This PDF file contains the front matter associated with SPIE Proceedings Volume 11678, including the Title Page, Copyright information, and Table of Contents.

Welcome and Introduction to SPIE Photonics West LASE conference 11678: Free-Space Laser Communications XXXIII

Multi-point free space optical communication (FSOC) has been identified as a valuable and promising technology for meeting high-capacity and -density demands of future space and terrestrial communication networks. FSOC’s point-topoint nature has boosted extensive research on technologies and methods that support multi-user optical networks. An FSOC system platform is essential for fully characterizing, testing, and evaluating state-of-the-art, multi-user prototypes and technologies developed by both businesses and academic communities. This paper presents an experimental FSOC testbed that demonstrates next generation FSO systems and allows cognitive, multi-point communication. These systems provide a significant improvement over those with currently hampered with single-user limitations. The FSOC testbed is multi-node, modular, and high-speed with real-time ability to test O-PHY modules and O-MAC schemes. The testbed consists of multiple, independently tunable optical transmitters and receivers that can be configured to emulate various communication scenarios (e.g., point-to-point (P2P), point-to-multipoint (P2MP), and multi-point-to-multipoint (MP2MP). At the receiver side, a cognitive controller performs real time, blind processing of received signals for identifying the number of concurrent transmissions. Accordingly, the controller drives an optical switch to route detected signals to pre-defined paths. Given that a single-user transmission is detected on multiple paths, diversity combining will be performed to improve received signal-to-noise ratio (SNR). If multiple-user transmissions are identified, signals are routed into separate high-speed photodetectors for processing. The work described below details hardware components integrated in the platform, as well as software development for the cognitive controller. Furthermore, this work provides an experimental demonstration of the testbed capabilities for single-user and multiple-user scenarios.

Free-space optical communication (FSOC) have directional light beams which makes communication links very sensitive to movement. A major challenge in mobile settings is handling of this fragility of FSOC links due to the highly directional distribution of light intensity within the light beams. Differing from previous studies using mechanical steering of the transceivers to remedy the brittleness of FSOC links, we use a square array of stationary elements for each transceiver for better directionality, optimum combination of elements (transmitter/receiver ratio along with location), and robustness to mobility. Based on previous studies showing the optimum transmit to receive area ratio in a square array layout, we locate the transmitters as a box around the center with extra elements on the four corners of the transceiver plane with the rest of the elements on the array being receivers. We design a hardware prototype using the same optimum transmit/receive ratio and a lOxlO square array layout with size, weight and power, cost, and geometric simplicity appropriate for a low-flying multi-copter drone. A full link margin analysis was completed for the lOxlO array, using commercial off-the-shelf components, with the same optimum transmit and receive combination. The range for the system was found to be rv 150 m operating at 1 Mbps. The outcome of this work will give us insight of how the tiling of transmit/receive elements affects a transceiver system to implement a directional wireless link in the optical spectrum for mobile settings, particularly for emerging use of low-flying drones.

We present the status of ongoing work at NASA-Goddard Space Flight Center (GSFC) to build a low-cost flexible ground terminal for optical communication. Previous laser communication missions at NASA have been supported by one-of-akind ground terminals built specifically for each mission. If NASA is to build a global network of optical terminals to enable widespread use of optical communications, then a blueprint for an economical ground terminal able to support a variety of missions is needed. With this goal in mind, NASA is constructing a ground terminal in Greenbelt, Maryland to enable testing of new ground terminal technologies from industry to academia.

The need for data storage is rising exponentially – 90% of the world’s data has been created in the last two years and almost 5% of the world’s electricity consumption is currently devoted to storing this massive amount of data. LyteLoop is developing a disruptive alternative to data storage technology using optical communications to store data in motion between objects with higher security and lower power consumption. The technology relies on a patented method of dramatically extending optical path length called Angle Multiplexing, as well as spatial division multiplexing with orbital angular momentum modes to increase data rate in order to store data in space, in optical fiber, or in free-space vacuum cavities. A free-space prototype is under development, which stores data using a high bit rate signal continuously circulating through an Angle Multiplexing system.

