This PDF file contains the front matter associated with SPIE Proceedings Volume 11860, including the Title Page, Copyright information, and Table of Contents.

Long-term observations of the main characteristics of the turbulent atmosphere in the Monin – Obukhov similarity theory were carried out in the urban environment of the Tomsk Academgorodok. A mobile ultrasonic station AMK-03-4, developed to measure the characteristics of turbulent meteorological fields, was used for the measurements. The data obtained in the urban environment are compared with the data of long-term observations in mountainous terrain, in different climatic and geographical regions. Under urban conditions, data were obtained for the turbulent scales of velocity V* (friction velocity) and temperature (friction velocity) and temperature T*, which are important characteristics of the Monin-Obukhov similarity theory. The range of experimentally observed Monin-Obukhov numbers in the positive region, where previously there were no experimental data, has been significantly expanded. The main results of this work include the experimental confirmation of the positions of the similarity theory in the urban atmospheric boundary layer.

Many applications such as ground to satellite optical communications or astronomy require precise knowledge of cloud cover, turbulence and absorption. In the case of telecoms, this data is critical for the initial ground station sites survey; during ground station operation to inform link availability and bandwidth; and finally, to predict atmospheric conditions over different ground stations for network planning. Historically the turbulence by night time has been measured by astronomers with research class solutions installed on observatories sites. Many implementations exist using either the moon or the stars as reference target. One of them is the Differential Image Motion Monitor (DIMM) from M. Sarazin and F. Roddier with the first implementations back in the 80’s for the ESO. All these turbulence monitors have in common the integration of a small telescope in the 20 to 40cm aperture range with various aperture masks on an automatic tracking mount within a protective dome. This form factor and cost is not in line with the requirement of a more industrial utilization as expected by telecom operators or for atmospheric studies. Since 2018, Miratlas has been using a simpler implementation of the image motion monitor (NSM) with a fixed outdoor system using a single aperture aiming at Polaris. Nevertheless this single aperture system requires a very stable fixture which is not always available and doesn’t apply in southern hemisphere. Therefore, Miratlas has developed a small outdoor implementation of a legacy DIMM named the C(ompact)-DIMM. It uses two different optical assemblies and two identical synchronized cameras to fulfil the same features. The C-DIMM is small enough to be installed anywhere, is not sensitive to vibration and therefore can be installed either on a fixed mount aiming at Polaris, or on a small outdoor tracking mount to operate on any sufficiently bright star and therefore under any latitude.

The effectiveness of a laser weapon systems (LWS) in the field largely depends on the weather conditions. The simple, budget method based on CMOS camera to determine the atmospheric refractive-index structure parameter Cn2 was developed. The measurement procedure consists of two independent measurements: beam width and resolution test contrast at target plane. The procedure of determining the beam diameter is based on the registration of the beam incident on the scattering surface. The Power in Bucket method was used to determine the beam diameter on previously processed frames of the recorded film. The effective beam spot value was used to calculate the Cn2 parameter from the Andrews model. Independently, the measurement of the contrast of the images of the group of resolution tests placed in the plane of the target was performed and the Fried's radius based on the Modulation Transfer Function was estimated. The results of the determined Cn2 parameter from two independent measurements coincide with an acceptable error. The developed method cannot compete with commercially available devices on the market but allows to determine the atmospheric conditions and thus the potential effectiveness of laser weapon.

Insight in the performance of laser systems as function of weather is relevant for both civilian and military applications. This paper focuses on a long-term experiment to characterize laser propagation conditions over a near-surface, across-water optical link over the outer Bay of Eckernförde in the German part of the Baltic Sea. Turbulence was characterized with a boundary layer scintillometer (BLS), and it is shown that saturation effects occurred over this long optical path. The turbulence is primary driven by thermal forces and correlates best with the air-sea temperature difference. Simulations exploiting numerical mesoscale weather prediction tools agree favorably with the observations. The effect of the environment on beam divergence and power-in-the-bucket is discussed.

We report the analysis of radio soundings launched at our permanent measurement site in North-western Germany. The data of potential temperature and wind speed are used to derive vertical profiles of the gradient Richardson number 𝑅𝑖𝑔 in the free troposphere, and subsequently, the strength of optical turbulence 𝐶𝑛 2. These values are compared to a numerical framework, which derives 𝐶𝑛 2 from mesoscale weather prediction data by two schemes based on the gradient Richardson number and Monin-Obukhov similarity theory, respectively.

Numerical simulations of a Rayleigh-Bénard turbulent convective flow are examined to determine the optical and mechanical turbulence properties and resulting index of refraction and temperature structure function fields with the goal of understanding the propagation characteristics of a laser beam carrying orbital angular momentum. Beams carrying orbital angular momentum are a topic of interest for secure high data density free-space communications systems in both the atmosphere and underwater environment. The choice of Rayleigh-Bénard convection provides a highly controllable configuration for studying optical turbulence and once the flow reaches a steady state, it may be treated as homogeneous. With a well characterized turbulent state provided by the simulations, attention is focused on the mechanics of beam propagation through the turbulence. Simulations are performed using the open source computational fluid dynamics package OpenFoam, a finite volume solver, and an in-house developed code that uses spectral methods. In the case of each solver, the Boussinesq approximation is used to model buoyancy and both the Navier-Stokes equations and the thermal energy equation are simultaneously solved. The outcome from the two computational schemes will be cross compared for result fidelity, spatial resolution, and computation time. The initial effort will examine air as the working medium in a domain with dimensions of 0.5 m on a side and a height of 0.1 m.

