This PDF file contains the front matter associated with SPIE Proceedings Volume 11272 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

The limited bandwidth and security provided by radio frequency communications between the ground and space can be overcome with optical communications. The smaller beam divergence and high carrier frequency increase the bandwidth and brings with it the potential of achieving a global communications network with absolute security using quantum states to transmit encryption keys, also known as Quantum Key Distribution (QKD). A drawback of ground-to-satellite optical communications, however, is that clouds provide effectively complete blockage of the beam. This can be mitigated by means of receiver site diversity, in which a network of geographically dispersed receivers provides far higher link availability.

We present a proposal for a network of optical ground stations in Australia and New Zealand for optical communications to provide secure satellite links for the growing space-based market. Optical ground station nodes in the Australian Capital Territory and South Australia have been funded and are currently being planned. Partial funding for other nodes in Western Australia and New Zealand has also been achieved. Funding for infrastructure is being sought to tie these stations together to produce a world leading optical communication network. This presents an opportunity for our nations to become a space-to-ground data highway and become a leading provider of secure satellite links for a large and growing market. In order to take advantage of hardware currently in orbit and planned (including quantum communication) each network node will be capable of communications with optical and current radio-frequency methods. This has the added benefit of future proofing optical communications hardware and building industry with the accessibility of an optical ground station network.

The aerospace network connectivity market is currently experiencing a renewed period of intense activity in space, driven by numerous companies planning constellations of thousands of low Earth orbit satellites. These constellations - globe-spanning networks of interconnected satellites - promise to enhance Earth observation, facilitate the Internet of Things, and bring connectivity to the estimated half of the world's population still without internet. The consequent demand for data bandwidth that will follow from these developments can only be delivered through the implementation of optical inter-satellite links, routing user information on-demand and with minimum delay. The economic feasibility of these endeavors is dependent on the size, weight, and power requirements (SWaP) of the laser terminals that will provide the backbone connectivity of these constellations. Given that several optical terminals will be required per satellite, they need to not only establish gigabit data links over thousands of kilometers but also possess minimal SWaP and mostly be design-engineered for serial production. This paper discusses the requirements for optical terminals derived from constellation modeling. We update on the qualification status of the inter-satellite link terminal program, and introduce our roadmap for large scale serial production.

With the EDRS program, the European Data Relay System, a private public partnership program between the European Space Agency ESA and Airbus Defence and Space ADS, laser communication has entered the commercial service since November 2016 [1]. Currently four Earth Observation satellites named Sentinels equipped with TESAT Laser Communication Terminals from the Copernicus program of the European Union are served by 40 data relay links per day, already accumulating to more than 31000 links in total. We report on the performance of the systems in space and detail on other activities of Tesat.

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 architecture is expandable regarding total output power and can support links from various orbits up to and including GEO-GEO as predicted from the amplifier testing and link budget analysis. The amplifier is based off of a TRL9 system originally used by GA-EMS for airborne applications that has been redesigned for space applications and is currently TRL6 based on a TVAC test conducted in 2018. The system utilizes a software defined modulation scheme that can change between non-return to zero (NRZ) and return to zero (RZ) to support various cross link distances by transitioning between RZ and NRZ. While the current LCT uses OOK, the architecture can support multiple modulation schemes including DPSK, which was tested as part of the 2018 T-VAC test. The LCT uses a novel acquisition scheme which is introduced here that enables rapid acquisition for systems even when the bus level pointing accuracy is in excess of 350urad. This results in a bus agnostic LCT architecture that can be used on multiple mission without necessitating extensive redesign and qualification. GA-EMS plans to launch two of these terminals in cubesats to host an on orbit demonstration of crosslinks between the two terminals and downlinks to a ground station.

Free-space optical communications (FSO) systems have gained increasing interest for both defense and commercial applications due to their ability to provide secure, long-distance, high-capacity communications on the move. In terrestrial environments, because clouds and strong weather effects can limit FSO systems performance, integrating them with directional radio frequency (RF) links can yield a system that leverages the best of both modalities - the high capacity of FSO when available with the reliability of the RF link to ensure the highest priority data can be sent even during degraded weather conditions. This paper will present the development of a highly integrated FSO/RF link architecture implementing three key functionalities: (1) operation at data transfer rates up to 10 Gbps, (2) seamless failovers between the FSO and RF modalities, and (2) the necessary quality of service (QoS) mechanisms to handle the rate disparity between the two links while providing priority to critical data. This architecture utilizes a network transport system that provides layer 2 data transport and QoS arbitration across the FSO and RF modalities. Results from testing in lab as well as at outdoor ranges of up to 30 km will be presented.

The development of large Low Earth Orbit (LEO) satellite constellations is driving the need for autonomous satellite orbit determination. The present state-of-the art is the use of global positioning system (GPS) receivers on individual satellites that provide the position and time information necessary to determine the satellite orbit. There is a growing realization of potential threats to GPS and Global Navigation Satellite System (GNSS) constellations in general. Loss of GNSS service would have significant implications to these large satellite constellations. At the time of writing, the Galileo GNSS system has been down for a week, and there is very little public information as to why and how long before services will be restored. Free Space Optical Communications (FSOC) between satellites and satellite-to-ground links has the potential to provide an independent source for position and time data necessary for orbit determination. FSOC systems can provide highly accurate ranging and time synchronization between both ends of an FSOC link. In the case of pulse-position-modulation (PPM), commonly used in FSOC systems, precise range and time synchronization is required to make the link work properly. The combination of inter-satellite ranging and ranging to fixed ground stations with access to precise timing can provide significant GPS and GNSS independent autonomous orbit determination of spacecraft and spacecraft constellations.

