This PDF file contains the front matter associated with SPIE Proceedings Volume 12266, including the Title Page, Copyright information, Table of Contents, and Conference Committee Page.

In the advent of Free-Space Optical Communications, laser links between feeder stations and satellites are foreseen to surpass radiofrequency (RF) in terms on low-latency, high bandwidth and reliable security thanks to a narrow angle of emission and possibility en encrypt communications with quantum keys. Because Earth’s atmosphere is subject to wind and temperature gradients, turbulence skews laser beams over divergent and random paths, eventually corrected with Adaptive Optics (AO). Demonstrators have shown need for gateway selection optimization based on turbulence monitoring, to reduce onboard telescope re-pointing manoeuvrers, link-power budget, data errors correction and handover times. To assess day-time atmosphere optical quality, Miratlas has developed an autonomous and passive daytime turbulence monitor based on sunlight scintillation. This so-called SHAdow BAnd Ranger (SHABAR) was designed with remote-site prospective analysis requirements as well as operational feeder station monitor. Design of this small footprint, durable and low-cost device is presented, along a preliminary result campaign obtained at various locations. Power-spectral densities and autocorrelations of sunlight scintillation showed a clear effect of lower atmosphere wind gusts, while higher-layers also produced low-frequency scintillation. Air refractive-index structure parameter, C²_N (h) obtained from such scintillation measurements is presented. A Machine-Learning algorithm, fed by numerous environmental sensors embedded in our Integrated Sky Monitor (ISM) is foreseen to offer short-time turbulence prediction. Further experimental campaigns at reference site with calibrated instruments is expected later this year for commissioning of our turbulence profiler.

Fielding TNO’s LPT (Laser Propagation Testbed) for the first time in an international trial, a 3.6 km optical propagation path was created in a maritime environment between the city of Den Helder and the island of Texel in the Netherlands. Using a 1 W, 1556 nm laser beam, atmospheric propagation was investigated by capturing the resulting beam profile on a capture plate, which was imaged with a high-speed SWIR camera. Meteorological conditions were monitored using standard meteo stations, two visibility meters, two scintillometers and aerosol equipment. Over two weeks of measurements, propagation conditions varied from windy with clear, blue skies to significantly limited visibility. In this paper, the setup is introduced and a first discussion of the relation between beam behaviour and meteorological conditions is presented.

An over-water propagation link of 3.6 km was set up between the Dutch city of Den Helder and the island of Texel. For 6 days in November 2021, a laser beam was propagated along this path. An analysis of the beam wander resulted in an estimate of the strength of optical turbulence C²_n. Estimates compare quite well with independent measurements of C²_n by two boundary layer scintillometers. Regional maps of C²_n were produced by a numerical tool consisting of the WRF model coupled to a micrometeorological module. Regional differences in C²_n could be explained in terms of surface conditions, and acceptable agreement was found between the numerical values of C²_n for the trial site and the values provided by the scintillometers.

The investigation of transmission, refraction, and turbulence over the False Bay in South Africa and their influence on wave propagation was the main topic of First European South African Transmission ExpeRiment (FESTER). It yielded a 9-month continuous dataset of turbulence (Cn2) data, acquired by three Boundary Layer Scintillometers (BLS) and one ultrasonic anemometer. The data is analysed in terms of atmospheric stability, and relations are sought between the atmospheric state and the power spectrum of turbulence on the one hand, and the vertical gradients of turbulence strength on the other hand. This allows us to test various parameterizations of the z/L function, not only against experimental data, but also against numerical weather prediction (NWP) data. This work extends our previous analysis of specific case studies of the FESTER dataset.

In the last decade, a nascent trend of characterizing turbulence from observing features of distant targets through ground-layer turbulence have been relentless growing. Either from observing regular geometrical features of buildings or arrays of LEDs, it is possible to retrieve the structure constant of the refractive index fluctuations. On the other hand, because of the lack of a definitive theoretical model describing anisotropic or inhomogeneous turbulence, most experimental observations have been reduced to mere descriptions in the event of deviations from expected Obukhov-Kolmogorov predictions. Our group has been able to retrieve power-spectrum exponents, without a prior knowledge of a subjacent model, and henceforth determine anisotropic behavior in controlled optical turbulence; furthermore, under convective turbulence, an exponent can be obtained from time series of the occurrence of power drops in optical communication links: extreme events.

