This PDF file contains the front matter associated with SPIE Proceedings Volume 11532, including the Title Page, Copyright information, and Table of Contents.

Opening remarks for the Environmental Effects on Light Propagation and Adaptive Systems III conference

Military operations in arid regions of the world are becoming more and more regular. The atmospheric conditions in these regions impose severe restrictions on the performance of optical systems. In contrast to regions, where many airports are located and therefore the monitoring network of ground stations is very dense, only few ground measurements are available for arid regions. To a certain extent, measurements can be collected and generalized with large-scale measurement campaigns, but they are very cost-intensive and partly not achievable due to the political situation. Another possibility to close this gap of data is provided by satellite measurements. For various measurement parameters such as humidity, wind, solar radiation and aerosols, this works quite well with some limitations. For this reason, models are a good complement to fill the lack of data in these regions. The study is concerned with identifying the turbulence in Western Sahara. The models used WRF (Weather Research and Forecasting Model) and ICON (Icosahedral Nonhydrostatic Model) have been sufficiently tested in different regions of the world. As there are no turbulence measurements in the Sahara, this is the first test to estimate the magnitude of the turbulence in order to discuss the need for an extensive measurement campaign. The models can be validated with previous trials of IOSB such as White Sands Missile Range (WSMR) in the USA, (New Mexico).

Our aim is to characterize the optical turbulence over the urban areas. Since it is difficult to measure 𝐶2/𝑛 (see PDF) continuously over an urban area, we explore the possibility of using a mesoscale weather prediction model to predict 𝐶2/𝑛 (see PDF) over the urban area. To this end, the output of the Weather Research and Forecast model (WRF) was coupled with a micrometeorological parametrization, which allowed calculation of 𝐶2/𝑛 (see PDF) at each numerical grid point in the surface layer. Numerical results are compared to data of path-averaged measurements of optical turbulence performed with a large aperture scintillometer (BLS900) over the city of Ettlingen (southwestern Germany) during two time periods in Spring and Summer 2013. Effects of the heat island effect are revealed by high turbulence values, observed at night-time.

Exploration of oil and gas by the geochemical method is somewhat underestimated. For its full disclosure, more modern and highly accurate methods of sample analysis are required. One of the promising methods is the Raman spectroscopy method. In this paper, we propose the concept of a portable Raman ultra-spectral resolution gas analyzer for recording low concentrations of hydrocarbons at a level of 2 ppm. The main idea is to use two radiation sources of 266 nm and 785 nm, with this configuration the total noise level is noticeably reduced, by comparing the data, Raman shifts of gases can be more accurately distinguished, an additional nitrogen channel is used for more accurate measurements in the system. The work shows a diagram of the optical channel, and the energy calculations performed show the possibility of detecting methane with a concentration of 2 ppm at a distance of one meter.

Optical communication between satellites and optical ground stations is a promising technology for high-speed data transfer between Space and Earth. However, the atmospheric turbulence present in the last few tens of kilometres near the ground has a significant impact on the quality of the link and reduces the transfer rate due to signal fading. To explore the robustness of adaptive optics (AO) systems to mitigate the turbulence effects at low elevation angles, we have developed a bench representative of the propagation channel between the ground and the satellite. The aim of the paper is to present the system analysis, the definition choices and the simulated performance that have conducted to the actual bench. We also describe the set-up and the characterisation of the main components.

The results of long-term studies of the formation of atmospheric turbulence over the territories and in closed rooms of mountain observatories are systematized: the Sayan Solar SB RAS, the Baikal Astrophysical SB RAS, the Special Astrophysical RAS, the Kolyvan Ridge Observatory, as well as at the Center for Laser Sounding of the Institute of Atmospheric Optics SB RAS in Tomsk. The regularity of changes in the local structure of turbulence depending on the terrain and the direction of the wind speed has been established. The mechanism of formation of long-lived regions of atmospheric coherent turbulence over the territories of mountain observatories is described. These studies are necessary to predict the propagation of electromagnetic waves in the atmosphere of mountainous regions.