Optical satellite communications is a maturing technology to enable word-wide access to high throughput internet. In the past years a lot of effort has been taken to increase the applicability and the TRL of this technology. In collaboration with industry, TNO initiated several developments for space and ground technologies. Many of these technologies have already passed critical design review (CDR) and are in an advanced state. A missing piece of the puzzle is an in orbit demonstration (IOD), which proves the technologies to be working. This paper presents the plans for an IOD with CubeCAT on the NorSat-TD. As ground segment the TNO optical communications lab is equipped with an 80 cm diameter telescope. By an successful IOD, worldwide available internet at high throughput is yet one step closer.

General Atomics Electromagnetics (GA-EMS) has developed a free space optical laser communication terminal (LCT) for space applications. The system operates at 1550 nm and utilizes on-off keying to support a data rate of up to 5 Gbps. The system has undergone assembly, integration, and testing and is being integrated into the host spacecraft in preparation for an Optical Inter-Satellite Link (OISL) demonstration with the Space Development Agency. The OISL demonstration will consists of two GAEMS LCT systems on two separate cubesats to perform cross-link demonstrations at distances of up to 2500 km and also serve as an on-orbit platform to characterize pointing, acquisition, and tracking capabilities between the two terminals as well as out of plane links to other satellites. While the 2021 mission will highlight an OISL demonstration between to identical spacecraft, the hardware is designed for easy integration into various satellites by using common electrical and mechanical interfaces with the overall goal of having a bus agnostic LCT architecture in support of interoperability.

This paper describes Fibertek’s progress toward developing high performance, compact, space laser communications terminals and lasers for commercial and government customers. Previous developments have addressed a variety of applications including deep space CubeSats, LEO, GEO and SmallSats. We are currently working on lunar and beyond optical com terminals. Fibertek has also developed high TRL space laser for space optical communications ranging from small CubeSat 0.5 to 3W up to 50 W WDM amplifiers supporting deep space PPM CCSDS modulations and WDM Tbps WDM space networks.

The European Data Relay System is now in it´s fifth year of operation. Until now (March 2021) more than 50000 data relay laser links have been successfully executed. We report on in orbit performance of the data relay LCTs and other activities related to Lasercom at Tesat.

SOLISS is designed for technology demonstration of small laser communication terminal connecting to optical ground station from International Space Station (ISS) and its name came from Small Optical Link for ISS. It was successfully launched on 25th Sep., 2019 and achieved to demonstrate bi-directional communication as its extra success. SOLISS is jointly developed with Japan Aerospace Exploration Agency (JAXA) and also jointly developed special functions of optical ground station with National Institute of Information and Communications Technology (NICT). We discuss and report in-orbit demonstration and its result.

A method of fine pointing of laser beams by using laser arrays has been developed. The telescope system combines a lens system and a VCSEL/Photodetector Array. It does not use moving parts. In computer simulations, it is applied to CubeSats that use body pointing. Body pointing was used by the Aerospace Corporation for CubeSats in LEO in NASA’s Optical Communications and Sensors Demonstration (OCSD) program. Computer simulations of this fine pointing capability have been applied previously to CubeSats in the OCSD program. In this paper, computer simulations of laser pointing using this telescope design are applied to CubeSats in LLO, at 100 km. These CubeSats could form part of the LunaNet, the lunar communications and navigation network, part of the NASA ARTEMIS Program. With more accurate pointing, a laser beam with smaller divergence can be used. For the case of the AeroCube-7B vehicle that was used in the OCSD program, computer simulations will show, for example, that the divergence of the output beam can be reduced from approximately 0.06° FWHM to 0.014°. For the proposed electro-optical system, reaction times to pointing changes and vibrations are on a nanosecond time scale, much faster than those for fine pointing systems that use moving parts such as fast steering mirrors, including MEMS, or that use quad-cell photodetectors to improve the body pointing of the CubeSat. Other possible applications are to Optical Multiple Access (OMA) for simultaneously communicating with ground stations at different locations and to Wavelength-Division Multiplexing (WDM) for increasing data rate transmission.

