Szymon Gladysz

Fraunhofer Institute of Optronics, System Technologies and Image Exploitation IOSB | IOSB · Department of Signatorics (SIG)

This paper presents an atmospheric optical turbulence mitigation method that uses a sequence of short-exposure frames. An iterative block matching registration method is proposed for image dewarping. The dewarped frames are combined in a least squares lucky look (LL) fusion process. Here image patches are weighted so as to produce a fused image tha...

The paper by Y. Li, W. Zhu, X. Wu, and R. Rao entitled “ Equivalent refractive-index structure constant of non-Kolmogorov turbulence ,” Opt. Express 23 ( 18 ), 23004 ( 2015 ). 10.1364/OE.23.023004 relates the non-Kolmogorov turbulence structure constant to the classical structure constant for Kolmogorov turbulence by imposing equality of their resp...

Adaptive optics systems are used to compensate for wavefront distortions introduced by atmospheric turbulence. The distortions are corrected by an adaptable device, normally a deformable mirror. The control signal of the mirror is based on the measurement delivered by a wavefront sensor. Relevant characteristics of the wavefront sensor are the meas...

The Adaptive Optics Research Group at Fraunhofer IOSB has developed several novel solutions to problems faced by terrestrial, space-based and underwater laser-based communications. These solutions will be presented and their efficacy demonstrated in real turbulence.

The modal holographic wavefront sensor enables fast measurements of individual aberration modes without the need for time-consuming calculations. To minimize the effect of inter-modal crosstalk on the measurement accuracy, we present sensor optimization and show its performance in closed-loop adaptive optics in simulation.

The development of coherent free-space optical communications systems could provide an opportunity for significantly increased data rates compared to the traditional intensity-modulation-based communications. In this work, we present experiments demonstrating coherent free-space optical communications over an outdoor free-space link with data rates...

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. O...

This focus issue offers a glimpse into the breadth and depth of research in optics and image processing at the Fraunhofer Institute of Optronics, System Technologies and Image Exploitation (abbreviated Fraunhofer IOSB). The institute belongs to the Fraunhofer Society, which is Europe's largest organization for applied research. The society comprise...

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,...

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 t...

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...

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 consid...

Coherent free-space optical (FSO) communications systems present an opportunity for significantly increased data rates in wireless communications due to the abundance of bandwidth available in the optical domain. To implement coherent FSO systems, degradation of received signals due to atmospheric propagation must be mitigated. This can be achieved...

We report on results of the field trial carried out in the dome of the 2.5 m Isaac Newton Telescope on the island of La Palma (Canary Islands, Spain). The goal of the trial was to characterize optical turbulence in the confined space of the observatory.

We have designed and built a transmitter and a receiver positioned at two ends of a 7.2-km free-space path. Path height is 20-30 m above the terrain with slant differential of ~20 m over the entire distance. The link, operating at the 1550 nm wavelength is producing first measurements.

Large apertures are used in FSOC systems to increase signal and reduce scintillation. Based on simulations of a realistic FSOC system using a Gaussian beam we found significant changes in the empirical PDF with increasing aperture size. We compare the fit of PDFs to data and the effect of PDF choice on and fade probability of a direct detection FSO...

Performance metrics such as mean-bit-error rate and probability of fade for free-space optical communication (FSOC) applications using intensity-modulation direct detection are theoretically calculated based on probability density functions (PDFs) describing irradiance fluctuations. Theoretical calculations using common PDF models can result in sig...

Over the years, there has been much interest in the use of optical wavelengths for communication because of the potential for high data rates. However, the performance of these systems can become significantly degraded due to turbulence-induced signal fluctuations. These fluctuations can be minimized by enlarging the receiving aperture, thereby ave...

We theoretically investigate the effectiveness of adaptive optics correction for Gaussian beams affected by oceanic turbulence. Action of an idealized adaptive optics system is modeled as a perfect removal of a certain number of Zernike modes from the aberrated wavefront. We focused on direct detection systems and we used the aperture-averaged scin...