For some free-space laser communications (lasercom) links impacted by atmospheric-induced scintillation, the principle of reciprocity provides knowledge of a remote terminal's received intensity fluctuations based on measurements of the local terminal's received intensity fluctuations. We evaluate a reciprocity-enhanced technique using optical switching between multiple spatially-diverse transmit apertures to mitigate against atmospheric-induced scintillation. Experimental bit-error rate measurements are presented to quantify the expected performance of this approach in comparison to standard static diversity approaches. We investigate system constraints of this reciprocity-enhanced transmitter diversity approach, and identify candidate optical switches and digital logic for implementation.

For low-Earth-orbit (LEO) satellite communication networks, free space optical (FSO) communication offers high data capacity and security without the spectrum limitations of more conventional RF approaches. However, receive signal power in FSO-LEO links can be highly variable based on multiple dynamic loss mechanisms occurring at different time scales. As a LEO pass moves from higher to lower elevation angles, propagation and scattering losses can vary by more than 10dB over a timescale of minutes. Separately, signal fading caused by atmospheric turbulence can also contribute greater than 10dB variation except at a much faster timescale on the order of milliseconds. Rather than implementing a modulation scheme based on the worst case link margin for a given FSO-LEO link, here we consider intensity modulated, direct detection (IM/DD) digital waveforms that can be dynamically adapted to the changing link conditions to provide increased bandwidth efficiency. In this work, we describe the development of IM/DD waveform modems and a waveform characterization test-bed which incorporates a scintillation playback system. BPSK, QPSK, 8PSK, and 16- QAM waveform performance will be presented under varying scintillation profiles.

HydRON ambition is to seamlessly integrate the space optical transport network into the terrestrial high capacity network infrastructure: the “Fibre in the Sky”. In HydRON, it is envisaged “All-Optical payloads” being interconnected by means of optical inter-satellite links in the Tbps regime (Terabit per second) furnishing the “bridges” for a truly “Fibre in the Sky” network. Technically speaking HydRON aims at Tbps “All-Optical Network” solutions, dividing the satellite payload into (i) a network part and (ii) an application / service part, equivalent to the backbone part and the access part of optical fibre networks on ground. The application / service part (i.e., the Customer’s payload) has access to the network part (i.e., the HydRON elements), in a similar way as computers are plugged into the terrestrial network.

HydRON encompasses optical feeder links connecting to a space network of in-orbit technology demonstrator payloads, which are interconnected by means of Tbps laser inter-satellite links. WDM (Wavelength Division Multiplexing) laser communication terminals (on ground and in space), optical switching / routing capabilities and high-speed interface electronics will be implemented on-board the network nodes in space to enable a high throughput network connection to the application / service part (i.e., the Customer’s payload). The space network concept will reduce the dependency on atmospheric conditions of single ground stations as all HydRON nodes can get their particular data via the network they are interfacing with. A combination of new optical technologies, novel photonics equipment and efficient network concepts will be proven in orbit. The system architecture must be adaptable to the changing network conditions.

The current status of the above mentioned investigations will be summarised in the present paper.

Recent technology trends of cost reduction of launch and satellite miniaturization are leading cutting-edge applications such as earth observation and communication with satellite constellation. Miniaturization of the communication terminal, light weight and low power consumption are required to the communication terminal on small satellites. Free space optical communication is expected approach to realize high data rate communication system on small satellites. Sony and Japan Aerospace Exploration Agency (JAXA) have experimentally verified the fundamental functions of the small optical communication terminal with optical disk technology for small satellites since 2016. Following this basic study, Sony Computer Science Laboratories, Inc. (Sony CSL) has jointly studied in orbit experiment of the small optical communication terminal with JAXA. To verify its functions in-orbit promptly, we have tried to utilize IVA-replaceable Small Exposed Experiment Platform(i-SEEP) attached to Japanese Experiment Module (JEM) on International Space Station (ISS). The developed small optical communication terminal (SOLISS) is connected to i-SEEP and SOLISS is designed to verify bi-directional communication with free-space optical technology capable of 100 Mbps Ethernet frame data transfer between SOLISS and the optical ground station. For the optical ground station connecting with SOLISS, Sony CSL also conducts a joint research project with National Institute of Information and Communications Technology (NICT). To establish the optical communication link, telemetry and commanding through ISS, controlling of the optical ground station and SOLISS are designed. This article discusses the system architecture for in-orbit experimentation of bi-directional optical communication between SOLISS and the optical ground station.

In this paper, we model the use of free space optical (FSO) links that employ adaptive waveforms such as n-PSK in the context of links to and from low earth orbit (LEO) satellite systems. FSO link models are described that can be used to predict dynamic link quality to and from a surface terminal to a LEO terminal. Because the link margin can vary by 10-20 dB in a LEO pass, we consider the use of adaptive waveforms. Higher order waveforms are used when the link margin is high, stepping back down to on-off keying when the margin is low. We also describe the design and development of network emulation and simulation capabilities to help analyze the use of dynamic FSO-LEO link models within existing network experimentation frameworks. We present a network emulation demonstration of the designed FSO-LEO link models involving data transfer between two surface observers and multiple opportunistic LEO satellite events. We discuss both the emerging opportunities and ongoing challenges of effectively using such FSO-LEO capabilities within a larger data network context.