Laser communication is an expected approach to realize a high data rate communication on small, micro and even cube satellites. Sony and Japan Aerospace Exploration Agency (JAXA) had experimentally verified fundamental functions of the small optical communication terminal with optical disk technology that is needed for miniaturization of light-weight and low power consumption laser communication terminals. To verify these functions from the optical disk technology in orbit as a laser communication system, Sony Computer Science Laboratories, Inc. (Sony CSL) and JAXA had jointly developed the small optical communication terminal called SOLISS from late 2017 that designed to be attached to the exposed facility of International Space Station (ISS) and it was successfully launched from Tanegashima in Japan on 25th September 2019. This experimentation aims to confirm 100 Mbps Ethernet-based laser communication establishment between low-Earth orbit and the ground and the availability of pointing control with the optical disk technology in-orbit. To achieve the goal, SOLISS continuously controls the accurate pointing with a coarse and fine pointing mechanism to keep the establishment of the optical link with a counterpart. In this experimentation, SOLISS successfully established the bidirectional Ethernet-based link with a PC connected to the optical ground station of National Institute of Information and Communications Technology (NICT) by its pointing mechanism. The result demonstrated the availability of the proposed pointing mechanism. This article discusses the pointing performance of SOLISS with the optical ground station.

Free-space laser links traditionally utilize an independent spatial tracking channel with a beacon laser and tracking sensors to meet stringent pointing requirements. In this work, we propose a miniaturized monostatic beaconless fiber transceiver that infers fine tracking information using existing receiver optoelectronics and a small injected pointing dither (nutation). A single MEMS steering mirror is used to both fine-point the beams and inject nutation. While this results in some additional link loss due to disturbed fiber coupling and transmit beam pointing, our analysis shows the loss becomes negligible for sufficient SNR. Links without point-ahead correction need an SNR of about 35 dB to minimize the dither loss below 0.1 dB and also maintain the RMS spatial tracking noise below a tenth of the beam divergence. Since the pointing and tracking bandwidth is typically many orders of magnitude slower than the receiver communication bandwidth, such SNR is usually achievable on the receiver with appropriate filtering. If point-ahead correction is needed, we show that depending on the available link margin, a transceiver based on single-mode fiber can reach up to about 1 beamwidth of correction, while a few-mode fiber design can reach up to about 1.75 beamwidths due to improved coupling sensitivity at higher point-ahead offsets. Finally, we propose the use of double-clad fiber with a secondary detector to help further minimize the incurred coupling loss.

The Deep Space Optical Communication (DSOC) project will demonstrate free-space optical communication at almost 3 AU, or 3 orders of magnitude further than any previous attempt. DSOC will utilize the 5m Palomar Hale Telescope to receive the downlink signal, which will couple the downlink light onto an optical table and into a superconducting nanowire single photon detector (SNSPD). The output of the SNSPD is digitized by the Ground Laser Receiver Signal Processing Assembly (GSPA) using a high throughput streaming time to digital converter (TDC). The GSPA is a scalable FPGA-based receiver which demodulates and decodes the DSOC downlink signal through novel signal processing algorithms implemented on Xilinx UltraScale+ FPGAs, as well as Python-based software monitor and control routines. Exploiting the unique TDC-based architecture, the GSPA supports over four orders of magnitude of downlink data rates across multiple orders of magnitude of signal and background powers. In this paper we present an overview of the hardware, firmware and software architectures to implement this system, as well as performance analysis for links ranging from near-Earth to 2.8 AU.