In this manuscript, we present a technique identifying as extreme events sudden changes in morphological characteristics of an array of point sources observed through real controlled anisotropic turbulence assisted by a deep-learning ad-hoc. This approach provides an effective approach to reduce high-volume data from imaging targets into a real-time stream of parameters to fully characterize optical turbulence.

Optical turbulence induces distortions in amplitude and phase in any beam propagating through it, resulting in beam spreading, beam wandering, and irradiance fluctuations among other effects. Due to the dynamic nature of these effects, the complex field reconstruction of a perturbed beam presents a great experimental challenge. Interferometric wavefront reconstruction techniques require very sophisticated assemblies prone to alignment errors due to their high sensitivity to environmental disturbances. This hinders its experimental implementation. New complex phase retrieval methods overcome most of the limitations of interferometric methods: they are suitable for amplitude or phase objects (or both) and their reconstruction algorithms—based on propagation equations—make unnecessary any a-priori knowledge of the beam to be reconstructed. We propose an experimental implementation of a complex phase retrieval technique for the characterization of Gaussian beams propagating through turbulence. This technique is based on binary amplitude modulation using a digital micro-mirror device (DMD) which has proven to be suitable for dynamic applications. To our knowledge, this is the first experimental high-speed complex wavefront reconstruction of optical beams—by binary amplitude modulation—through controlled real turbulence. This experiment represents the first step in our research focused on understanding optical turbulence from an experimental point of view.

Dissipation of heat can be a major challenge when applying sensor systems outdoors under varying environmental conditions. Typically, complex software and knowledge is needed to optimize thermal management. In this paper it is shown how the thermal optimization of a LiDAR (light detection and ranging) sensor can be performed efficiently. This approach uses standard CAD (computer aided design) software, which is readily available, and saves time and cost as the thermal design can be optimized before experimental realisation. A four-step process was developed and realized: (i) Measurement of the thermal energy distribution of the current sensor design; (ii) Simulation of the time-dependant thermal behaviour using standard CAD software; (iii) Simulation of a thermally optimized design. This was compared quantitatively with the original design and was also used for verification of sufficient increase in heat dissipation; (iv) Experimental realisation and verification of the optimized design. It could be shown that the optimized prototype shows significantly improved thermal behaviour in accordance with the predictions from the simulations. The new LiDAR sensor shows lower heat generation and optimized dissipation of thermal energy which proofs the applicability of the approach to complex sensors.

This article describes an experimental study of the optical vortex formation using beams reflected from a combination of two cube-corner reflectors with a special interference phase-shifting coating. As predicted earlier, if arranged properly, these cube-corner reflectors create a spatial polarization structure, that can be called an optical vortex, since the plane of oscillations of thе vector E rotates with azimuth variation in the transverse plane. Our previous studies concluded that such configuration works as intended, however only near-field images were obtained. This time we preset experimental results in the far field.

We theoretically investigate how adaptive optics (AO) can reduce symbol error probability (SEP) of free-space optical communications systems based on phase-shift keying heterodyne coherent receivers. In our analysis we use an updated model for describing the optical turbulence-induced fading present in coherent communications systems and it is assumed that AO is implemented at the receiver side. We consider the ideal case of partial phase conjugation, or in other words, we assume that the Zernike modes (up to the specific number Nmodes) can be corrected with 100% accuracy. Results show that removal of several modes from the turbulence-affected wavefront allows to reach values of SEP that are typically required in coherent laser communications.