Free space communications (FSO) is interesting for distant applications where high bandwidth is needed while using a fiber is not possible. However these links have to face several issues, and the most important one is the beam scintillation due to the propagation through a turbulent media, the atmosphere. Several mitigation strategies have been developed, but the best way to suppress scintillation is to use adaptive optics, widely used now in astronomy. The main difficulty for FSO is to probe the wavefront fast enough to have a good turbulence correction. This was not possible due to the lack of wavefront sensors working in the SWIR. C-RED 3 is a 640x512 SWIR camera running at 600FPS full frame and has the legacy of all the developments of astronomical infrared fast wavefront sensors on top of specific features for FSO (Low SWaP, Low Cost). We will present the performances of this new camera and demonstrate how it fulfills the needs of FSO adaptive optics.

We have comprehensively tested uncooled, free space coupled, InGaAs Quad Photoreceivers having 0.5 mm, 1 mm, and 2 mm diameter integrated with a low noise transimpedance amplifier (TIA) using 30 MeV Protons, 100 MeV Protons, 662 keV Gamma Rays, 1 GeV/n Helium, and 1 GeV/n Iron at room temperature of ~20°C. These devices find multiple applications in space for differential wavefront sensing as part of a Gravitational Wave Observatory, as well as instrumentation and control for next generation space telescopes. The bandwidth of all receivers was 20 MHz which was TIA limited.

All 0.5 mm and 1 mm devices were found to be fully functional at normal operating conditions and at room temperature for Protons, Gamma Rays, 1 GeV/n Helium, and 1 GeV/n Iron. Only one quadrant of a 2 mm InGaAs Quad had hard failure due to 1 GeV/n Helium Ions; otherwise it too survived all other radiation tests. Detailed test results follow in the manuscript including recommendations for future space flights. These radiation test results, combined with the earlier successful mechanical shock and vibration testing mean these devices have passed preliminary testing for space qualification.

Maintaining a stable and high quality laser wavefront is pivotal for efficient laser communications in deep space networks. In this presentation, we describe the design and expected optical and structural performance of the afocal beam expanding telescope for the NASA DSOC mission. This 22 cm aperture, 11x magnification telescope must survive the stresses of launch and maintain alignment through solar illumination, laser irradiance, thermal transients, and temperature extremes during the DSOC mission life from Earth to Mars. Structural-Thermal-OPtical (STOP) analysis predict very stable downlink wavefront error (< 122 nm RMS) and beam divergence (< 14.5 microradians). Furthermore, we present additional telescope link loss contributions that will be minimized through particulate contamination control, high spectral throughput, and polarization purity. Successful performance of this telescope will support NASA’s ongoing efforts to extended high data rate communications into deep space.

Satellite laser communication hardware design that supports space-based optical communications, and successful hardware demonstrations, are presented for Low Earth Orbit (LEO) terminals. For inter-satellite links (ISL), the design of an optical module has been optimized to support satellite-to-satellite relays. Providing optical line-of-sight (LOS) stabilization, the Optical Bench Assembly (OBA) is the modular component that includes the LOS jitter rejection control loop system, which stabilizes the transmit (Tx) and receive (Rx) data channels. The jitter rejection system design of the OBA is described. The demonstrated performance is reported for the nested control loop rejecting the host platform’s on-orbit vibration profile.

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 availability of an optical communication link between a deep space transmitter and an Earth-based receiver is limited by the location of the sun relative to the line of sight. The sun could block the line of sight entirely, account for sufficient background radiation to degrade the system performance, or, the receiver telescope may form an image of the sun resulting in a safety hazard. The large diameter ground telescopes capable of supporting high rate optical links over solar-system distances exacerbate these challenges. We present experimental results bounding the safety threshold for solar-induced damage upon a Deep Space Network antenna and predict system level performance.

Free-space optical communication (FSO) enables high-bandwidth data links that are difficult to detect, intercept, and jam. In this paper we provide an overview of a small form factor FSO prototype intended for UAVs called OCELOT (Optical Communication Efficient Low-profile Terminal). NRL designed, developed, and tested an OCELOT prototype, and demonstrated a 1 Gbps duplex link 16 km across the Chesapeake Bay. We will discuss the design decisions and tradeoffs, highlighting the low-SWaP FSO technologies used in the prototype.

We present a mission proposal to provide a space based quantum key distribution service. Using entangled photon pairs distributed to two ground stations with a simultaneous double downlink establishes a secret key directly at the user locations. The satellite and the payload do not generate or process any secret key information and the security design, certification and operation of the key management can be restricted to the user locations on ground. Positioning the satellite in GEO enables covering Europe and intercontinental connections, allows flexible service planning to cope with weather constraints and removes the need for coarse tracking telescopes in space and on ground while allowing long integration times. The large link distance and the combined losses of two down links require large optical terminals in space and on ground to obtain a usable secure key generation rate. We will present results from an ESA internal Phase 0 study concentrating on the payload design and performance.