Fibertek has developed a space qualifiable 50W 8Ch. WDM Amplifier prototype that is designed to meet all the environmental and optical requirements of a DSOC mission. The deliverd amplifier is optimized for efficiency and athermal performance achieving 22% e-o efficiency. The high TRL 1.5-μm high TL fiber amplifier supports up to 6W/channel, with >128-ary pulse-position-modulation (PPM) format, and with 25-nm gain-flat bandwidth. Output electro-optic characteristics, the System Reliability Analysis, Mechanical Thermal analysis and Mechanical Structural and Vibration analysis of the high TRL delivered laser prototype are presented. A power efficient TDM based FWM mitigation technique that improves PEV performance of Tx, is demonstrated.

We present recent progress in developing miniaturized optical transmitters and receiver amplifiers for space communications. Three C-band high-speed optical transmitter designs are presented: a bespoke 300 mW version as part of TNO’s “SmallCat” terminal to fly on-board NordSat and two variants that provide 300 mW and 3 W of optical power complying to standard cubesat form factors. In addition to these transmitters, an ultra-small form factor, high gain, low noise amplifier, for boosting received signals is presented.

Distributed satellite mesh networks utilizing low-cost small satellites require communications components that have low impact on the cost, size, weight and power (SWaP-c) while long range, high bandwidth communications can reduce the number of required satellites in the mesh network. Free space laser communications provide a potential for low-SWaP, long-range communications links by leveraging high aperture gains due to short wavelengths yielding narrow divergence. Additionally, wide band optical booster amplifiers frequently operate with an average power limit rather than a peak power limit enabling low-duty cycle formats to take advantage of high peak powers. Full realization of these benefits depends on the format being utilized. New-space laser communications terminals leverage much of the existing fiber optical telecommunication infrastructure to repurpose products for long range free space applications that currently only have a launch amplifier and, potentially, a preamplified receiver. While long haul fiber applications favor binary phase shift keying formats, low cost applications of optical fiber telecommunications links that require low-SWaP on the transmitter and receiver ends of the link frequently drive designs towards intensity modulated direct drive (IM-DD) links. We investigate extending the range of a free space optical link through use of three different variable data rate methods including, reducing receiver bandwidth, utilizing burst waveforms, and pulse position modulation formats (PPM). Our results indicate that although a higher SNR is required for PPM formats, orders higher than 64 can acquire links at comparable average power and data rate as differential phase shift keying formats under similar receiver conditions.

For high capacity free space optical (FSO) communication systems, expected be used to support extended coverage for the sixth generation mobile service, the digital coherent technology and wavelength division multiplexing used in optical fiber communications are promising technologies. These technologies can generate optical signals supporting Tb/s level capacity. However, to achieve the link budget required for Tb/s optical links, transmit power in the order of 100 W is required, and achieving 100 W power output with an optical fiber amplifier is challenging. In this work we propose parallel optical amplification of channel groups split out from the WDM signal, instead of amplification of the undivided WDM signal, and the transmission of the amplified signals as separate beams passed through multi-aperture optics. This configuration can reduce the required output power from the individual optical fiber amplifiers. We designed the FSO terminals for the proposed configuration with 3 transmitter apertures, such that the apertures fell within the directivity of the FSO receiver terminal. We evaluated the configuration in an outdoor experiment with a 500 m FSO link and wavelength division multiplexed real-time 100 Gb/s digital coherent QPSK signals. The experimental results show that the proposed configuration can increase the total capacity by 3 times, from 200 Gb/s to 600 Gb/s, without needing to increase the output power from the individual optical fiber amplifiers.

High photon efficiency (HPE) techniques, using pulse modulation, capacity approaching error correcting codes, and photon counting receivers, can be used to significantly reduce the size, weight, and power (SWaP) of an optical communication system by reducing aperture size and transmit laser power requirements. Low-SWaP systems also require compact, low-power receivers capable of photon-limited performance. We investigate the use of semiconductor optical amplifiers (SOAs) to achieve near-photon limited performance in low-SWaP HPE systems. SOAs are significantly smaller and require less external support compared with Erbium-doped fiber amplifiers (EDFAs). We build and test a low-noise optical pulse receiver using a semiconductor optical amplifier and avalanche photodiode (APD). Pulse-to-pulse variations, amplified spontaneous emission (ASE) levels, and background noise levels are measured and used to evaluate an expected bit error rate (BER) as a function of signal photons per pulse. Overall power draw of the system, including SOA drive current, thermo-electric cooler (TEC) drive, and APD bias and temperature control and trans-impedance amplifier, are evaluated to assess the overall impact of the SOA receiver on optical terminal power requirements. We show that SOA receivers can be used as near-photon limited receivers in HPE optical communication systems.

