This PDF file contains the front matter associated with SPIE Proceedings Volume 11506, including the Title Page, Copyright information, and Table of Contents.

The generated amount of data on high flying platforms like aircrafts, satellites and Unmanned Aerial Vehicles (UAV) increases continuously, due to the technical improvement of modern sensor systems. The resulting demands for higher data rates on airborne and space platforms motivates the development of Laser Communication terminals for aircrafts and satellites in the last years. DLR’s Institute of Communications and Navigation has a successful track record in developing Free Space Optical (FSO) terminals for flying platforms like stratospheric balloons, aircrafts and small satellites to transfer data from moving platforms down to earth in real-time. Beside the advantages of FSO such as high data rates and a secure transfer channel against Radio Frequency (RF) interferences, a direct line of sight is mandatory for a successful link. Traditional RF-Communication is more robust and less effected by atmospheric disturbances or weather conditions. Thus, new system concepts have been developed to benefit from the provided high data rates of the FSO and the reliability of RF-Communication technologies. As part of this trend, DLR has developed and demonstrated a Hybrid FSO/RF-communication system capable of overcoming the spurious effects of the atmosphere. This paper gives an overview about DLR’s current studies and developments with the goal to combine the advantages of FSO and RF-Communication. It discusses possible implementation concepts on different platforms and presents experimental results of the implemented FSO/RF hybrid communication system operating for airborne, optical downlinks at 1Gbps.

Space industry has undergone a significant change over the last years. The development moved from large and costly spacecrafts to cost-efficient designs and shorter development times. While the satellites became smaller, the resolution of high compact sensors increased which led to a high data-volume to be transmitted and increasing demands for higher data rates on small satellites. This motivated for a highly compact version of DLR’s optical communication payload OSIRIS for small LEO satellites. DLR’s Institute of Communications and Navigation has developed the OSIRIS (Optical Space Infrared Downlink System) program starting with payloads on the satellites Flying Laptop of Univ. of Stuttgart and BiROS of DLR. Combining miniaturization to the flight-proven developments with novel concepts, OSIRIS4CubeSat allows integration in a standard CubeSat bus. The development of OSIRIS4CubeSat (industrialized under the product name “CubeLCT”) is conducted in close collaboration with Tesat Spacecom, DLR’s commercialization partner. The first implementation will be demonstrated within the PIXL-1-Mission on a 3Unit CubeSat. Furthermore, OSIRIS4CubeSat (O4C) has been chosen to support scientific missions together with university partners in the field of Quantum Key Distribution (QUBE). In the future, the modular design will enable extensions for optical inter-satellite links. This paper will give an overview about the development of the O4C payload and the current status of the PIXL-1- Mission. Furthermore, it will show the adaptation of the payload for the scientific mission QUBE. Besides these projects, the paper will give an outlook for future extensions of the O4C payload and the necessity of high data-rates in other scenarios such as inter-satellite links.

The technological development in the field of optical communication has nearly addressed the concern for availability of bandwidth at higher data rates to meet the user requirement. The high speed multimedia applications have put an impetus for seamless transmission and reception of error free data at desired bit rates. The presence of recurring inherent errors in data transmission and processing makes Free Space Optical (FSO) communication susceptible to errors and loss of information. For efficient communication and effective service delivery in FSO, the mechanism for detecting errors and then correcting them has to be evolved. In this research work, we have focused on the application of Reed Solomon (RS) codes over the finite Galois field (GF) for detecting and correcting maximum received errors. The RS (n, k, d_min) code scheme is applied to the designed FSO link by taking the data bits (k) 239, 223, 191 and 127, code length (n) 255 with a code rate (R) 14/15, 7/8, 3/4 and 1/2 under the influence of moderate to strong turbulences and weather conditions. The proposed FSO system has been modeled using the Modified Gamma-Gamma model and the performance is evaluated for Bit Error Rate (BER), number of errors corrected and Geometric path losses as a function of link distance with and without the application of RS codes. The designed FSO system with RS code rate=1/2 performs optimally and shows an improvement of 8dB in coding gain as compared to the conventional FSO system. The geometric path losses have reduced from -8.76dB to -28.04dB along with maximum possible error correction of 399 errors in the transmitted data frame for the link distance ranging from 500m to 5km.