In the framework of NATO task group SET 226 on turbulence mitigation techniques for OA systems, a trial was conducted in the premises of RDDC-Valcartier, using indoor and outdoor facilities in September 2016. Images data sets were collected under various turbulence conditions, both controllable (indoor) and natural (outdoor). The imagery of this tr...

The aim of this research is to develop a new automated blood vessel (BV) detection algorithm for optical coherence tomography (OCT) scans and corresponding fundus images. The algorithm provides a robust method to detect BV shadows (BVSs) using Radon transformation and other supporting image processing methods. The position of the BVSs is determined...

This article aims to discuss the general equivalence between two main metrics used in laser beam propagation through random media and imaging: the Fried parameter and the coherence radius. In particular, we show that their relationship deduced for Kolmogorov and non-Kolmogorov turbulence stays valid for other media as long as their spectral power l...

We carried out differential measurements of temperature and image motion in a convective Rayleigh-Bénard water tank to test the assumption of Kolmogorov turbulence. Careful selection of the experimental methodology is required to arrive at the agreement between theory and experiment.

Reliable no-reference image quality metrics are indispensable in many research areas. In this paper, a metric for image warping due to atmospheric turbulence is presented. Its main mechanism is the quantification of “straightness” in distorted images.

A Shack-Hartmann sensor which utilizes diffractive lens arrays displayed on a spatial light modulator was built. First measurements with different lens array configurations were carried out. The sensor holds the premise of increased robustness against scintillation.

We theoretically investigate the effectiveness of adaptive optics to correct phase aberrations for laser beams affected by oceanic turbulence. Action of adaptive optics is modeled as removal of a certain number of Zernike modes from the distorted wavefront

The use of adaptive optics, wavefront sensing and wavefront correction beyond the large astronomical telescopes or vision science behoves careful analysis of the media being corrected for, and identification of non-traditional systems and devices.

A possibility of extracting information about atmospheric turbulence from video sequences taken with a mobile phone’s on-board camera is presented. A field trial over a 100-m path was conducted and the results from the technique are shown to agree with those from a scintillometer.

Correction of atmospheric effects on the propagation of light can be done by adaptive optics (AO). Nevertheless, challenging scenarios like strong turbulence near the ground lead to high failure rates of the traditional AO systems. Unconventional wavefront sensors and AO strategies are developed at Fraunhofer IOSB to provide alternatives for measur...

We use a non-Kolmogorov power spectrum for theoretical investigation of laser beam propagation in the double-passage problem: transmitter-target-receiver. The major application of our work is the light detection and ranging (LIDAR) system operating in the zone of the atmosphere where non-Kolmogorov turbulence may be present. We show that the scinti...

This letter aims to discuss the general equivalence between two main metrics used in laser beam propagation through random media and imaging: the Fried parameter and the coherence radius. In particular, we show that their relationship deduced for Kolmogorov and non-Kolmogorov turbulence stays valid for other media as long as their spectral power la...

We theoretically investigate the performance of adaptive-optics correction for Gaussian beams affected by oceanic turbulence. Action of adaptive optics is modeled as removal of a certain number of Zernike modes from the aberrated wavefront. We found that, similarly to atmospheric turbulence, adaptive optics is very effective to improve optical syst...

We introduce a method for creating temporally evolving wavefront distortions that uses Karhunen-Loève decomposition and the associated temporal power spectra. We demonstrate that the method is able to produce dynamic wavefronts that follow the behavior predicted by the theory while introducing key advantages in terms of calculation speed and storag...

We investigate theoretically the performance of adaptive-optics correction for Gaussian beams affected by oceanic turbulence. Action of adaptive optics is modeled as removal of a certain number of Zernike modes from the aberrated wavefront. We found that, similarly to atmospheric turbulence, adaptive optics is very effective in improving optical sy...

The concept of differential motion allows in principle for very precise characterization of optical turbulence. The models for motion decorrelation with increasing distance have to be validated before the technique becomes established. This validation is done here with the help of Monte-Carlo simulations.

We introduce a method for creating temporally evolving wavefront distortions that uses Karhunen–Loève (K-L) decomposition and the associated temporal power spectra. We model the single- and multilayer cases for horizontal and vertical paths, for which spherical and plane-wave geometries must be assumed, respectively. Simulation results confirming c...