Free-Space Optical (FSO) communication can play an important role in meeting the demands of future high throughput and high data rate communication applications. However, atmospheric turbulence induced effects degrade the performance of FSO communication links resulting in high volume data losses. Adaptive Optics (AO) can be used to mitigate the effects of atmospheric turbulence in FSO links. A key challenge is the fact that turbulence scenarios in FSO links are stronger and FSO links are expected to remain operational in all conditions. This requires a robust AO controller that can cope with the more extreme turbulence. In this work, the design and simulation of one such advanced controller based on Linear Quadratic Gaussian control (LQG) is presented. The operation of the controller is demonstrated with an end-to-end simulation. The simulation uses multi-layer phase screens for representing the turbulent atmosphere and angular spectrum propagation for accuracy. We present here the performance of the AO controller through an analysis of the Strehl ratio, the fiber coupling efficiency and the power scintillation index on the fiber achieved.

Free-space optical communication systems always require a precise focusing on the receiver to maximize the fiber coupling efficiency. Unfortunately, atmospheric turbulence causes scintillation at the receiver. Ground to ground receivers over short distances (up to 500m) have usually small aperture (about 50mm). In this case the main aberration is tip/tilt and its correction is of fundamental importance for high bandwidth data transmission.

We present a new concept of Fast Steering Prism (FSP) for the correction of tilt, suitable for optical communication and optical tracking. The system consists in the use of a novel design of a tunable prism with a variable angle based on the usage of piezoelectric actuators. A system with a FSP has the advantage to be more compact and simpler with respect to the one with a fast-steering mirror. The entire setup has been tested in a 200m outdoor transmission with promising results.

Long distance imaging and free space optical communications are largely affected by atmospheric turbulence. To attenuate turbulence effects, Adaptive Optics (AO) has been the main answer and, in the case of large FOV Multi Conjugate AO (MCAO) using two deformable mirrors (DM) has been proven to be an effective solution. We present a study and some preliminary results on the use of a stack of Adaptive Lenses (AL) in a MCAO setup with the main advantage of compactness end easiness of installation.

Satellite based quantum key distribution (QKD) enables the delivery of keys for quantum secure communications over long distances. Maturity of the technology as well as industrial interest are ever increasing. Same is true for satellite free-space optical communications (FSOC). In order to enable a robust channel for transmission it is indispensable to account for static and dynamically changing misalignments between the transmitter and receiver pair. This work will focus on the transmitter terminal (Alice) and the design and verification process of the active beam steering system. The novelty is a recently developed variable reluctance fine steering mirror (FSM) including eddy current sensors (ECS) to measure its tip and tilt. A cascaded architecture was chosen in order to combine the optical stabilization objective with the dynamics of the mirror platform. The inner control loop makes use of an observer model whose estimated output is fed into a state controller allowing for an increased responsiveness. While high gains increase the closed loop bandwidth the eigenfrequency of the system introduces a pole to the plant which has to be avoided by the controller output. A digital notch filter was introduced to reject the excitation of the critical frequency band which gets obsolete in a system with high frequency sampling capabilities. The outer loop is engaged when a valid optical signal is received and a transition from a closed loop pointing to a closed loop tracking mode is performed. A proportional-integral (PI) controller keeps the received beam at the 4-quadrant-diode (4QD) whose center is used as the main reference through prior calibration with the transmit beam launching on the same path. The presented cascaded control scheme allows improvements in system performance and reliability.

Free-space optical communications is an emerging field for a variety of use cases in the domain of satellite communications. It is deployed in Earth observation missions to downlink payload data, in telecom missions (GEO or LEO based) to up- and downlink data streams and in quantum communication missions to implement the needed quantum and classical channels. Part of the propagation path goes through the atmosphere where the propagating optical wave experiences wave-front distortions and consequently distortions of the irradiance field. These distortions lead to scintillation of the optical power in the focal plane of the optical ground stations receiver front-end which can cause signal outages. Characterization of power scintillation is very important to assess the needed fading margin in the communication link design. Thus, these power scintillations are matter of investigation in this paper. Measurement campaigns were conducted to experimentally characterize the power scintillation. Received power was measured with a 40 cm telescope located at the DLR site Oberpfaffenhofen, i.e. in a suburban area. Satellite source is the OSIRISv1 laser terminal on the LEO satellite Flying Laptop. The campaigns were conducted in the years 2018, 2019 and 2020 where data of 15 passes could successfully be recorded. The power scintillation statistics are analyzed over elevation. The power scintillation index shows different behavior from pass to pass which is due to the different environmental conditions during the individual passes. Median values and quartiles of power scintillation index over elevation are given. Furthermore, statistics of fading margin for link budget calculation are derived. Both can be used to define best, nominal and worst cases in the design of the LEO-ground communication link.