The communication with Earth satellites depends on propagation of a laser beam through the atmosphere. The advantages of an optical wave system over a conventional radio frequency system were analyzed earlier [1]. When using optical transmitters there are a number of problems. We must know the structure of turbulent fluctuations in the atmosphere and beam distortions that occur due to those fluctuations. Recently we set forth the solution for electromagnetic field of a non-paraxial Gaussian beam [2]. The main computational difficulty is the fact that the solution includes highly oscillatory integrands. The data were revised and supplemented. Some of the calculations were performed again. We have considered the propagation of a laser beam in both homogeneous and inhomogeneous media. The components of the electromagnetic field at different distances from the source were calculated. In addition to the results that refer to the Gaussian beam the paper contains data that refer to the Hermite-Gaussian beam. The task of beam propagation in an inhomogeneous atmosphere is reduced to solving the equation E=F{n(r),E(r). Here n is the refractive index and F is a known function. The equation can be solved by the method of successive approximations. We used only the first approximation. We supposed that permeability is equal to unity. A function describing the dependence of the refractive index on coordinates was selected. An example of the calculation is given in the paper. The solution may be generalized to the case when the refractive index depends on time.

Laser communications are expected to enable wide deployment of high capacity telecommunication networks. To ensure that components used in such architectures are competitive both in terms of costs and performances, coupling into a single-mode fiber at the reception side is mandatory as most off the shelf components (COTS) have already been developed for optical fiber networks. However, in free-space optical communication through the atmosphere, turbulences modify the wavefront profile which degrades coupling towards a single-mode fiber. Multi-Plane Light Conversion (MPLC) is proving to be a new effective technique to mitigate turbulence effect. The degraded beam is decomposed on a mode basis, typically Hermite-Gaussian modes, each mode being passively demultiplexed towards a single-mode fiber. The incoming turbulent spatial mode undergoes phase and intensity fluctuations, but, as the MPLC is a passive component, this only leads to phase and intensity fluctuations of the signal inside the corresponding single-mode fiber. The complexity of the architecture is transferred from correcting actively the wavefront to signal processing inside single-mode fibers. Here, we investigate the performance improvement of the MPLC technique and mode collection compared to direct single-mode fiber coupling. We evaluate theoretical and experimental collection efficiency for SMF only and the summation of the 15 first Hermite-Gaussian modes for D/r0 from 1 to 14. Results show that 15 modes MPLC appear to be a good compromise between the number of modes and the complexity of the device. This configuration typically improves the collection efficiency by >7 dB in the case of strong turbulence when D/r0 >4. Moreover, the minimum collection efficiency that would correspond to a link failure is dramatically improved compared to SMF fiber alone. Finally, power distribution over the modes seems to be similar which will facilitate the implementation of this technique.

There has been an ever-increasing demand for high data rates and security in underwater communication networks as well as sensor applications; the use of lasers has proven to be a realistic option given its inherently high bandwidth as well as low probability of intercept and detection by opponents. That said, laser light experiences degradation when propagated through random media, such as experienced underwater due to optical turbulence, absorption, and scattering. We are motivated to investigate the encoding, propagation, and performance of beams carrying orbital angular momentum through underwater turbulence as a mechanism for utilization in a high-performance communication system. Additionally, we seek to explore and quantify the performance characteristics of neural network techniques to classify and decode the received beams.

Recent works revealed that transmission of light beams carrying orbital-angular-momentum (OAM) through turbulence causes the optical vortex defining these beams to split into multiple vortices with unit topological charge. Here, we consider the numerical propagation of orbital-angular-momentum (OAM) modes through a horizontal atmospheric channel. By analysing the beam's phase front after transmission through turbulence, we confirm the occurence of vortex splitting, but we also witness the emergence of vortex-antivortex pairs. Moreover, by performing performing a decomposition of the transmitted wave into OAM modes, we show that while adaptive optics cannot cancel vortex splitting, it still is pretty efficient in diminishing the turbulence-induced crosstalk between different OAM modes.

