be used for the fabrication of microstructures by Two-Photon Polymerization. Typically used Ti:Sapphire or fiber lasers can be replaced by mode-locked diode lasers for Two-Photon- Polymerization. This allows for much cheaper Two-Photon-Polymerization systems and therefore, may open this field for more application-based research groups. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2555890
  • Self-mode-locking and chirp compensation in an external cavity diode laser at 1550 nm
    Alloush, A. and Bassal, A. and Brenner, C. and Fortin, C. and Mekhazni, K. and Gamarra, P. and Calò, C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 11356 (2020)
    In this work, self-mode-locking of 100 GHz mode-locked pulses from a single-section InP quantum-dash-based laser chip whilst employed in external cavity geometry at 1550nm is investigated. The chip is operated at a forward current marginally above its monolithic operation's lasing threshold. Ultrashort pulses with 1 ps pulse- width were obtained by compensating the chirp by a single mode fiber (SMF). © 2020 SPIE.
    view abstract10.1117/12.2555946
  • Spin-lasers: spintronics beyond magnetoresistance
    Žutić, I. and Xu, G. and Lindemann, M. and Faria Junior, P.E. and Lee, J. and Labinac, V. and Stojšić, K. and Sipahi, G.M. and Hofmann, M.R. and Gerhardt, N.C.
    Solid State Communications 316-317 (2020)
    Introducing spin-polarized carriers in semiconductor lasers reveals an alternative path to realize room-temperature spintronic applications, beyond the usual magnetoresistive effects. Through carrier recombination, the angular momentum of the spin-polarized carriers is transferred to photons, thus leading to the circularly polarized emitted light. The intuition for the operation of such spin-lasers can be obtained from simple bucket and harmonic oscillator models, elucidating their steady-state and dynamic response, respectively. These lasers extend the functionalities of spintronic devices and exceed the performance of conventional (spin-unpolarized) lasers, including an order of magnitude faster modulation frequency. Surprisingly, this ultrafast operation relies on a short carrier spin relaxation time and a large anisotropy of the refractive index, both viewed as detrimental in spintronics and conventional lasers. Spin-lasers provide a platform to test novel concepts in spin devices and offer progress connected to the advances in more traditional areas of spintronics. © 2020
    view abstract10.1016/j.ssc.2020.113949
  • Transversally travelling ultrasound for light guiding deep into scattering media
    Cherkashin, M.N. and Brenner, C. and Schmitz, G. and Hofmann, M.R.
    Communications Physics 3 (2020)
    The application of optical methods for tissue diagnosis, activation, and treatment suffers dramatically from the low accessible depths due to strong light scattering in tissues. Here we demonstrate a method to address this issue by utilizing transient ultrasound waves, travelling transversally to the light propagation direction, to guide light into deeper tissue regions. We study the formation of the ultrasound-induced refractive index structures and waveguides using simple ultrasound field configurations and analyze their effects on the propagation of short light pulses. As a proof of concept, we demonstrate using 5 ns pulses of 532 nm light and Intralipid-20%-based phantoms with μs′ up to 4.5 cm−1 a waveguide supported light intensity increase up to the depths of at least 90 mean free paths. © 2020, The Author(s).
    view abstract10.1038/s42005-020-00443-w
  • Two-photon polymerization with diode lasers emitting ultrashort pulses with high repetition rate
    Zyla, G. and Surkamp, N. and Gurevich, E.L. and Esen, C. and Klehr, A. and Knigge, A. and Hofmann, M.R. and Ostendorf, A.
    Optics Letters 45 (2020)
    In this Letter, we investigate the resolution of two-photon polymerization (2PP) with an amplified mode-locked external cavity diode laser with adjustable pulse length and a high repetition rate. The experimental results are analyzed with a newly developed 2PP model. Even with low pulse peak intensity, the produced structural dimensions are comparable to those generated by traditional 2PP laser sources. Thus, we show that a compact monolithic picosecond laser diode without amplification and with a repetition rate in the GHz regime can also be applied for 2PP. These results show the high application potential of compact mode-locked diode lasers for low-cost and compact 2PP systems. © 2020 Optical Society of America
    view abstract10.1364/OL.401738
  • Analysis of in vivo optical coherence tomography images of human peripheral nerves using texture analysis
    Lenz, M. and Möller, J. and Miemiec, R. and Carolus, A. and Brenke, C. and Schmieder, K. and Welp, H. and Gerhardt, N.C. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 11073 (2019)
    In this work we demonstrate optical coherence tomography (OCT) to support peripheral nerve surgery which provides superior resolution than intraoperative sonography. For that purpose, a handheld probe covered with a sterile foil was utilized. In total 34 patients were measured with the OCT device during surgery. In this study eight different peripheral nerves have been accessed at various positions of the human body. It could be observed that near-surface nerves provided the best contrast. In the acquired images it was possible to identify single fascicles within the volumetric images of the peripheral nerves. However, due to the use of a sterile foil, the sensitivity was decreased compared to ex vivo results. In order to highlight different areas within a nerve, e.g. the perineurium, texture analysis was performed. Thus, chronic nerve compression with an enhanced amount of connective tissue can be precisely located. We are confident that in the future this methodology can provide high resolution images of a peripheral nerve's microstructure in real time, which leads to multiple possible applications, e.g. the revealing of scar tissue or the direct optical biopsy without the need of a pathological analysis. © 2019 SPIE.
    view abstract10.1117/12.2526755
  • Birefringent surface gratings for ultrafast spin-VCSELs
    Pusch, T. and Debernardi, P. and Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R. and Michalzik, R.
    2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 (2019)
    The data rates required for optical interconnection in datacenters continue to increase. On the other hand, intensity-modulated vertical-cavity surface-emitting lasers (VCSELs), which are the preferred light source for this task, have a limited bandwidth of at present about 35 GHz [1]. This limit can be overcome with spin-VCSELs that show much faster polarisation and modulation dynamics [2]. In these devices, spin-polarised electrons are injected, by which the degree of circular polarisation of the emitted light can be changed and thus information be encoded. The characteristic eigenfrequency of the laser, corresponding to the resonance frequency in conventional laser diodes, is determined by the birefringence splitting B, namely the frequency difference between the two fundamental polarisation modes of a single-mode VCSEL. Recently a polarisation oscillation frequency of 212 GHz was demonstrated [2]. In that laser the birefringence was induced by bending the sample and employing the elasto-optic effect. Alternatively we have shown that the cavity birefringence can be tuned by mounting VCSELs on piezoelectric substrates or by asymmetric heating, which, however, leads to much lower B. Up to now no truly integrated B-inducing technique existed for a spin-VCSEL that would allow to maintain great similarity to the successful high-efficiency device platform of present oxide-confined VCSELs. In this work, for the first time, we present a VCSEL in which high B of almost 100 GHz stems from a tailored integrated surface grating. © 2019 IEEE.
    view abstract10.1109/CLEOE-EQEC.2019.8873122
  • Continuous wave thz source based on an electrically tunable monolithic two-color semiconductor diode laser
    Gwaro, J.O. and Brenner, C. and Sumpf, B. and Klehr, A. and Fricke, J. and Hofmann, M.R.
    2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 (2019)
    We present a compact coherent CW THz source, based on optical heterodyning of a monolithically integrated two-color distributed Bragg reflector (DBR) semiconductor laser emitting at 785 nm with fiber coupled ion-implanted gallium Arsenide (GaAs) photoconductive antennas. © 2019 IEEE.
    view abstract10.1109/CLEOE-EQEC.2019.8872901
  • Contrast enhancement for topographic imaging in confocal laser scanning microscopy
    Schnitzler, L. and Finkeldey, M. and Hofmann, M.R. and Gerhardt, N.C.
    Applied Sciences (Switzerland) 9 (2019)
    The influence of the axial pinhole position in a confocal microscope in terms of the contrast of the image is analyzed. The pinhole displacement method is introduced which allows to increase the contrast for topographic imaging. To demonstrate this approach, the simulated data of a confocal setup as well as experimental data is shown. The simulated data is verified experimentally by a custom stage scanning reflective microscopy setup using a semiconductor test target with low contrast structures of sizes between 200nm and 500 nm. With the introduced technique, we are able to achieve a contrast enhancement of up to 80% without loosing diffraction limited resolution. We do not add additional components to the setup, thus our concept is applicable for all types of confocal microscopes. Furthermore, we show the application of the contrast enhancement in imaging integrated circuits. © 2019 by the authors.
    view abstract10.3390/app9153086
  • Correction: Luminescent Nd2S3 thin films: A new chemical vapour deposition route towards rare-earth sulphides (Dalton Transactions (2019) 48 (2926-2938) DOI: 10.1039/C8DT04317E)
    Cwik, S. and Beer, S.M.J. and Schmidt, M. and Gerhardt, N.C. and De Los Arcos, T. and Rogalla, D. and Weßing, J. and Giner, I. and Hofmann, M. and Grundmeier, G. and Wieck, A.D. and Devi, A.
    Dalton Transactions 48 (2019)
    The authors regret an incorrect assignment of proton signals in the 1H NMR spectrum of complex 2 [Nd(dpamd)3] (right spectrum in Fig. 2) which was published in their original submission. The correct NMR assignment is explained in the following text and depicted in Fig. 2: (Figure Presented) In the 1H-NMR spectrum (Fig. 2) of the presented complex 2 [Nd(dpamd)3] the proton signal for the methyl moieties of the iPr-groups (a) is detected at -4.5 ppm exhibiting a total integral of 36 H. The lone protons of the CH moiety within the iPr group are shifted to the low-field regime at 20.9 ppm due to their close spatial distance to the Nd metal center. Furthermore, the integral of 6 H is in accordance to the suggested structure. This proton signal is assigned as b. The signal at 12.76 ppm (total integral of 9 H) can be unambiguously assigned to the CH3 moiety located at the NCN backbone of the ligand. The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c9dt90217a
  • Digital holographic microscopy for sub-µm scale high aspect ratio structures in transparent materials
    Besaga, V.R. and Saetchnikov, A.V. and Gerhardt, N.C. and Ostendorf, A. and Hofmann, M.R.
    Optics and Lasers in Engineering 121 (2019)
    This paper discusses a digital holographic microscopy approach for comprehensive and non-destructive inspection of spatially confined structures, which are characterized by the high aspect ratio of their lateral dimensions ( ∼ 1:30) and are fabricated out of transparent materials. Two-photon polymerization is chosen as a high-resolution lithographic technology for manufacturing of corresponding structures out of positive photoresin. The proposed holographic inspection method does not require any numerical methods for aberration correction and thus demonstrates rapid phase map evaluation ( ∼ 250 ms per cycle). Applicability and performance of the method has been successfully tested on a set of samples modelled to resemble step-index structures similar to the lab-on-chip basic elements, e.g. waveguides. In terms of lateral resolution a diffraction-limited operation of the proposed imaging system is ensured while vertical accuracy of 1.61 nm has been demonstrated. © 2019 Elsevier Ltd
    view abstract10.1016/j.optlaseng.2019.05.007
  • Golomb ruler based discrete frequency multimodal continuous wave THz spectroscopy system
    Gerling, A. and Becke, L. and Tonder, S. and Hofmann, M.R. and Balzer, J.C. and Brenner, C.
    2019 2nd International Workshop on Mobile Terahertz Systems, IWMTS 2019 (2019)
    Photonic Terahertz systems covering a broad frequency range are a versatile tool for non-destructive testing and thickness determination. The advantage of these systems to measure amplitude and phase is usually accompanied by a certain complexity of the laser source or the THz parts of the setup. A cost-effective THz system which enables a fast scan over a broad frequency range would be beneficial for many applications in the THz regime. We build a system with standard telecommunication components to accomplish a frequency scan on discrete frequencies over a broad frequency range, which is usually not feasible with non-tunable monolithic diode lasers. This paper describes the main considerations for this setup and shows a proof of principle operation, as well as the requirements for upscaling the system to wider frequencies ranges and more emission lines. It is shown that with 5 diode lasers we were able to generate 10 discrete emission lines in the frequency range from 50GHz to 1170GHz. The system has the potential to accomplish high scanning speeds with no tunable elements. © 2019 IEEE
    view abstract10.1109/IWMTS.2019.8823651
  • Luminescent Nd 2 S 3 thin films: A new chemical vapour deposition route towards rare-earth sulphides
    Cwik, S. and Beer, S.M.J. and Schmidt, M. and Gerhardt, N.C. and De Los Arcos, T. and Rogalla, D. and Weßing, J. and Giner, I. and Hofmann, M. and Grundmeier, G. and Wieck, A.D. and Devi, A.
    Dalton Transactions 48 (2019)
    Neodymium sulphide (Nd 2 S 3 ) belongs to the exciting class of rare earth sulphides (RES) and is projected to have a serious potential in a wide spectrum of application either in pure form or as dopant. We demonstrate a facile and first growth of Nd 2 S 3 thin films via metal-organic chemical vapour deposition (MOCVD) at moderate process conditions using two new Nd precursors, namely tris(N,N′-diisopropyl-2-dimethylamido-guanidinato)Nd(iii) and tris(N,N′-diisopropyl-acetamidinato)Nd(iii). The promising thermal properties and suitable reactivity of both Nd precursors towards elemental sulphur enabled the formation of high purity γ-Nd 2 S 3 . While the process temperature for film growth ranged from 400 °C to 600 °C, the films were crystalline above 500 °C. We also demonstrate that the as-deposited γ-Nd 2 S 3 are luminescent, with the optical bandgap ranging from 2.3 eV to 2.5 eV. The process circumvents post-deposition treatments such as sulfurisation to fabricate the desired Nd 2 S 3 , which paves the way for large scale synthesis and also opens up new avenues for exploring the potential of this class of materials with properties for functional applications. © 2019 The Royal Society of Chemistry.
    view abstract10.1039/c8dt04317e
  • Modelocked diode lasers for THz asynchronous optical sampling
    Surkamp, N. and Döpke, B. and Brenner, C. and Orend, K. and Baer, C. and Musch, T. and Prziwarka, T. and Klehr, A. and Knigge, A. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10917 (2019)
    Even though Terahertz Time Domain Spectroscopy (TDS) setups have been available for decades in laboratories worldwide and possible applications have been shown in many research papers, a breakthrough of applications actively used on an industrial scale is yet to come. Apart from the THz sources, such as photoconductive antennas (PCAs), a conventional TDS system consists of an ultrafast laser source, a mechanical delay line used for the sampling, and a data acquisition system. While the femtosecond laser makes up for the majority of the system cost, the mechanical delay is accountable for the long acquisition time. In order to push pulsed THz systems one step further towards industrial applications, this work addresses both: the optical source and the sampling mechanism. To overcome the necessity of the delay line an asynchronous optical sampling (ASOPS) approach is chosen. Here, the sampling mechanism is obtained by operating two femtosecond lasers with slightly different repetition rates Δf resulting in an inherent sampling of the THz transient. While this was shown with ultrafast solid state lasers such as Ti:Sapphire or fiber lasers, we use edge emitting mode locked semiconductor quantum well lasers operating in the 830nm wavelength regime. In a first step, two laser diodes operated in compact external cavity configurations are hybridly modelocked at repetition rates around 390MHz with a RF synthesizer each in order to obtain a stable pulse scanning. In a second step, we evaluate hybridly modelocked monolithic edge emitting laser diodes at 12:8 GHz for THz TDS ASOPS. © 2019 SPIE.
    view abstract10.1117/12.2508396
  • Monitoring of photochemically induced changes in phase-modulating samples with digital holographic microscopy
    Besaga, V.R. and Saetchnikov, A.V. and Gerhardt, N.C. and Ostendorf, A. and Hofmann, M.R.
    Applied Optics 58 (2019)
    This paper analyzes the performance of single-shot digital holographic microscopy for rapid characterization of static step-index structures in transparent polymer materials and for online monitoring of the photoinduced polymerization dynamics. The experiments are performed with a modified Mach–Zehnder transmission digital holographic microscope of high stability (phase accuracy of 0.69°) and of high magnification (of ≈90×). Use of near-infrared illumination allows both nondestructive examination of the manufactured samples and monitoring of optically induced processes in a photosensitive material concurrently with its excitation. The accuracy of the method for a precise sample’s topography evaluation is studied on an example of microchannel sets fabricated via two-photon polymerization and is supported by reference measurements with an atomic force microscope. The applicability of the approach for dynamic measurements is proved via online monitoring of the refractive index evolution in a photoresin layer illuminated with a focused laser beam at 405 nm. High correlation between the experimental results and a kinetics model for the photopolymerization process is achieved. © 2019 Optical Society of America
    view abstract10.1364/AO.58.000G41
  • Monolithic Mode-Locked Laser Diode for THz Communication
    Gerling, A. and Tybussek, K. and Gaimard, Q. and Merghem, K. and Ramdane, A. and Hofmann, M.R. and Brenner, C. and Balzer, J.C.
    International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2019-September (2019)
    A need for higher data rates in wireless applications necessitates fast technological development to meet this. One option for advancement is the use of higher carrier frequencies as this technique can incorporate previously established methods of data transfer such as modulation techniques. We show that generation and detection of multiple carrier frequencies in the THz range is possible using standard telecom equipment and a single mode-locked laser diode. © 2019 IEEE.
    view abstract10.1109/IRMMW-THz.2019.8874373
  • Near real-time digital holographic imaging on conventional central processing unit
    Besaga, V.R. and Saetchnikov, A.V. and Gerhardt, N.C. and Ostendorf, A. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 11056 (2019)
    In this paper we discuss a near real time digital holographic imaging algorithm achieving 4 fps operation speed on a common central processing unit. The hologram recording is performed in the off-axis geometry in the transmission mode. The algorithm follows a standard angular spectrum method routine and utilizes experimental calibration of the optical instrument for aberration correction. The main limiting factor is related to the size of the initial hologram and its Fourier transform (57% of the total execution duration). The performance of the approach is tested on different transparent and semi-transparent samples for reconstruction of sample topography and object in-depth allocation. © SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2526112
  • Three-dimensional particle localization with common-path digital holographic microscopy
    Neutsch, K. and GöÂ-Ring, L. and Tranelis, M.J. and Hofmann, M.R. and Gerhardt, N.C.
    Proceedings of SPIE - The International Society for Optical Engineering 10944 (2019)
    In this paper, we present a common-path digital holographic microscope for 3D particle localization. Due to the common-path geometry, our setup is self-referencing, which reduces mechanical sensitivity to environmental disturbances. Thus, the system provides high stability and phase sensitivity down to 10nm. For system testing, different particles of varying size and material are distributed in a phantom material. Holograms are recorded by a camera and reconstructed with the angular spectrum method, phase unwrapping and Zernike polynomials. Numerical propagation of a single-shot hologram to multiple focal planes allow for optimal focusing on the sample. The resulting images show the different positions and distribution of particles. © 2019 SPIE.
    view abstract10.1117/12.2509448
  • Transducer-matched multipulse excitation for signal-to-noise ratio improvement in diode laser-based photoacoustic systems
    Cherkashin, M.N. and Brenner, C. and Hofmann, M.R.
    Journal of Biomedical Optics 24 (2019)
    We analyze transducer-matched multipulse excitation as a method for improving of the signal-to-noise ratio (SNR) for diode laser-based photoacoustic systems. We discuss the principle of the technique, its advantages, and potential drawbacks and perform measurements to analyze the obtainable SNR increase. We show in experiment and computationally that a lower boundary estimate of 1.2 to 1.8 fold SNR improvement can be provided using transducer-matched pulse bursts, depending on the transducer and particular arrangement. Finally, we analyze implications that the transducer resonance effects may have on the recently introduced advanced photoacoustic techniques. The findings are of immediate interest to modalities utilizing dense pulse sequences and systems possessing limited pulse energy. In particular, transducer-matched multipulse excitation may be beneficial for diode-based photoacoustic systems operated with transducers in the range of 1 to 5 MHz since the required hardware is readily available. © The Authors. Published by SPIE under a Creative Commons Attribution 4.0 Unported License.
    view abstract10.1117/1.JBO.24.4.046001
  • Ultrafast spin-lasers
    Lindemann, M. and Xu, G. and Pusch, T. and Michalzik, R. and Hofmann, M.R. and Žutić, I. and Gerhardt, N.C.
    Nature 568 (2019)
    Lasers have both ubiquitous applications and roles as model systems in which non-equilibrium and cooperative phenomena can be elucidated 1 . The introduction of novel concepts in laser operation thus has potential to lead to both new applications and fundamental insights 2 . Spintronics 3 , in which both the spin and the charge of the electron are used, has led to the development of spin-lasers, in which charge-carrier spin and photon spin are exploited. Here we show experimentally that the coupling between carrier spin and light polarization in common semiconductor lasers can enable room-temperature modulation frequencies above 200 gigahertz, exceeding by nearly an order of magnitude the best conventional semiconductor lasers. Surprisingly, this ultrafast operation of the resultant spin-laser relies on a short carrier spin relaxation time and a large anisotropy of the refractive index, both of which are commonly viewed as detrimental in spintronics 3 and conventional lasers 4 . Our results overcome the key speed limitations of conventional directly modulated lasers and offer a prospect for the next generation of low-energy ultrafast optical communication. © 2019, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstract10.1038/s41586-019-1073-y
  • Ultrafast spin-lasers for optical data communication
    Lindemann, M. and Xu, G. and Pusch, T. and Michalzik, R. and Hofmann, M.R. and Zutic, I. and Gerhardt, N.C.