In order to mitigate atmospheric turbulence effects such as increased blur, reduced contrast, and image motion as well as geometric deformations, a wide variety of reconstruction techniques has been developed. Such techniques have proven reasonably successful in mitigating one or several turbulence effects but frequently at the cost of introducing unwanted artefacts such as ringing or noise amplification, depending on the algorithm's properties. The application of image quality metrics (IQMs) as a means of comparing the results of various reconstruction algorithms as objectively as possible is a widely used practice. However, added noise and artefacts affect IQMs which rely on information like high frequency components, disproportionately, since noisy results are invariably interpreted as "higher quality". The underlying goal of this article is to define a methodology for comparing the performance of structurally differing algorithms by a combination of select quality metrics. As different metrics will likely yield different ratings for the same algorithm's performance, a combination of suitable metrics is proposed. Therefore the main focus here is foremost on a survey of current methods for assessing image quality in general and on appraising their suitability for evaluating the quality of images processed by turbulence mitigation algorithms in particular.

As European optronic industries are facing an increasing demand for high-definition imagery, atmospheric turbulence becomes one of the main factors hindering the performance of military equipment used for intelligence, surveillance and reconnaissance. The TURBO project, which is funded by European Defence Agency and has Fraunhofer IOSB, TNO and Adimec as partners, aims to develop a real-time demonstrator capable of selecting the optimal algorithm for the given dataset from a repertoire of software-based turbulence correction algorithms. The work to be presented illustrates Fraunhofer IOSB's contribution to the first stage of the project. It consists of the selection of the most promising candidate methods for the demonstrator, as well as a detailed assessment of their limitations. A total of eight algorithms were selected. The first method performed global registration. For the correction of local motion, optical ow, block matching and iterative image generation methods were applied. To correct blur, we investigated various deconvolution approaches, including Richardson-Lucy approach and a blind deconvolution algorithm, and a Lucky fusion method. Finally, we focused on turbulence correction for scenes with moving content (people, vehicles). All methods were applied to several video sequences, which are recordings from field trials performed under different atmospheric conditions. The resulting corrected sequences were analysed first regarding their quantitative improvement in relation to the uncorrected sequences, and afterwards considering the execution time for each method. Three no-reference metrics were chosen for the quantitative analysis, namely Fisher information, edge variance and total variation. It was demonstrated that the selected methods achieved good correction results in many sequences. In particular optical ow and deconvolution achieved a high increase in image quality. Quantitative analysis was shown to be accurate, in the sense that it agreed systematically with visual perception. It was also demonstrated that certain methods produce good results within a short computation time, deconvolution for instance, which is of great interest for real-time use and it provides a good basis for future work.

The article proposes an approach to improve the visual quality of generated images based on the use of primary data processing methods. To reduce the effect of noise associated with the appearance of dust or water droplets, a multi-criteria filtering method is proposed for use. The proposed method allows two-pass processing. At the first stage, the method uses a criterion that forming strong noise suppression with blurring of the boundaries. Based on the generated data, we perform filtering while preserving and strengthening the boundaries of objects. Next, we perform the formation of a combined approach to fusion of image. In the case of low illumination, the noise component is strong and its suppression is an important problem. In the case of fog, the task is complicated, since the boundaries of objects are blurred. An adaptive non-linear alpha-contrasting algorithm is used for elimination and compensation. Pairs of test images obtained by sensors with a resolution of 6000x4000 (8 bit, color image, visible range) are used as test data used to evaluate the effectiveness, pairs of images with the same illumination but different fog density are used for the test. Images of simple shapes are used as analyzed objects.