This PDF file contains the front matter associated with SPIE Proceedings Volume 10524, including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Free space optics (FSO) offers a high bandwidth wireless communication solution that operates outside of the traditional radio frequency spectrum. With the recent auction of spectrum in accordance with the 2010 National Broadband Plan, access to alternative spectrum is a critical need in order to allow our military forces to maintain operational, training and test capability. While FSO offers access to a very high bandwidth outside of traditional RF spectrum, challenges posed by the optical channel require careful consideration of the tactical FSO system design. Over the past decade, significant research efforts have been expended to demonstrate FSO technologies in ground, maritime, airborne, and space environments. At the tactical level, the Office of Naval Research focused their effort at the development of modest, robust FSO terminals in the TALON Future Naval Capability program. The challenges of developing tactically-relevant FSO terminals encompass a diverse set of requirements inclusive of more than link performance alone, such as operating environment, platform integration, automation and user interface, network architecture, and the balance of these requirements with size, weight, power and cost. NRL, in partnership with the National Spectrum Consortium, continues the prototyping and evaluation of tactical FSO systems to meet user needs. As a result of the execution of the National Broadband Plan, the need for communication systems operating in alternate spectrum has become more acute.

Robust and agile reconfigurable free space optical communication (FSO) links over dynamic traffic play a key role in next generation flexible wireless datacenter inter-rack networks, in terms of high throughput, dynamic robustness, cable complexity and energy efficiency. In this paper, we propose and demonstrate an agile reconfigurable 10Gbps FSO system incorporated with intelligent beam acquisition and tracking mechanism based on gimbal-less two-axis MEMS micromirror and retro-reflective film optics. Steering latency and alignment accuracy of reconfigurable FSO links are evaluated over various distances and directions, and get enhanced by intelligent adaptive acquisition and tracking schemes. Optical power loss and bit error rate of reconfigurable 10Gbps FSO links with mobility are reported over a distance of 13 m. Results show that it is feasible to use MEMS retroreflective acquisition and tracking system to achieve agile reconfigurable FSO links for flexible wireless datacenter inter-rack networks.

This work puts forward new technologies for free-space optical communications, with emphasis on deployments between ground and aerial transceivers. The proposed system targets the challenges of these aerial-ground links by applying direct laser transmission for the ground-to-aerial active uplink and applying all-optical retro-modulation (AORM) for the aerial-to-ground passive downlink. It is shown that such a system can function with multiple ground transceivers, over wide service coverage, and one aerial transceiver, with low demands for its mass and power. The AORM architecture applied in the passive downlink implements glass S-LAH79 hemispheres for effective retroreflection and CuO nanocrystal semiconductor thin film layer for all-optical modulation on ultrafast timescale. The fabricated AORM architecture is demonstrated to have an system response time of 770 fs, which limits the aggregate data rate. Such a fast system response establishes the possibility of terabit-per-second data rates. Ultimately, these findings can lay the foundation for future laser-based terabit-per-second links between satellites, unmanned aerial vehicles, and high-altitude platforms.

Lasers, modulators and photodetectors are key components to configure transmitter and transceiver in optical communication links or optical information processing systems. There are a few optical transmission windows with low attenuation (< 0.2 dB/km) located around 1550nm, 1060nm and 850nm. The 1550nm and 850nm bands are wider explored than 1060nm window owing to more available components and optics in these regimes that used nowadays in telecom/datacom network facilities. This work presents design, fabrication and characterizations of III-V semiconductor quantum structures based optoelectronic components produced at RISE Acreo’s ISO9001 certified clean-room laboratory. The 850nm and 1060nm devices have been designed and fabricated using the quantum structures grown on GaAs substrate, while the 1550nm components are based on the epi structures lattice matched to InP substrate. Desired operating wavelength can be turned by bandgap engineering, and the format of the devices can be arranged according customized specifications. As a demonstration, a 2x64 spatial light modulator (SLM) array with gradually increase operating wavelengths of each pixel at about 850nm will be detailed, which was used to achieve planar-integrated free-space optics for signal splitting, interconnection and processing. For 1550nm window, two components will be presented, one is a small aperture electroabsorption modulator (EAM) with diameter of 150µm that reached data rate at about 1 Gbps in a FSO link, another is an avalanche photodiode (APD) linear array. In addition, the lasers operating at about 1060nm utilizing quantum wells or quantum dots will be addressed.