Quantum entanglement based communication schemes have many advantages. High-dimensional encoding has been shown to improve robustness and channel capacities in secure quantum communications. The transmission of genuine high-dimensional entanglement under real-world atmospheric link conditions, however, is an ongoing experimental challenge. We report on a proof-of-concept experiment in which we, for the first time, use energy-time and polarization hyperentanglement to transmit 4-dimensional entanglement via a 1.2-km-long intra-city free-space link. We discuss how this approach can be adapted for high-dimensional free-space QKD and report on the progress of ongoing experiments.

A combination of the coherent detection and digital signal processing (DSP) deployed in spectrally-efficient optical fiber communications is being applied to free space optical (FSO) communications. The DSP enables adaptive frequency offset compensation between the transmitter and receiver laser diodes (LDs), and also the adaptive equalization of the non-ideal frequency response of the optical and electrical devices, by continuously updating values for the phase rotation angles and the coefficients of equalization. Due to atmospheric turbulence, the SNR can suddenly be reduced, so that the adaptation will diverge from its optimum. Then, even if the SNR recovers, it will take much longer than usual for the adaptation to re-converge because it will be starting from a diverged value. In this paper, we propose to control the calculations for updating the adaptation with a state machine based on a SNR estimated from the extracted clock amplitude. The updated values are periodically written into FIFO registers when the SNR is higher than the receiver threshold, and when the SNR degrades, we fix the values using the output of the FIFO registers. This prevents divergence of the adaptation and enables reuse of the values before divergence, taking into account the fact that estimating the SNR takes a finite time. We designed the proposed DSPs, and confirmed that these designs can be implemented in field programmable gate arrays (FPGA). In an offline experiment we evaluated this model of the proposed DSP design using a 2.5 Gbaud quadrature phase shift keying (QPSK) signal. The experimental results showed that the proportion of error free time is increased from 91% to 98% by the proposed technique.

Optical communication links, operating in low photon flux conditions, rely on an array of single photon counting detectors to receive the signal. Due to the reset time of these detectors, many separate detectors must be used to receive a continuous signal. Photon counting, channel combining, channel alignment, and digitization of the detected signal can be complex and expensive due to the parallel hardware required for each channel. This issue is compounded as the system scales to greater numbers of detectors due to the amount of hardware required and alignment requirements between each channel. The purpose of this research is to examine a photon counting and channel combining method which allows for photon detection channels to be summed into a single signal before digitization, eliminating the need for parallelized hardware. This reduction in parallel hardware has the potential to reduce the cost and complexity of the system. In this paper, a single layer fully connected neural network architecture is explored as a possible solution for the photon counting of summed photon detection channels. The signal to noise ratio of the combined signal was lower than that of the individual channels and was inversely proportional to the root of the number of channels being summed. The neural network signal processing implementation produced signal gain when the symbol phase remains constant. This is most likely due to the network exploiting the modulation structure of the signal and possibly offsets the losses incurred during analog summation.

Global coverage of internet access is essential for digitalization in society, becoming a disruptive technology in industry, education or political participation for example. Satellite communications is a complementary approach to the terrestrial fiber network, which can provide world-wide coverage with few satellites in geostationary orbit or with low-earth-orbit constellations. Optical wavelengths offer multiple THz of available spectrum that can be used to connect satellites to the ground network with high-throughput links, solving the radiofrequency bandwidth bottleneck, without regulations. Cloud covereage and atmospheric turbulence are the main challenge in guaranteeing the same availability as in terrestrial fiber-based systems. While the former can be addressed by site diversity, for the latter, other mitigation strategies are required. Adaptive optics is a common approach to correct for atmospheric phase distortions and ensure stable fiber coupling. However, this approach requires a relatively complex active setup and therefore a collaboration between DLR Institute of Communications and Navigation and Cailabs has been formed to investigate alternative passive solutions for low-complexity ground stations. Coupling into multimode fibers does not require adaptive optics due to the large fiber core, however the coupled signal is distributed into multiple fiber-modes and is therefore incompatible with standard telecommunications components. Cailabs Multi-Plane Light Conversion (MPLC) technology overcomes this issue, selectively demultiplexing the fibermodes into single-mode fibers. Here, DLR’s adaptive optics system and the MPLC technology in a turbulence-relevant environment for GEO communications are compared, investigating the advantages of the MPLC approach for compensating strong turbulence. This paper presents an overview of the measurement setup and analyzes the single-mode fibers outputs of the spatial demultiplexer and the measured phase-distortions from a wavefront sensor.

The National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) is developing a low cost, scalable, photon-counting receiver prototype for space-to-ground optical communications links. The receiver is being tested in a test bed that emulates photon-starved space-to-ground optical communication links. The receiver uses an array of single-pixel fiber-coupled superconducting nanowire single-photon detectors. The receiver is designed to receive the high photon efficiency serially concatenated pulse position modulation (SCPPM) waveform specified in the Consultative Committee for Space Data Systems (CCSDS) Optical Communications Coding and Synchronization Blue Book Standard. The optical receiver consists of an array of single-pixel superconducting nanowire detectors, analog phase shifters for channel alignment, digitizers for each detector channel, and digital processing of the received signal. An overview of the test bed and arrayed receiver system is given. Simulation and system characterization results are presented. The data rate increase of using a four-channel arrayed detector system over using one single pixel nanowire detector is characterized. Results indicate that a single-pixel detector is capable of receiving data at a rate of 40 Mbps and a four-channel arrayed detector system is capable of receiving data at a rate of 130 Mbps.