    2019 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2019 (2019)
    An increasing amount of high-bit rate applications such as video streaming and cloud computing more and more leads to a bandwidth bottleneck limitation in the world-wide telecommunication infrastructure [1]. Inside server centers, vertical-cavity surface-emitting lasers (VCSELs) are commonly used for short-haul optical data transmission. A maximum intensity modulation bandwidth of around 35 GHz (e.g. [2]) is currently achievable. However, reviewing the VCSEL development during the past years, the demand on bandwidth increases more rapidly than the available modulation bandwidth. Thus, alternative concepts are required. © 2019 IEEE.
    view abstract10.1109/CLEOE-EQEC.2019.8873073
  • Vertical-cavity surface-emitting laser with integrated surface grating for high birefringence splitting
    Pusch, T. and Debernardi, P. and Lindemann, M. and Erb, F. and Gerhardt, N.C. and Hofmann, M.R. and Michalzik, R.
    Electronics Letters 55 (2019)
    Increasing the birefringence splitting in single-mode vertical-cavity surface-emitting lasers (VCSELs) enables high-speed polarisation dynamics which can be the basis to overcome the current bandwidth limitations in short-haul data transmission. The authors observe large birefringence splittings of up to 98 GHz in an oxide-confined AlGaAs-based VCSEL with a tailored integrated surface grating. Since surface gratings are routinely used in VCSEL production, there is a great potential of this technique to realise spin-VCSELs for ultrafast optical communication. © The Institution of Engineering and Technology.
    view abstract10.1049/el.2019.1441
  • A direct-view customer-oriented digital holographic camera
    Besaga, V.R. and Gerhardt, N.C. and Maksimyak, P.P. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10612 (2018)
    In this paper, we propose a direct-view digital holographic camera system consisting mostly of customer-oriented components. The camera system is based on standard photographic units such as camera sensor and objective and is adapted to operate under off-axis external white-light illumination. The common-path geometry of the holographic module of the system ensures direct-view operation. The system can operate in both self-reference and self-interference modes. As a proof of system operability, we present reconstructed amplitude and phase information of a test sample. © 2018 SPIE.
    view abstract10.1117/12.2304923
  • Automated differentiation between meningioma and healthy brain tissue based on optical coherence tomography ex vivo images using texture features
    Lenz, M. and Krug, R. and Dillmann, C. and Stroop, R. and Gerhardt, N.C. and Welp, H. and Schmieder, K. and Hofmann, M.R.
    Journal of Biomedical Optics 23 (2018)
    Brain tissue analysis is highly desired in neurosurgery, such as tumor resection. To guarantee best life quality afterward, exact navigation within the brain during the surgery is essential. So far, no method has been established that perfectly fulfills this need. Optical coherence tomography (OCT) is a promising three-dimensional imaging tool to support neurosurgical resections. We perform a preliminary study toward in vivo brain tumor removal assistance by investigating meningioma, healthy white, and healthy gray matter. For that purpose, we utilized a commercially available OCT device (Thorlabs Callisto) and measured eight samples of meningioma, three samples of healthy white, and two samples of healthy gray matter ex vivo directly after removal. Structural variations of different tissue types, especially meningioma, can already be seen in the raw OCT images. Nevertheless, an automated differentiation approach is desired, so that neurosurgical guidance can be delivered without a-priori knowledge of the surgeon. Therefore, we employ different algorithms to extract texture features and apply pattern recognition methods for their classification. With these postprocessing steps, an accuracy of nearly 98% was found. © 2018 Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstract10.1117/1.JBO.23.7.071205
  • Brain tissue analysis using texture features based on optical coherence tomography images
    Lenz, M. and Krug, R. and DIllmann, C. and Gerhardt, N.C. and Welp, H. and Schmieder, K. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 10483 (2018)
    Brain tissue differentiation is highly demanded in neurosurgeries, i.e. tumor resection. Exact navigation during the surgery is essential in order to guarantee best life quality afterwards. So far, no suitable method has been found that perfectly covers this demands. With optical coherence tomography (OCT), fast three dimensional images can be obtained in vivo and contactless with a resolution of 1-15 μm. With these specifications OCT is a promising tool to support neurosurgeries. Here, we investigate ex vivo samples of meningioma, healthy white and healthy gray matter in a preliminary study towards in vivo brain tumor removal assistance. Raw OCT images already display structural variations for different tissue types, especially meningioma. But, in order to achieve neurosurgical guidance directly during resection, an automated differentiation approach is desired. For this reason, we employ different texture feature based algorithms, perform a Principal Component Analysis afterwards and then train a Support Vector Machine classifier. In the future we will try different combinations of texture features and perform in vivo measurements in order to validate our findings. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2292032
  • Classification of brain tissue with optical coherence tomography by employing texture analysis
    Lenz, M. and Krug, R. and Dillmann, C. and Gerhardt, N.C. and Welp, H. and Schmieder, K. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10679 (2018)
    Biomedical tissue classification is of great interest in many fields, e.g. for finding a clear boundary for cancer resections. At the moment, no sufficient tool has been found to fulfill the needs for intraoperative surgical guidance. For a precise tumor removal, the resolution has to be as high as possible and the delivered information should be in real time. Otherwise intraoperative guidance cannot be done accurately. Optical coherence tomography (OCT) has already demonstrated its benefits in ophthalmology, dermatology and endoscopy. Providing μm resolution for a penetration depth of 1-2 mm at acquisition rates in the MHz regime, OCT is a perfect tool for contactless investigations during surgery. Additional benefit can be provided if the obtained images are analyzed and the tissue type is immediately classified. Usually, the histopathological analysis of ex vivo samples directly after removal conforms the tissue classification. A common practice for that is the histopathological analysis, where the samples are embedded in paraffin, stained with, e.g. hematoxylin and eosin and then, classified by an experienced pathologist, who analyzes tissue slices of approximately 10 μm thickness. By employing OCT for classification, an entire three-dimensional image can be classified without further preparation of the tissue. Here we present a texture feature based approach by utilizing local binary patterns, run length analysis, Haralicks texture features and Laws texture energy measures. After applying all these texture features, a principal component analysis (PCA) was performed, which decreased the dimensionality of the data set. This step was necessary in order to enhance the performance of the employed support vector machines (SVM) classifier. To find the best parameters for the kernel, a grid search and a 10 fold cross validation were done. As a first step, the texture analysis based post processing approaches were applied on 13 ex vivo brain tissue samples, which were diagnosed as meningioma (8), healthy white tissue (3) and healthy gray tissue (2). The samples were imaged with a commercial OCT system (Thorlabs Callisto). On the raw OCT images, some structural differences between healthy tissue and meningioma may already be recognized. However, an automated approach that does not require interpretation of the result, would certainly help the surgeon during surgery. At the end, we trained a SVM classifier, which was able to differentiate between healthy tissue and meningioma, with an accuracy of nearly 98%. As the next logical step, these findings will be validated intraoperatively and further application for texture based classification will be investigated. © 2018 SPIE.
    view abstract10.1117/12.2307701
  • Compact continuous wave THz source based on monolithic two-color laser diode
    Gwaro, J.O. and Brenner, C. and Sumpf, B. and Klehr, A. and Fricke, J. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10684 (2018)
    We report a stable and compact CW THz source, based on fiber coupled photoconducting antennas pumped with monolithically integrated dual mode distributed Bragg reflector semiconductor laser diode (DBR LD). Two DBR lasers are monolithically integrated on single substrate with a Y-shaped waveguide structure and made to emit two wavelengths simultaneously at 785nm center wavelength, with stable spectral wavelength difference of 0.6 nm. Ion implanted GaAs log spiral antennas are used to generate and detect THz radiation in homodyne set up. The detected THz frequency corresponds well to the value obtained for the optical beat frequency of the two modes. We analyze the use of this system for simple THz non-destructive testing applications like moisture measurement on leaves and different papers. The results obtained demonstrate the feasibility of a compact, simple, and cost-efficient CW THz system which could gain application in industrial non-destructive testing measurements. © 2018 SPIE.
    view abstract10.1117/12.2306866
  • Continuous Wave Multimode Amplitude THz Spectroscopy
    Gerling, A. and Dulme, S. and Schrinski, N. and Stöhr, A. and Hofmann, M.R. and Brenner, C.
    International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2018-September (2018)
    THz spectroscopy with continuous wave lasers is typically based on the frequency tuning of the lasers. This process is rather slow, and some systems require additional sophisticated postprocessing. We propose a multimode system without postprocessing requirements to reduce measurement time. For this, we use a modulation scheme with multiple laser sources to enable snapshot amplitude THz spectroscopy. As the detection is performed with a Schottky barrier diode the scheme is very fast but discards phase information. This approach makes amplitude THz spectroscopy possible without frequency tuning or any moving mechanical parts. © 2018 IEEE.
    view abstract10.1109/IRMMW-THz.2018.8510486
  • Demonstrating ultrafast polarization dynamics in spin-VCSELs
    Lindemann, M. and Pusch, T. and Michalzik, R. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10552 (2018)
    Vertical-cavity surface-emitting lasers (VCSELs) are used for short-haul optical data transmission with increasing bit rates. The optimization involves both enhanced device designs and the use of higher-order modulation formats. In order to improve the modulation bandwidth substantially, the presented work employs spin-pumped VCSELs (spin-VCSELs) and their polarization dynamics instead of relying on intensity-modulated devices. In spin-VCSELs, the polarization state of the emitted light is controllable via spin injection. By optical spin pumping a single-mode VCSEL is forced to emit light composed of both orthogonal linearly polarized fundamental modes. The frequencies of these two modes differ slightly by a value determined by the cavity birefringence. As a result, the circular polarization degree oscillates with their beat frequency, i.e., with the birefringence-induced mode splitting. We used this phenomenon to show so-called polarization oscillations, which are generated by pulsed spin injection. Their frequency represents the polarization dynamics resonance frequency and can be tuned over a wide range via the birefringence, nearly independent from any other laser parameter. In previous work we demonstrated a maximum birefringence-induced mode splitting of more than 250 GHz. In this work, compared to previous publications, we show an almost doubled polarization oscillation frequency of more than 80 GHz. Furthermore, we discuss concepts to achieve even higher values far above 100 GHz. © 2018 SPIE.
    view abstract10.1117/12.2289560
  • Digital holography for the investigation of buried structures with a common-path reflection microscope
    Göaring, L. and Finkeldey, M. and Hofmann, M.R. and Gerhardt, N.C.
    Proceedings of SPIE - The International Society for Optical Engineering 10558 (2018)
    Digital holographic microscopy (DHM) is an established technique for the investigation of biological samples and very promising for non-destructive testing. As DHM is an optical metrology technique it enables a non-contact, non-destructive and fast measurement which can be used for material characterization and quality control. DHM provides amplitude and phase information which originate from optical path differences and refractive index changes of the material, thus it is able to measure topographic structures. Especially the non-destructive inspection of buried or capped structures such as microelectromechanical systems (MEMS) is still challenging, due to the absorption in the covering layer and interference of light between the different layers. In this paper we present a common-path digital holographic setup for the investigation of reflective samples. By choosing a wavelength at which the covering layer is transparent, laser light can pass this surface. Furthermore, we employ a coherence gating effect utilizing a laser diode below threshold with reduced temporal coherence to diminish speckle patterns and to suppress interference between the layers. At the same time the spatial coherence remains high which improves the image quality. Using the angular spectrum method and a quality guided phase unwrapping algorithm, we successfully reconstruct 3D images of buried structures. In addition Zernike polynomials reduce the effect of wave distortions within the setup. Overall a lateral resolution of about 1.5 μm can be achieved. Copyright © 2018 SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2289524
  • Electrical birefringence tuning of VCSELs
    Pusch, T. and Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R. and Michalzik, R.
    Proceedings of SPIE - The International Society for Optical Engineering 10552 (2018)
    The birefringence splitting B, which is the frequency difference between the two fundamental linear polarization modes in vertical-cavity surface-emitting lasers (VCSELs), is the key parameter determining the polarization dynamics of spin-VCSELs that can be much faster than the intensity dynamics. For easy handling and control, electrical tuning of B is favored. This was realized in an integrated chip by thermally induced strain via asymmetric heating with a birefringence tuning range of 45 GHz. In this paper we present our work on VCSEL structures mounted on piezoelectric transducers for strain generation. Furthermore we show a combination of both techniques, namely VCSELs with piezo-thermal birefringence tunability. © 2018 SPIE.
    view abstract10.1117/12.2295917
  • High Speed Single Point THz Phase Measurement Based on Dual Channel Lock-in Technique
    Gerling, A. and Hofmann, M.R. and Brenner, C.
    2018 1st International Workshop on Mobile Terahertz Systems, IWMTS 2018 (2018)
    The generation of radiation in the range of hundreds of GHz with electronic systems is challenging. Using photonic systems makes it possible to generate and detect signals in the complete frequency range. To measure amplitude and phase with so-called time domain spectroscopy (TDS) systems usually requires the acquisition and Fourier analysis of complete pulse transients. Simplifying this process helps to design systems which allow for fast scanning in the THz range. We modified a commercial TDS system with a two-channel lock-in amplifier and a bandpass filter in the range of 800-900GHz to completely remove the post processing which is necessary to acquire phase and amplitude information. In this paper we present a system which reduces the acquired data to two output parameters for amplitude and phase which can be measured at the same time. This makes precise and fast measurements of phase objects at a target frequency possible. This is shown by acquiring a qualitative phase image of a technical sample. © 2018 IEEE.
    view abstract10.1109/IWMTS.2018.8454689
  • High-resolution 3D light fluence mapping for heterogeneous scattering media by localized sampling
    Cherkashin, M.N. and Brenner, C. and Hofmann, M.R.
    Applied Optics 57 (2018)
    We demonstrate an innovative concept for three-dimensional optical fluence mapping in heterogeneous highly scattering media as, e.g., biomedical tissues. We propose to use the relative light extinction analysis principle together with a miniaturized collection fiber in a direct fluence measurement setup as a method to obtain the spatially resolved light intensity distribution under transversally inhomogeneous light propagation conditions and provide local characterization of the transport medium. System performance is validated in two extreme conditions: an optically thin scattering medium and an absorption-dominated light transport. Both extremes demonstrate good agreement to theoretical expectations. Finally, we successfully prove the ability of the system to deliver high-resolution fluence maps through a model study of the light distribution induced in a scattering medium by a vertical diode laser stack with individual bars pitched only 500 μm apart. © 2018 Optical Society of America.
    view abstract10.1364/AO.57.010441
  • Mode-locked diode laser with resonant ring amplifier
    Ali Alloush, M. and Pilny, R.H. and Brenner, C. and Prziwarka, T. and Klehr, A. and Knigge, A. and Tränkle, G. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10682 (2018)
    We present a ring semiconductor amplifier system which is seeded by ultrashort pulses for additive amplification. An external cavity diode laser configuration is built to generate the ultrashort pulses based on a hybrid modelocking scheme. A monolithic multi-segment diode laser is utilized as a light source in the operating oscillator. It has the advantage that the gain and absorber are integrated on one chip. The oscillator operates at a fundamental repetition-rate of 206MHz and can be driven on various harmonics of this frequency. The generated pulses are injected into a tapered amplifier (TA) which consists of a ridge waveguide section (RWS) for coupling and a tapered section (TS) for amplification. The amplified pulses are coupled back after amplification towards the TAs RWS forming a ring resonator setup. By matching the cavity lengths of the oscillator and ring resonator, we can obtain additively amplified pulses. The emission spectrum of the chosen TA is centered around 850nm which is in the wavelength range of the oscillator. The spectrum of the additively amplified pulses is observed for different pumping parameters of the TA using an optical spectrum analyzer. Additionally, we characterized the system for the best seeding parameters by monitoring the output signal with an autocorrelator. We figured out that the best performance is achieved when the amplifier is seeded by pulses at the second harmonic of 412 MHz. When blocking the seeding pulses the amplifier operates in continuous wave (CW) regime. By comparing the obtained spectra for CW and additively amplified pulses, we conclude that the system operates with a CW background also in pulsed operation. However, from the comparison of the spectra, we estimate that the amplified pulsed power is about 120mW for a seed power of 1:1mW. Thus, the ring amplifier provides a significantly higher amplification than a single pass amplifier. In future work the CW background has to be suppressed, e.g. by synchronous modulation of the current into the amplifiers ridge waveguide section. © 2018 SPIE.
    view abstract10.1117/12.2307220
  • Near Infrared Diode Laser THz Systems
    Brenner, C. and Hu, Y. and Gwaro, J. and Surkamp, N. and Döpke, B. and Hofmann, M.R. and Kani, B. and Stöhr, A. and Sumpf, B. and Klehr, A. and Fricke, J.
    Advances in Radio Science 16 (2018)
    The generation and detection of radiation in the THz frequency range can be achieved with many different electronic and photonic concepts. Among the many different photonic THz systems the most versatile are based on diode lasers. In this paper we describe and review the different concepts and optimization ideas for diode laser based THz systems in order to achieve the best performance for different types of THz setups. © Author(s) 2018.
    view abstract10.5194/ars-16-167-2018
  • Optical metrology for the investigation of buried technical structures
    Göring, L. and Finkeldey, M. and Schellenberg, F. and Brenner, C. and Hofmann, M.R. and Gerhardt, N.C.
    Technisches Messen 85 (2018)
    In this paper, we present different optical metrology approaches for the investigation of buried technical structures. Contactless, potentially fast and non-destructive techniques such as optical beam induced current (OBIC), confocal laser scanning microscopy (CLSM) and digital holographic microscopy (DHM) are described. Their properties are illustrated by investigating the buried structures of a microcontroller. © 2017 Walter de Gruyter Berlin/Boston.
    view abstract10.1515/teme-2017-009
  • Passive, active, and hybrid mode-locking in a self-optimized ultrafast diode laser
    Alloush, M.A. and Pilny, R.H. and Brenner, C. and Klehr, A. and Knigge, A. and Tränkle, G. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10553 (2018)
    Semiconductor lasers are promising sources for generating ultrashort pulses. They are directly electrically pumped, allow for a compact design, and therefore they are cost-effective alternatives to established solid-state systems. Additionally, their emission wavelength depends on the bandgap which can be tuned by changing the semiconductor materials. Theoretically, the obtained pulse width can be few tens of femtoseconds. However, the generated pulses are typically in the range of several hundred femtoseconds only. Recently, it was shown that by implementing a spatial light modulator (SLM) for phase and amplitude control inside the resonator the optical bandwidth can be optimized. Consequently, by using an external pulse compressor shorter pulses can be obtained. We present a Fourier-Transform-External-Cavity setup which utilizes an ultrafast edge-emitting diode laser. The used InGaAsP diode is 1 mm long and emits at a center wavelength of 850 nm. We investigate the best conditions for passive, active and hybrid mode-locking operation using the method of self-adaptive pulse shaping. For passive mode-locking, the bandwidth is increased from 2.34 nm to 7.2 nm and ultrashort pulses with a pulse width of 216 fs are achieved after external pulse compression. For active and hybrid mode-locking, we also increased the bandwidth. It is increased from 0.26 nm to 5.06 nm for active mode-locking and from 3.21 nm to 8.7 nm for hybrid mode-locking. As the pulse width is strongly correlated with the bandwidth of the laser, we expect further reduction in the pulse duration by increasing the bandwidth. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2290086
  • Phase Delay of Terahertz Fabry-Perot Resonator characterized by a Photonic Two-Tone Spectroscopy System with Self-Heterodyne Receiver
    Dulme, S. and Schrinski, N. and Steeg, M. and Lu, P. and Khani, B. and Brenner, C. and Hofmann, M.R. and Stöhr, A.