Numerous Free Space Optical Communications (FSOC) applications use fine tracking to achieve precise jitter stabilization necessary for high data rate communications. In addition, precision pointing mechanism are often required for point-ahead of the transmit laser. Prior systems have used either fast steering mirrors (FSMs), piezoelectric fiber optic positioners, or inertially stabilized platforms each of which has its own advantages and disadvantages for different applications.

We developed a small form-factor, high performance FSM capable of meeting both high bandwidth stabilization requirements as well as high precision pointing necessary for the point-ahead function. The current design achieves a 2.5 kHz closed-loop optical track bandwidth, <5 μrad/mrad accuracy, and better than 15 nm rms surface figure error. Because there is no single approach to FSOC architecture, we designed the FSM to be easily scaled and customized for various applications ranging from FSOC, image stabilization, and scanning. Simple choices and customization of the FSM components including the mirror substrate, flexure, feedback sensors, and actuator design can provide custom designs for various applications. Analysis tools were developed to quickly trade the multitude of design parameters that influence performance. This paper reviews the FSM design, performance, and qualification test results, and trade space available to customize the FSM. We present analysis and test data from a couple of design variations to show how our design and analysis approach allows the FSM to be quickly adapted to various performance and environmental requirements.

For terrestrial free space optical (FSO) communication applications, large area photodetectors not only allow for more efficient power coupling but also ease the effects of atmospheric turbulence. When fabricated in array form, these devices have the added capability of operating as both data reception sensors and position sensitive detectors (PSD). Concentric five element impact ionization engineered (I2E) avalanche photodetector (APD) arrays have been developed to operate as combinational sensors at gigabit data rates. While the excellent sensitivity performance of these detectors allows for enhanced link range in calm atmospheric conditions, link availability with a high quality of service for an FSO system is strongly impacted by its ability to dynamically counter-act atmospheric fading with varying temporal scale. In this paper we present initial results using a combinational sensor in tandem with an advanced FSO modem which utilizes low-density parity check coding (LDPC) in an incremental redundancy (IR) hybrid automatic repeat request (HARQ) protocol. System performance enhancements as function of varying atmospheric scintillation will be discussed.

Deep-space optical communications systems often utilize an uplink optical beacon in order to provide a reference for optical flight terminal stabilization and downlink pointing. The single-photon-counting detector arrays that are used to sense this beacon under photon-starved conditions may be count-rate limited due to blocking from detector quench times and other readout circuit constraints. This blocking effect reduces the effective detection efficiency in a non-uniform manner over the array, changing the photon counting channel model and degrading the performance of uplink centroiding algorithms. In this paper, we discuss an uplink beacon centroiding algorithm and the impact of detector blocking on this algorithm. Statistical models of detector output data and centroiding observables are presented based upon analysis, Monte Carlo simulations, and measured laboratory data, incorporating realistic effects such as beacon signal parameters and Earth background. A simple blocking compensation algorithm is presented and shown to mitigate the effect of blocking, allowing one to maintain sub-microradian centroiding errors under channel conditions representative of deep space optical links.

We investigate theoretically the efficiency of deep-space optical communication in the presence of background noise. With decreasing average signal power spectral density, a scaling gap opens up between optimized simple-decoded pulse position modulation and generalized on-off keying with direct detection. The scaling of the latter follows the quantum mechanical capacity of an optical channel with additive Gaussian noise. Efficient communication is found to require a highly imbalanced distribution of instantaneous signal power. This condition can be alleviated through the use of structured receivers which exploit optical interference over multiple time bins to concentrate the signal power before the detection stage.

Missions will continue to need very capable communications systems to transfer data to and from Deep Space, both for science and human exploration. Despite the amazing accomplishments of NASA’s Deep Space Network to date, even higher data rates will be desired in the future. The optical bands have long held out the potential for delivering these higher data rates without having SWaP and cost grow too high. In fact, recent and ongoing laboratory development and space demonstrations make the optical technologies look very promising. A question has arisen recently, though, as to whether the technology will truly be able to meet the needs of many different kinds of missions, especially as the missions go deeper and deeper into space. We present here a communications analysis of the links in question which results in an insight into how we can make such links work. We then propose some feasible system architectures, affecting both the space and ground segments, which can lead to systems that could outperform present-day and future RF systems by from one to four orders of magnitude. We feel that optical technology should be able to support such high-rate communications throughout the Solar System, and even far beyond Pluto.