Free Space Optical (FSO) Communications data link offers high data rates with low system complexity but atmospheric attenuation, such as fog, alters signal integrity. In the paper, we present a novel low-cost visible band FSO system design and its performance evaluations in foggy conditions. Using LTSPICE design tools, we proposed a low-cost transmitter and receiver circuits. We built a fog-testing chamber and FSO systems to measure different transmitter sources by analyzing DC carrier powers and digital AC signals at 1MHz. A He-Ne laser at 630nm was used to calibrate chamber fog density and measure the steady state performance. Concurrently, we developed a CAD model of our FSO systems using Synopsys Optsim and obtained FSO system performance at different foggy levels to compare with our experimental results. The red laser diode (LD) at 658nm has the best performance in foggy environments with experimental attenuation of -0.20dB and simulated attenuation of -0.63dB. The green LD at 520nm and violet LD at 405nm experience experimental attenuation of -0.35dB and -0.53dB respectively. Despite beam divergence, the red LED at 615nm has good performance in foggy environments with experimental attenuation of -0.33dB and simulated attenuation of -5.2dB. The yellow LED at 590nm, green LED at 522nm, and blue LED at 470nm experience an experimental attenuation of -0.51dB, -2.2dB, and - 0.72dB respectively.

The results of numerical simulations and experiments on the correction of turbulent distortions of a laser beam are presented. The experiments were carried out using an adaptive optical system with a bandwidth of 2000 Hz. It was shown that for effective correction the bandwidth of the adaptive optical system should be an order of magnitude larger than the bandwidth of turbulent distortions.

The inherent and apparent optical properties (IOPs and AOPs) of seawater limit the performance of free-space optical (FSO), underwater wireless optical communication (UWOC), and imaging systems. Absorption, scattering, and downwelling irradiance are three such properties that influence system performance and often evolve independently. In situ measurements of multiple IOPs and AOPs would provide environmental sensing for fielded optical systems, but such comprehensive measurements are typically expensive or impractical. This effort analyzed existing oceanographic data sets to uncover wavelength-dependent correlations between IOPs, AOPs, test depths, and ocean depths. We then employed machine learning (ML) methods to predict the optical properties of diffuse attenuation (Kd) and backscatter (bb) using beam attenuation (c) and compared these results to ground-truth values. Predicted values of Kd and bb were well matched to their ground-truth data. Furthermore, we demonstrate ML-based Jerlov optical water type classification using beam attenuation as the optical data input. With our methods validated, we collected new optical data sets and processed them using our ML models. Results are promising and indicate future in situ classification and prediction capability. To highlight one practical application, we present a preliminary ML-enabled performance estimator for a generic FSO or UWOC system.

Laser beams carrying orbital angular momentum (OAM) in underwater environments have been a topic of research for underwater communications and remote sensing applications. When a laser beam propagates through turbid water, the dominant form of attenuation and spatial dispersion is scattering due to small particles. The goal of this experimental study is to measure the transmitted OAM mode and its intermodal crosstalk via measurements of the OAM spectrum after propagation through turbid water. An initial beam is encoded with a single OAM state using a spiral phase hologram displayed on a high-resolution spatial light modulator. The optical receiver performs a phase cancellation measurement to decode the OAM on the incident beam. After recording images of the phase canceled beam, the OAM spectrum is found in post-processing. Three methods of post-processing are presented and compared to account for beam wander and an astigmatism in the experiment. After determining which method of post-processing gives the most accurate results, our results are compared to those in the literature. Our results show that an OAM beam maintains mode purity up to an optical depth (OD) of 12, whereas previous literature saw a loss of mode purity at an OD of 6. This is attributed to differences in receiver field of view, scattering volume, scattering length, and beam size.

