Fraunhofer-Institut für Optronik, Systemtechnik und Bildauswertung IOSB

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  • Publication
    Influence of bandwidth error on the performance of adaptive optics systems for uncooperative beacons
    ( 2021)
    Toselli, Italo
    ;
    Gladysz, Szymon
    We discuss the capability of adaptive optics to increase the performance of laser systems operating in atmospheric turbulence. Our approach is based on the Zernike filter functions, Taylor's frozen-flow hypothesis, and bandwidth limitations of a realistic servo control system. System performance is analyzed in terms of the Strehl ratio on target. Our results for plane-wave geometry indicate that adaptive optics can be effective even when engaging fast moving targets and that moderate closed-loop bandwidths of ∼100Hz would suffice for most analyzed scenarios. Applications of interest are beam delivery systems and free-space optical communications.
  • Publication
    Wavefront sensing for terrestrial, underwater, and space-borne free-space optical communications
    ( 2021)
    Gladysz, Szymon
    ;
    Zepp, Andreas
    ;
    Segel, Max
    ;
    McDonald, Douglas
    ;
    Bellossi, Raphaël
    ;
    Lechner, Daniel
    ;
    Galicia Gasperin, Osvaldo Javier
    ;
    Stein, Karin
    We present several solutions to problems particular to adaptive optics for free-space laser-based communications. Specifically, for scenarios where strong scintillation is present, we have developed a digital, adaptable Shack-Hartmann wavefront sensor, as well as the modal holographic wavefront sensor based on the Karhunen-Loève modes. Additionally, using the same modal basis and optimization algorithms from deep learning, we have improved upon stochastic parallel gradient descent wavefront-sensorless approach. For underwater communications, we have set up a water tank and demonstrated real-time adaptive optics in the visible. For deep-space downlinks, we have investigated several wavefront sensing modalities with respect to their robustness to very low signal-to-background ratios expected during daytime. We also present results of data transmission experiments using coherent modulation over a 400-m double-pass horizontal link.
  • Publication
    Optimization of the holographic wavefront sensor for open-loop adaptive optics under realistic turbulence. Pt.I: Simulations
    ( 2021)
    Zepp, Andreas
    ;
    Gladysz, Szymon
    ;
    Stein, Karin
    ;
    Osten, Wolfgang
    The modal holographic wavefront sensor enables fast measurement of individual aberration modes without the need for time-consuming calculations. However, the measurement accuracy suffers greatly from intermodal crosstalk, caused when the wavefront contains more aberrations than the one to be measured. In this paper, we present sensor optimization to minimize this effect and show the improvement when using Karhunen-Lòeve instead of Zernike modes as the basis. Finally, we show in simulation that an open-loop adaptive optics system based on the optimized sensor can be used to correct the effect of realistic, dynamic atmospheric turbulence on a wavefront and increase its Strehl ratio significantly.
  • Publication
    Simulation of an optimized holographic wavefront sensor for realistic turbulence scenarios
    ( 2021)
    Zepp, Andreas
    ;
    Gladysz, Szymon
    ;
    Stein, Karin
    ;
    Osten, Wolfgang
    Atmospheric turbulence limits the performance of laser systems operating within the atmosphere. Therefore, adaptive optics systems are designed to measure and correct the effects of turbulence in real-time. A crucial part of such a system is the wavefront sensor. The modal holographic wavefront sensor is a promising alternative to well-established sensor types (e.g. Shack-Hartmann wavefront sensor). It measures the strength of individual aberration modes directly. Since there is no need for complex calculations, bandwidths in the megahertz range are possible. However, different aberration modes present in the laser beam influence each other's measurements. This inter-modal crosstalk has a very significant impact on the performance of the sensor. Careful design of the holographic sensor can reduce this influence. In this paper we show a method to optimize the sensor design for a given turbulence strength. We use a merit function to find the optimal combination of two design parameters: the detector size and the phase bias. This optimization is done on a mode-by-mode basis. We simulate realistic turbulence scenarios and evaluate the performance of the optimized holographic sensor. By considering the expected turbulence strength during the design process, we can increase the measurement accuracy significantly. We also compare two different modal bases and achieve a further improvement in accuracy when using Karhunen-Lòeve instead of Zernike modes. We evaluate the efficiency of an open-loop adaptive optics system based on the optimized holographic sensor and show that it can be used to correct the effects of realistic dynamic atmospheric turbulence.