We present WDM-scalable low-loss and narrow-bandwidth optical filtering techniques for high-sensitivity photon-counting applications. A cascaded approach using two filtering elements is leveraged that can efficiently receive multi-spatial-mode WDM optical signals through the turbulent free-space optical channel: a periodic narrowband background-rejection filter in which each passband can be well matched to the incoming transmitter waveform; and a WDM separation filter that distributes received WDM channels to detection elements within a receiver array. This combination enables large-constellation modulation options with improved sensitivity and provides a scalable high-performance optical-filtering solution that is particularly attractive for bandwidth-constrained photon-counting applications.

Within the frame of European Data Relay System (EDRS) upgrades, ESA is developing technologies for reliable optical communication links through atmospheric turbulence. This includes adaptive optics for wave-front correction on the downlink (and pre-distortion on the uplink), but a promising alternative to adaptive optics is a self-homodyne multimode differential phase shift interferometer, which doesn’t require phase-integrity of the incoming signal. Moreover, a custom-made avalanche photo detector (APD) with 80μm active area was developed for efficient conversion into an electrical signal. Field test were carried out from ESA’s optical ground station (OGS) with a laser communication terminal (LCT) onboard the Alphasat satellite in geosynchronous orbit.

Photonic lanterns are being evaluated as a component of a scalable photon counting real-time optical ground receiver for space-to-ground photon-starved communication applications. The function of the lantern as a component of a receiver is to efficiently couple and deliver light from the atmospherically distorted focal spot formed behind a telescope to multiple small-core fiber-coupled single-element super-conducting nanowire detectors. This architecture solution is being compared to a multimode fiber coupled to a multi-element detector array. This paper presents a set of measurements that begins this comparison. This first set of measurements are a comparison of the throughput coupling loss at emulated atmospheric conditions for the case of a 60 cm diameter telescope receiving light from a low earth orbit satellite. The atmospheric conditions are numerically simulated at a range of turbulence levels using a beam propagation method and are physically emulated with a spatial light modulator. The results show that for the same number of output legs as the single-mode fiber lantern, the few-mode fiber lantern increases the power throughput up to 3.92 dB at the worst emulated atmospheric conditions tested of D/r₀=8.6. Furthermore, the coupling loss of the few-mode fiber lantern approaches the capability of a 30 micron graded index multimode fiber chosen for coupling to a 16 element detector array.

51W average power, 7 Channel WDM Fiber Laser Transmitter with 25nm flat gain has been demonstrated for optical space communication applications. Power Amplifier supports >10kW/channel SBS limited peak power and achieves o-o efficiency 44%. Pulse energy variation (PEV) due to gain dynamics and four wave mixing of the PPM tx output is characterized. Significant improvement in PEV with wavelength dependent pre-pulse shaping is demonstrated. A high reliability 50W 8 WDM channel amplifier design is described. The amplifier will be housed in a high TRL small SWAP space laser package with dimensions 10.6x13.8x 5.3” and weighs 28.7 lbs

Combined radiation and temperature effects on the performances of three different Erbium-doped fiber amplifiers (EDFAs) have been evaluated for free-space optical communications. Each EDFA has been tested during and after an exposure to 40 keV X-rays up to a cumulated dose of 3 kGy (SiO₂) (300 krad) and at a dose rate of ~0.27 Gy (SiO₂)/s. Tests have been done at different temperatures of irradiation ranging from -40°C to 120°C showing that this parameter does not significantly affect the EDFA radiation response, comforting the pertinence of room temperature tests for system vulnerability evaluation.

This paper reports on the design and manufacture of C-band multi-Watt optical fibre amplifiers for satellite laser communications. Three module types have been developed and are presented, outputting optical power of 1W, 3W and 5W respectively. A modular design was adopted for the amplifier so that it can be scaled to different power levels and achieve cost-effectiveness and mass manufacturing, thus enabling application in satellite constellations. Radiation testing demonstrates the robust performance against ionizing radiation levels found in LEO and GEO orbits. All degradations are within system requirements and are recoverable in operation by changing the operational parameters, whilst still complying with end of life power consumption and component de-rating specifications. The amplifiers are able to deliver >1W, >3W and >5W output power at 1550 nm for 100 krad TID based on the measurements performed. An environmental test campaign has demonstrated the robustness of the 5W module against sine and random vibration. Additionally, thermal cycling in vacuum has been performed on an optical breadboard, demonstrating a stable optical output, verifying the robustness of the optical design. G and H unit design and AIT space processes are capable of delivering robust and qualified fibre optic units for deployment in satellite laser communication missions.

Free-space optical links are being considered for high throughput feeder links in satellite communication. Optical power up to 500 W is required at the optical ground terminal, which represents a challenge for current technologies. One way to scale power whilst preserving beam quality is coherent beam combining. Here, we present a novel technique based on Multi-Plane Light Conversion, consisting in a spatial multiplexer whose output modes form a Gaussian beam when superimposed constructively. The beam combiner is fully reflective, enabling kW optical power. We demonstrate a mode purity and a total efficiency higher than the conventional tiled apertures technique.