Atmospheric propagation experiments have been conducted over the 1km TISTEF laser range to examine the effects of turbulence, absorption and scattering on a supercontinuum laser beam. This supercontinuum laser beam has been band limited to transmit only visible wavelengths (400-850 nm). In this work, the effects of atmospheric turbulence on the supercontinuum laser will be examined.

We compare femtosecond hollow-core multifilament arrays created in the air with a TEM₁₁ phase plate and a Dammann diffraction grating under additional loose focusing. Phase shifts introduced into the beam by the phase plate lead to zero intensity lines, which prevent transverse energy flow and filament merging. The Dammann grating forms four spatially separated energy reservoirs near the focus due to the interference. Transverse multifilament structure obtained using the Dammann grating is more resistant to phase and amplitude distortions of the initial laser beam. Plasma density inside the multifilament arrays does not exceed the value in a single filament, obtained without DOEs.

We review and compare recent methods for generating and detecting orbital angular momentum states in the context of free-space optical data transmission, considering their actual contribution in terms of aggregate capacity, photon efficiency, and flexibility. Based on simulation-based and experimental evaluations, we propose practical metrics to evaluate the performance of such systems under different scenarios, including those that require quantum security.

The study of light carrying complex phase profiles, specifically orbital angular momentum (OAM), has been of interest for its use in free-space optical communications and remote sensing systems. Each of these applications requires a beam to propagate through the atmosphere, where optical turbulence is the main distorter of the beam. In this computational study, coherent Laguerre-Gaussian (LG) beams and partially coherent I_m Bessel beams are propagated through atmospheric turbulence. The LG beams are propagated through turbulence using a split-step method for solving the Fresnel diffraction integral. Whereas for the the I_m Bessel beams, the coherent mode representation is used, where each eigenmode is individually propagated through turbulence. The split-step algorithm is then modified to simulate optical turbulence by the use of phase screens. Beam metrics, in the form of intensity, scintillation, spot size, and OAM spectrum, are then calculated over a number of turbulence realizations. Three turbulence regimes are simulated that include the weak, moderate, and strong turbulence regimes along with two different initial beam sizes. The I_m Bessel beam is simulated using three values of overall coherence ξ. The results for the metrics are plotted against propagation distance and OAM mode l. The resulting beam metrics show a strong dependence on turbulence strength, a weak dependence on OAM mode due to LG modes expanding with an extra prefactor of ι + 1, and no strong dependence on the overall coherence ξ.

The creation and detection of light carrying orbital angular momentum (OAM) has been of great interest for applications that require a beam to propagate through atmospheric turbulence such as for free-space optical communications and remote sensing. In this experiment, Laguerre-Gaussian (LG) beams are created using a high-resolution deformable micromirror device (DMD) and then propagated through two artificial turbulence strengths. To measure the OAM encoded on the LG beams, the wavefront is decomposed into an orthogonal basis set denoted as the OAM spectrum. The OAM spectrum is measured using two forms of Mach-Zehnder interferometers (MZI). The first interferometer is a modified MZI that is used to measures the OAM spectrum by first measuring the angular correlation function with a single interferogram. The second interferometer is a traditional MZI used to record an interferogram that can then be processed to extract the phase of the LG beam. The measured phase is then used to find the OAM spectrum by applying modal decomposition. To improve the OAM spectrum measurement, a singularity tracking algorithm is used to correct for the turbulence distortions. Each interferometric technique is compared with and without the presence of artificial turbulence. Both interferometric methods were significantly affected by turbulence, but the traditional MZI was able to measure the spreading of the OAM spectrum well with the support of a singularity tracking algorithm.