  • Publication
    Straightness metric for measurement of turbulence-induced distortion in long-range imaging
    ( 2021)
    Hofmann, Julia
    ;
    Goelzer, Rilene
    ;
    Wegner, Daniel
    ;
    Gladysz, Szymon
    ;
    Stein, Karin
    Algorithms used for mitigation of the effects of atmospheric turbulence on video sequences often rely on a process for creating a reference image to register all of the frames. Because such a pristine image is generally not available, no-reference image quality metrics can be used to identify frames in a sequence that have minimum distortion. Here, we propose a metric that quantifies image warping by measuring image straightness based on line detection. The average length of straight lines in a frame is used to select best frames in a sequence and to generate a reference frame for a subsequent dewarping algorithm. We perform tests with this metric on simulated data that exhibits varying degrees of distortion and blur and spans normalized turbulence strengths between 0.75 and 4.5. We show, through these simulations, that the metric can differentiate between weak and moderate turbulence effects. We also show in simulations that the optical flow that uses a reference frame generated by this metric produces consistently improved image quality. This improvement is even higher when we employ the metric to guide optical flow that is applied to three real video sequences taken over a 7 km path.
  • Publication
    Optimal, blind-search modal wavefront correction in atmospheric turbulence. Part I: Simulations
    ( 2021)
    Segel, Max
    ;
    Gladysz, Szymon
    Modal control is an established tool in adaptive optics. It allows not only for the reduction in the controllable degrees of freedom, but also for filtering out unseen modes and optimizing gain on a mode-by-mode basis. When Zernike polynomials are employed as the modal basis for correcting atmospheric turbulence, their cross-correlations translate to correction errors. We propose optimal modal decomposition for gradient-descent-based wavefront sensorless adaptive optics, which is free of this problem. We adopt statistically independent Karhunen-Loève functions for iterative blind correction and analyze performance of the algorithm in static as well as in dynamic simulated turbulence conditions.
  • Publication
    Demonstration of GBit/s coherent free-space optical communications over an 800 m outdoor path
    ( 2021)
    McDonald, Douglas
    ;
    Bellossi, Raphaël
    ;
    Gladysz, Szymon
    ;
    Lambert, Andrew
    Coherent free-space optical (FSO) communications systems offer both an opportunity for significantly increased data rates and improved security compared to conventional radio frequency (RF) systems. A key challenge in implementing FSO systems is the characterization and mitigation of atmospheric turbulence present along the optical channel. In this work, we present experiments demonstrating coherent free-space optical communications over a two-pass 800 m link with data rates on the order of gigabits per second (Gbit/s). The link consists of a single telescope and retroreflector. At the start/end point of the monostatic link we have built an optical transceiver capable of coherent communications. We present here design considerations and results from transmission in moderate turbulence.
  • Publication
    Modelling, Measurement and Correction of Underwater Turbulence Effects on Optical Communications
    ( 2021)
    Gladysz, Szymon
    ;
    Segel, Max
    ;
    Montoya, Jose P.
    ;
    Toselli, Italo
    ;
    Gasperin, Osvaldo Javier Galicia
    ;
    Stein, Karin
    We report on progress made in the last few years at Fraunhofer IOSB with regards to understanding the effects that underwater turbulence poses for laser-based communications and also with regards to design and implementation of efficient wavefront correction strategies. We have pursued, through parallel efforts, analytical modelling of light propagation through oceanic turbulence, conception and validation of turbulence characterization methods, and construction of "wavefront sensorless" adaptive optics for underwater communications.
  • Publication
    Adaptable Shack-Hartmann wavefront sensor with diffractive lenslet arrays to mitigate the effects of scintillation
    ( 2020)
    Lechner, Daniel
    ;
    Zepp, Andreas
    ;
    Eichhorn, Marc
    ;
    Gladysz, Szymon
    Adaptive optics systems are used to compensate for distortions of the wavefront of light induced by turbulence in the atmosphere. Shack-Hartmann wavefront sensors are used to measure this wavefront distortion before correction. However, in turbulence conditions where strong scintillation (intensity fluctuation) is present, these sensors show considerably worse performance. This is partly because the lenslet arrays of the sensor are designed without regard to scintillation and are not adaptable to changes in turbulence strength. Therefore, we have developed an adaptable Shack-Hartmann wavefront sensor that can flexibly exchange its lenslet array by relying on diffractive lenses displayed on a spatial light modulator instead of utilizing a physical microlens array. This paper presents the principle of the sensor, the design of a deterministic turbulence simulation test-bed, and an analysis how different lenslet arrays perform in scintillation conditions. Our experiments with different turbulence conditions showed that it is advantageous to increase the lenslet size when scintillation is present. The residual phase variance for an array with 24 lenslets was up to 71% lower than for a 112 lenslet array. This shows that the measurement error of focal spots has a strong influence on the performance of a Shack-Hartmann wavefront sensor and that in many cases it makes sense to increase the lenslet size. With our adaptable wavefront sensor such changes in lenslet configurations can be done very quickly and flexibly.