Optical coherent technology has been attractive for realizing optical satellite communication, optical beam-former and photonic payload in the future. The radiation resistant test of onboard components was also evaluated as the change of the optical output power, optical spectra and optical frequency noise before and after proton irradiation. As a result, there was no performance degradation due to an aluminum shield with thickness of 4 mm against the proton irradiation corresponding to 15 years of geostationary satellite orbit.

We have studied the in-situ radiation-induced degradation and recovery of the output optical power of actively-pumped erbium-doped fiber amplifiers (EDFAs) pumped at both 980 and 1480 nm during and after radiation exposure. The EDFs have high Er concentrations corresponding to 20 – 60 dB/m peak Er³⁺ absorptionat 1.532 μm suitable for high power cladding-pumped applications. The fibers were made with a wide range of glass compositions, including the addition of specific dopants to reduce clustering and improve efficiency. Radiation tolerance of nanoparticle doped fibers is superior to those fabricated by solution doping. The effects of photo-annealing have been explicitly studied, and an n^th-order kinetic model has been used to predict end-of-life degradation. More robust statistical kinetics modeling of accelerated lifetime data has been used to predict on-orbit performance in more realistic scenarios, such as Geosynchronous Earth Orbit.

Optical satellite communication is growing fast and among various applications it requires higher throughput optical feeder links. Optical feeder links for satellite communication necessitate very high data throughput, up to 1 Terabit/s and beyond. Amongst several multiplexing strategies, dense wavelength division holds a key position to enable overall throughput rates above 1 Terabit/s. As a consequence, hardware architectures capable of handling high throughput links must be devised. Complementary to the high throughput requirement, the devices should also cope with the high optical power levels needed in optical ground stations. Combination of spatial aperture multiplexing and free space bulk optics configurations of multiplexers with transmission diffraction gratings are presented as possible concepts. Besides wavelength multiplexing, it is essential to include the beam propagation effects in the performance analysis, since this may affect the overall feeder link properties. A modelling framework is presented that covers the multiplexing behavior as well as the beam propagation of the transmission gratings based concept. The modelling framework based on first principles of optical diffraction is general, and independent of the grating choice. The results suggest that the design of a free space bulk multiplexer for optical feeder link must be approached already at system level. Decisions about telescope sizing, channels distribution and modulation formats may affect the performance of the multiplexer, resulting in severe effects on the link performance. The work discusses the effect of each design parameter and proposes design guidelines for high power satellite communication beam multiplexing.

A fine pointing capability has been developed for laser beam pointing to augment body pointing by CubeSats. An application is made to CubeSats in Low Lunar Orbit (LLO), at 100 km. Body pointing was used by 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 to the OCSD program. With fine pointing, the spot size on the Earth could be reduced by a factor of eight with a reduction in laser output power by a factor of sixty-four, thereby mitigating the thermal load challenge on the CubeSats. The same reductions in spot size and laser output power can be achieved for CubeSats in LLO. The new method uses laser arrays for fine laser beam pointing and does not use moving parts. It combines a lens system and a VCSEL/Photodetector Array. For these electro-optical systems, reaction times to pointing changes and vibrations are on a nanosecond time scale, much faster than those for mechanical systems. Results from computer simulations will be presented.

Modeling the effects of atmospheric turbulence on optical beam propagation is a key element in the design and analysis of free-space optical communication systems. Numerical wave optics simulations provide a particularly useful technique for understanding the degradation of the optical field in the receiver plane when the analytical theory is insufficient for characterizing the atmospheric channel. Motivated by such an application, we use a splitstep method modeling the turbulence along the propagation path as a series of thin random phase screens with modified von Karman refractive index statistics using the Hufnagel-Valley turbulence profile to determine the effective structure constant for each screen. In this work, we employ a space-to-ground case study to examine the irradiance and phase statistics for both uniformly and non-uniformly spaced screens along the propagation path and compare to analytical results. We find that better agreement with the analytical theory is obtained using a non-uniform spacing with the effective structure constant for each screen chosen to minimize its contribution to the scintillation in the receiver plane. We evaluate this method as a flexible alternative to other standard layered models used in astronomical imaging applications.

With conventional optical concentrators high gains can only be achieved by restricting their field-of-view. In contrast, it has recently been shown that fluorescent concentrators can combine a relatively high gain with a wide field-of-view. However, a method to determine the benefits of using a concentrator is not available. Experiments have therefore been performed with a white LED and two receivers; an APD coupled to a fluorescent concentrator and an APD with a blue filter. When the concentrator is used its gain increases the receiver output voltage signal (ROVS) and so with each of these receivers, the incident light intensity was varied and both the peak-to-peak voltage at the receiver output and the data rate that can be achieved with a bit error rate of 10-3 were determined. These peak-to-peak voltages were used to determine the ROVS that corresponds to each data rate. An empirical relationship between the data rate and the ROVS is described. Results are presented which show that this relationship accurately predicts both sets of experimental data. In the future, this method can be used to predict the increase in data rate that will be obtained by adding any optical concentrator to a VLC receiver.