    International Conference on Infrared, Millimeter, and Terahertz Waves, IRMMW-THz 2018-September (2018)
    Terahertz phase delay of high-resistive Silicon based Fabry-Perot resonators (FPR) is experimentally investigated using a photonic two-tone THz spectroscopy system with a self-heterodyne receiver. The photonic spectroscopy system consists of two free-running lasers of which one is externally modulated and an InP-based photodiode for two-tone THz signal generation. A Schottky barrier diode (SBD) is employed as self-heterodyne THz receiver. This way, spectroscopic THz phase and amplitude characterization is enabled without bulky delay lines and the phase noise of the two free running lasers is canceled out. By employing the developed photonic spectroscopy system, phase delay and transmission of a THz FPR is experimentally characterized between 270 GHz and 305 GHz. It is furthermore shown, that the measured THz phase delay difference and FSR of 23 degrees and 13 GHz, respectively, as well as the measured THz transmission coefficient agree well with an analytic model for the phase delay of a Fabry-Perot resonator. © 2018 IEEE.
    view abstract10.1109/IRMMW-THz.2018.8510021
  • Spin lasers for optical data communication
    Lindemann, M. and Pusch, T. and Michalzik, R. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10682 (2018)
    For short-haul optical interconnects, state-of-the-art technology are vertical-cavity surface-emitting lasers (VCSELs). To transmit data, direct current modulation is used. The corresponding intensity modulation resonance frequency is determined by design and material parameters of the laser and therefore practically limited to a few tens of GHz. To overcome this limitation, an alternative approach is the utilization of spin-VCSELs. In this case, the information carrier is no longer represented by the intensity, but instead by the polarization. The polarization can be controlled by the carrier spin. The birefringence in the cavity has the strongest impact on the polarization modulation resonance frequency. This can be explained by the generation of resonant polarization oscillations in the circular polarization degree in a spin-VCSEL. The circular polarization is composed of the two orthogonal linearly polarized cavity modes. The electromagnetic fields emitted from the two modes are coupled in phase by birefringence and in amplitude by dichroism. However, dependent on the birefringence in the cavity, their frequencies may differ. Spin pumping, i.e., circularly polarized optical pumping pulses, causes the fact that both modes become active. This results in an oscillation of the circular polarization degree of the emitted light, representing the polarization dynamics resonance frequency of the spin-VCSEL device. We demonstrate that the birefringence can be manipulated in actual VCSEL devices over a broad tuning range. Employing this parameter tuning, we demonstrate a polarization dynamics resonance frequency of 89 GHz, which is much faster than currently obtained intensity dynamics resonance frequencies. Not only the maximum frequency, but also the amplitude of the polarization effects should be optimized. An important factor for the amplitude damping is the dichroism, which represents the difference in the gain of the two orthogonal modes. We investigate the influence of birefringence on dichroism and the polarization oscillation amplitude. © 2018 SPIE.
    view abstract10.1117/12.2306464
  • Terahertz Time-Domain Spectroscopy by Asynchronous Sampling with Modelocked Semiconductor Lasers
    Surkamp, N. and Döpke, B. and Hu, Y. and Brenner, C. and Hofmann, M.R. and Klehr, A. and Knigge, A. and Tränkle, G.
    2018 1st International Workshop on Mobile Terahertz Systems, IWMTS 2018 (2018)
    Broadband terahertz spectroscopy measurements are commonly recorded with time domain systems that require a mechanical delay stage to obtain sampling of the terahertz transient. The principle limitation in measurement time for such detection schemes arises from the necessary stage movement to acquire the terahertz signal. The fastest systems use two ultrafast fiber or solid-state lasers for asynchronous sampling, which results in a fast system at the drawback of high cost. In our proof of principle study, we synchronize two hybridly modelocked semiconductor lasers to perform terahertz time domain spectroscopy by asynchronous sampling. The two external cavity lasers operate at repetition frequencies of 392.8 MHz with an offset of 125 Hz and emit in the wavelength range of 830 nm. We obtain Terahertz transients with spectral components of up to 250 GHz which is mainly limited by the detection system. This enables very compact and highly cost effective terahertz spectroscopy systems and may prepare the way for industrial applications. © 2018 IEEE.
    view abstract10.1109/IWMTS.2018.8454698
  • Thermally-induced birefringence in VCSELs: Approaching the limits
    Pusch, T. and Scherübl, S. and Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R. and Michalzik, R.
    Proceedings of SPIE - The International Society for Optical Engineering 10682 (2018)
    Polarization dynamics in vertical-cavity surface-emitting lasers (VCSELs) are much faster than their intensity-driven counterparts and can be a potential approach to overcome the bandwidth limitation in short-distance data transmission. The birefringence splitting B as the frequency difference between the two fundamental polarization modes is an important factor determining the polarization dynamics in spin-VCSELs. Although B can be strongly influenced by mechanical bending, for later applications an on-chip solution for birefringence tuning is favored. With an electrically driven asymmetric heating device we have demonstrated a thermally induced tuning range of ΔB = 45GHz. The maximum achievable birefringence tuning was not limited by the laser but by material parameters and the fabrication process. In this paper we present an optimized design for thermally induced birefringence tuning and additional possibilities to increase the efficiency of the mechanism. © 2018 SPIE.
    view abstract10.1117/12.2306215
  • Backside imaging of a microcontroller with common-path digital holography
    Finkeldey, M. and Göring, L. and Schellenberg, F. and Gerhardt, N.C. and Hofmann, M.
    Proceedings of SPIE - The International Society for Optical Engineering 10127 (2017)
    The investigation of integrated circuits (ICs), such as microcontrollers (MCUs) and system on a chip (SoCs) devices is a topic with growing interests. The need for fast and non-destructive imaging methods is given by the increasing importance of hardware Trojans, reverse engineering and further security related analysis of integrated cryptographic devices. In the field of side-channel attacks, for instance, the precise spot for laser fault attacks is important and could be determined by using modern high resolution microscopy methods. Digital holographic microscopy (DHM) is a promising technique to achieve high resolution phase images of surface structures. These phase images provide information about the change of the refractive index in the media and the topography. For enabling a high phase stability, we use the common-path geometry to create the interference pattern. The interference pattern, or hologram, is captured with a water cooled sCMOS camera. This provides a fast readout while maintaining a low level of noise. A challenge for these types of holograms is the interference of the reflected waves from the different interfaces inside the media. To distinguish between the phase signals from the buried layer and the surface reflection we use specific numeric filters. For demonstrating the performance of our setup we show results with devices under test (DUT), using a 1064 nm laser diode as light source. The DUTs are modern microcontrollers thinned to different levels of thickness of the Si-substrate. The effect of the numeric filter compared to unfiltered images is analyzed. © 2017 SPIE.
    view abstract10.1117/12.2250903
  • Common-path holographic objective for conventional photographic camera
    Besaga, V.R. and Gerhardt, N.C. and Maksimyak, P.P. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10376 (2017)
    In this paper, we introduce a digital holographic camera objective based on conventional customer-oriented components for off-axis external white light illumination. The interferometric module based on a modified common-path point diffraction interferometer provides a direct view of the system and admits self-reference and self-interference operation modes. The proposed system is designed for self-emitting and reflective samples. Its modular assemblage provides easy scalability and up-grade possibilities. The operability of the suggested camera system has been proven for both coherent and low-coherent broadband sources, and reconstructed amplitude and phase information of test samples under white light illumination is presented. © 2017 SPIE.
    view abstract10.1117/12.2273890
  • Depth-filtering in common-path digital holographic microscopy
    Finkeldey, M. and Göring, L. and Brenner, C. and Hofmann, M. and Gerhardt, N.C.
    Optics Express 25 (2017)
    We demonstrate a method to select different layers in a sample using a low coherent gating approach combined with a stable common-path quantitative phase imaging microscopy setup. The depth-filtering technique allows us to suppress the negative effects generated by multiple interference patterns of overlaying optical interfaces in the sample. It maintains the compact and stable common-path setup, while enabling images with a high phase sensitivity and acquisition speed. We use a holographic microscope in reflective geometry with a non-tunable low coherence light source. First results of this technique are shown by imaging the hardware layer of a standard micro-controller through its thinned substrate. © 2017 Optical Society of America.
    view abstract10.1364/OE.25.019398
  • Dynamics of the photoacoustic response of single-element PZT transducers to pulse burst excitation
    Cherkashin, M. and Brenner, C. and Putro, W.D. and Döpke, B. and Gerhardt, N.C. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 10064 (2017)
    Achieving a good signal-to-noise ratio at increased depths remains a challenge, even for photoacoustic imaging, which stimulates the search for possible contrast improvements. Both double-pulse and pulse burst excitation are shown beneficial for increasing the signal-to-noise ratio or acquiring additional information about the sample. We use the advantage of semiconductor laser diodes offering great opportunities regarding both number of pulses in the burst and inter-pulse delay times to study the dynamics of the pulse burst excitation responses of the single-element PZT transducers, looking for possibilities towards contrast improvement. We concentrate on inter-pulse delay ranges of few hundred nanoseconds and low central frequency transducers as they are mainly used for clinical applications We show that using pulse burst excitation with up to five pulses per burst and transducer-matched inter-pulse delays can increase the registered maximum amplitude, leading to signal-to-noise ratio improvement. The multi-pulse response amplitude increase amounted to 20% of the amplitude of a single-pulse response in the performed measurement. © 2017 SPIE.
    view abstract10.1117/12.2253051
  • Femtosecond semiconductor laser system with resonator-internal dispersion adaptation
    Pilny, R.H. and Döpke, B. and Balzer, J.C. and Brenner, C. and Klehr, A. and Knigge, A. and Tränkle, G. and Hofmann, M.R.
    Optics Letters 42 (2017)
    We present a femtosecond laser diode system that is capable of autonomously adjusting itself to compensate for the external dispersion in an arbitrary application. The laser system contains a spatial light modulator inside the cavity which is controlled by an evolutionary algorithm in order to allow for phase and amplitude shaping of the laser emission. The cavity-internal dispersion control is shown to be much more efficient than an external control with a pulse shaper. © 2017 Optical Society of America.
    view abstract10.1364/OL.42.001524
  • High-frequency polarization dynamics in spin-lasers: Pushing the limits
    Gerhardt, N.C. and Lindemann, M. and Pusch, T. and Michalzik, R. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10357 (2017)
    While the high-frequency performance of conventional lasers is limited by the coupled carrier-photon dynamics, spin-polarized lasers have a high potential to overcome this limitation and to push the direct modulation bandwidth beyond 100 GHz. The key is to utilize the ultrafast polarization dynamics in spin-polarized vertical cavity surface-emitting lasers (spin-VCSELs) which is decoupled from the intensity dynamics and its fundamental limitations. The polarization dynamics in such devices, characterized by the polarization oscillation resonance frequency, is mainly determined by the amount of birefringence in the cavity. Using an approach for manipulating the birefringence via mechanical strain we were able to increase the polarization dynamics to resonance frequencies of more than 40 GHz. Up to now these values are only limited by the setup to induce birefringence and do not reflect any fundamental limitations. Taking our record results for the birefringence-induced mode splitting of more than 250 GHz into account, the concept has the potential to provide polarization modulation in spin-VCSELs with modulation frequencies far beyond 100 GHz. This makes them ideal devices for next-generation fast optical interconnects. In this paper we present experimental results for ultrafast polarization dynamics up to 50 GHz and compare them to numerical simulations. © 2017 SPIE.
    view abstract10.1117/12.2272972
  • Investigations on polarization oscillation amplitudes in spin-VCSELs
    Lindemann, M. and Pusch, T. and Michalzik, R. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10122 (2017)
    Compared to conventional vertical-cavity surface-emitting lasers (VCSELs), spin-pumped VCSELs offer the possibility of polarization control and fast polarization dynamics. It has been demonstrated that oscillations in the circular polarization degree can be excited. In short, the frequency of these polarization oscillations is determined by the frequency splitting between the two orthogonal linearly polarized cavity modes and therefore by the cavity birefringence. The polarization oscillation frequency is the resonance frequency of the VCSEL's polarization dynamics and can be compared to the conventional resonance frequency for intensity modulation. We have demonstrated polarization oscillations up to 44 GHz, exceeding the direct intensity resonance frequency of the investigated devices by far. As the polarization oscillation frequency can be increased by increasing the cavity birefringence and a VCSEL cavity birefringence of more than 250 GHz has been demonstrated, using polarization dynamics is a possible way of substantially increasing the modulation speeds of VCSELs. This is for instance interesting for high-bandwith short-haul optical interconnects. The experimental results associated with the polarization oscillation effects can be simulated by the widely used spin-flip model. In this work we focus on the amplitude of the polarization oscillations. Previous publications have shown a decrease with increasing oscillation frequency. Here, we show amplitude dependencies on several system parameters like the photon and carrier lifetimes as well as pumping conditions. Based on this, we investigate how to increase the polarization oscillation amplitude, since a significant amplitude is necessary for, e.g., data transmission applications. © 2017 SPIE.
    view abstract10.1117/12.2252490
  • Lensless digital holographic microscope using in-line configuration and laser diode illumination
    Göring, L. and Finkeldey, M. and Adinda-Ougba, A. and Gerhardt, N.C. and Hofmann, M.
    Proceedings of SPIE - The International Society for Optical Engineering 10127 (2017)
    In this paper we present a lensless transmission digital holographic microscope for the investigation of transparent samples. The setup consists of a laser diode, an object positioned on a cover slip and a CMOS sensor. We use a laser diode for illumination which emits a divergent beam and acts as a point source, so that additional components such as a pinhole are not required. The laser diode is operated below the lasing threshold to decrease the coherence length and thus to reduce speckle noise. Due to the compact and small size of the setup, it requires minimized effort for applications in field operation. The lensless setup was characterized by using an USAF-target for determining the resolution of the system which is 2.2 μm. In the following, transparent or semitransparent samples are investigated. Microstructured plastic samples are placed on the specimen holder and characterized by the holographic microscope. By applying the angular spectrum method on the recorded images, we are able to reconstruct the investigated objects. The in-line geometry of the setup facilitates the simplicity of the setup but also induces optical errors, for instance twin images. Twin images superimpose with the object's signal and require additional numerical reconstruction algorithms. For reducing the effect of the twin image problem, we apply an iterative phase retrieval algorithm. In the conclusion, we discuss the resolution and quality of the recorded images and evaluate the numerical reconstruction process. © 2017 SPIE.
    view abstract10.1117/12.2250927
  • Linking transducer transfer function with multi-pulse excitation photoacoustic response
    Cherkashin, M. and Brenner, C. and Putro, W.D. and Döpke, B. and Gerhardt, N.C. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 10139 (2017)
    It has been shown recently, that varying the excitation sequence could deliver additional benefits for photoacoustic imaging, for instance, bringing additional information on the sample under study, or reducing the total acquisition time. However, for the typically used solid state laser systems, such modification requires significant increase of the systems' complexity. We are taking an advantage of high pulse repetition rates that semiconductor laser diodes could offer. That allows the usage of dense pulse bursts with varied number of pulses and inter-pulse delays in the range of the transducer waveform duration to study the effects of the overlay of the single pulse photoacoustic responses. In this study, we conduct a pump-probe experiment, using multi-pulse excitation sequences with varied inter-pulse delays while registering the acoustic response. We show that pulse burst excitation can be beneficial for increasing the registered amplitude and suitable inter-pulse delay values can be obtained from the transducer transfer function, either known or measured. Additionally, we examine the frequency content of the multi-pulse photoacoustic response and show that it is dominated by the pulse repetition rate used. We focus on low central frequency transducers as being widely used for clinical applications. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2254577
  • Monolithic vertical-cavity surface-emitting laser with thermally tunable birefringence
    Pusch, T. and La Tona, E. and Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R. and Michalzik, R.
    Applied Physics Letters 110 (2017)
    The birefringence splitting in vertical-cavity surface-emitting lasers offers an opportunity for spintronic-based high-frequency operation. By means of coupling of the carrier spin in the active region with the photons of the laser mode, the device can be excited to oscillations in the degree of circular polarization with a frequency corresponding to the birefringence splitting. On-chip frequency tunability of those oscillations is desirable for future applications. By asymmetric current-induced heating using the elasto-optic effect, we demonstrate a reversible tuning of the birefringence splitting of 45 GHz with less than 3 dB output power penalty. © 2017 Author(s).
    view abstract10.1063/1.4980025
  • Multimodal backside imaging of a microcontroller using confocal laser scanning and optical-beam-induced current imaging
    Finkeldey, M. and Göring, L. and Schellenberg, F. and Brenner, C. and Gerhardt, N.C. and Hofmann, M.
    Proceedings of SPIE - The International Society for Optical Engineering 10110 (2017)
    Microscopy imaging with a single technology is usually restricted to a single contrast mechanism. Multimodal imaging is a promising technique to improve the structural information that could be obtained about a device under test (DUT). Due to the different contrast mechanisms of laser scanning microscopy (LSM), confocal laser scanning microscopy (CLSM) and optical beam induced current microscopy (OBICM), a combination could improve the detection of structures in integrated circuits (ICs) and helps to reveal their layout. While OBIC imaging is sensitive to the changes between differently doped areas and to semiconductor-metal transitions, CLSM imaging is mostly sensitive to changes in absorption and reflection. In this work we present the implementation of OBIC imaging into a CLSM. We show first results using industry standard Atmel microcontrollers (MCUs) with a feature size of about 250nm as DUTs. Analyzing these types of microcontrollers helps to improve in the field of side-channel attacks to find hardware Trojans, possible spots for laser fault attacks and for reverse engineering. For the experimental results the DUT is placed on a custom circuit board that allows us to measure the current while imaging it in our in-house built stage scanning microscope using a near infrared (NIR) laser diode as light source. The DUT is thinned and polished, allowing backside imaging through the Si-substrate. We demonstrate the possibilities using this optical setup by evaluating OBIC, LSM and CLSM images above and below the threshold of the laser source. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.
    view abstract10.1117/12.2250912
  • Nondestructive evaluation of protective coatings for the conservation of industrial monuments
    Welp, H. and Lenz, M. and Mazzon, C. and Dillmann, C. and Gerhardt, N.C. and Prange, M. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 10331 (2017)
    For the conservation of cultural monuments standard anti-corrosion coatings are not applicable because the historical character of the objects would be lost. Alternative transparent coatings have to be evaluated and monitored nondestructively with respect to their effectiveness in protecting metal surfaces. We demonstrate that Optical Coherence Tomography (OCT) can be an alternative to the currently used method of Electrochemical Impedance Spectroscopy (EIS) for the characterization of coating defects and corrosion processes. © 2017 SPIE.
    view abstract10.1117/12.2270170
  • Optical metrology for the investigation of buried technical structures
    Göring, L. and Finkeldey, M. and Schellenberg, F. and Brenner, C. and Hofmann, M.R. and Gerhardt, N.
    Technisches Messen (2017)
    In this paper, we present different optical metrology approaches for the investigation of buried technical structures. Contactless, potentially fast and non-destructive techniques such as optical beam induced current (OBIC), confocal laser scanning microscopy (CLSM) and digital holographic microscopy (DHM) are described. Their properties are illustrated by investigating the buried structures of a microcontroller. © 2017 Walter de Gruyter Berlin/Boston.
    view abstract10.1515/teme-2017-0096
  • Spectral domain optical coherence tomography for non-destructive testing of protection coatings on metal substrates
    Lenz, M. and Mazzon, C. and Dillmann, C. and Gerhardt, N.C. and Welp, H. and Prange, M. and Hofmann, M.R.
    Applied Sciences (Switzerland) 7 (2017)
    In this paper we demonstrate that optical coherence tomography (OCT) is a powerful tool for the non-destructive investigation of transparent coatings on metal substrates. We show that OCT provides additional information which the common practice electrical impedance spectroscopy (EIS) cannot supply. First, coating layer thicknesses were measured and compared with reference measurements using a magnetic inductive (MI) measurement technique. After this validation of the OCT measurements, a customized sectioned sample was created to test the possibility to measure coating thicknesses with underlying corrosion, which cannot be analyzed accurately by MI or EIS measurements. Finally, we demonstrate the benefit of OCT on a standard sample. The OCT measurements provide the correct coating layer thickness with high lateral resolution and even enable metal and corrosion layers to be distinguished from each other. © 2017 by the authors.
    view abstract10.3390/app7040364
  • Spectroscopic optical coherence tomography for ex vivo brain tumor analysis
    Lenz, M. and Krug, R. and Dillmann, C. and Gerling, A. and Gerhardt, N.C. and Welp, H. and Schmieder, K. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 10054 (2017)
    For neurosurgeries precise tumor resection is essential for the subsequent recovery of the patients since nearby healthy tissue that may be harmed has a huge impact on the life quality after the surgery. However, so far no satisfying methodology has been established to assist the surgeon during surgery to distinguish between healthy and tumor tissue. Optical Coherence Tomography (OCT) potentially enables non-contact in vivo image acquisition at penetration depths of 1-2 mm with a resolution of approximately 1-15 μm. To analyze the potential of OCT for distinction between brain tumors and healthy tissue, we used a commercially available Thorlabs Callisto system to measure healthy tissue and meningioma samples ex vivo. All samples were measured with the OCT system and three dimensional datasets were generated. Afterwards they were sent to the pathology for staining with hematoxylin and eosin and then investigated with a bright field microscope to verify the tissue type. This is the actual gold standard for ex vivo analysis. The images taken by the OCT system exhibit variations in the structure for different tissue types, but these variations may not be objectively evaluated from raw OCT images. Since an automated distinction between tumor and healthy tissue would be highly desirable to guide the surgeon, we applied Spectroscopic Optical Coherence Tomography to further enhance the differences between the tissue types. Pattern recognition and machine learning algorithms were applied to classify the derived spectroscopic information. Finally, the classification results are analyzed in comparison to the histological analysis of the samples. © 2017 SPIE.
    view abstract10.1117/12.2252141
  • Terahertz frequency generation with monolithically integrated dual wavelength distributed Bragg reflector semiconductor laser diode
    Gwaro, J. O. and Brenner, C. and Sumpf, B. and Klehr, A. and Fricke, J. and Hofmann, M. R.