The results of experiments conducted on a laboratory setup of a fast adaptive optical system based on the use of FPGA as the main control element and a bimorph mirror as a wavefront corrector are presented. The adaptive system bandwidth ranged from a dozen Hertz to 2,000 Hertz. For independent control of the quality of correction the intensity distribution in the far field was recorded. It is shown that for a good correction of the wavefront the system bandwidth should be an order of magnitude higher than the upper boundary of the spectrum of wavefront distortions caused by turbulence. A comparison of the model and experimental data is also presented.

Increasing the information capacity of the Deep Space Network, a global network of radio frequency receivers used to communicate with and track interplanetary spacecraft, will increase the number and complexity of future space explorations missions it can support. Adding optical communications capability will improve the information capacity of the Deep Space Network. The imaging resolution of the telescope is one of the key factors driving both system-level performance and cost. This report describes the control systems designed to maintain telescope alignment.

Laser communications can enable more efficient and higher bandwidth communications across longer distances than conventional radio frequency (RF) systems. However, beam divergence angles for laser systems are narrower than typical RF systems, and require precise pointing, acquisition, and tracking systems to establish and maintain the link. In addition, typical lasercom links are point-to-point, and not capable of multicast or broadcast. Conventional pointing and tracking (PAT) systems use mechanical gimbals or fast-steering mirrors. Mechanical gimbals may not meet the size, weight, and power (SWaP) constraints for small spacecraft, particularly for multiple concurrent spatially diverse beams. Fast-steering mirrors while compact and efficient have limited aperture size, and many would be needed to provide multiple links over a hemisphere. The Miniature Optical Steered Antenna for Intersatellite Communications (MOSAIC) aims to provide nonmechanical pointing and tracking using liquid lenses, allowing a wide field-of-view and support for multiple concurrent links. Initial work with commercially available liquid lenses showed that liquid lenses can be used in a space environment and assessed spatial coverage. In this work, we model a transmitter using three liquid lenses. One on-axis lens provides focusing capability. Two off-axis and perpendicular lenses provide beam steering, with a fisheye lens amplifying the effect. This provides near-hemispherical pointing up to 170 degrees. We investigate beam quality and divergence using a Zemax model and conduct a link analysis dependent on the beam steering angle and rotation angle. A 25 Mbps link with 200 mW transmit power at 1550 nm (optical C band) and 16-ary pulse position modulation (16-PPM) can be maintained up to 28 km separation with 3 dB margin for an Optotune EL-16-40-TC liquid lens. Losses are primarily due to the liquid lenses limiting aperture size to 16 mm. We also consider the impact of diffusers for increasing numerical aperture through a simple ray transfer analysis and experimental results.

Mitigation of turbulence-induced disturbances is crucial for high data rate optical links in the atmosphere. Sensorless adaptive optics, based on the optimization of the coupling in a single mode fiber, is a promising solution as it circumvents the limitations of conventional wavefront sensing in strong perturbations. We propose the use of a spatial multiplexer to reduce the bandwidth of the temporal modulation required with such a technique. In this approach, after correction by a deformable mirror, the residual perturbations are analyzed thanks to the multiplexer. The concept and first results of laboratory tests are presented.