In this paper, lens design-based optimization of the optical system for a blue laser diode downconverted with remote phosphor based indoor visible light communication link is studied using optical raytracing and experimentally characterized for illumination/ communication performance. Data modulated 450nm blue laser-diode is used to excite a remote-phosphor to down-convert the blue spectrum to white which is transmitted over a direct line-of-sight free-space link and detected using an amplified p-i-n detector. The combination of transmit and receive lenses are optimized in Zemax ray-tracing software with the objective of minimizing the path loss or maximizing the light collection efficiency within the detector area when placed at different transmitter-to-receiver separation distance. It is found that contrary to the typical 1/d² dependence typically used in VLC system models, the path loss can be minimized at the required link distance by choosing the lens to phosphor and p-i-n detector distance at the transmitter and receiver side respectively. At the optimized location, the VLC link is experimentally characterized by transmitting digital data at a maximum rate of 700 Mbps and bit error rates (BER) obtained is much below 10^-3 . BER versus distance is also found to follow the inverse relation of the path-loss versus distance indicating that the optimized lens positions help in achieving improved data through-put due to minimization of the path losses. Optical spectra and color content measurements indicate that the optimized lens positioning results in enhanced blue content at the receive side due to efficient collection of the data modulated blue components at the expense of the green and red components of the down-converted white light. Further improvements to this link can be achieved by simultaneously optimizing performance at multiple wavelengths spanning blue, green and red wavelengths or using lower color temperature phosphors to improve illumination performance, albeit at the expense of some deterioration to the communication performance.

This report presents our model for atmospheric turbulence fade for an Earth/Space system, and our physical emulation test bed components. For modeling the atmosphere, we have used the Hufnagel-Valley model, in combination with Cn² measurement parameters and MATLAB software. From these models, power fluctuation time series were generated and subsequently converted to voltages that were uploaded into an acousto-optic modulator and signal generator. The acousto-optic modulator is a compact, fiber-based device that has a maximum 55 MHz response and 45 dB of range, making it a viable component for future integration into a laboratory atmospheric emulation test bed. Results from our fade model implementation with the acousto-optic modulation system will be offered and discussed.

For high photon-efficiency deep space or low power optical communications links, such as the Orion Artemis-2 Optical Communications System (O2O) project, the received optical signal is attenuated to the extent that single- photon detectors are required. For direct-detection receivers operating at 1.55 μm wavelength, single-photon detectors including Geiger-mode InGaAs avalanche photon diodes (APDs), and in particular superconducting nanowire single-photon detectors (SNSPDs) offer the highest sensitivity and fastest detection speeds. However, these photon detectors exhibit a recovery time between registered input pulses, effectively reducing the detection efficiency over the recovery interval, resulting in missed photon detections, reduced count rate, and ultimately limiting the achievable data rate. A method to overcome this limitation is to divide the received optical signal into multiple detectors in parallel. Here we analyze this approach for a receiver designed to receive a high photon efficiency serially concatenated pulse position modulation (SCPPM) input waveform. From measured count rate and efficiency data using commercial SNSPDs, we apply a model from which we determine the effective detection efficiency, or blocking loss, for different input signal rates. We analyze the scalability of adding detectors in parallel for different modulation orders and background levels to achieve desired data rates. Finally we show tradeoffs between the number of detectors and the required received optical power, useful for real link design considerations.

The orbital angular momentum (OAM) of photons is a promising degree of freedom for high-dimensional quantum key distribution (QKD). Due to the greater flexibility in applications and the lower loss, QAM QKD over the free-space channel is still significant. However, effectively mitigating the adverse effects of atmospheric turbulence is a persistent challenge. In contrast to previous works focusing on correcting static simulated turbulence, we investigate the performance of OAM QKD in real atmospheric turbulence with real-time adaptive optics (AO) correction. We show that, it is possible to mitigate the errors induced by weak turbulence and establish a secure channel under some conditions. The cross-talk induced by turbulence and the performance of AO systems are investigated in a lab-scale link with controllable turbulence. The relations between the crosstalk and AO specifications is also studied. Our experimental results suggest that an advanced AO system with fine beam tracking, reliable beam stabilization, precise wavefront sensing and accurate wavefront correction is necessary to adequately correct turbulence-induced error.

Optical Wireless Power Transmission (OWPT) is one of Wireless Power Transmission (WPT) technique which are developed to deliver electric power to long distance without using copper cable. In OWPT, laser or LED is used as the transmitter to convert electric power to light and solar cell is used as the receiver convert back light to electric power. Among the techniques of WPT, OWPT has advantage in term of its wide possible applications such as to send power through water and through human skin to charge implantable medical devices and its potential to deliver high intensity power to long distance by taking advantage of small beam size and small divergence angle of laser beam. However, additional device is needed to steer the laser beam. In this research, Galvano mirror is used to steer the beam to power up moving object. Camera with color segmentation method which is one of the techniques of OpenCV is used to recognize the target. Color marker is attached at the target. This color marker is recognized by the camera. In OWPT to 2-dimensional moving object, 2 color markers are attached at the moving target to solve position mismatch problem between camera and mirror. By taking advantage of the fast reaction of the Galvano mirror, OWPT to multiple moving objects using only one set of mirrors has been demonstrated. Periodic color segmentation method in which the recognized color is switched periodically is used in this demonstration.