We present a numerical wave propagation method for simulating imaging of an extended scene under anisoplanatic conditions. While isoplanatic simulation is relatively common, few tools are specifically designed for simulating the imaging of extended scenes under anisoplanatic conditions. We provide a complete description of the proposed simulation t...

Artificial motion and warping of images taken at long range is one of the most significant and troublesome effects of atmospheric turbulence. It is important to understand and model this effect correctly in order to: 1) fully characterize turbulence between the target and the observer, 2) devise efficient post-processing strategies for artificial m...

In this paper we introduce the use of Karhunen-Loève functions as a basis set to decompose atmospheric phase aberrations in a digital holographic wavefront sensor (HWS). We show that the intermodal crosstalk when using Karhunen-Loève functions is reduced in comparison to the Zernike decomposition. Additionally, the sensor’s response showed an impro...

Differential tilt variance is a useful metric for interpreting the distorting effects of turbulence in incoherent imaging systems. In this paper, we compare the theoretical model of differential tilt variance to simulations. Simulation is based on a Monte Carlo wave optics approach with split step propagation. Results show that the simulation close...

Laser beam propagation underwater is becoming an important research topic because of high demand for its potential applications. Namely, ability to image underwater at long distances is highly desired for scientific and military purposes, including submarine awareness, diver visibility, and mine detection. Optical communication in the ocean can pro...

Atmospheric effects limit the performance of any electro-optical (EO) system. Tasks such as laser communication and delivery of directed energy are significantly affected by turbulence and refraction. A correction of atmospheric effects on the propagation of light can be done by adaptive optics (AO). Especially for astronomical applications, AO tec...

Unconventional wavefront sensing strategies are being developed to provide alternatives for measuring the wavefront deformation of a laser beam propagating through strong turbulence and/or along a horizontal-path. We present results from two “wavefront-sensorless” approaches: stochastic parallel gradient descent (SPGD) and its modal version.

The complex amplitude in the focal plane of adaptive optics system is modelled as an elliptical complex random variable. The geometrical properties of the probability density function (pdf) of it relate directly to the statistics of the residual phase. Building solely on the 2D geometry, the expression for the pdf of speckle intensity is derived.

We propose the Karhunen-Loève functions as a set of basis to decompose atmospheric phase aberrations in holographic wavefront sensing. We evaluate the sensor’s performance in the presence of atmospheric turbulence and compare it with the sensor based on Zernike decomposition.

Adaptive optics PSF can be described as a weighted sum of modes. These weights are estimated using a marginal blind deconvolution algorithm. Results in the context of turbulence-degraded observations are shown. Our method can be extended to anisoplanatic imaging.

Reconstruction of point spread function (PSF) using information from the wavefront sensor is a classic problem in adaptive optics. Here we present an alternative solution which does not require any extra information from the wavefront sensor, nor does it rely on knowledge of the turbulence profile; the method requires merely a sequence of images .

Correction of atmospheric turbulence effects on images involves mainly mitigation of distortion (“de-warping”) and removal of image blur. One of the approaches for correcting atmospheric blurring involves the use of deconvolution. The ill-posed nature of the problem and the number of unknowns makes this problem hard to solve. This is why methods li...

We have performed a series of experiments in order to simultaneously validate several devices and methods for measurement of the path-averaged refractive index structure constant ( 𝐶𝑛 2). The experiments were carried out along a horizontal urban path near the ground. Measuring turbulence in this layer is particularly important because of the prospe...

A simple grid of 10×10 white-light LEDs allows for simultaneous measurement of several characteristics of atmospheric turbulence. With this device, an imaging sensor and the model of tilt anisoplanatism one can determine turbulence strength, anisotropy, outer scale and spectral slope of turbulence. We describe the theory and present preliminary res...

The efficiency of laser communications systems is significantly limited by atmospheric effects. Notably challenging scenarios like a long horizontal path or strong scintillation lead to high failure rates of the electro-optical systems. Adaptive optics (AO) methods and components developed for astronomical applications cannot fulfill these higher r...