We carry out a numerical analysis of the spatial structure of the eigenmodes of light in atmospheric turbulence and assess the distribution of the singular values under variable turbulence conditions characterized by the Fried parameter and Rytov variance. Under weak scintillation, the highly transmitting eigenmodes found here possess a modal structure that is reminiscent of Laguerre-Gaussian (LG) modes and their simple superpositions. When scintillation becomes significant, we establish that the optimal eigenmodes for communication differ substantially from LG modes and tend to have highly localized transverse intensity distributions.

Secure optical communications networks are a functional requirement for many military, government, and civilian applications. Optical free space links provide security due to the small ground footprint of highly collimated laser beam patterns. Space-based optical communications provide an additional layer of security due to dynamic angular tracking requirements and remote orbiting infrastructure. The addition of Quantum Key Distribution (QKD) adds a third layer of safety through the use of physically unbreakable keys. The QEYSSat mission is scheduled to launch in 2022. This mission will carry a primary QKD science payload and a secondary high-speed optical communications demonstration payload. The QEYSSat secondary communications payload (QP2) is the latest space-based optical communications terminal designed to be amenable to low cost mass production methods, meeting the price targets of many planned low-earth-orbit optical communications constellations. The successful demonstration of both technologies on a single micro-satellite platform demonstrates the key technologies necessary to enable next generation high speed secure communications networks. In this paper we present an overview of the QEYSSAT optical payloads and describe secure architectures for QKD-enabled optical communications network applications.

Coherent free-space optical (FSO) communications systems present an opportunity for significantly increased data rates in wireless communications. To implement coherent FSO systems, degradation of received signals due to atmospheric propagation must be mitigated. This can be achieved by deploying adaptive optics (AO) systems at the optical receiver to reconstruct wavefronts deformed during propagation. In this work, we present the design and implementation of a coherent FSO communications system which utilizes a wavefront sensorless AO system to improve received signal quality. The performance of the communications system is quantified with and without the use of AO by measuring bit error ratio (BER) and error vector magnitude (EVM).

FSO systems present many advantages like high data rate, license-free bandwidth and tap-proof communication allowing the download of vast amounts of data from LEO satellites. However, the atmospheric channel is quite challenging, because of spurious effects such as absorption, scattering and scintillation that in turn vary the link losses in correspondence to the elevation. In order to maximize the downlink throughput, it should start around 5° elevation. This leads to a design with suboptimal performance for higher elevations when constant data rates are used. Therefore, DLR is developing a system to adjust the data rate according to elevation and atmospheric channel conditions. This data rate variation is achieved by determining the maximum rate for higher elevation and then for lowering the data rates a bit-level repetition is performed. The presented system enables a fast transition between the different data rates. Additionally, this system allows the satellite to transmit data at rates even lower than those nominally supported by the physical transceiver. At the receiver side, the system complexity increases as it should be able to acquire, detect, and filter the signal for different data rates. DLR proposes a system that mirrors the operation of its transmitter counterpart by sampling the acquired signal at the maximum data rate. Then an FPGA processes the signal by majority decision algorithm followed by voting system that filters and detects the intended data rate in real-time. This enables replication and parallelization of the filtering and detection processes enabling the automatic detection of the received data rate. In order to provide noise stability, the transition between data rates is governed by a hysteresis process. This scheme allows the detection and selection of the proper data rate in the range of few microseconds for a system operating between 10 Gbps and 1.25 Gbps in steps of factor of 2, ignoring the propagation delays.

The performance of laser weapon system based on Coherent Beam Combining (CBC) depends on its propagation properties in atmosphere. The appropriate semi-analytical model based on partial coherent beam combination for assumed coherence coefficients between beams in CBC lattice was developed. To simulate atmospheric turbulences the Kolmogorov-Fried model of partial coherence and Hufnagel-Valley model of C_n² dependence on atmosphere parameters were implemented. The approximated formula on CBC performance in dependence on Fried radius was proposed. The results of CBC modeling were compared to known analytical solutions of Gaussian beam propagation in turbulent atmosphere. The dependence of CBC performance on C_n² parameter, elevation angle and range was analyzed. The general conclusion is that application of CBC has not practical sense for propagation on medium and long ranges without effective adaptive optics system.