    Iet Optoelectronics 11 (2017)
    Optoelectronic terahertz (THz) generation techniques have helped to narrow the THz gap and have opened up a wealth of new applications for THz technology. However, the development of THz systems into mass market is a major technical challenge, which is attributed to high cost of THz hardware components including sources and detectors. Here, the authors report THz generation from a distributed Bragg reflector (DBR) semiconductor laser diode together with fibre coupled photoconducting antennas. Two fibre coupled ion-implanted gallium arsenide photoconducting antennas were employed to generate and detect THz radiation. Two DBR lasers connected with a Y-shaped waveguide structure were monolithically integrated and used to simultaneously emit two wavelengths in the range of 785 nm. These lasers were employed as pumping source for the photomixers. An optical beat frequency of 286 GHz of the dual wavelengths was obtained from optical characterisation. A corresponding THz frequency was confirmed via photomixing in a homodyne set up. By variation of the operation parameters of the laser, the difference frequency was tuned in the range between 286 GHz to 320 GHz. In summary, they report the implementation of a compact and cost effective fiber coupled Terahertz source based on a monolithically integrated dual wavelength DBR semiconductor laser diode.
    view abstract10.1049/iet-opt.2016.0054
  • Ultrafast diode laser with self-adapting pulse-shaping in passive, active and hybrid mode-locking operation
    Pilny, R.H. and Döpke, B. and Brenner, C. and Klehr, A. and Knigge, A. and Tränkle, G. and Hofmann, M.R.
    Optics InfoBase Conference Papers Part F43-CLEO_AT 2017 (2017)
    We present an ultrafast edge-emitting diode laser system, which is able to self-adapt the resonator internal phase and amplitude. The best operating conditions for passive, active and hybrid mode-locking are analyzed. © 2017 OSA.
    view abstract10.1364/CLEO_AT.2017.JTh2A.128
  • Birefringence tuning of VCSELs
    Pusch, T. and Bou Sanayeh, M. and Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9892 (2016)
    Using the elasto-optic effect we increase the frequency difference between the two orthogonally polarized modes, the so-called birefringence splitting, in standard single-mode oxide-confined GaAs-based vertical-cavity surface-emitting lasers (VCSELs). The birefringence may play an important role in the realization of ultrafast polarization modulation for high-speed data transmission. For practical implementation it is necessary to miniaturize the strain-inducing mechanism for birefringence tuning in VCSELs. The goal is the realization of integrated structures on the VCSEL chip. In this paper we discuss our work on miniaturized bending devices as the next step in achieving extremely high birefringence splitting. Furthermore measurements with integrated hotspot structures on VCSEL chips were made to reach much smaller scales for birefringence fine-tuning. © 2016 SPIE.
    view abstract10.1117/12.2228091
  • Birefringent vertical cavity surface-emitting lasers: Toward high-speed spin-lasers
    Gerhardt, N.C. and Lindemann, M. and Pusch, T. and Michalzik, R. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9892 (2016)
    Spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) provide novel opportunities to overcome several limitations of conventional, purely charge-based semiconductor lasers. Presumably the highest potential lies in the spin-VCSEL's capability for ultrafast spin and polarization dynamics which can be significantly faster than the intensity dynamics in conventional devices. By injecting spin-polarized carriers, these coupled spin-photon dynamics can be controlled and utilized for high-speed applications. While relaxation oscillations provide insights in the speed and direct modulation bandwidth of conventional devices, resonance oscillations in the circular polarization degree step in for the spin and polarization dynamics in spin-VCSELs. These polarization oscillations can be generated using pulsed spin injection and achieve much higher frequencies than the conventional intensity relaxation oscillations in these devices. Furthermore polarization oscillations can be switched on and off and it is possible to generate short polarization pulses, which may represent an information unit in polarization-based optical communication. The frequency of polarization oscillations is mainly determined by the birefringence-induced mode splitting between both orthogonal linearly polarized laser modes. Thus the polarization modulation bandwidth of spin-VCSELs can be increased by adding a high amount of birefringence to the cavity, for example by incorporating mechanical strain. Using this technique, we could demonstrate tunable polarization oscillations from 10 to 40 GHz in AlGaAs-based 850nm VCSELs recently. Furthermore a birefringence-induced mode splitting of more than 250 GHz could be demonstrated experimentally. Provided that this potential for ultrafast dynamics can be fully exploited, birefringent spin-VCSELs are ideal devices for fast short-haul optical interconnects. In this paper we review our recent progress on polarization dynamics of birefringent spin-VCSELs and investigate numerically how ultrafast polarization oscillations can be utilized for data communication using simulations based on the spin-flip model. © 2016 SPIE.
    view abstract10.1117/12.2229356
  • Common-path depth-filtered digital holography for high resolution imaging of buried semiconductor structures
    Finkeldey, M. and Schellenberg, F. and Gerhardt, N.C. and Paar, C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9771 (2016)
    We investigate digital holographic microscopy (DHM) in reflection geometry for non-destructive 3D imaging of semiconductor devices. This technique provides high resolution information of the inner structure of a sample while maintaining its integrity. To illustrate the performance of the DHM, we use our setup to localize the precise spots for laser fault injection, in the security related field of side-channel attacks. While digital holographic microscopy techniques easily offer high resolution phase images of surface structures in reflection geometry, they are typically incapable to provide high quality phase images of buried structures due to the interference of reflected waves from different interfaces inside the structure. Our setup includes a sCMOS camera for image capture, arranged in a common-path interferometer to provide very high phase stability. As a proof of principle, we show sample images of the inner structure of a modern microcontroller. Finally, we compare our holographic method to classic optical beam induced current (OBIC) imaging to demonstrate its benefits. © 2016 SPIE.
    view abstract10.1117/12.2212454
  • Dynamics of double-pulse photoacoustic excitation
    Cherkashin, M. and Brenner, C. and Göring, L. and Döpke, B. and Gerhardt, N.C. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 9708 (2016)
    In contrast to the well-established and widely used theory of photoacoustic signal generation by single delta-like pulses, the field of multiple pulse excitation is not yet studied well. Using double-pulse excitation can be beneficial, but as ultrasound transducers have a certain waveform duration, the inter-pulse delays used might be limited. In order to assess the strength of the transducer influence at short delay times and develop data analysis procedure, we investigate the photoacoustic responses of a phantom sample to double-pulse excitation measured with different transducers. Both focused and flat surface single element transducers are used in the study. The central frequencies are chosen in the low-frequency band as they are most widely used in clinical ultrasound and one higher frequency transducer is taken for comparison. Despite not observing signal amplification due to Grueneisen relaxation effect, we show that transducer influence is not exceeding measurement error. Additionally we prove that single pulse subtraction procedure can be used to restore the second pulse waveform in double pulse excitation scheme. We believe using this procedure can be beneficial when transducer's waveform duration is longer than used inter-pulse delays. © 2016 SPIE.
    view abstract10.1117/12.2213476
  • Ex vivo brain tumor analysis using spectroscopic optical coherence tomography
    Lenz, M. and Krug, R. and Welp, H. and Schmieder, K. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 9697 (2016)
    A big challenge during neurosurgeries is to distinguish between healthy tissue and cancerous tissue, but currently a suitable non-invasive real time imaging modality is not available. Optical Coherence Tomography (OCT) is a potential technique for such a modality. OCT has a penetration depth of 1-2 mm and a resolution of 1-15 μm which is sufficient to illustrate structural differences between healthy tissue and brain tumor. Therefore, we investigated gray and white matter of healthy central nervous system and meningioma samples with a Spectral Domain OCT System (Thorlabs Callisto). Additional OCT images were generated after paraffin embedding and after the samples were cut into 10 μm thin slices for histological investigation with a bright field microscope. All samples were stained with Hematoxylin and Eosin. In all cases B-scans and 3D images were made. Furthermore, a camera image of the investigated area was made by the built-in video camera of our OCT system. For orientation, the backsides of all samples were marked with blue ink. The structural differences between healthy tissue and meningioma samples were most pronounced directly after removal. After paraffin embedding these differences diminished. A correlation between OCT en face images and microscopy images can be seen. In order to increase contrast, post processing algorithms were applied. Hence we employed Spectroscopic OCT, pattern recognition algorithms and machine learning algorithms such as k-means Clustering and Principal Component Analysis. Copyright © 2016 SPIE.
    view abstract10.1117/12.2214704
  • Frequency tuning of polarization oscillations in spin-lasers
    Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R. and Pusch, T. and Michalzik, R.
    Proceedings of SPIE - The International Society for Optical Engineering 9931 (2016)
    Polarization oscillations can be observed as resonant oscillations of the coupled spin-photon system in spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs). They are a reasonable measure of the polarization dynamics and provide insights to the polarization modulation bandwidth of these devices. These oscillations can be generated using pulsed spin injection and have proven to be much faster than the relaxation oscillations for the intensity dynamics under the same conditions. The oscillation frequency mainly depends on the cavity birefringence, which can be tuned by applying mechanical strain to the VCSEL structure. This provides a direct tool to considerably increase the polarization oscillation frequency and thus the modulation bandwidth. Following this approach we were able to experimentally tune the frequency over a range of 34 GHz. We demonstrated polarization oscillations in spin-VCSELs with frequencies up to 44 GHz recently, only limited by the used mechanical strain setup.1 By measuring the polarization oscillation frequency and the birefringence-governed mode splitting as a function of the applied strain simultaneously, we investigated the correlation between birefringence and polarization oscillations. Here we use an optimized and simplified mount, which potentially allows for larger strain values. The experimental findings are compared to numerical calculations based on the spin-flip model. Taking our previously reported record value of more than 250 GHz for the birefringence splitting in VCSEL cavities into account,2 this technique may pave the road toward high-speed polarization modulation in VCSELs for bit rates above 100 Gb/s. © 2016 SPIE.
    view abstract10.1117/12.2237566
  • Frequency tuning of polarization oscillations in spin-polarized vertical-cavity surface-emitting lasers
    Lindemann, M. and Pusch, T. and Michalzik, R. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9892 (2016)
    Controlling the coupled spin-photon dynamics in vertical-cavity surface-emitting lasers (VCSELs) is an attractive opportunity to overcome the limitations of conventional, purely charge based semiconductor lasers. Such spin-controlled VCSELs (spin-VCSELs) offer several advantages, like reduced threshold, spin amplification and polarization control. Furthermore the coupling between carrier spin and light polarization bears the potential for ultrafast polarization dynamics. By injecting spin-polarized carriers, the complex polarization dynamics can be controlled and utilized for high-speed applications. Polarization oscillations as resonance oscillations of the coupled spin- photon system can be generated using pulsed spin injection, which can be much faster than the intensity dynamics in conventional devices. We already demonstrated that the oscillations can be switched in a controlled manner. These controllable polarization dynamics can be used for ultrafast polarization-based optical data communication. The polarization oscillation frequency and therefore the possible data transmission rate is assumed to be mainly determined by the birefringence-induced mode-splitting. This provides a direct tool to increase the polarization dynamics toward higher frequencies by adding a high amount of birefringence to the VCSEL structure. Using this technique, we could recently demonstrate experimentally a birefringence splitting of more than 250 GHz using mechanical strain. Here, we employ the well-known spin-flip model to investigate the tuning of the polarization oscillation frequency. The changing mechanical strain is represented by a linear birefringence sweep to values up to 80πGHz. The wide tuning range presented enables us to generate polarization oscillation frequencies exceeding the conventional intensity modulation frequency in the simulated device by far, mainly dependent on the birefringence in the cavity only. © 2016 SPIE.
    view abstract10.1117/12.2229358
  • Frequency tuning of polarization oscillations: Toward high-speed spin-lasers
    Lindemann, M. and Pusch, T. and Michalzik, R. and Gerhardt, N.C. and Hofmann, M.R.
    Applied Physics Letters 108 (2016)
    Spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) offer a high potential to overcome several limitations of conventional purely charged-based laser devices. Presumably, the highest potential of spin-VCSELs lies in their ultrafast spin and polarization dynamics, which can be significantly faster than the intensity dynamics in conventional devices. Here, we experimentally demonstrate polarization oscillations in spin-VCSELs with frequencies up to 44 GHz. The results show that the oscillation frequency mainly depends on the cavity birefringence, which can be tuned by applying mechanical strain to the VCSEL structure. A tuning range of about 34 GHz is demonstrated. By measuring the polarization oscillation frequency and the birefringence governed mode splitting as a function of the applied strain simultaneously, we are able to investigate the correlation between birefringence and polarization oscillations in detail. The experimental findings are compared to numerical calculations based on the spin-flip model. © 2016 AIP Publishing LLC.
    view abstract10.1063/1.4940713
  • High-frequency operation of spin vertical-cavity surface-emitting lasers: Towards 100 GHz
    Gerhardt, N.C. and Lindemann, M. and Pusch, T. and Michalzik, R. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9931 (2016)
    Spin-controlled vertical-cavity surface-emitting lasers (spin-VCSELs) have a high potential to overcome limitations of conventional purely charge-based lasers. Probably the most important feature of such spin-lasers lies in their ultrafast spin and polarization dynamics which are decoupled from the intensity dynamics and their limitations. This yields the potential to modulate the polarization state of spin-VCSELs with frequencies far above the barriers known for the intensity modulation dynamics of conventional VCSELs. Such a quality makes them ideal devices for fast optical interconnects. While in conventional devices relaxation oscillations provide insights in the intensity dynamics and modulation bandwidth, in spin-VCSELs oscillations in the circular polarization degree are an ideal measure for investigating the dynamics of the coupled spin-photon system. These polarization oscillations (POs) can be generated using pulsed spin injection and have been proven to be much faster than intensity dynamics in the devices. Their frequency is mainly dependent on the birefringence in the cavities and can be increased by adding mechanical strain. Using an approach for manipulating the birefringence via mechanical strain we demonstrated tunable POs with frequencies up to 44 GHz, recently. Taking our results for strain-induced birefringence splitting of more than 250 GHz into account, the concept has the potential to overcome conventional limitations and to provide polarization modulation in VCSELs with bit rates beyond 100 Gbit=s. In this paper we investigate numerically the in uence of the spin decay rate on the PO amplitude and frequency in order to investigate potential limitations for future ultrafast polarization modulation schemes. © 2016 SPIE.
    view abstract10.1117/12.2237163
  • Influence of birefringence splitting on ultrafast polarization oscillations in VCSELs
    Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R. and Pusch, T. and Michalzik, R.
    Proceedings of SPIE - The International Society for Optical Engineering 9766 (2016)
    Spin-VCSELs offer numerous advantages over conventional lasers like reduced threshold, spin amplification and ultrafast polarization dynamics. The latter have the potential to generate polarization modulation frequencies far above the conventional intensity relaxation oscillation frequency of one and the same device and thus can be an interesting basis for ultrafast optical data transmission. We have shown that fast polarization oscillations can be generated by pulsed spin injection. Furthermore the oscillation frequency can be tuned via modification of the VCSEL's cavity strain. Using this technique, oscillation frequencies with a tuning range from nearly zero up to 40 GHz can be demonstrated. In the device under study, this is more than six times the intensity relaxation oscillation frequency, which is nearly independent of the strain. Now we demonstrate the influence of the strain-induced birefringence splitting on the oscillation frequency. We find that the polarization oscillation frequency is directly corresponding to the birefringence splitting. The reason is that the polarization oscillates according to the beating frequency of the two orthogonal linearly polarized cavity modes in the VCSEL. In the case of spin-pumping, those two modes form the circular polarization output of the laser by superposition. Their frequencies are shifted by birefringence manipulation and form the basis of birefringence splitting. The measurement results are compared with simulations employing the spin-flip model. Our results show that high-frequency polarization oscillations can not only be generated with the help of external strain but with high birefringence splitting in general. © 2016 SPIE.
    view abstract10.1117/12.2212413
  • Interaction of phase and amplitude shaping in an external cavity semiconductor laser
    Pilny, R.H. and Döpke, B. and Balzer, J.C. and Brenner, C. and Klehr, A. and Erbert, G. and Tränkle, G. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9767 (2016)
    Ultrashort pulse generation with semiconductor lasers poses a promising alternative to currently available femtosecond laser sources like solid state and fiber lasers. Semiconductor devices can be produced inexpensively, are energy efficient and their wavelength can be designed by band gap engineering. Furthermore they feature a tunable repetition rate. Yet pulse duration and peak power of those devices limit their potential for applications so far. However, recent research demonstrated a reduction of the pulse width from 534 fs (full width half maximum) to 216 fs by shaping the spectrally resolved spectral phase and amplitude inside the cavity. The utilized system consisted of a mode-locked edge emitting semiconductor laser diode, a spatial light modulator inside the external cavity to carry out the pulse shaping and an evolutionary algorithm to optimize the phase and amplitude. Here we present the results of separate phase and amplitude shaping as well as their interaction if optimized together at the same time. Furthermore we demonstrate the flexibility of the phase and amplitude shaping with respect to each other. Thus we expect of our system to enable adaptation to a resonator external dispersion. © 2016 SPIE.
    view abstract10.1117/12.2212906
  • Large laser spots and fault sensitivity analysis
    Schellenberg, F. and Finkeldey, M. and Gerhardt, N. and Hofmann, M. and Moradi, A. and Paar, C.
    Proceedings of the 2016 IEEE International Symposium on Hardware Oriented Security and Trust, HOST 2016 (2016)
    Laser Fault Injection (LFI) is a powerful method of introducing faults into a specific area of an integrated circuit. Because the minimum spot size of the laser spot is physically bounded, many recent publications investigate down to which technology node individual transistors can be targeted. In contrast, we develop a novel attack that is applicable even when a large number of gates is affected at the smallest feature sizes. To achieve this, we adapt Fault Sensitivity Analysis to the laser setting. Such attacks require reasoning about the critical path of a combinatorial circuit and were previously only considered for clock glitches. Indeed, we show that this prerequisite is available for LFI as well. This leads to a very relaxed fault model, especially in terms of the required laser spot size. We conclude that there is no intrinsic protection for the latest technology nodes and LFI remains a serious threat for embedded devices. Experimental results are provided by targeting the combinatorial AES Sbox of an Atmel ATxmega microcontroller with an artificially large laser spot. Finally, we discuss why this attack is still applicable to the smallest structure sizes. © 2016 IEEE.
    view abstract10.1109/HST.2016.7495583
  • Recognition of pharmaceuticals with compact mini-Raman-spectrometer and automized pattern recognition algorithms
    Jähme, H. and Di Florio, G. and Conti Nibali, V. and Esen, C. and Ostendorf, A. and Grafen, M. and Henke, E. and Soetebier, J. and Brenner, C. and Havenith, M. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9899 (2016)
    Robust classification of pharmaceuticals in an industrial process is an important step for validation of the final product. Especially for pharmaceuticals with similar visual appearance a quality control is only possible if a reliable algorithm based on easily obtainable spectroscopic data is available. We used Principal Component Analysis (PCA) and Support Vector Machines (SVM) on Raman spectroscopy data from a compact Raman system to classify several look-alike pharmaceuticals. This paper describes the data gathering and analysis process to robustly discriminate 19 different pharmaceuticals with similar visual appearance. With the described process we successfully identified all given pharmaceuticals which had a significant amount of active ingredients. Thus automatic validation of these pharmaceuticals in a process can be used to prevent wrong administration of look-alike drugs in an industrial setting, e.g. patient individual blistering. © 2016 SPIE.
    view abstract10.1117/12.2228070
  • Terahertz wave generation from dual wavelength monolithic integrated distributed Bragg reflector semiconductor laser diode
    Gwaro, J.O. and Brenner, C. and Hofmann, M.R. and Sumpf, B. and Klehr, A. and Fricke, J.