Free Space Optics (FSO) has the potential to offer fast broad-bandwidth communication, but experiences signal loss due to atmospheric attenuation. Our study assesses the performance of different lens configurations to mitigate the fog scattering loss in a low-cost visible-band FSO communication system. We built a fog-testing chamber and novel transmitter board to evaluate our FSO link at four different visible-band wavelengths and three different lens configurations. We also analyze the receiver signal strength to determine the fog-induced attenuation and compare each lens performance in the system. To design and evaluate the optimum lens system, several novel transmitterreceiver lens configurations are analyzed and compared: plano-convex to plano-convex (P-P), bi-convex to planoconvex (B-P), and bi-convex to bi-convex (B-B). We observe that the visible-band wavelength can minimizes the amount of fog-induced signal loss. The lens configuration in conjunction with the most optimal visible-band wavelength is then analyzed with various fog levels. We determine the most efficient double-lens configuration in the FSO system with fog-induced noise. On average, the biconvex-planoconvex system performed 63.85% better than the planoconvex-planoconvex system and 50.42% better than the biconvex-biconvex system. These results can be attributed to the spherical aberration from the transmitter lens and will be discussed in the paper.

Unmanned Aerial Vehicles (UAVs) are used in numerous applications ranging from defense, law enforcement, environmental monitoring, disaster recovery, aerial photography, and delivering consumer packages. Securing wireless communication between drones in-flight is critical to ensure safe operation during flight and avoid multiple types of attacks, e.g., eavesdropping, spoofing, jamming, etc. Quantum communication protocols offer enhancements over classical approaches. In this effort, we present progress towards demonstrating Quantum Key Distribution (QKD) between two drones in flight. A significant challenge includes achieving system performance using compact Size, Weight, and Power (SWaP) constraints of the drone vehicle. We introduce and evaluate critical subsystems including the QKD source, which is based on resonant-cavity Light Emitting Diodes (LED) controlled by an FPGA, and we discuss a secondary QKD source based on a fiber-coupled polarization modulator. The Pointing, Acquisition and Tracking (PAT) system is comprised of several cascading subsystems, which provide course alignment using based on Infrared (IR) beacons/cameras with gimbals, and fine alignment using Fast Steering Mirrors (FSM) with absolute encoders and feedback position sensors. We discuss both transmit and receive optics including custom designed 3D-printed optical benches. Finally, we introduce single-photon detectors, FPGA-based time-tagger, and a novel statistical post-processing synchronization algorithm. Establishing a quantum communications link between drones in-flight is an important prerequisite for future drone-based quantum applications such as entanglement distribution, distributed quantum sensing, and Quantum Positional Verification (QPV).

The fundamental innovative design of managed optical communications array (MOCA) features some distinct advantages over other lasercomm approaches: (1) MOCA relies on proven, Commercial off-the-shelf (COTS) technologies, as opposed to more exotic optical phase array or beam-forming approaches. MOCA has already been demonstrated (2) the MOCA technology array supports a low-profile conformal terminals for low-drag; (3) MOCA eliminates failure-prone gimbals (4) the modular approach allows development dollars to be focused at the shared subaperture level, from which unique platform terminals can be developed at lower cost; and (5) use of commoditized components and elimination of gimbals reduces size, weight, and cost.

Infrared satellite links are examined to measure earth surface temperatures. The low earth orbits would offer low cost and Brandon orbits would offer the convenience of stationary ground antennas. A square array of ground stations is considered (‘Quad Diversity’) to relieve laser attenuation, and higher order diversity is considered. Jalali’s ten micron silicon laser links are studied for high reliability, terrestrial temperature measurements, and low cloud attenuation for clean terrestrial power. Even lower attenuation is found for satellite-aircraft links.

Optical space data relay systems will require Terabit per second (Tbps) capacities to match or exceed RF communication capabilities. A previous study1 has shown that implementing these high-capacity optical feeder links through a traditional geostationary constellation will pose significant development challenges. Meshed satellite constellations in low earth orbit (LEO), operating at significantly shorter ranges, were shown to have the potential to support Tbps feeder links using the technical capabilities of current free-space optical communication systems. Meshed LEO constellations, however, provide unique challenges, including complex constellation maintenance, dynamic meshing and data routing, and short contact periods with ground stations. We evaluate constellation geometries and ground station sites to establish minimum system requirements to maintain space-to-ground feeder links for a meshed LEO constellation. System requirements include site diversity and redundancy to compensate for local weather. We examine historical weather data to test the conclusions of the constellation and site diversity evaluation. Simulations of data collection and transfer through meshed LEO constellations and space-to-ground feeder links are performed.