TNO and DLR envision optical free-space communication between ground stations and geostationary telecommunication satellites to replace the traditional RF links for the next generation of Very High Throughput Satellites. To mitigate atmospheric turbulence, an Adaptive Optics (AO) system will be used. TNO and DLR are developing breadboards to validate Terabit/s communication links using an AO system. In this paper the breadboard activities and first results of the sub-systems will be presented. Performance of these subsystems will be evaluated for viability of terabit/s optical feeder links.

We report the design of OPTEL-D which is an optical communication system for direct data transfer from deep space to earth. OPTEL-D extends the link distances of current optical communication systems to the deep space. The design covers the electrical, thermal, mechanical and optical challenges that such a link distance imposes to both the ground and the space segment. The detailed conceptual system design is presented together with its trade-offs and performance analyses. The space terminal is based on a modular approach which groups the fiber-optical, the opto-mechanical, and the electrical functions into separated modules for independent accommodation. The transmitter beam is provided by an amplitude modulated MOPA system operating at 1.5um. The opto-mechanical unit accomplishes beam forming and steering of the transmit beam as well as reception of the uplink channel. An electrical unit comprises power-conditioning, data-handling, and terminal control through its interfaces to the spacecraft. It is concluded that the OPTEL-D space terminal mass and power are 34 kg and 116 Watts. Depending on the ground station configuration, a data rate of several Mbps is reached over distances of >75 Mio kilometers. Critical assumptions on the receiver sensitivity were verified by tests and the results will be presented. The design of OPTEL-D was derived in collaboration with the European Space Agency to serve the Asteroid Impact Mission. Due to its modular approach using commercially available technology, it can be realized in a competitive schedule and cost frame providing flexible yet powerful data downlinks to most deep space missions.

Laser communications (Lasercomm) systems have gained increasing interest for both defense and commercial applications due to their ability to provide secure, long-distance, high-bandwidth communications on the move, without the need for RF spectrum management. This paper will present field test results from the U.S. Navy’s Trident Warrior 2017 fleet exercise, where a compact Lasercomm system was evaluated. As compared to previously demonstrated long range Lasercomm terminals, this compact terminal design leverages simultaneous transmit and receive spatial diversity to mitigate scintillation fades in a smaller form factor. In addition, a 10 Gbps retransmission capability has been integrated and tested to assure error-free data transport even through short duration path blockages and optical fades. The system was operated at full functionality over seven test days with network traffic loads ranging from 1 - 7.5 Gbps in bidirectional configurations. The system was exercised to a line of sight limited range of 45 km and showed capability through haze and even some levels of fog on multiple days.

Recently, satellite broadband communication services using Ka-band are emerging all over the world, some of them with capacities in excess of 100 Gbps. However, as the radio bandwidth resources become exhausted, high-speed optical communication can be used instead to achieve ultra-broadband communications. The National Institute of Information and Communications Technology (NICT) in Japan has more than 20 years of experience in R&D of space laser communications, with important milestones like ETS-VI (Engineering Test Satellite VI), OICETS, and SOTA. We are currently developing a laser-communication terminal called “HICALI”, which goal is to achieve 10 Gbps-class space communications in the 1.5-μm band between Optical Ground Stations (OGSs) and a next generation high-throughput satellite (called ETS-IX) with a hybrid communication system using radio and optical frequencies, which will be launched into a geostationary orbit in 2021. The development of test and a breadboard model for HICALI has been conducted for several years and we are now carrying out an engineering model as well as designing the OGSs segment. In this paper, we describe concepts and current design status of the HICALI system.

Coherent, free-space optical communication technology offers near-quantum-limited receiver sensitivity and high spectral efficiency compared to conventional direct detection systems. In this paper, we will present the initial results from a bidirectional air-to-ground demonstration of a coherent optical link.

Free-space optical communications links have the perpetual challenge of coupling light from free-space to a detector or fiber for subsequent detection. It is especially challenging to couple light from free-space into single-mode fiber (SMF) in the presence of atmospheric tilt due to its small acceptance angle; however, SMF coupling is desirable because of the availability of extremely sensitive digital coherent receivers developed by the fiber-telecom industry. In this work, we experimentally compare three-mode and single-mode coupling after propagating through 1.6 km of free-space with and without the use of a fast-steering mirror (FSM) control loop to mitigate atmospherically induced tilt. Here, the 3-mode fiber is a 3-mode photonic lantern multiplexer (PLM) that passively couples light into three SMF outputs. With the FSM control loop active, coupling into the PLM and the SMF yielded nearly identical coupling efficiencies, as expected. Experimental results with the FSM control loop off show that coupling from free-space to PLM increases the average power received, and mitigates the negative impacts of tilt-induced fading relative to coupling directly to SMF.