    GeMiC 2016 - 2016 German Microwave Conference (2016)
    THz wave generation from a dual wavelength distributed Bragg reflector (DBR) semiconductor diode laser connected by an Y-shaped waveguide with photoconductive antennas is demonstrated. Two fibre coupled photoconductive antennas were employed to generate and detect the beat frequency of the two lasers by lock in technique. Terahertz frequencies of approximately 300GHz were demonstrated via photomixing with a tuning range between 290 GHz to 320GHz. © 2016 Institut fur Mikrowellen und Antennentechnik-IMA.
    view abstract10.1109/GEMIC.2016.7461543
  • Timing jitter performance of mode-locked external cavity multi-quantum-well semiconductor lasers
    Döpke, B. and Pilny, R.H. and Horstkemper, H. and Brenner, C. and Klehr, A. and Erbert, G. and Tränkle, G. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9767 (2016)
    Mode-locked semiconductor lasers are a promising source for applications such as ultrafast optical sampling. For such an application, the reduction of timing jitter of the pulse source in a cost-effective manner is a key challenge. While monolithic devices have been the source of much recent interest, external cavity lasers have been less well studied. In this work, the noise of an external cavity laser under passively mode-locked operation is evaluated. A ridge-waveguide super-large optical cavity material system is used. © 2016 SPIE.
    view abstract10.1117/12.2213042
  • Compact low-cost lensless digital holographic microscope for topographic measurements of microstructures in reflection geometry
    Adinda-Ougba, A. and Kabir, B. and Koukourakis, N. and Mitschker, F. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9628 (2015)
    Digital holography is capable of providing surface profiles of samples with axial resolution in the nanometer range. Lensless digital holography is a well-established microscopic method providing diffraction limited resolution of the order of the wavelength of the used light source. It is based on inline holography and usually allows imaging only in transmission geometry. In this contribution we propose a compact low cost lensless digital holographic microscope capable of performing measurements on reflective microstructures. The novelty of the system consists on a direct use of a laser diode without any need of coupling optics as light source. This simplifies the setup and provides sufficient magnification to measure microstructures. We evaluate our setup by imaging reflective microstructures. We have achieved ∼ 6 mm2 field of view amplitude images with ∼ 2.5μm lateral resolution and phase images with axial resolution in nanometer range. The phase image provides a full-field profile measurement of the sample in nanometer range. © 2015 SPIE.
    view abstract10.1117/12.2191073
  • Controlled switching and frequency tuning of polarization oscillations in vertical-cavity surface-emitting lasers
    Lindemann, M. and Höpfner, H. and Gerhardt, N.C. and Hofmann, M.R. and Pusch, T. and Michalzik, R.
    Proceedings of SPIE - The International Society for Optical Engineering 9551 (2015)
    Spintronic lasers offer promising perspectives for novel concepts and characteristics superior to conventional purely charge-based devices. This applies in particular to spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs), which exhibit ultrafast spin and polarization dynamics. Using pulsed spin-injection, oscillations in the circular polarization degree can be generated, which have the potential to be much faster than conventional relaxation oscillations and may exceed frequencies of 100 GHz. The oscillations originate from the coupled carrier-spin-photon system in birefringent VCSEL cavities. The polarization oscillations are independent from conventional relaxation oscillations and thus can be the cornerstone for ultrafast directly modulated spin-VCSELs in the near future. It is possible to switch the oscillations on and off, depending on phase and amplitude conditions of two consecutive excitation pulses. Even half-cycles can be generated, which is the basis for short polarization pulses, only limited by the polarization oscillation resonance frequency. Experimental results of oscillation switching are given using an 850 nm oxide-confined single-mode VCSEL. In order to increase the polarization oscillation frequency, the birefringence has to be tuned to higher values. We demonstrate a method to manipulate the birefringence by adding mechanical strain to the substrate in vicinity of the VCSEL. With this method the polarization oscillation frequency can be tuned over a wide range. The results are compared to the theory with simulations using the spin-flip-model. © 2015 SPIE.
    view abstract10.1117/12.2197970
  • Laser-diode-based photoacoustic setup to analyze Grüneisen relaxation-effect induced signal enhancement
    Cherkashin, M. and Brenner, C. and Göring, L. and Döpke, B. and Gerhardt, N.C. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 9539 (2015)
    In this paper we investigate the possibilities to use a pulsed laser diode based setup to achieve the photoacoustic signal amplification via the Grueneisen relaxation effect. It is shown that the system is capable of producing the required multiple pulses burst with pulse widths of 12 ns and pulse inter-delays down to approximately 135 ns. With additional fluence considerations we expect no improvement from this technique for photoacoustic tomography setting, while our laser diode based setup is a highly promising compact alternative for Grueneisen relaxation related studies in photoacoustic microscopy. © 2015 SPIE.
    view abstract10.1117/12.2183957
  • On the complexity reduction of laser fault injection campaigns using OBIC measurements
    Schellenberg, F. and Finkeldey, M. and Richter, B. and Schapers, M. and Gerhardt, N. and Hofmann, M. and Paar, C.
    Proceedings - 2015 Workshop on Fault Diagnosis and Tolerance in Cryptography, FDTC 2015 (2015)
    Laser Fault Injection (LFI) is one of the most powerful methods of inducing a fault as it allows targeting only specific areas down to single transistors. The downside compared to non-invasive methods like introducing clock glitches is the largely increased search space. An exhaustive search through all parameters including dimensions for correct timing, intensity, or length might not be not feasible. Existing solutions to this problem are either not directly applicable to the fault location or require additional device preparation and access to expensive equipment. Our method utilizes measuring the Optical Beam Induced Current (OBIC) as imaging technique to find target areas like flip-flops and thus, reducing the search space drastically. This measurement is possible with existing laser scanning microscopes or well-equipped LFI setups. We provide experimental results targeting the Advanced Encryption Standard (AES) hardware accelerator of an Atmel ATXMega microcontroller. © 2015 IEEE.
    view abstract10.1109/FDTC.2015.10
  • Passively mode-locked diode laser with optimized dispersion management
    Balzer, J.C. and Pilny, R.H. and Döpke, B. and Klehr, A. and Erbert, G. and Tränkle, G. and Brenner, C. and Hofmann, M.R.
    IEEE Journal on Selected Topics in Quantum Electronics 21 (2015)
    We investigate passively mode-locked diode lasers with external cavity for ultrashort pulse generation. Our strategy to achieve ultrashort pulses is to generate strongly chirped pulses with a maximized bandwidth and to compress them externally. By managing intracavity dispersion with an evolutionary algorithm, we obtain pulse widths as short as 278 fs following this approach. We analyze the bandwidth of the optimized pulses in comparison to the available net gain bandwidth of the diode laser device to derive further strategies for achieving shorter pulses. © 2015 IEEE.
    view abstract10.1109/JSTQE.2015.2418225
  • Phase and amplitude calibration of dualmask spatial light modulator for highresolution femtosecond pulse shaping
    Döpke, B. and Balzer, J.C. and Hofmann, M.R.
    Electronics Letters 51 (2015)
    A new calibration method for dual-mask liquid crystal spatial light modulators (SLMs) is presented, which can be applied while the SLM is in the setup of a pulse shaper optimised for semiconductor laser sources. An evolutionary algorithm is used to retrieve the phase retardances from the transmitted spectral intensity. © The Institution of Engineering and Technology 2015.
    view abstract10.1049/el.2015.0078
  • Quantitative phase imaging by wide field lensless digital holographic microscope
    Adinda-Ougba, A. and Koukourakis, N. and Essaidi, A. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9529 (2015)
    Wide field, lensless microscopes have been developed for telemedicine and for resource limited setting [1]. They are based on in-line digital holography which is capable to provide amplitude and phase information resulting from numerical reconstruction. The phase information enables achieving axial resolution in the nanometer range. Hence, such microscopes provide a powerful tool to determine three-dimensional topologies of microstructures. In this contribution, a compact, low-cost, wide field, lensless microscope is presented, which is capable of providing topological profiles of microstructures in transparent material. Our setup consist only of two main components: a CMOSsensor chip and a laser diode without any need of a pinhole. We use this very simple setup to record holograms of microobjects. A wide field of view of ~24 mm, and a lateral resolution of ~2 μm are achieved. Moreover, amplitude and phase information are obtained from the numerical reconstruction of the holograms using a phase retrieval algorithm together with the angular spectrum propagation method. Topographic information of highly transparent micro-objects is obtained from the phase data. We evaluate our system by recording holograms of lines with different depths written by a focused laser beam. A reliable characterization of laser written microstructures is crucial for their functionality. Our results show that this system is valuable for determination of topological profiles of microstructures in transparent material. © 2015 SPIE.
    view abstract10.1117/12.2184471
  • Self-optimizing femtosecond semiconductor laser
    Döpke, B. and Pilny, R.H. and Brenner, C. and Klehr, A. and Erbert, G. and Tränkle, G. and Balzer, J.C. and Hofmann, M.R.
    Optics Express 23 (2015)
    A self-optimizing approach to intra-cavity spectral shaping of external cavity mode-locked semiconductor lasers using edge-emitting multi-section diodes is presented. An evolutionary algorithm generates spectrally resolved phase- and amplitude masks that lead to the utilization of a large part of the net gain spectrum for mode-locked operation. Using these masks as a spectral amplitude and phase filter, a bandwidth of the optical intensity spectrum of 3.7 THz is achieved and Fourier-limited pulses of 216 fs duration are generated after further external compression. © 2015 Optical Society of America.
    view abstract10.1364/OE.23.009710
  • Spectral domain optical coherence tomography for ex vivo brain tumor analysis
    Lenz, M. and Krug, R. and Jaedicke, V. and Stroop, R. and Schmieder, K. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 9541 (2015)
    Non-contact imaging methods to distinguish between healthy tissue and brain tumor tissue during surgery would be highly desirable but are not yet available. Optical Coherence Tomography (OCT) is a non-invasive imaging technology with a resolution around 1-15 μm and a penetration depth of 1-2 mm that may satisfy the demands. To analyze its potential, we measured ex vivo human brain tumor tissue samples from 10 patients with a Spectral Domain OCT system (Thorlabs Callisto: center wavelength of 930 nm) and compared the results with standard histology. In detail, three different measurements were made for each sample. First the sample was measured directly after surgery. Then it was embedded in paraffin (also H and E staining) and examined for the second time. At last, the slices of each paraffin block cut by the pathology were measured. Each time a B-scan was created and for a better comparison with the histology a 3D image was generated, in order to get the corresponding en face images. In both, histopathological diagnosis and the analysis of the OCT images, different types of brain tumor showed difference in structure. This has been affirmed by two blinded investigators. Nevertheless the difference between two images of samples taken directly after surgery is less distinct. To enhance the contrast in the images further, we employ Spectroscopic OCT and pattern recognition algorithms and compare these results to the histopathological standard. © 2015 SPIE-OSA.
    view abstract10.1117/12.2183614
  • Towards high frequency operation of polarization oscillations in spin vertical-cavity surface-emitting lasers
    Lindemann, M. and Höpfner, H. and Gerhardt, N.C. and Hofmann, M.R. and Pusch, T. and Michalzik, R.
    Proceedings of SPIE - The International Society for Optical Engineering 9551 (2015)
    Compared to purely charge based devices, spintronic lasers offer promising perspectives for new superior device concepts. Especially vertical-cavity surface-emitting lasers with spin-polarization (spin-VCSELs) feature ultrafast spin and polarization dynamics. Oscillations in the circular polarization degree can be generated using pulsed spin-injection. The oscillations evolve due to the carrier-spin-photon system that is coupled for the linear modes in the VCSEL's cavity via the birefringence. The polarization oscillations are independent of the conventional relaxation oscillations and have the potential to exceed frequencies of 100 GHz. The oscillations are switchable and can be the basis for ultrafast directly modulated spin-VCSELs for, e.g., communication purposes. The polarization oscillation frequency is mainly determined by the birefringence. We show a method to tune the birefringence and thus the polarization oscillation frequency by adding mechanical strain to the substrate in the vicinity of the laser. We achieved first experimental results for high-frequency operation using 850 nm oxide-confined single-mode VCSELs. The results are compared with simulations using the spin- IP-model for high birefringence values. © 2015 SPIE.
    view abstract10.1117/12.2197969
  • Ultrafast polarization dynamics with controlled polarization oscillations in vertical-cavity surface-emitting lasers
    Lindemann, M. and Höpfner, H. and Gerhardt, N.C. and Hofmann, M.R. and Pusch, T. and Michalzik, R.
    Proceedings of SPIE - The International Society for Optical Engineering 9381 (2015)
    Spintronic lasers offer promising perspectives for new concepts superior to options of purely charge-based devices. Especially spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) exhibit ultrafast spin and polarization dynamics. Using pulsed spin-injection, oscillations in the circular polarization degree can be generated, which have the potential to exceed frequencies of 100 GHz. The oscillations evolve due to coupling of the carrier-spin-photon system for linear modes via birefringence in the VCSEL's cavity. They are independent of the conventional relaxation oscillations and thus their usage can be the cornerstone for ultrafast directly modulated spin-VCSELs in the near future. After giving a short overview of the state of scientific and technical knowledge we will outline a method to control the polarization oscillations by multiple spin-injection pulses. It is possible to switch these oscillations on and off, depending on phase and amplitude conditions of two consecutive excitation pulses. Even half-cycles can be generated, which is the basis for short polarization pulses, only limited by the polarization oscillation resonance frequency. We investigate influences of the birefringence, which directly determines the oscillation frequency, by means of calculations with the spin-flip-model and experimental verification using 850 nm VCSELs. Furthermore we discuss experimental possibilities of increasing the birefringence and therefore the oscillation frequency, such that ultrashort pulses come into reach. © 2015 SPIE.
    view abstract10.1117/12.2076920
  • Ultrashort pulse generation with semiconductor lasers using intracavity phase- and amplitude pulse shaping
    Döpke, B. and Balzer, J.C. and Pilny, R.H. and Brenner, C. and Klehr, A. and Erbert, G. and Tränkle, G. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9382 (2015)
    We present intra-cavity pulse shaping of external cavity mode-locked semiconductor lasers. In our approach, a pulse shaper utilizing a dual LC-panel spatial light modulator is used in the cavity of a mode-locked multi-quantum-well semiconductor laser to introduce spectrally resolved phase manipulation and losses to the pulse propagating in the cavity. Utilizing this, we generate pulses with broader spectra than obtained in conventional external cavity geometries without pulse shaping. The pulses can be compressed near to the transform limit using a grating compressor. © 2015 SPIE.
    view abstract10.1117/12.2079040
  • Vertical-cavity surface-emitting lasers with birefringence splitting above 250 GHz
    Pusch, T. and Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R. and Michalzik, R.
    Electronics Letters 51 (2015)
    Using the elasto-optic effect, increase of the frequency difference between the two orthogonally polarised modes, the so-called birefringence splitting, in standard single-mode oxide-confined AlGaAs-based vertical-cavity surface-emitting lasers is achieved to values beyond 250 GHz. A large birefringence is required for the generation of ultra-fast polarisation oscillations for potential future high-speed communication applications. © 2015 The Institution of Engineering and Technology.
    view abstract10.1049/el.2015.2149
  • Axial scanning in confocal microscopy employing adaptive lenses (CAL)
    Koukourakis, N. and Finkeldey, M. and Stürmer, M. and Leithold, C. and Gerhardt, N.C. and Hofmann, M.R. and Wallrabe, U. and Czarske, J.W. and Fischer, A.
    Optics Express 22 (2014)
    In this paper we analyze the capability of adaptive lenses to replace mechanical axial scanning in confocal microscopy. The adaptive approach promises to achieve high scan rates in a rather simple implementation. This may open up new applications in biomedical imaging or surface analysis in micro- and nanoelectronics, where currently the axial scan rates and the flexibility at the scan process are the limiting factors. The results show that fast and adaptive axial scanning is possible using electrically tunable lenses but the performance degrades during the scan. This is due to defocus and spherical aberrations introduced to the system by tuning of the adaptive lens. These detune the observation plane away from the best focus which strongly deteriorates the axial resolution by a factor of ∼2.4. Introducing balancing aberrations allows addressing these influences. The presented approach is based on the employment of a second adaptive lens, located in the detection path. It enables shifting the observation plane back to the best focus position and thus creating axial scans with homogeneous axial resolution. We present simulated and experimental proof-of-principle results. © 2014 Optical Society of America.
    view abstract10.1364/OE.22.006025
  • Coherent switching of polarization oscillations in vertical-cavity surface-emitting lasers
    Höpfner, H. and Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9001 (2014)
    Spin polarized lasers, especially spin polarized vertical-cavity surface-emitting lasers (VCSEL) provide improved performance when compared to conventional, purely charge-based lasers. Advantages of these spin-enhanced devices lie in their reduced laser threshold, increased emission intensity, amplification of spin information, chirp control and possibilities for ultrafast modulation due to their faster dynamics. Utilizing a commercially available conventional VCSEL and additional spin polarized optical pumping we are able to enhance the modulation dynamics of a conventional VCSEL with new spin effects. Our experiments show polarization oscillations in the spin-photon system that result in oscillations of the circular polarization of the VCSEL emission. The resulting polarization oscillations are of significantly higher frequency than the direct modulation bandwidth of the VCSEL and persist for durations longer than the spin lifetime in the active region. Simulations based on a rate-equation model show that with an improved VCSEL layout it should be possible to reach oscillation frequencies well above 100 GHz. Here, we show that with multiple optical spin polarized pulses these oscillations can be coherently excited, amplified and also stopped. Using this excitation scheme, polarization oscillations faster than the purely charge-based dynamics can be achieved with half-cycle to multi-cycle duration. Various influences of unpolarized electrical bias, optical excitation power and delay between pulses will be discussed both theoretically and experimentally. Additionally, we analyze the qualification of this new concept for ultrafast optical communication. © 2014 SPIE.
    view abstract10.1117/12.2039196
  • Controlled switching of ultrafast circular polarization oscillations in spin-polarized vertical-cavity surface-emitting lasers
    Höpfner, H. and Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R.
    Applied Physics Letters 104 (2014)
    We demonstrate a scheme for controlled switching of polarization oscillations in spin-polarized vertical-cavity surface-emitting lasers (spin-VCSEL). Under hybrid electrical and optical pumping conditions, our VCSEL devices show polarization oscillations with frequencies far above the VCSEL's electrical modulation bandwidth. Using multiple optical pulses, we are able to excite and amplify these polarization oscillations. When specific phase and amplitude conditions for the optical excitation pulses are met, destructive interference leads to switch-off of the polarization oscillation, enabling the generation of controlled short polarization bursts. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4862330
  • Development and evaluation of a device for simultaneous uniaxial compression and optical imaging of cartilage samples in vitro
    Steinert, M. and Kratz, M. and Jaedicke, V. and Hofmann, M.R. and Jones, D.B.
    Review of Scientific Instruments 85 (2014)
    In this paper, we present a system that allows imaging of cartilage tissue via optical coherence tomography (OCT) during controlled uniaxial unconfined compression of cylindrical osteochondral cores in vitro. We describe the system design and conduct a static and dynamic performance analysis. While reference measurements yield a full scale maximum deviation of 0.14% in displacement, force can be measured with a full scale standard deviation of 1.4%. The dynamic performance evaluation indicates a high accuracy in force controlled mode up to 25 Hz, but it also reveals a strong effect of variance of sample mechanical properties on the tracking performance under displacement control. In order to counterbalance these disturbances, an adaptive feed forward approach was applied which finally resulted in an improved displacement tracking accuracy up to 3 Hz. A built-in imaging probe allows on-line monitoring of the sample via OCT while being loaded in the cultivation chamber. We show that cartilage topology and defects in the tissue can be observed and demonstrate the visualization of the compression process during static mechanical loading. © 2014 AIP Publishing LLC.
    view abstract10.1063/1.4898669
  • Effects of axial scanning in confocal microscopy employing adaptive lenses (CAL)
    Koukourakis, N. and Finkeldey, M. and Stürmer, M. and Gerhardt, N.C. and Wallrabe, U. and Hofmann, M.R. and Czarske, J.W. and Fischer, A.
    Proceedings of SPIE - The International Society for Optical Engineering 9132 (2014)
    We analyze axial scanning in Confocal microscopy based on Adaptive Lenses (CAL). A tunable lens located in the illumination path of a confocal setup enables scanning the focus position by applying an electrical voltage. This opens up the possibility to replace mechanical axial scanning which is commonly used. In our proof-of-principle experiment, we demonstrate a tuning range of about 380 μm. The range can easily be extended by using the whole possible tuning range. During the scan the axial resolution degrades by a factor of about 2.3. The deterioration is introduced by aberrations that strongly depend on the scanning process. Therefore a second lens is located in the detection path of the CAL setup to balance the aberration effects. Both experiments and simulations show that this approach allows creating a homogeneous axial resolution throughout the scan. This is at the cost of tuning range which halves to about 200 μm. The lateral resolution is not noticeably affected and amounts to 500 nm. © 2014 SPIE.
    view abstract10.1117/12.2052152
  • Femtosecond semiconductor laser system with arbitrary intracavity phase and amplitude manipulation
    Balzer, J.C. and Döpke, B. and Klehr, A. and Erbert, G. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9002 (2014)
    In this work we present a laser cavity with a spatial light modulator (SLM), which allows for arbitrary phase and amplitude manipulation. In comparison to previous setups, it allows the manipulation of spectral components inside the laser cavity without the introduction of spatial chirp. An electrically driven ultrafast semiconductor laser system is used for proper alignment of the laser cavity. We were able to demonstrate that the gain of the laser supports mode-locking operation over a spectral range greater than 12 nm with a central wavelength of 850 nm. This bandwidth has the potential to generate sub 150 fs pulses.
    view abstract10.1117/12.2039206
  • In-situ calibration of spatial light modulators in femtosecond pulse shapers
    Döpke, B. and Balzer, J.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9004 (2014)
    Femtosecond pulse shapers are an important tool for the manipulation of ultrashort pulses. Two-Liquid-Crystal (LC)- Panel Spatial Light Modulator (SLM) pulse shapers5 are especially useful, as they permit simultaneous control of phase and amplitude of spectral components, thereby allowing for a wide range of possible manipulations of the temporal shape of ultrashort pulses. In this work, a detailed description of the alignment of a pulse shaper optimized for modelocked semiconductor lasers is presented. Several methods to calibrate the phase-retardance-LC-voltagerelationship are reviewed and a calibration method is presented which is robust to non-idealities of the components of the setup. © 2014 SPIE.
    view abstract10.1117/12.2039451
  • Mode-locked semiconductor laser system with intracavity spatial light modulator for linear and nonlinear dispersion management
    Balzer, J.C. and Döpke, B. and Brenner, C. and Klehr, A. and Erbert, G. and Tränkle, G. and Hofmann, M.R.