Phase-based techniques to measure atmospheric turbulence have potential advantages when used over long ranges since they do not suffer from saturation issues as the irradiance-based techniques. The present work uses time-lapse imagery of a non-cooperative target from two spatially separated cameras to extract turbulence distribution along a path. By measuring the differential motion of pairs of extended features on the target, sensed by a single camera or between cameras, turbulence profiles can be obtained. Tracking the motion of extended features rather than point features allows estimation over a longer range. The approach uses a derived set of path weighting functions for differential tilt variances. The mathematical framework is discussed and the technique is applied to images collected of a multi-story building. Turbulence profiles over different slant paths are extracted from features at multiple levels of the building. This work will ultimately help in a better understanding of how turbulence varies with altitude in the surface layer.

We present generalized expressions for the piston-removed and piston-and-tilt-removed anisoplanatic error in non-Kolmogorov turbulence with a finite outer scale. We use these expressions to investigate the behavior of the anisoplanatic error when imaging over long horizontal and slant-paths paths, through a Hufnagel-Valley turbulence profile, where the angular extent of the scene is often many times the isoplanatic angle. By evaluating these expressions we first found that the removal of piston and tilt contributions causes the isoplanatic angle to increase in all cases. Relatedly, when turbulence is weighted nearer to the aperture the anisoplanatic error is more likely to saturate to a value less than 1 rad² for larger outer scales, regardless of piston and tilt removal.

Speckle imaging has long been used for recovery images of static scenes distorted by atmospheric optical turbulence. One limitation of speckle imaging is the need for multiple image frames where the scene is static and corrupted by independent turbulence realizations. This is a critical limitation when observing scenes containing moving objects. In this work, I use view-point images from simulated light-fields as inputs to a bispectrum-based speckle imaging algorithm. I find that near diffraction-limited imagery can be recovered from a single light-field exposure.

We propose new techniques for the reconstruction of complex fields through the development of new iterative algorithms based on propagation equations and the representation of complex objects on orthogonal bases. These techniques use the advantages of the latest phase retrieval techniques and single pixel (SP) detection to retrieve the complex field. These techniques as a whole form a suitable tool for the characterization of dynamically perturbed scalar beams with singular phase profiles.

In an optical phased array (OPA) free space laser communication system, one of the main issues is to control the phase of each beam to acquire fast and stable phase-locking for phase consistency, in which the number of the elements in the array is of vital importance. In this paper, we studied the method on improving the performance of phase-locking about speed and stability of optical phased array with large number of elements. The performance of OPA with different number of emitting elements was analyzed with the simplified stochastic parallel gradient descent (SPGD) algorithm for closed-loop phase-locking. And the algorithm showed a poor performance when the elements of the array were in large amounts. An optimal algorithm, the distributed SPGD (DSPGD) algorithm, was proposed to improve the performance of the array, which revealed fast speed and stability in phase-locking of a large number of emitting elements of optical phased array, suggesting the algorithm is effective.

Recently, communication capacity has increased significantly due to diversification of contents, and online classes and teleworks emerging from the spread of COVID-19. Furthermore, technological development of sixth-generation mobile communication system (6G) has planned in 2030, it is predicted that communication capacity increase more in the future. For the realization of 6G, not only a large capacity of backhaul, but also an all-optical network is required owing to high frequencies. To satisfy these requirements, orbital angular momentum (OAM) in optical wireless communication has been investigated. The OAM is a part of the Laguerre-Gaussian (LG) beam, which possesses modes that are determined by the radial order n and the azimuthal order m. The OAM is modes when the radial order n is equal to zero. Because crosstalk between the modes are inevitable, it is necessary to separate orders during multiplexing. Therefore, the LG mode that has been extended to the radial order n must be used for mode multiplexing in order to achieve large communication capacity. For realizing multiplex communication, the multiplexed signal must be separated into individual detectors. Previous studies have reported a method for separating signals using light as the carrier wave in multiplex communication, by preparing the same number of filters as the number of multiplexes. However, the system becomes more complex and reception efficiency deteriorates as the number of multiplex increases. In this study, we achieved mode-demultiplexing with one filter using kinoform-type computer generated holograms as multiplexed holograms in LG mode multiplex communication.