Recent advances in image processing for atmospheric propagation have provided a foundation for tackling the similar but perhaps more complex problem of underwater imaging, which is impaired by scattering and optical turbulence. As a result of these impairments underwater imagery suffers from excessive noise, blur, and distortion. Underwater turbule...

We present a monostatic system to characterize optical turbulence along a horizontal path in the lower boundary layer. The apparatus is a telescope focusing a beam into a retro-reflector and collecting the reflected beam. Two approaches for estimation of Fried’s parameter are presented: Zernike mode variances and the modulation transfer function.

Decorrelation of tip/tilt is important in imaging and laser propagation through the atmosphere without a cooperative beacon. Applications include laser-guide-star AO and multi-laser-beam communications. We compute gradient-tilt anisoplanatism for spherical-wave propagation. The model is to be validated against experiments over a 110-m path.

Research activities in the Adaptive Optics Group at the Fraunhofer Institute of Optronics, System Technologies and Image Exploitation (IOSB) in Ettlingen, Germany, revolve around imaging and laser propagation through strong turbulence, especially along horizontal paths. We are developing simulations, theoretical models, image processing software an...

Ground-level turbulence is usually strong and can thus be a limiting factor for many applications such as laser communications or astronomical observations. In this paper, the working principle of a setup for characterization of optical turbulence along a horizontal path in the lower boundary layer is described. The system includes a telescope focu...

Correction of atmospheric effects on the propagation of laser light can be achieved with adaptive optics (AO) by relying on adequate wavefront sensors. For free-space laser communications and for tracking of high-speed airborne objects Shack-Hartmann sensor is not effective. Its performance is severely limited by scintillation. We characterize a pr...

It is well known that a laser beam propagating through optical atmosphere is affected by atmospheric turbulence. In this paper, we describe an experimental double-passage system for laser beam propagation along a horizontal urban path that can be useful for applications such as free-space laser communications. The setup includes a telescope to focu...

Correction of atmospheric effects on the propagation of laser light can be achieved with adaptive optics (AO) by relying on adequate wavefront sensors. For free-space laser communications and for tracking of high-speed airborne objects, conventional wavefront sensing methods e.g. those based on the Shack-Hartmann sensor (SHS), are not always effect...

We study the influence of each Zernike mode on the propagation of a laser beam through the atmosphere by two different numerical methods. In the first method, an idealized adaptive optics system is modeled to subtract a certain number of Zernike modes from the beam. The effect of each aberration is quantified using the Strehl ratio of the longterm...

The use of remote sensing techniques such as adaptive optics and image restoration post processing to correct for aberrations in a wavefront of light propagating through turbulent environment has become customary for many areas including astronomy, medical imaging, and industrial applications. EO imaging underwater has been mainly concentrated on o...

Atmospheric effects limit the performance of any electro-optical (EO) system. Tasks such as laser communications or horizontal-path imaging for long-range surveillance are highly affected by environmental effects. In the majority of cases, effects like turbulence impose a fundamental limitation to the capability of EO systems. In this paper, we giv...

Deconvolution through atmospheric turbulence requires regularization. Many object priors have been proposed but their utility might be limited to one class of real objects. Optical effects of atmospheric turbulence on the PSF form a viable alternative.

In this paper we investigate theoretically the performance of adaptive-optics correction for Gaussian beams in weak non-Kolmogorov turbulence. Action of adaptive optics is modeled as removal of a certain number of Zernike modes from the aberrated wavefront.

Atmospheric effects significantly influence the propagation of light. Conventional adaptive optics systems, based on Shack-Hartmann sensors (SHS), work well for vertical-path propagation. However, for more challenging scenarios like horizontal-path imaging or free-space laser communications through extended-volume turbulence and strong scintillatio...

We have performed a series of experiments aiming at understanding the statistics of deep turbulence over cities. The experimental setup consisted of a Shack-Hartmann wavefront sensor and an imaging camera that simultaneously recorded wavefront-, and focal-plane data, respectively. At the same time, measurements of deep optical turbulence were perfo...

When collecting images through turbulence it is always useful to have an estimate of turbulence strength during the time of the observations. We show how Fried& rsquo;s parameter can be estimated from images of extended objects in the case of vertical-, and horizontal-path imaging scenarios.