    Optics Express 22 (2014)
    We analyze the influence of second and third order intracavity dispersion on a passively mode-locked diode laser by introducing a spatial light modulator (SLM) into the external cavity. The dispersion is optimized for chirped pulses with highest possible spectral bandwidth that can be externally compressed to the sub picosecond range. We demonstrate that the highest spectral bandwidth is achieved for a combination of second and third order dispersion. With subsequent external compression pulses with a duration of 437 fs are generated. © 2014 Optical Society of America.
    view abstract10.1364/OE.22.018093
  • Multiwavelength phase unwrapping and aberration correction using depth filtered digital holography
    Jaedicke, V. and Goebel, S. and Koukourakis, N. and Gerhardt, N.C. and Welp, H. and Hofmann, M.R.
    Optics Letters 39 (2014)
    In this Letter, we present a new approach to processing data from a standard spectral domain optical coherence tomography (OCT) system using depth filtered digital holography (DFDH). Intensity-based OCT processing has an axial resolution of the order of a few micrometers. When the phase information is used to obtain optical path length differences, subwavelength accuracy can be achieved, but this limits the resolvable step heights to half of the wavelength of the system. Thus there is a metrology gap between phase-and intensity-based methods. Our concept addresses this metrology gap by combining DFHD with multiwavelength phase unwrapping. Additionally, the measurements are corrected for aberrations. Here, we present proof of concept measurements of a structured semiconductor sample. © 2014 Optical Society of America.
    view abstract10.1364/OL.39.004160
  • Performance comparison of different metrics for spectroscopic optical coherence tomography
    Jaedicke, V. and Agcaer, S. and Robles, F.E. and Steinert, M. and Jones, D. and Goebel, S. and Gerhardt, N.C. and Welp, H. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 8952 (2014)
    When light interacts with a scattering medium, the spectrum of the incident light undergoes changes that are dependent on the size of the scatterers in the medium. Spectroscopic Optical Coherence Tomography (S-OCT) is a method that can be used to ascertain the resulting spatially-dependent spectral information. In fact, S-OCT is sensitive to structures that are below the spatial resolution of the system, making S-OCT a promising tool for diagnosing many diseases and biological processes that change tissue structure, like cancer. The most important signal processing steps for S-OCT are the depth-resolved spectral analysis and the calculation of a spectroscopic metric. While the former calculates the spectra from the raw OCT data, the latter analyzes the information content of the processed depth-resolved spectra. We combine the Dual Window spectral analysis with different spectroscopic metrics, which are used as an input to colorize intensity based images. These metrics include the spectral center of mass method, principal component (PCA) and phasor analysis. To compare the performance of the metrics in a quantitative manner, we use a cluster algorithm to calculate efficiencies for all methods. For this purpose we use phantom samples which contain areas of microspheres of different sizes. Our results demonstrate that PCA and phasor analysis have the highest efficiencies, and can clearly separate these areas. Finally we will present data from cartilage tissue under static load in vitro. These preliminary results show that S-OCT can generate additional contrast in biological tissue in comparison to the pure intensity based images. © 2014 SPIE.
    view abstract10.1117/12.2038891
  • Polarization dynamics in spin-polarized vertical-cavity surface-emitting lasers
    Gerhardt, N.C. and Höpfner, H. and Lindemann, M. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9167 (2014)
    Spin-polarized lasers and especially spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) are attractive novel spintronic devices providing functionalities and characteristics superior to their conventional purely charge-based counterparts. This applies in particular to ultrafast dynamics, modulation capability and chirp control of directly modulated lasers. Here we demonstrate that ultrafast oscillations of the circular polarization degree can be generated in VCSELs by pulsed spin injection which have the potential to reach frequencies beyond 100 GHz. These oscillations are due to the coupling of the carrier-spin-photon system via the optical birefringence for the linearly polarized laser modes in the micro-cavity and are principally decoupled from conventional relaxation oscillations of the carrier-photon system. Utilizing these polarization oscillations is a very promising path to ultrafast directly modulated spin-VCSELs in the near future as long as an effective concept can be developed to modulate or switch these polarization oscillations. After briefly reviewing the state of research in the emerging field of spin-VCSELs, we present a novel concept for controlled switching of polarization oscillations by use of multiple optical spin injection pulses. Depending on the amplitude and phase conditions of the excitation pulses, constructive or destructive interference of polarization oscillations leads to an excitation, stabilization or switch-off of these oscillations. Furthermore even short single polarization bursts can be generated with pulse widths only limited by the resonance frequency of the polarization oscillation. Consequently, this concept is an important building block for using spin controlled polarization oscillations for future communication applications. © 2014 SPIE.
    view abstract10.1117/12.2063723
  • Spin-controlled ultrafast vertical-cavity surface-emitting lasers
    Höpfner, H. and Lindemann, M. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 9134 (2014)
    Spin-controlled semiconductor lasers are highly attractive spintronic devices providing characteristics superior to their conventional purely charge-based counterparts. In particular, spin-controlled vertical-cavity surface emitting lasers (spin-VCSELs) promise to offer lower thresholds, enhanced emission intensity, spin amplification, full polarization control, chirp control and ultrafast dynamics. Most important, the ability to control and modulate the polarization state of the laser emission with extraordinarily high frequencies is very attractive for many applications like broadband optical communication and ultrafast optical switches. We present a novel concept for ultrafast spin-VCSELs which has the potential to overcome the conventional speed limitation for directly modulated lasers by the relaxation oscillation frequency and to reach modulation frequencies significantly above 100 GHz. The concept is based on the coupled spin-photon dynamics in birefringent micro-cavity lasers. By injecting spin-polarized carriers in the VCSEL, oscillations of the coupled spin-photon system can by induced which lead to oscillations of the polarization state of the laser emission. These oscillations are decoupled from conventional relaxation oscillations of the carrier-photon system and can be much faster than these. Utilizing these polarization oscillations is thus a very promising approach to develop ultrafast spin-VCSELs for high speed optical data communication in the near future. Different aspects of the spin and polarization dynamics, its connection to birefringence and bistability in the cavity, controlled switching of the oscillations, and the limitations of this novel approach will be analysed theoretically and experimentally for spin-polarized VCSELs at room temperature. © 2014 SPIE.
    view abstract10.1117/12.2049684
  • Colliding pulse modelocked lasers for terahertz photomixing
    Brenner, C. and Horstkemper, H. and Mayorga, I.C. and Klehr, A. and Erbert, G. and Hofmann, M.R.
    2013 Conference on Lasers and Electro-Optics Europe and International Quantum Electronics Conference, CLEO/Europe-IQEC 2013 (2013)
    THz time domain spectroscopy systems (TDS) have been finding their way into the market quite recently. Most of these systems use a laser source delivering ultra short laser pulses into a homodyne detection scheme based on photomixers. These sources are usually characterized by pulse lengths below 200fs, bandwidths of several THz and round trip frequencies / repetition rates below 1GHz. All in all this gives remarkable results in terms of signal to noise ratio in the frequency range up to 4THz. But, unfortunately, these sources are also the main cost factor in such THz TDS systems which hampers the possibilities for mass applications. © 2013 IEEE.
    view abstract10.1109/CLEOE-IQEC.2013.6800754
  • Comparison of different metrics for analysis and visualization in spectroscopic optical coherence tomography
    Jaedicke, V. and Agcaer, S. and Robles, F.E. and Steinert, M. and Jones, D. and Goebel, S. and Gerhardt, N.C. and Welp, H. and Hofmann, M.R.
    Biomedical Optics Express 4 (2013)
    Spectroscopic Optical Coherence Tomography (S-OCT) extracts depth resolved spectra that are inherently available from OCT signals. The back scattered spectra contain useful functional information regarding the sample, since the light is altered by wavelength dependent absorption and scattering caused by chromophores and structures of the sample. Two aspects dominate the performance of S-OCT: (1) the spectral analysis processing method used to obtain the spatially-resolved spectroscopic information and (2) the metrics used to visualize and interpret relevant sample features. In this work, we focus on the second aspect, where we will compare established and novel metrics for S-OCT. These concepts include the adaptation of methods known from multispectral imaging and modern signal processing approaches such as pattern recognition. To compare the performance of the metrics in a quantitative manner, we use phantoms with microsphere scatterers of different sizes that are below the system's resolution and therefore cannot be differentiated using intensity based OCT images. We show that the analysis of the spectral features can clearly separate areas with different scattering properties in multi-layer phantoms. Finally, we demonstrate the performance of our approach for contrast enhancement in bovine articular cartilage. © 2013 Optical Society of America.
    view abstract10.1364/BOE.4.002945
  • Contrast enhancement methods in Optical Coherence Tomography using spectral features
    Jaedicke, V. and Aǧcaer, S. and Goebel, S. and Gerhardt, N.C. and Welp, H. and Hofmann, M.R.
    Biomedizinische Technik 58 (2013)
    Spectroscopic Optical Coherence Tomography (SOCT) is an extension of standard OCT. In this approach also the spectrum that is backscattered from the sample is analyzed. Light that has travelled through tissue is altered by the structure and molecular composition of the sample. Hence an analysis of the spectroscopic information is an additional contrast mechanism for imaging of tissue. In our work we compare different approaches to translate the information content of depth resolved spectra obtained by SOCT into a color map that is used to colorize the standard OCT images. The spectroscopic information is strongly affected by noise, caused by various error sources. Therefore the processing has to be insensitive for noise. First results indicate that our approach using pattern recognition algorithms is superior to other methods. © 2013 by Walter de Gruyter.
    view abstract10.1515/bmt-2013-4269
  • High peak power pulses from dispersion optimised modelocked semiconductor laser
    Balzer, J.C. and Schlauch, T. and Klehr, A. and Erbert, G. and Tränkle, G. and Hofmann, M.R.
    Electronics Letters 49 (2013)
    Presented is an electrically pumped passively modelocked edge-emitting semiconductor laser system in an external cavity setup with intracavity dispersion management. This concept, in combination with a pulse compressor, provides pulses as short as 158 fs. By expanding the setup with a tapered diode laser amplifier, peak powers up to 6.5 kW were achieved in the 850 nm wavelength range. © 2013 The Institution of Engineering and Technology.
    view abstract10.1049/el.2013.1447
  • Nonexponential photoluminescence transients in a Ga(NAsP) lattice matched to a (001) silicon substrate
    Jandieri, K. and Kunert, B. and Liebich, S. and Zimprich, M. and Volz, K. and Stolz, W. and Gebhard, F. and Baranovskii, S.D. and Koukourakis, N. and Gerhardt, N.C. and Hofmann, M.R.
    Physical Review B - Condensed Matter and Materials Physics 87 (2013)
    Ga(NAsP) grown lattice-matched on (001) silicon substrate is a very promising material for future integrated, electrically pumped lasers on silicon. Here, we present experimental and theoretical studies of the time-resolved photoluminescence in Ga(NAsP)/Si quantum well structures. The experimental results obtained at 10 K show a strong nonexponential transient behavior for the photoluminescence signal. A detailed comparison with theoretical calculations based on rate equations and on straightforward Monte Carlo simulations reveals that this effect is controlled by an interplay between the fast capture of carriers on nonradiative centers and the slow radiative recombination via localized states. We demonstrate that the measurement of the time-resolved photoluminescence can serve as a convenient tool for estimating the relative concentrations of nonradiative and radiative centers in compound materials. © 2013 American Physical Society.
    view abstract10.1103/PhysRevB.87.035303
  • Quantitative phase analysis through scattering media by depth-filtered digital holography
    Goebel, S. and Jaedicke, V. and Koukourakis, N. and Wiethoff, H. and Adinda-Ougba, A. and Gerhardt, N.C. and Welp, H. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 8589 (2013)
    Digital holography (DH) is capable of providing three-dimensional topological surface profiles with axial resolutions in the nanometer range. To achieve such high resolutions requires an analysis of the phase information of the reflected light by means of numerical reconstruction methods. Unfortunately, the phase analysis of structures located in scattering media is usually disturbed by interference with reflected light from different depths. In contrast, low-coherence interferometry and optical coherence tomography (OCT) use broadband light sources to investigate the sample with a coherence gate providing tomographic measurements in scattering samples with a poorer depth-resolution of a few micrometers. We propose a new approach that allows recovering the phase information even through scattering media. The approach combines both techniques by creating synthesized interference patterns from scanned spectra. After applying an inverse Fourier transform to each spectrum, we yield three-dimensional depth-resolved images. Subsequently, contributions of photons scattered from unwanted regions are suppressed by depth-filtering. The back-transformed data can be considered as multiple synthesized holograms and the corresponding phase information can be extracted directly from the depthfiltered spectra. We used this approach to record and reconstruct holograms of a reflective surface through a scattering layer. Our results demonstrate a proof-of-principle, as the quantitative phase-profile could be recovered and effectively separated from scattering influences. Moreover, additional processing steps could pave the way to further applications, i.e. spectroscopic analysis. © 2013 Copyright SPIE.
    view abstract10.1117/12.2001159
  • Spectroscopic optical coherence tomography with graphics processing unit based analysis of three dimensional data sets
    Jaedicke, V. and Aǧcaer, S. and Goebel, S. and Gerhardt, N.C. and Welp, H. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 8592 (2013)
    Spectroscopic optical coherence tomography (OCT) is an extension of the standard backscattering intensity analysis of OCT. It enables depth resolved monitoring of molecular and structural differences of tissue. One drawback of most methods to calculate the spectroscopic data is the long processing time. Also systematic and stochastic errors make the interpretation of the results challenging. Our approach combines modern signal processing tools with powerful graphics processing unit (GPU) programming. The processing speed for the spectroscopic analysis is nearly 3 mega voxel per second. This allows us to analyze multiple B-Scans in a few seconds and to display the results as a three dimensional data set. Our algorithm contains the following steps in addition to the conventional processing for frequency domain OCT: a quality map to exclude noisy parts of the data, spectral analysis by short time Fourier transform, feature reduction by Principal Component Analysis, unsupervised pattern recognition with K-means and rendering of the gray scale backscattering OCT data which is superimposed with a color map that is based on the results of the pattern recognition algorithm. Our set up provides a spectral range from 650-950nm and is optimized to suppress chromatic errors. In a proof-of-principle attempt, we already achieved additional spectroscopic contrast in phantom samples including scattering microspheres of different sizes and ex vivo biological tissue. This is an important step towards a system for real time spectral analysis of OCT data, which would be a powerful diagnosis tool for many diseases e.g. cancer detection at an early stage. © 2013 Copyright SPIE.
    view abstract10.1117/12.2000464
  • Spin injection, transport, and relaxation in spin light-emitting diodes: Magnetic field effects
    Höpfner, H. and Fritsche, C. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8813 (2013)
    Efficient electrical spin injection into semiconductor based devices at room temperature is one of the most important requirements for the development of applicable spintronic devices in the near future and is thus an important and very active research field. Here we report experimental results for the electrical spin injection in spin light-emitting diodes (spin-LEDs) without external magnetic fields at room temperature. Our devices consist of a Fe/Tb multilayer spin injector with remanent out-of-plane magnetization, an MgO tunnel barrier for efficient spin injection and an InAs quantum dot light-emitting diode. Using a series of samples with different injection path lengths allows us to experimentally determine the spin relaxation during vertical transport from the spin injector to the active region at room temperature. In combination with our concept for remanent spin injection, we are additionally able to investigate the influence of an external magnetic field on the spin relaxation process during transport. While the spin relaxation length at room temperature without external magnetic field is determined to be 27 nm, this value almost doubles if an external magnetic field of 2 Tesla is applied in Faraday geometry. This demonstrates that the results for spin injection and spin relaxation obtained with or without magnetic field can hardly be compared. The efficiency of spin-induced effects is overestimated as long as magnetic fields are involved. Since strong magnetic fields are not acceptable in application settings, this may lead to wrong conclusions and potentially impairs proper device development. © 2013 Copyright SPIE.
    view abstract10.1117/12.2023324
  • Spin relaxation in spin light-emitting diodes: Effects of magnetic field and temperature
    Höpfner, H. and Fritsche, C. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8623 (2013)
    We report experimental results on the electron spin relaxation length during vertical transport in spin lightemitting diodes (LEDs). Our devices are GaAs based LEDs with InAs quantum dots in the active region, an MgO tunnel barrier and an Fe/Tb multilayer spin injector with perpendicular magnetic anisotropy, i.e. remanent out-of-plane magnetization, enabling efficient electrical spin injection in magnetic remanence. Additionally, our devices can be operated at room temperature. A series of samples with different injection path lengths allows us to experimentally determine the spin relaxation length in our devices. In combination with operation in magnetic remanence, we are able to determine the spin relaxation length without the influence of external magnetic fields and at room temperature and find it to be 27 nm. Applying an additional external magnetic field, we find that at a field strength of 2 T, this relaxation length almost doubles, which is in good agreement with spin relaxation times in GaAs. Temperature control of our samples allows us to measure the temperature dependence of the spin relaxation length. At 200 K, the spin relaxation length doubles to 50 nm and reaches 80 nm at 30 K, in good agreement with theoretic calculations. Our results show that polarization values obtained with spin-LEDs inside strong magnetic fields and at low temperatures are not comparable to those in remanence and at room temperature. However, the transfer of efficient spintronic devices to such applicationenabling settings is absolutely necessary and will be a major challenge considering the enormous differences in spin relaxation. © 2013 SPIE.
    view abstract10.1117/12.2001511
  • Spin relaxation length in quantum dot spin LEDs
    Höpfner, H. and Fritsche, C. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Physica Status Solidi (C) Current Topics in Solid State Physics 10 (2013)
    We analyse the spin relaxation length during vertical electron transport in spin light-emitting diode devices at room temperature. We obtain a spin relaxation length of 27 nm in remanence. When a magnetic field is applied, spin relaxation is significantly reduced during transport to the active region of the device. This results in a nearly doubled spin relaxation length at 2T magnetic field strength. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/pssc.201200689
  • Ultrafast spin-polarized vertical-cavity surface-emitting lasers
    Gerhardt, N.C. and Höpfner, H. and Lindemann, M. and Li, M. and Jähme, H. and Ackemann, T. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8813 (2013)
    Spin-polarized lasers are highly attractive spintronic devices providing characteristics superior to their conventional purely charge-based counterparts. Spin-polarized vertical-cavity surface emitting lasers (spin-VCSELs) promise to offer lower thresholds, enhanced emission intensity, spin amplification, full polarization control, chirp control and ultrafast dynamics. In particular, the ability to control and modulate the polarization state of the laser emission with extraordinarily high frequencies is very attractive for many applications like broadband optical communication and ultrafast optical switches. After briefly reviewing the state of research in this emerging field of spintronics, we present a novel concept for ultrafast spin-VCSELs which has the potential to overcome the conventional speed limitation for directly modulated lasers and to reach modulation frequencies significantly above 100 GHz. The concept is based on the coupled spin-photon dynamics in birefringent micro-cavity lasers. By injecting spin-polarized carriers in the VCSEL, oscillations of the coupled spin-photon system can by induced which lead to oscillations of the polarization state of the laser emission. These oscillations are decoupled from conventional relaxation oscillations of the carrier-photon system and can be much faster than those. Utilizing these polarization oscillations is thus a very promising approach to develop ultrafast spin-VCSELs for high speed optical data communication in the near future. © 2013 Copyright SPIE.
    view abstract10.1117/12.2022720
  • All semiconductor high power fs laser system with variable repetition rate
    Balzer, J.C. and Schlauch, T. and Klehr, A. and Erbert, G. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8277 (2012)
    Laser diodes offer an interesting alternative to commercially available light sources for the generation of ultrashort pulses. They have the unique feature that they can be directly electrically pumped and that the emission wavelength can be controlled over a huge spectral range by changing the composition of the laser material. Hence they have the potential of being a highly flexible, compact and cost effective light source. However there is a considerable chirp of the pulses generated by a diode laser as a consequence of the strong coupling of real and imaginary part of the susceptibility in the semiconductor. This problem is solved by using an external cavity with intracavity dispersion management. By applying this technique we are able to generate pulse durations with less then 200 fs if an additional external pulse compressor is used. By using such a cavity in a master oscillator power amplifier setup the peak power can be increased up to 6.5 kW. This enables a huge field of possible applications like time domain terahertz spectroscopy or material processing. Anyway for some applications like fluorescence lifetime imaging even the repetition rate of an external cavity laser is too high. To solve this problem an ultrafast semiconductor pulse picking element is implemented to reduce the repetition rate into the kHz region. In conclusion we will demonstrate a compact all semiconductor laser system which is capable to generate sub ps pulses with a high peak power and a variable repetition rate at central wavelength of approximately 840 nm. © 2012 SPIE.
    view abstract10.1117/12.907953
  • Depth-filtered digital holography
    Koukourakis, N. and Jaedicke, V. and Adinda-Ougba, A. and Goebel, S. and Wiethoff, H. and Höpfner, H. and Gerhardt, N.C. and Hofmann, M.R.