We proposed and evaluated the effectiveness of a simple rate control technique of Adaptive Distributed Frame Repetition (ADFR), which achieves both high speed link in the stable condition and suppresses the influence of turbulence-induced fading in the unstable condition. In order to estimate its performance in the LEO-to-Ground link with 10-Gb/s modulation speed, we developed a numerical simulator which enables us to calculate the frame error rate and the throughput under the specific turbulence condition emulated by random phase screens. We confirmed that ADFR can increase the link duration and the capacity by increasing the tolerance to the fading especially in the region of lower elevation angle. In addition, we evaluated its effectiveness through the comparison with the general adaptive rate control, in which the receiver sensitivity can be manipulated in accordance with the link conditions. The results show that ADFR provides similar performances with the sufficient received optical power.

Pulsed optical modulations with photon-counting receivers have been shown to be capable of very high photon efficiencies. With channel capacity as the relevant metric, we review the mathematical models of free-space receivers with photon-counting detectors in varying amounts of background noise. Although historically, such direct detection receivers have been designed to cover relatively wide fields of view because of turbulence, modern systems may be able to achieve performance with many fewer spatial modes, as well as near-matched filtering. We investigate the channel capacity in those cases and show the relative benefits of reducing both spatial and temporal modes of noise. In addition, because the exact capacity formulas are calculation-intensive and not intuitively helpful, we propose a particularly simple yet accurate mathematical approximation that greatly simplifies the analysis when the Poisson model is relevant, and is useful in link budgeting exercises.

The availability of free space optics (FSO) links remains an important question in wide-scale deployment of the technology. As with microwave links, for any given terminal design there will be a correlation between the range between nodes and the link availability. This correlation will change depending on location and season. The key variables are the atmospheric transmission and the level of scintillation, which may themselves be correlated.

The Naval Research Laboratory’s Chesapeake Bay (CBD) Lasercom Test Facility (LCTF) maintains a full suite of optical instruments characterizing a path that goes 16 km across Chesapeake Bay. In addition, a 100 Mbps FSO link is continuously run to gather link quality statistics.

In this work we examine data from CBD including scintillation index, transmission, and packet error rate, as well as local weather data. With this data, as well as link models, we determine to what extent we can predict the actual link availability at CBD and at other locations.

Space-based optical links can, in principle, support high data rates by using power efficient communication schemes and unconstrained spectrum. In particular, direct links from low-Earth orbit (LEO) to ground have the potential to support very high rates due to the short link distances involved. In this work, we consider an architecture for LEO-to-ground links that operate at peak rates of 100+Gb=s. Such rates are routinely achieved over fiber channels using power-efficient, fiber-coupled transceivers; however, free-space systems that use these devices may need additional error control to ensure reliable communication over an atmospheric channel. We analyze the data volume, or throughput, that can be delivered by a LEO-to-ground system using fiber-coupled transceivers in conjunction with automatic repeat request (ARQ) protocols. We show that many terabytes per day can be delivered error-free from LEO to a single ground terminal for a variety of orbit and ground terminal geometries.

Quantum key distribution (QKD) can be used to produce a cryptographic key whose security is unconditionally guaranteed by quantum mechanics. The range of fiber-based QKD links is limited, by loss, to a few hundred kilometers, and cannot be used between mobile platforms. Free space QKD can, in principle, overcome these limitations. In practice, very narrow beam divergences must be used, requiring highly accurate pointing of the transmitting terminal to the receiver. This makes deployment very difficult. Here we describe an entirely new type of free space QKD link, using modulating retro-reflectors (MRR). The MRR-QKD link eases the pointing requirements A laboratory-based BB84 QKD link using multiple quantum well MRRs is demonstrated, and link budgets for possible applications are discussed.

Over the last couple of years, NASA has been making changes to the Laser Communications Relay Demonstration (LCRD) project, a joint effort spanning NASA’s Goddard Space Flight Center (GSFC), the Jet Propulsion Laboratory California Institute of Technology (JPL) and the Massachusetts Institute of Technology Lincoln Laboratory (MIT LL). The changes make LCRD more like a future Earth relay system that has both high-speed optical and radio frequency links, enabling LCRD to demonstrate a more detailed concept of operations for a future operational, mission-critical Earth relay. LCRD is planned to launch in June 2019 and is expected to be followed a couple of years later by a prototype user terminal on the International Space Station. LCRD’s architecture will allow it to serve as a testbed in space, and this paper will provide an update of its planned capabilities and experiments.