In recent years, optical communication protocols resilient to atmospheric disturbances for near-Earth have been discussed in Consultative Committee for Space Data System (CCSDS). We proposed our Forward Error Correction (FEC) format to the committee to provide a better option with a balance of its complexity and performance. The FEC consists of Reed-Solomon product code (RS-PC) which is robust to burst error and has been adopted as an optical disc format because they are commonly suffering from burst error caused by scratch, fingerprint, and other materials which disturb laser light. We considered the feature was also be able to be applied to the burst error correction of fading due to atmospheric disturbance and successfully demonstrated optical Ethernet communication by using proposed FEC between International Space Station and optical ground station which is located in Japan. However, the performance of RS-PC for free-space optical communication was not discussed quantitatively yet. Here we discuss the proposed FEC format structure in detail and its Bit Error Rate (BER) curve simulated under the condition of the fading channel. We also discuss the robustness of burst error can be adjusted in correspondence with the condition of the atmosphere channel by concatenating multiple FEC blocks. Furthermore, the BER performance can be improved without changing the FEC format itself by applying iterative correction and erasure correction. The simulation result shows the proposed FEC can realize better performance compared with a single Reed- Solomon code. In terms of error correction capability, soft-decision codes such as Low Density Parity Check (LDPC) and Serial Concatenated Convolutional Codes (SCCC) provide a better performance, but the advantage of RS-PC is that it can be implemented with smaller and less power consuming circuits than these FECs. It shows the proposed FEC format can be a promising approach especially for an in-orbit solution which supports limited power resource.

In this work, we develop a numerical model to propagate a Gaussian beam through Kolmogorov phase screen under weakmoderate turbulence regime. The numerical method uses the Second-Order Split-Step Algorithm (SSM) to perform the numerical calculations. The Kolmogorov power spectrum model is used to characterize the atmosphere turbulence conditions. The proposed model should also provide the flexibility to enhance the computing accuracy, speed, and memory usage. We found the FWHM of Gaussian profile dropped to about 25% of the maximum intensity of the initial beam, but the shape is not deformed after a propagating through a single-phase screen under weak turbulence regime. On the other hand, when the beam is sent through multiple phase screens and under moderate atmosphere fluctuation, the interference between the beam and the two-phase screen led to a minor deformation of the beam shape and more coherence. The SSM method showed an extremely efficient performance in propagating the beam using Fourier transform and inverse Fourier transform.

As satellite constellations continue to expand and capacity demands grow, free-space optical (FSO) communication offers a complementary alternative to RF systems for low-Earth-orbit satellite communication networks. FSO systems can support higher link bandwidths and provide high data security without RF spectral constraints. The performance of FSO-LEO links, however, can be significantly impacted by receive power variation caused by propagation and scattering losses along with losses due to atmospheric turbulence. Here, we investigate intensity modulated, direct detection (IM/DD) digital waveforms that can be adapted to dynamically changing link conditions to optimize bandwidth utilization. Using a laboratory scintillation playback system, the performance of BPSK, QPSK, and 8PSK waveforms will be presented and compared to theoretical modelling. The impact of adaptive equalization will be characterized and initial performance of a multi-channel IM/DD architecture will be presented.

In this talk, we examine the use of free space optical (FSO) links to and from low earth orbit (LEO) satellite systems and their incorporation within larger network architecture models. FSO link models are presented to predict dynamic link quality to and from a surface terminal to a LEO terminal. We also outline the design and development of network emulation and simulation capabilities to analyze the use of dynamic FSO-LEO link models within broader communication network architectures. We present an early network emulation demonstration of FSO-LEO link models involving data transfer between two surface observers and multiple opportunistic LEO satellite events.