    Optics Express 20 (2012)
    We introduce depth-filtered digital holography (DFDH) as a method for quantitative tomographic phase imaging of buried layers in multilayer samples. The procedure is based on the acquisition of multiple holograms for different wavelengths. Analyzing the intensity over wavelength pixel wise and using an inverse Fourier transform leads to a depth-profile of the multilayered sample. Applying a windowed Fourier transform with a narrow window, we choose a depth-of interest (DOI) which is used to synthesize filtered interference patterns that just contain information of this limited depth. We use the angular spectrum method to introduce an additional spatial filtering and to reconstruct the corresponding holograms. After a short theoretical framework we show experimental proof-of-principle results for the method. © 2012 Optical Society of America.
    view abstract10.1364/OE.20.022636
  • Erratum: Room temperature spin relaxation length in spin light-emitting diodes (Applied Physics Letters (2011) 99 (051102))
    Soldat, H. and Li, M. and Gerhardt, N.C. and Hofmann, M.R. and Ludwig, Ar. and Ebbing, A. and Reuter, D. and Wieck, A.D. and Stromberg, F. and Keune, W. and Wende, H.
    Applied Physics Letters 100 (2012)
    view abstract10.1063/1.4728990
  • High room-temperature optical gain in Ga(NAsP)/Si heterostructures
    Koukourakis, N. and Bückers, C. and Funke, D.A. and Gerhardt, N.C. and Liebich, S. and Chatterjee, S. and Lange, C. and Zimprich, M. and Volz, K. and Stolz, W. and Kunert, B. and Koch, S.W. and Hofmann, M.R.
    Applied Physics Letters 100 (2012)
    We analyze the modal gain of Ga(NAsP) multi quantum-well heterostructures pseudomorphically grown on (001) silicon substrate by metal-organic vapor-phase epitaxy. Using the variable stripe length method, we obtain high modal gain values up to 78 cm -1 at room temperature that are comparable to the values of common high quality III-V laser material. We find good agreement between experimental results and theoretically calculated gain spectra obtained using a microscopic model. The results underline the high potential of Ga(NAsP) as an active material for directly electrically pumped lasers on silicon substrate. © 2012 American Institute of Physics.
    view abstract10.1063/1.3690886
  • Magnetic field dependence of the spin relaxation length in spin light-emitting diodes
    Höpfner, H. and Fritsche, C. and Ludwig, As. and Ludwig, Ar. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Applied Physics Letters 101 (2012)
    We investigate the spin relaxation length during vertical electron transport in spin-light emitting diode devices as a function of magnetic field strength at room temperature. In most publications on spin relaxation in optoelectronic devices, strong magnetic fields are used to achieve perpendicular-to-plane magnetization of the spin injection contacts. We show experimentally that high magnetic field strengths significantly reduce spin relaxation during transport to the active region of the device. We obtain a spin relaxation length of 27(3) nm in magnetic remanence and at room temperature, which nearly doubles at 2 T magnetic field strength. © 2012 American Institute of Physics.
    view abstract10.1063/1.4752162
  • Multispectral photoacoustic coded excitation using pseudorandom codes
    Beckmann, M.F. and Friedrich, C.-S. and Mienkina, M.P. and Gerhardt, N.C. and Hofmann, M.R. and Schmitz, G.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 8223 (2012)
    Photoacoustic imaging (PAI) combines high ultrasound resolution with optical contrast. Laser-generated ultrasound is potentially beneficial for cancer detection, blood oxygenation imaging, and molecular imaging. PAI is generally performed using solid state Nd:YAG lasers in combination with optical parametric oscillators. An alternative approach uses laser diodes with higher pulse repetition rates but lower power. Thus, improvement in signal-to-noise ratio (SNR) is a key step towards applying laser diodes in PAI. To receive equivalent image quality using laser diodes as with Nd:YAG lasers, the lower power must be compensated by averaging, which can be enhanced through coded excitation. In principle, perfect binary sequences such as orthogonal Golay codes can be used for this purpose when acquiring data at multiple wavelengths. On the other hand it was shown for a single wavelength that sidelobes can remain invisible even if imperfect sequences are used. Moreover, SNR can be further improved by using an imperfect sequence compared to Golay codes. Here, we show that pseudorandom sequences are a good choice for multispectral photoacoustic coded excitation (MSPACE). Pseudorandom sequences based upon maximal length shift register sequences (m-sequences) are introduced and analyzed for the purpose of use in MSPACE. Their gain in SNR exceeds that of orthogonal Golay codes for finite code lengths. Artefacts are introduced, but may remain invisible depending on SNR and code length. © 2012 SPIE.
    view abstract10.1117/12.907427
  • Photoluminescence and optical gain of Ga(NAsP) heterostructures pseudomorphically grown on silicon (001) substrate
    Koukourakis, N. and Klimasch, M. and Funke, D.A. and Gerhardt, N.C. and Hofmann, M.R. and Liebich, S. and Zimprich, M. and Kunert, B. and Volz, K. and Stolz, W.
    Proceedings of SPIE - The International Society for Optical Engineering 8266 (2012)
    We present photoluminescence and modal gain measurements in a Ga(NAsP) single-quantum well sample pseudomorphically grown on silicon substrate. The temperature dependence indicates that disorder induced localization effects dominate the low temperature photoluminescence spectra. Nevertheless, using the variable stripe length method, we observe modal gain values up to 15 cm-1 at room temperature. These values are very promising, demonstrate the high optical quality of the new dilute nitride material Ga(NAsP) and underline its candidacy for electrically pumped lasers on silicon substrate. © 2012 SPIE.
    view abstract10.1117/12.907677
  • Room temperature spin relaxation in quantum dot based spin-optoelectronic devices
    Höpfner, H. and Li, M. and Ludwig, Ar. and Ludwig, As. and Stromberg, F. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8260 (2012)
    Spin-optoelectronic devices have become a field of intensive research in the past few years. Here we present electrical spin injection into spin light-emitting diodes both at room temperature and in magnetic remanence. Our devices consist of a Fe/Tb multilayer spin injection structure with remanent out-of-plane magnetization, a MgO tunnel barrier for efficient spin injection and an InAs quantum dot light-emitting diode. The ground state emission and first excited state emission both show circularly polarized emission in remanence, i.e. without external magnetic fields which is due to spin injection from our ferromagnetic contact. Using a series of samples with varying transport path lengths between the spin injector and the active region, we investigate the spin relaxation length during vertical carrier transport through our devices. Due to our spin injector with remanent out-of-plane magnetization this spin relaxation can be investigated without the need for external magnetic fields which would possibly influence the spin relaxation process. The decrease in circular polarization with increasing injection path length is found to be exponential, indicating drift-based transport which is in accordance with theoretic calculations. From the exponential decay the spin relaxation length of 26 nm as well as a lower bound for the spin injection efficiency of 25% are calculated. Additionally, influences of magnetic field, temperature and current density in the devices on the spin relaxation process are discussed. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstract10.1117/12.907821
  • Spin-controlled vertical-cavity surface-emitting lasers
    Gerhardt, N.C. and Hofmann, M.R.
    Advances in Optical Technologies (2012)
    We discuss the concept of spin-controlled vertical-cavity surface-emitting lasers (VCSELs) and analyze it with respect to potential room-temperature applications in spin-optoelectronic devices. Spin-optoelectronics is based on the optical selection rules as they provide a direct connection between the spin polarization of the recombining carriers and the circular polarization of the emitted photons. By means of optical excitation and numerical simulations we show that spin-controlled VCSELs promise to have superior properties to conventional devices such as threshold reduction, spin control of the emission, or even much faster dynamics. Possible concepts for room-temperature electrical spin injection without large external magnetic fields are summarized, and the progress on the field of purely electrically pumped spin-VCSELs is reviewed. Copyright © 2012 Nils C. Gerhardt and Martin R. Hofmann.
    view abstract10.1155/2012/268949
  • Birefringence and spin controlled ultrafast polarization oscillations in vertical-cavity surface-emitting lasers
    Li, M.Y. and Jaehme, H. and Soldat, H. and Gerhardt, N.C. and Hofmann, M.R. and Ackemann, T.
    2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 (2011)
    Optical selection rules for the interaction between light and matter lead to a direct connection between the spin of the carriers, e.g. in GaAs, and the polarization of the emitted or absorbed light. This enables spin-controlled photonic devices for the generation, transport and processing of spin information. For applications, room-temperature, electrically pumped devices are sought after. The spin-controlled vertical-cavity surface-emitting laser (spin-VCSEL) is a much more promising concept for applications than the spin-LED, since cavity feedback effects can amplify the relatively small spin polarization usually achievable under electrical injection at room temperature. In this contribution we present results on the transient spin and polarization dynamics of commercial VCSELs combining electrical with optical pumping, the latter representing a small spin-polarized electrical injection anticipated for future spin-VCSELs. © 2011 IEEE.
    view abstract10.1109/CLEOE.2011.5942549
  • Colliding pulse mode-locked lasers as light sources for single-shot holography
    Grosse, D. and Koukourakis, N. and Gerhardt, N.C. and Schlauch, T. and Balzer, J.C. and Klehr, A. and Erbert, G. and Tränkle, G. and Hofmann, M.R.
    Proceedings of SPIE - The International Society for Optical Engineering 8001 (2011)
    So far, concepts for three dimensional biomedical imaging rely on scanning in at least one dimension. Single-shot holography1, in contrast, stores three-dimensional information encoded in an electro-magnetic wave scattered back from a sample in one single hologram. Single-shot holography operates with simultaneous recordings of holograms at different wavelengths. While the lateral sample information is stored in the interference patterns of individual holograms, the depth information is obtained from the spectral distribution at each lateral image point, similar to Fourier-domain optical coherence tomography2. Consequently, the depth resolution of the reconstructed image is determined by the bandwidth of the light source, so that a broadband light source is needed to obtain high depth resolution. Additionally, the holographic material, in which the holograms are stored, restricts the useable bandwidth. A thick photorefractive crystal can store several holograms of different wavelengths at once. As the crystal works best when using a source with a discrete spectrum, a light source is needed that has a spectrum with well distinguishable laser lines. In a proof-of-principle experiment, we use colliding pulse mode-locked (CPM)3 laser diodes as light sources with a comb-like spectrum to demonstrate the concept of single-shot holography by storing multiple holograms at the same time in a photorefractive Rh:BaTiO3 crystal. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstract10.1117/12.892185
  • Electrical spin injection in InAs quantum dots at room temperature and adjustment of the emission wavelength for spintronic applications
    Ludwig, Ar. and Roescu, R. and Rai, A.K. and Trunov, K. and Stromberg, F. and Li, M. and Soldat, H. and Ebbing, A. and Gerhardt, N.C. and Hofmann, M.R. and Wende, H. and Keune, W. and Reuter, D. and Wieck, A.D.
    Journal of Crystal Growth 323 (2011)
    We have observed room temperature (RT) electrical spin injection in an InAs quantum dot (QD) light emitting diode (LED) grown on a p-type GaAs substrate from a ferromagnetic Fe/Tb electrode with strong out-of-plane anisotropy in remanence, i.e. without applied magnetic field. The QDs in the LED emit at 1275 nm (ground state luminescence), which is beyond the range for highly sensitive detectors, and therefore not optimum for various applications, e.g. quantum information studies. We will present two different ways to blue-shift the emission wavelength and discuss the advantages and drawbacks of the experiments. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jcrysgro.2010.09.087
  • Enhanced photoelectrochemical properties of WO3 thin films fabricated by reactive magnetron sputtering
    Vidyarthi, V.S. and Hofmann, M. and Savan, A. and Sliozberg, K. and König, D. and Beranek, R. and Schuhmann, W. and Ludwig, Al.
    International Journal of Hydrogen Energy 36 (2011)
    Polycrystalline WO3 thin films were fabricated by reactive magnetron sputtering at a substrate temperature of 350 °C under different Ar/O2 gas pressures. In order to study the thickness dependence of photoelectrochemical (PEC) behavior of WO3, the thickness-gradient films were fabricated and patterned using a micro-machined Si-shadow mask during the deposition process. The variation of the sputter pressure leads to the evolution of different microstructures of the thin films. The films fabricated at 2 mTorr sputter pressure are dense and show diminished PEC properties, while the films fabricated at 20 mTorr and 30 mTorr are less dense and exhibit enhanced water photooxidation efficiency. The enhanced photooxidation is attributed to the coexistence of porous microstructure and space charge region enabling improved charge carrier transfer to the electrolyte and back contact. A steady-state photocurrent as high as 2.5 mA cm-2 at 1 V vs. an Ag/AgCl (3 M KCl) reference electrode was observed. For WO3 films fabricated at 20 mTorr and 30 mTorr, the photocurrent increases continuously up to a thickness of 600 nm. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
    view abstract10.1016/j.ijhydene.2011.01.087
  • Fastly tunable external-cavity diode-lasers controlled by electro-absorption modulators
    Jähme, H. and Bülters, M. and Jäger, D. and Hofmann, M.
    2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 (2011)
    External-cavity diode-lasers are useful for numerous applications, because of their compactness, low cost, and broad tuning range. However, the tuning-speed of typical external-cavity setups (i.e. Littman) is typically limited to the kHz range by their mechanical components. © 2011 IEEE.
    view abstract10.1109/CLEOE.2011.5942626
  • High modal gain in Ga(NAsP)/(BGa)((As)P) heterostructures grown lattice matched on (001) silicon
    Koukourakis, N. and Funke, D.A. and Gerhardt, N.C. and Hofmann, M.R. and Liebich, S. and Bückers, C. and Zinnkann, S. and Zimprich, M. and Beyer, A. and Chatterjee, S. and Koch, S.W. and Kunert, B. and Volz, K. and Stolz, W.
    Proceedings of SPIE - The International Society for Optical Engineering 7939 (2011)
    We present modal gain measurements in Ga(NAsP) heterostructures pseudomorphically grown on silicon substrate. Using the variable stripe length method we analyze the modal gain performance of an unprocessed single quantum well sample for different excitation densities. We obtain high modal gain values up to 55 cm-1 at room temperature. These values are comparable to those of common high quality laser material. This demonstrates the high optical quality of the new dilute nitride material Ga(NAsP) and underlines its candidacy for electrically pumped lasing on silicon substrate. © 2011 SPIE.
    view abstract10.1117/12.873170
  • High-throughput characterization of film thickness in thin film materials libraries by digital holographic microscopy
    Lai, Y.W. and Krause, M. and Savan, A. and Thienhaus, S. and Koukourakis, N. and Hofmann, M.R. and Ludwig, Al.
    Science and Technology of Advanced Materials 12 (2011)
    A high-throughput characterization technique based on digital holography for mapping film thickness in thin-film materials libraries was developed. Digital holographic microscopy is used for fully automatic measurements of the thickness of patterned films with nanometer resolution. The method has several significant advantages over conventional stylus profilometry: it is contactless and fast, substrate bending is compensated, and the experimental setup is simple. Patterned films prepared by different combinatorial thin-film approaches were characterized to investigate and demonstrate this method. The results show that this technique is valuable for the quick, reliable and high-throughput determination of the film thickness distribution in combinatorial materials research. Importantly, it can also be applied to thin films that have been structured by shadow masking. © 2011 National Institute for Materials Science.
    view abstract10.1088/1468-6996/12/5/054201
  • Modelocked semiconductor laser system with pulse picking for variable repetition rate
    Balzer, J.C. and Schlauch, T. and Hoffmann, Th. and Klehr, A. and Erbert, G. and Hofmann, M.R.
    Electronics Letters 47 (2011)
    The generation of picosecond pulses from a hybrid modelocked semiconductor master oscillator power amplifier system with a repetition rate as low as 340kHz is presented. The fundamental repetition rate of the external cavity laser was 348MHz and is reduced by an integrated ultrafast semiconductor pulse picker element which picks and amplifies only every 2nth pulse. The central wavelength was around 850nm. © 2011 The Institution of Engineering and Technology.
    view abstract10.1049/el.2011.3378
  • Passively mode-locked two section laser diode with intracavity dispersion control
    Schlauch, T. and Balzer, J.C. and Hofmann, M.R. and Klehr, A. and Erbert, G. and Tränkle, G.
    Proceedings of SPIE - The International Society for Optical Engineering 7937 (2011)
    Ultrashort laser pulses with a duration of 200 fs were obtained from a passively modelocked external cavity diode laser at 830 nm emission wavelength. By intracavity dispersion control the spectral bandwidth is increased and the emitted pulses are compressed externaly by a grating compressor. A tapered amplifier is used to achieve peak powers of up to 2.5 kW. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).
    view abstract10.1117/12.873831
  • Photoacoustic coded excitation using periodically perfect sequences
    Beckmann, M.F. and Mienkina, M.P. and Schmitz, G. and Friedrich, C.-S. and Gerhardt, N.C. and Hofmann, M.R.
    IEEE International Ultrasonics Symposium, IUS (2011)
    Photoacoustic imaging is based on the generation of ultrasound using laser irradiation. Nd:YAG laser systems are commonly employed for this purpose, but cheap and handy pulsed laser diodes can be an attractive alternative. They emit significantly lower pulse energies, but fast averaging is feasible due to high repetition rates. Averaging is limited by the time-of-flight of the acoustic signal, but coded excitation can be used to overcome this limit. Here, we examine the performance of difference set based sequences with perfect correlation properties (periodically perfect sequences, PPS). PPS can be used for continuous, artifact free acquisition, the acquisition scheme is simpler than for all previously reported coding strategies. The coding gain reached for periodic imaging is higher than for Golay codes and Legendre sequences. © 2011 IEEE.
    view abstract10.1109/ULTSYM.2011.0290
  • Photorefractive two-wave mixing for image amplification in digital holography
    Koukourakis, N. and Abdelwahab, T. and Li, M.Y. and Höpfner, H. and Lai, Y.W. and Darakis, E. and Brenner, C. and Gerhardt, N.C. and Hofmann, M.R.
    Optics Express 19 (2011)
    We use photorefractive two-wave mixing for coherent amplification of the object beam in digital holographic recording. Both amplitude and phase reconstruction benefit from the prior amplification as they have an increased SNR. We experimentally verify that the amplification process does not affect the phase of the wavefield. This allows for digital holographic phase analysis after amplification. As the grating formation in photorefractive crystals is just driven by coherent light, the crystal works as a coherence gate. Thus the proposed combination allows for applying digital holography for imaging through scattering media, after the image bearing light is coherence gated and filtered out of scattered background. We show experimental proof-of principle results. © 2011 Optical Society of America.
    view abstract10.1364/OE.19.022004
  • Quantitative photoacoustic blood oxygenation measurement of whole porcine blood samples using a multi-wavelength semiconductor laser system
    Friedrich, C. S. and Mienkina, M. P. and Brenner, C. and Gerhardt, N. C. and Jorger, M. and Strauss, A. and Beckmann, M. F. and Schmitz, G. and Hofmann, M. R.
    Diffuse Optical Imaging Iii 8088 (2011)
    We present a photoacoustic measurement system based on semiconductor lasers for blood oxygenation measurements. It permits to use four different optical wavelengths (650nm, 808nm, 850nm, 905nm) to generate photoacoustic signals. As the optical extinction coefficient of oxygenated hemoglobin and deoxygenated hemoglobin is different at specific wavelengths, a blood oxygenation measurement by a multi-wavelength photoacoustic laser system is feasible. Especially at 650nm, the clear difference between the extinction coefficients of the two hemoglobin derivates permits to determine the blood oxygenation in combination with other near infrared wavelengths. A linear model based on tabulated values of extinction coefficients for fully oxygenated and fully deoxygenated hemoglobin is presented. We used heparin stabilized whole porcine blood samples to model the optical behavior of human blood, as the optical absorption behavior of porcine hemoglobin does not differ significantly from human hemoglobin. To determine the real oxygen saturation values of the blood samples, we measured the partial oxygen pressure with an IRMA Trupoint Blood Analysis System. The oxygen saturation values were calculated from a dissociation curve for porcine blood. The results of the photoacoustic measurement are in qualitatively good agreement with the predicted linear model. Further, we analyze the abilities and the limitations of quantitative oxygenation measurements.
    view abstract10.1117/12.889682
  • Room temperature spin relaxation length in spin light-emitting diodes
    Soldat, H. and Li, M. and Gerhardt, N.C. and Hofmann, M.R. and Ludwig, Ar. and Ebbing, A. and Reuter, D. and Wieck, A.D. and Stromberg, F. and Keune, W. and Wende, H.