Laser based applications including optical communications, LIDAR and Raman spectroscopy benefit from ultra-narrow (< 1.0 nm) bandpass and high edge slope dichroic optical filters by rejecting off-band ambient and scattered light. However, applications for these filters are limited by shifts in wavelength due to temperature and angle of incidence, system f-number, doppler shift and pointing error of the gimbal as well as the stability of the source. Passive design techniques such as athermalization, use of high refractive index materials and widening the passband are compared with active tuning options. Adding thermal or tilt tuning can expand the operational range of the filter and mitigate the compromise to signal to noise which follows from widening the passband.

In tasks related to free-space communications, a significant role has a turbulent atmosphere which influences lead to a decrease in the efficiency of systems. Since the characteristic turbulence spectrum rarely exceeds 100 Hz for typical paths, it is proposed to use a discrete adaptive optical system with a frequency of 1500 frames per second to reduce the influence of the atmosphere. The structure of the system based on the use of FPGA as a computing device as well as the main results associated with the correction of both static and dynamic components of aberrations are presented.

In free-space optical communication (FSO) photodetectors are used for both data reception and positioning. For most FSO systems, these functions are performed by two separate photodetectors. The development of a new avalanche photodiode (APD) array has allowed positioning and data reception to be done on a single device. This device takes the form of concentric cells with four cells forming a circle and a fifth cell in the center. The change from a standard quadrant cell format to a concentric cell format affects the tracking algorithm an FSO system needs to use. Analytical characterization of the effects of a combinatorial sensor of this type on an FSO tracking algorithm has been done but it has not been verified experimentally. Here we test different tracking algorithms in an existing FSO system to determine the optimal way to use a combinatorial detector for position sensing.

In free space optical communications (FSO) systems, fast steering mirrors (FSMs) are commonly used for closed loop tracking. Newer, lighter-weight and lower-power consumption, MEMS devices are also being considered for beam steering applications. Since FSO systems have to be capable of operating under a variety of conditions, robust components are needed to withstand the various temperatures and other environmental factors. This paper reports on thermal measurements over a temperature range of -40°C to 60°C that were made to show operational capability and to highlight any systematic issues associated with the operation of various commercially available, high-speed beam steering mirrors.

Free space optical (FSO) communication systems have a myriad of applications, from secure high-speed links to use in radio frequency (RF) challenged or restricted environments. Because FSO communication systems have improved performance when the transceivers are well aligned, and since many practical applications are nonstationary, low-latency acquisition, tracking and pointing (ATP) is crucial. In this paper, we will present an ATP system based on computer vision, suitable for various scenarios including robotics, vehicular communications, and indoor optical wireless. For this system, a camera is integrated with a gimbal-less two-axis micro-mirror array driven by a micro-electro-mechanical system (MEMS) on the transmitter (base station) side. The receiver (mobile device) is marked by a landmark, so the direction and relative position from the transmitter to the receiver can be estimated by applying image recognition and computer vision techniques (note that the latency of the MEMS beam steering is low compared to the execution time of these image recognition algorithms). Once the receiver is acquired, optical flow algorithms (which are much less complicated than image recognition) are used to track the recognized landmark in real-time. When the camera and micro-mirror array are well calibrated, the transmitter is able to track the landmarked receiver in a room-sized space. We also analyze the trade-off among field-of-view (FOV), latency and reliability in this paper. Finally, we present FSO communication system link performance (e.g., latency, signal-to-noise ratio, bit error rate, reliability) between moving platforms incorporating the new ATP capability.

Small inexpensive satellite platforms, such as cubesats, offer opportunities for pathfinder experiments, space qualification of components and systems, and enhancements of larger assets. The Aerospace Corporation has been developing cubesats with lasercom transmitters for downlinking payload data to the ground from orbit. Recently we demonstrated a 200 Mbps link with a 1.5 U cubesat, AeroCube-7B, under NASA’s Optical Communications and Sensors Demonstration program [1]. The nearly error free link was accomplished using on-board star trackers for attitude determination and control and without any forward error correction. The capability developed under this effort has been implemented in follow-on missions for two 3U AeroCube- 11 vehicles where camera recorded data has been downlinked to the ground. While very modest data volumes (~1.5 GB) have been transmitted (to date) in a single pass under non-ideal conditions, the utility of this architecture for certain types of missions has been demonstrated. Additional experiments involving the illumination of the ISARA cubesat with the laser from AeroCube-7B demonstrated the ability to locate and illuminate a remote satellite in orbit, a necessary step towards realizing intersatellite links with this class of vehicles.

LyteLoop is developing data storage in motion using Petabyte per second data rate optical communications in 4 domains: space, fiber, and two free space vacuum cavities. The amount of data in the world is expanding exponentially. 90% of the world’s data has been created in the last 2 years. 4-5% of the world’s power is currently being used by data storage and computing. We need more efficient methods of data storage. Physical sites can be subject to attack, and to local laws. Data storage that avoids these constraints is desirable. This is a revolutionary approach to data storage, using optical communications as a base. Data is not stored in an object. It is stored in motion between objects. The speed of light is fast, so to store data in motion we need very high data rates, and very long paths. LyteLoop has had an operating system using fiber and is currently building a free-space storage prototype. The current fiber system stores > 1 GByte, using 2000 Km of fiber, and is allowing us data center like writing and retrieval experience. Multiple patents have been filed, and some granted. One of the patent-pending concepts is called angle multiplexing, a method of dramatically increasing path length over a free-space path among multiple optical apertures. Another patent pending concept is the savings that will occur because we can have optical loops in a single location.