Atmospheric turbulence causes scintillation, spatial and temporal blurring as well as global and local image motion and is responsible for geometric distortions in image data. The approaches that can be found in literature for correcting turbulence-related impairments of image quality are just as varied as the prevailing environmental and turbulence conditions can be. There is a variety of conceivable applications for such correction methods. Consequently, it is difficult to determine a suitable taxonomy that can cover all possible application cases. Therefore, in this paper a tabular approach is proposed on the basis of which similar assumptions can be summarized in order to make algorithms for typical scenarios comparable with each other. A profiling scheme is introduced for this purpose where points are assigned to a selected (and variable) number of criteria according to their priority in a given context. This point system has a dual function, enabling a given application to be systematically described in terms of its requirements as well as a given algorithm to be characterized in terms of its performance parameters. Thus, corresponding (point) profiles are obtained for applications as well as for correction methods, which can be used for meaningful comparison and methodological evaluation of the respective correction results.

Detailed analysis of optical cascades is usually performed using the matrix method of optics, which allows us to determine the transformation matrix of coordinates and the incidence angle of the beam for each element of the cascade. The convenience of matrix description lies in the unity of approach to both ray and wave propagation problems. A series of symmetrical and asymmetric optical cascades that are optimal for turning a wave beam in phase space and do not have forbidden rotation bands are considered using matrix optics methods. For the simplest symmetric two-lens cascades, the tuning characteristics are obtained that allow setting the rotation angle in the range ±π. Rotation control is performed by adjusting the external or internal "run" of the beam between the lenses. For specific cascades, the geometry of the forbidden zones of turns, physical analogs of turns at a complex angle are analyzed.

The amplitude distribution complex structure of the optical beam profile at the end of a long atmospheric path can be considered in terms typical of surface profile descriptions used in solid state physics or geophysics. The characteristic values for such approaches are based on the geometry analysis of constant level curves, directions of steepest descent from a given point on the surface and quadratic combinations describing the deformation of the analyzed surface. The matrix of local orientation values determined on the basis of the first spatial differentials is unique for the image. It can be used as a basis for describing the of the initial intensity distribution. Differential characteristics of the spatial structure make it possible to classify distortions of the beam profile by types of symmetry and direction of deformations. Differential characteristics of the spatial structure make it possible to classify distortions of the beam profile by types of symmetry and direction of deformations. It's possible to associate the observed spatial parameters with the meteorological conditions on the path and in the first approximation, restore the main directions of phase modulation in the observation plane.

The method of computer simulation is used to solve the problem of assessing the distortions of coherent structured light beams (with and without an optical vortex) during their propagation in a randomly inhomogeneous atmospheric medium. The methods of adaptive phase-conjugate correction of the light wave from the reference beacon source are used. Two schemes of an adaptive optical system with a beacon location (at the beginning of the propagation path or at the end of it) are considered. The propagation of focused Laguerre-Gaussian beams is investigated by numerically solving a scalar parabolic wave equation. Initially, all beams are reduced to the same effective radius. A dynamically time-varying randomly inhomogeneous medium with a power-law spectrum characteristic of atmospheric turbulence is modeled by a set of several dynamically changing turbulent phase screens uniformly distributed along the beam propagation path. For this, a dynamic algorithm for simulating a time-varying random environment is used, based on an autoregression model with a moving average. This also makes it possible to study the temporal spectra of fluctuations of the indicated beams under conditions of weak turbulence in cases without the use of preliminary phase correction and with its application.

Publisher’s Note: This paper, originally published on 20 September 2020, was replaced with a corrected/revised version on 11 November 2020. If you downloaded the original PDF but are unable to access the revision, please contact SPIE Digital Library Customer Service for assistance.