    Applied Physics Letters 99 (2011)
    We investigate the spin relaxation length in GaAs spin light-emitting diode devices under drift transport at room temperature. The spin-polarised electrons are injected through a MgO tunnel barrier from a Fe/Tb multilayer in magnetic remanence. The decrease in circular polarization with increasing injection path length is investigated and found to be exponential, supporting drift-based transport. The spin relaxation length in our samples is 26 nm, and a lower bound for the spin injection efficiency at the spin injector/GaAs interface is estimated to be 25±2%. © 2011 American Institute of Physics.
    view abstract10.1063/1.3622662
  • Semiconductor diode lasers for terahertz technology
    Brenner, C. and Friedrich, C.-S. and Hofmann, M.R.
    Journal of Infrared, Millimeter, and Terahertz Waves 32 (2011)
    We discuss different concepts for generating terahertz (THz) radiation with semiconductor diode lasers. Photomixing enables the generation of continuous wave THz radiation by difference frequency generation of two lasers or of two-colour lasers. Pulsed THz radiation for time domain THz spectroscopy is generated with modelocked diode laser systems including amplification and chirp compression. Finally, we analyse the concept of quasi time domain spectroscopy based on broadband diode laser systems. © 2011 Springer Science+Business Media, LLC.
    view abstract10.1007/s10762-011-9815-4
  • Shoe box all semiconductor high power femtosecond laser system
    Balzer, J.C. and Schlauch, T. and Hofmann, M.R. and Klehr, A. and Erbert, G.
    2011 Conference on Lasers and Electro-Optics Europe and 12th European Quantum Electronics Conference, CLEO EUROPE/EQEC 2011 (2011)
    Mode-locked laser diodes are attractive sources for the generation of ultra short pulses. These systems are particularly interesting as an alternative to conventional femtosecond lasers like Ti:sapphire lasers, which are rather complex and expensive. For example, a compact diode based system was already shown to replace successfully a Ti:sapphire laser in a Terahertz time-domain spectrometer [1]. © 2011 IEEE.
    view abstract10.1109/CLEOE.2011.5942508
  • Single-shot holography with colliding pulse mode-locked lasers as light source
    Grosse, D. and Koukourakis, N. and Gerhardt, N.C. and Schlauch, T. and Balzer, J.C. and Klehr, A. and Erbert, G. and Tränkle, G. and Hofmann, M.R.
    2011 Int. Quantum Electron. Conf., IQEC 2011 and Conf. Lasers and Electro-Optics, CLEO Pacific Rim 2011 Incorporating the Australasian Conf. Optics, Lasers and Spectroscopy and the Australian Conf. (2011)
    So far, concepts for three dimensional biomedical imaging rely on scanning in at least one dimension. Single-shot holography [1], in contrast, stores three-dimensional information encoded in an electromagnetic wave scattered back from a sample in one single hologram. Single-shot holography operates with simultaneous recordings of holograms at different wavelengths. While the lateral sample information is stored in the interference patterns of individual holograms, the depth information is obtained from the spectral distribution at each lateral image point, similar to Fourier-domain optical coherence tomography [2]. Consequently, the depth resolution of the reconstructed image is determined by the bandwidth of the light source, so that a broadband light source is needed to obtain high depth resolution. Additionally, the holographic material, in which the holograms are stored, restricts the useable bandwidth. A thick photorefractive crystal can store several holograms of different wavelengths at once. As the crystal works best when using a source with a discrete spectrum, a light source is needed that has a spectrum with well distinguishable laser lines. In a proof-of-principle experiment, we use colliding pulse mode-locked (CPM) laser diodes [3] as light sources with a comb-like spectrum to demonstrate the concept of single-shot holography by storing multiple holograms at the same time in a photorefractive Rh:BaTiO3 crystal. © 2011 IEEE.
    view abstract10.1109/IQEC-CLEO.2011.6194017
  • Spectroscopic optical coherence tomography for substance identification
    Jaedicke, V. and Wiethoff, H. and Agcaer, S. and Kasseck, C. and Gerhardt, N. C. and Welp, H. and Hofmann, M. R.
    Optical Coherence Tomography and Coherence Techniques V 8091 (2011)
    We use Spectroscopic Optical Coherence Tomography (S-OCT) to identify substances by their spectral features in multi layer non-scattering samples. Depth resolved spectra are calculated by a windowed Fourier Transform in the spatial regime at discrete layer borders. By dividing subsequent spectra in an iterative manner transfer functions of the samples layers are calculated. Estimating these spectral transfer functions with high accuracy is still challenging, since the system's transfer function introduces an error, which can be orders of magnitude higher than the spectroscopic information of the sample. We retrieve the buried spectroscopic information of the sample with high accuracy by correcting the spectral transfer functions with an identically structured reference sample. This spectral calibration method has many critical parameters and is in many cases not even possible. To perform substance identification without spectral calibration we implemented a pattern recognition algorithm, which allocates the transfer functions to known substances. Our results show that substance identification by spectral features with high performance without spectral calibration is feasible. Aside from that we modeled a simplified set up of our OCT system to minimize the error which is introduced by the optical system. The error can be reduced by orders of magnitude, when our improved optical set-up is used. This is an important step towards an improved system for S-OCT.
    view abstract10.1117/12.889651
  • Spin induced gigahertz polarization oscillations in vertical-cavity surface-emitting laser devices
    Li, M.Y. and Jaehme, H. and Soldat, H. and Gerhardt, N.C. and Hofmann, M.R. and Ackemann, T.
    Proceedings of SPIE - The International Society for Optical Engineering 7952 (2011)
    Spin-controlled vertical-cavity surface-emitting lasers (VCSELs) have been intensively studied in recent years because of the low threshold feasibility and the nonlinearity above threshold, which make spin-VCSELs very promising for spintronic devices. Here we investigate the circular polarization dynamics of VCSELs on a picosecond time scale after pulsed optical spin injection at room temperature. A hybrid excitation technique combining continuous-wave (cw) unpolarized electrical excitation slightly above threshold and pulsed polarized optical excitation is applied. The experimental results demonstrate ultrafast circular polarization oscillations with a frequency of about 11 GHz. The oscillations last inside the first undulation of the intensity relaxation oscillations. Via theoretical calculations based on a rate equation model we analyze these oscillations as well as the underlying physical mechanisms. © 2011 Copyright SPIE - The International Society for Optical Engineering.
    view abstract10.1117/12.873758
  • Time-resolved photoluminescence and optical gain of Ga(NAsP) heterostructures pseudomorphically grown on silicon (001) substrate
    Gerhardt, N.C. and Koukourakis, N. and Klimasch, M. and Funke, D.A. and Hofmann, M.R. and Kunert, B. and Liebich, S. and Trusheim, D. and Zimprich, M. and Volz, K. and Stolz, W.
    International Conference on Transparent Optical Networks (2011)
    The novel metastable dilute nitride material Ga(NAsP) is a very promising candidate for electrically pumped lasers on silicon because it can be pseudomorphically grown on silicon substrate. Here we investigate the optical properties of a series of multi-quantum well Ga(NAsP) samples grown lattice matched on GaP and Si substrates. Temperature and excitation resolved photoluminescence spectroscopy indicates a significant impact of disorder-induced carrier localization effects on the optical properties. On the other hand, optical gain measurements reveal high modal gain up to 80 cm -1 at room temperature and demonstrate the suitability of this new material as an active material for laser devices. A comparative analysis of optical gain and photoluminescence data demonstrates a strong impact of the barrier-growth conditions on the optical quality of the material. © 2011 IEEE.
    view abstract10.1109/ICTON.2011.5970794
  • Ultrafast spin-induced polarization oscillations with tunable lifetime in vertical-cavity surface-emitting lasers
    Gerhardt, N.C. and Li, M.Y. and Jhme, H. and Hpfner, H. and Ackemann, T. and Hofmann, M.R.
    Applied Physics Letters 99 (2011)
    We report spin-induced polarization oscillations in vertical-cavity surface-emitting lasers above threshold and at room temperature. The oscillation frequency is 11.6 GHz, which is significantly higher than the modulation bandwidth of less than 4 GHz in the device. The oscillation frequency is determined by an additional resonance frequency in birefringence containing microcavities, which is potentially much higher than the conventional relaxation oscillation frequency. The damping of the oscillations can be controlled by the current, allowing for oscillation lifetimes much longer than the spin lifetime in the device as well as for short bursts potentially interesting for information transmission. © 2011 American Institute of Physics.
    view abstract10.1063/1.3651339
  • Birefringence controlled room-temperature picosecond spin dynamics close to the threshold of vertical-cavity surface-emitting laser devices
    Li, M.Y. and Jähme, H. and Soldat, H. and Gerhardt, N.C. and Hofmann, M.R. and Ackemann, T.
    Applied Physics Letters 97 (2010)
    We analyze the spin-induced circular polarization dynamics at the threshold of vertical-cavity surface-emitting lasers at room-temperature using a hybrid excitation combining electrically pumping without spin preference and spin-polarized optical injection. After a short pulse of spin-polarized excitation, fast oscillations of the circular polarization degree (CPD) are observed within the relaxation oscillations. A theoretical investigation of this behavior on the basis of a rate equation model shows that these fast oscillations of CPD could be suppressed by means of a reduction of the birefringence of the laser cavity. © 2010 American Institute of Physics.
    view abstract10.1063/1.3515855
  • Compact diode-laser-based system for continuous-wave and quasi-time-domain terahertz spectroscopy
    Brenner, C. and Hofmann, M. and Scheller, M. and Shakfa, M.K. and Koch, M. and Mayorga, I.C. and Klehr, A. and Erbert, G. and Tränkle, G.
    Optics Letters 35 (2010)
    We present a multimodal diode-laser-based terahertz (THz) spectroscopy system. In contrast to other laser-based THz setups that provide either cw or broadband THz generation, our configuration combines the advantages of both approaches. Our low complexity setup enables fast switching from cw difference frequency generation to broadband THz emission, enabling sophisticated data analysis like much more complex time domain spectroscopy systems. © 2010 Optical Society of America.
    view abstract10.1364/OL.35.003859
  • Comparison of optical coherence tomography, microcomputed tomography, and histology at a three-dimensionally imaged trabecular bone sample
    Kasseck, C. and Kratz, M. and Torcasio, A. and Gerhardt, N.C. and Van Lenthe, G.H. and Gambichler, T. and Hoffmann, K. and Jones, D.B. and Hofmann, M.R.
    Journal of Biomedical Optics 15 (2010)
    We investigate optical coherence tomography (OCT) as a method for imaging bone. The OCT images are compared directly to those of the standard methods of bone histology and microcomputed tomography (μCT) on a single, fixed human femoral trabecular bone sample. An advantage of OCT over bone histology is its noninvasive nature. OCT also images the lamellar structure of trabeculae at slightly higher contrast than normal bone histology. While μCT visualizes the trabecular framework of the whole sample, OCT can image additionally cells with a penetration depth limited approximately to 1 mm. The most significant advantage of OCT, however, is the absence of toxic effects (no ionizing radiation), i.e., continuous images may be made and individual cell tracking may be performed. The penetration depth of OCT, however, limits its use to small animal models and small bone organ cultures. © 2010 Society of Photo-Optical Instrumentation Engineers.
    view abstract10.1117/1.3477193
  • Epitaxial growth and interfacial magnetism of spin aligner for remanent spin injection: [Fe/Tb]n /Fe/MgO/GaAs -light emitting diode as a prototype system
    Schuster, E. and Brand, R.A. and Stromberg, F. and Lo, F.-Y. and Ludwig, Ar. and Reuter, D. and Wieck, A.D. and Hövel, S. and Gerhardt, N.C. and Hofmann, M.R. and Wende, H. and Keune, W.
    Journal of Applied Physics 108 (2010)
    We have successfully grown and characterized [Fe/Tb]10 /Fe (001) / 57Fe (001) /MgO (001) multilayer contacts on a GaAs-based light emitting diode. Using 57Fe conversion-electron Mössbauer spectroscopy at room temperature (RT) and at 4.2 K, we provide atomistic proof of large perpendicular Fe spin components in zero external field at and below RT at the 57Fe (001) /MgO (001) interface. Further, indirect evidence of large interfacial Fe atomic moments is provided. Our contacts serve as a prototype spin aligner for remanent electrical spin injection at RT. © 2010 American Institute of Physics.
    view abstract10.1063/1.3476265
  • Experimental evaluation of photoacoustic coded excitation using unipolar golay codes
    Mienkina, M.P. and Friedrich, C.-S. and Gerhardt, N.C. and Wilkening, W.G. and Hofmann, M.R. and Schmitz, G.
    IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control 57 (2010)
    Q-switched Nd:YAG lasers are commonly used as light sources for photoacoustic imaging. However, laser diodes are attractive as an alternative to Nd:YAG lasers because they are less expensive and more compact. Although laser diodes deliver about three orders of magnitude less light pulse energy than Nd:YAG lasers (tens of microjoules compared with tens of millijoules), their pulse repetition frequency (PRF) is four to five orders of magnitude higher (up to 1 MHz compared with tens of hertz); this enables the use of averaging to improve SNR without compromising the image acquisition rate. In photoacoustic imaging, the PRF is limited by the maximum acoustic time-of-flight. This limit can be overcome by using coded excitation schemes in which the coding eliminates ambiguities between echoes induced by subsequent pulses. To evaluate the benefits of photoacoustic coded excitation (PACE), the performance of unipolar Golay codes is investigated analytically and validated experimentally. PACE imaging of a copper slab using laser diodes at a PRF of 1 MHz and a modified clinical ultrasound scanner is successfully demonstrated. Considering laser safety regulations and taking into account a comparison between a laser diode system and Nd:YAG systems with respect to SNR, we conclude that PACE is feasible for small animal imaging. © 2006 IEEE.
    view abstract10.1109/TUFFC.2010.1588
  • Femtosecond passively modelocked diode laser with intracavity dispersion management
    Schlauch, T. and Balzer, J.C. and Klehr, A. and Erbert, G. and Tränkle, G. and Hofmann, M.R.
    Optics Express 18 (2010)
    We report on the generation of ultrashort pulses by dispersion management of a passively modelocked external cavity diode laser. Pulse widths down to 200 fs are obtained at 830nm emission wavelength. We use intracavity dispersion management to increase the spectral bandwidth and compress the strongly chirped pulses externally with a grating compressor. © 2010 Optical Society of America.
    view abstract10.1364/OE.18.024316
  • Frequency domain optical coherence tomography with subsequent depth resolved spectroscopic image analysis
    Kasseck, C. and Jaedicke, V. and Gerhardt, N.C. and Welp, H. and Hofmann, M.R.
    Progress in Biomedical Optics and Imaging - Proceedings of SPIE 7554 (2010)
    We present a method to obtain additional depth resolved spectroscopic information from standard frequency domain optical coherence tomography (FDOCT) images. This method utilizes Fourier transforms of signal peaks within the complex FDOCT depth profiles to extract depth resolved spectroscopic information. For verification of the depth resolved spectroscopic image analysis method, theoretical simulations as well as experimental studies are demonstrated. Both show accurate depth resolved spectroscopic reconstruction enabling a depth allocation of material specific transmission spectra due to absorption. This analysis tool improves significantly the image contrast and allows image mapping of material specific spectral characteristics. © 2010 Copyright SPIE - The International Society for Optical Engineering.
    view abstract10.1117/12.840029
  • Integrity of micro-hotplates during high-temperature operation monitored by digital holographic microscopy
    Lai, Y.W. and Koukourakis, N. and Gerhardt, N.C. and Hofmann, M.R. and Meyer, R. and Hamann, S. and Ehmann, M. and Hackl, K. and Darakis, E. and Ludwig, Al.
    Journal of Microelectromechanical Systems 19 (2010)
    An investigation on the integrity of micro-hotplates using in situ digital holographic microscopy is reported. The surface topography and surface evolution of the devices during high-temperature operation (heating/cooling cycles) is measured with nanometer-scale resolution. A localized permanent out-of-plane surface deformation of 40% of the membrane thickness caused by the top measurement electrodes occurring after the first cycle is observed. The integrity-related issues caused by such a permanent deformation are discussed. © 2006 IEEE.
    view abstract10.1109/JMEMS.2010.2067442
  • Monospectral photoacoustic imaging using Legendre sequences
    Beckmann, M.F. and Mienkina, M.P. and Schmitz, G. and Friedrich, C.-S. and Gerhardt, N.C. and Hofmann, M.R.
    Proceedings - IEEE Ultrasonics Symposium (2010)
    Photoacoustic (PA) imaging is an imaging modality based on the generation of ultrasound using laser irradiation. Pulsed laser diodes are an attractive alternative to Q-switched Nd:YAG lasers since they are cheaper and handier. As acoustic time-of-flight limits the pulse repetition frequency (PRF) for averaging, photoacoustic coded excitation (PACE) can be used to enhance the diodes' low signal to noise ratio (SNR). Strategies exhibiting range side lobes can yield higher SNR than previously proposed perfect methods while using simpler code sending procedures. Here, we examine the performance of Legendre sequences (LGS) for PACE. The gain in SNR compared to time equivalent averaging (coding gain) is derived as a function of code length and verified experimentally. The main lobe to peak side lobe ratio (MPSR) of the codes' autocorrelation functions is used to quantify the artifacts introduced by the codes' range side lobes. The coding gain is asymptotically equal to that of previously proposed methods, such as Golay codes (GC) or Simplex codes (SC). For finite code sending time, it exceeds the gain of GC and SC. For a PRF of 500 kHz and an imaging depth of 4.5 cm, the coding gain ranges from about 2 dB (11 bit sequence) to about 5.8 dB (547 bit sequence). Range side lobes are introduced but remain invisible for sufficiently large code lengths, which are necessary for practical applications. © 2010 IEEE.
    view abstract10.1109/ULTSYM.2010.5935648
  • Multispectral photoacoustic coded excitation imaging using unipolar orthogonal Golay codes
    Mienkina, M.P. and Friedrich, C.-S. and Gerhardt, N.C. and Beckmann, M.F. and Schiffner, M.F. and Hofmann, M.R. and Schmitz, G.
    Optics Express 18 (2010)
    We present a method to speed up the acquisition of multispectral photoacoustic data sets by using unipolar orthogonal Golay codes as excitation sequences for the irradiation system. Multispectral photoacoustic coded excitation (MS-PACE) allows acquiring photoacoustic data sets for two irradiation wavelengths simultaneously and separating them afterwards, thus improving the SNR or speeding up the measurement. We derive an analytical estimation of the SNR improvement using MS-PACE compared to time equivalent averaging. We demonstrate the feasibility of the method by successfully imaging a phantom composed of two dyes using unipolar orthogonal Golay codes as excitation sequence for two high power laser diodes operating at two different wavelengths. The experimental results show very good agreement with the theoretical predictions. ©2010 Optical Society of America.
    view abstract10.1364/OE.18.009076
  • Substance identification by depth resolved spectroscopic pattern reconstruction in frequency domain optical coherence tomography
    Kasseck, C. and Jaedicke, V. and Gerhardt, N.C. and Welp, H. and Hofmann, M.R.
    Optics Communications 283 (2010)
    We analyze a method to extract additional depth resolved spectroscopic information from frequency domain optical coherence tomography (FDOCT) data. The reconstruction of depth resolved spectra is obtained by a Fourier transform of the individual peaks in the complex FDOCT depth profiles. We demonstrate a validation of this concept with theoretical simulations and with accurate experimental studies on a multilayer sample with four different characteristic absorbers. The spatially resolved spectroscopic patterns of all individual sample layers are calculated from the depth resolved reconstructed spectra. With an additional pattern recognition algorithm, these reconstructed patterns are compared automatically to the spectral characteristics of the expected substances. This provides an allocation of the reconstructed spectra to the substances with high reliability. Thus, we present an automated substance identification directly from conventional FDOCT data, which increases significantly the information content of the image. © 2010 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.optcom.2010.06.073
  • Ultrafast circular polarization oscillations in spin-polarized vertical-cavity surface-emitting laser devices
    Gerhardt, N. C. and Li, M. and Jaehme, H. and Soldat, H. and Hofmann, M. R. and Ackemann, T.
    Physics and Simulation of Optoelectronic Devices Xviii 7597 (2010)
    Spin-polarized lasers offer new encouraging possibilities for future devices. We investigate the polarization dynamics of electrically pumped vertical-cavity surface-emitting lasers after additional spin injection at room temperature. We find that the circular polarization degree exhibits faster dynamics than the emitted light. Moreover the experimental results demonstrate a strongly damped ultrafast circular polarization oscillation due to spin injection with an oscillation frequency of approximately 11GHz depending on the birefringence in the VCSEL device. We compare our experimental results with theoretical calculations based on rate-equations. This allows us to predict undamped long persisting ultrafast polarization oscillations, which reveal the potential of spin-VCSELs for ultrafast modulation applications.
    view abstract10.1117/12.841606
  • lasers

  • magnetoelectronics

  • microoptics

  • polarization

  • spin dynamics

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