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.

We consider a design of a high-speed wireless optical communication system that involves generating an alphabet using structured light by encoding the transmitted information using Laguerre-Gaussian (LG) beams carrying orbital angular momentum (OAM) [1]. LG beams carrying OAM are chosen for this system due to their resilience when propagated through complex environments. Following propagation through optical turbulence, light intensity patterns are decoded with high accuracy using a convolutional neural network (CNN), commonly used for image classification applications. CNNs only see and learn from the diversity of the received images, they do not recognize how the light intensity distribution has been generated. This motivates us to investigate how the CNN handles and reacts to different forms of structured light that have the same intensity pattern on reception and are transmitted through the same environmental conditions. To compare the effects of structured light on the performance of a CNN we constructed an alphabet using LG beams carrying OAM [2] and 2D projected Far Field Images (FFI) [5]. Under the conditions where there is no induced optical turbulence, the received images have approximately the same intensity pattern, despite differing formation methods. Each form of the structured light is generated using a spatial light modulator (SLM). The slight differences in the generated intensity patterns are a result of the optical artifacts of the SLM and the method used to create the SLM phase screen. The primary comparison metric in this research is the classification accuracy of the CNN when individually trained on images of each type of light. This will provide insight into which form of light is potentially the most resilient to be utilized in receivers supported by CNN. Our experimental set uses a 632.8 nm He-Ne laser and an SLM to generate structured light and propagate two forms of structured light in underwater optical turbulence over a ~2.5 m propagation path. Induced underwater optical turbulence is created using a heater that allows for the control and estimation of turbulence strength. The images are captured by a camera, and the CNN is comprised of 15 layers, reducing computational complexity [4]. The beams are propagated through strong optical turbulence, corresponding to 𝐶𝑛 2 values of approximately 10⁻¹¹ 𝑚^−2/3. Based on preliminary testing, the CNN has been able to accurately learn the resulting variations for all forms of light on as little as 50 images. This result further strengthens the evidence for the resilience of machine learning-based communication systems in harsh environments.

As known, lasers are used for atmospheric communication. There are advantages of an optical wave system over a radio frequency system. But there arise a number of problems in that case. Turbulent fluctuations of density in the atmosphere cause various deteriorative effectson signals. We investigated the propagation of the non-paraxial Gaussian beam through air inhomogeneity. The refractive index of air can be written in the form : n=1+n₁ , n₁ being a small quantity. In this case we can find the solution of the Maxwell equations. The paper represents results of computations for model inhomogeneity.

Approaches to constructing a mock-up of a system for focusing laser radiation on distant objects using both adaptive optics elements and nonlinear-optical wave-front reversal methods providing compensation for turbulent distortions are considered. Numerical calculations were preliminarily performed, in which the split-step method was used as a numerical method for solving a second-order partial differential wave equation for the complex amplitude of the wave field of a laser beam. This method, combined with methods of spectral-phase Fourier transforms and statistical tests, is the most effective way to obtain reliable quantitative results for solving engineering problems of atmospheric wave optics. Quantitative data are obtained on the effect of turbulent atmospheric distortions along propagation paths on the main parameters of coherent laser beams – focusing, effective average radius, and the proportion of the beam energy in its diffraction spot. The preliminary results obtained of the system mock-up performance confirm the conclusions of the theory.

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.

The fabrication of an analog holographic wavefront sensor, capable of detecting the low order defocus aberration, was achieved in an acrylamide-based photopolymer. While other implementations of holographic wavefront sensors have been carried out digitally, this process utilises a recording setup consisting only of conventional refractive elements so the cost and complexity of holographic optical element (HOE) production could be much reduced. A pair of diffraction spots, corresponding to a maximum and minimum amount of defocus, were spatially separated in the detector plane by multiplexing two HOEs with different carrier spatial frequencies. For each wavefront with a known aberration that was introduced during playback of the hologram, the resulting intensity ratio was measured in the expected pair of diffracted spots. A number of HOEs were produced with the diffraction efficiency of the multiplexed elements equalized, for a range of diffraction efficiency strengths, some as low as <5%. These HOEs were used to successfully classify four amounts of the defocus aberration through the observed intensity ratio.

Adaptive Optics (AO) systems for the compensation of optical turbulence in the atmosphere have been proven to work well within certain boundaries. Under strong turbulence conditions, AO based on conventional gradient wavefront sensors such as the Shack-Hartmann combined with linear least-squares reconstructors have shown to perform poorly due to the occurrence of phase singularities, that inherently cannot be reconstructed by the least-squares method. Directwavefront sensors, measuring phase differences directly rather than the gradient, avoid this problem of reconstruction. The self-referencing point-diffraction interferometer, a concept for direct-wavefront sensing that relies on the principle of spatial filtering to generate a (theoretically) unaberrated reference wave from the incoming aberrated wavefront, was early identified as a strong contender for an advanced wavefront sensor in strong turbulence conditions. Several authors have presented such systems. They make use of either the Fourier-transform method or instantaneous phase-shifted interferograms imaged by a complex optical set-up on a single image sensor. This paper evaluates a dynamic selfreferencing point-diffraction interferometer based on a pixelated polarization filter array imaging sensor for instantaneous spatial phase-shifting, promising a simpler optical set-up than other instantaneous phase-shifting approaches while retaining the advantage of less computational requirement compared with Fourier-transform methods.