For the last three years the European deep-space optical communications program had been based on the Asteroid Impact Missions (AIM), a rendezvous mission with the double-Asteroid Didymos. Unfortunately, the AIM mission was not approved by ESA council and efforts are now concentrated on the implementation of a Deep-space Optical Communication System (DOCS) in a Space Weather (SWE) mission to libration orbit L5 within the frame of ESA’s Space Situational Awareness (SSA) program. DOCS is an in-orbit technology demonstration that also serves a scientific objective, namely the transfer of high-resolution solar imagery. The characteristics of the SWE L5 mission allow significant simplifications in deep-space optical communication terminal design, because the equidistant triangular orbital geometry between the sun, the Earth and L5 (all three distances are 1 AU = 150 Mio km) ensures that the sun is always separated by 60 degrees from both, the space and the ground terminal. This allows for very efficient solar stray-light shielding and thermal management. It also minimizes the pointing requirements of the DOCS space terminal; a coarse pointing mechanism is not required, nor is a point ahead mechanism. The SSA SWE L5 mission Phase A, Phase B1 and B2 studies will be conducted 2017 - 2019. DOCS aims to demonstrate a data rate of 10 Mbps over 150 Mio km to a 4 m optical ground station. The paper will give an overview of deep-space optical communication technology developments and present the design and performance of the Deep-space Optical Communication System (DOCS).

With the increasing need for higher data rates on small LEO spacecraft, highly compact laser communication systems are required to overcome the limitations in the downlink channel. The Optical Communication Systems (OCS) group at the German Aerospace Center (DLR) has been working on the program OSIRIS (Optical Space Infrared Downlink System) since 10 years. The OSIRIS program started with the development of optical communication payloads for the Flying Laptop and BiROS satellite, which are already in orbit. The program covers a very broad range from scientific experiments and developments to demonstrations of relevant technologies. The range of developments and demonstrations covers an OSIRIS application on CubeSats with a demonstration in 2018, as well as very high data rate applications in the scope of the 3^rd OSIRIS generation with up to 100 Gbps. These developments are the basis for scientific measurements on the channel characteristics and new applications and technologies in space. In the process towards an industrial application, OSIRIS is also involved in standardization activities in the framework of CCSDS.

This paper will give an overview of scientific missions and developments in the OSIRIS program and give an outlook on the development path ahead.

In recent years, NASA has been developing a scalable, modular space terminal architecture to provide low-cost laser communications for a wide range of near-Earth applications. This development forms the basis for two upcoming demonstration missions. The Integrated Low-Earth Orbit Laser Communications Relay Demonstration User Modem and Amplifier Optical Communications Terminal (ILLUMA-T) will develop a user terminal for platforms in low-Earth orbit which will be installed on the International Space Station and demonstrate relay laser communications via NASA’s Laser Communication Relay Demonstration (LCRD) in geo-synchronous orbit. The Orion EM-2 Optical Communication Demonstration (O2O) will develop a terminal which will be installed on the first manned launch of the Orion Crew Exploration Vehicle and provide direct-to-Earth laser communications from lunar ranges. We describe the objectives and link architectures of these two missions which aim to demonstrate the operational utility of laser communications for manned exploration in cislunar space.

The paper reports on the deployment of the first commercial optical data relay system, the European Data Relay System (EDRS), the achieved performance so far, and the progress in the characterization of optical bi-directional space to ground links performed between the Laser Communication Terminal (LCT) on the Alphasat geostationary satellite and the transportable adaptive optics ground station (T-AOGS) currently co-located with the optical ground station from the European Space Agency (ESA) at Tenerife, Spain. Uplink results using homodyne binary phase shift keying at 1064 nm from the T-AOGS were examined. The performance of a packet erasure code according to the orange book CCSDS 131.5-O-1 optimized for the conditions of a laser uplink without adaptive optics correction of the phase front is analyzed.

The future demand for enhanced telecommunication capacity required to support human and robotic exploration from deep-space has motivated the advancement of free-space laser communication technologies for the past few decades. Steady advances in these technologies, validated through space-to-ground demonstrations, have resulted in incremental advances with the deep-space optical communications (DSOC) technology demonstration being one of the next milestones on NASA’s roadmap. NASA’s Psyche Mission to launch early next decade plans to host a DSOC flight laser transceiver for link demonstrations extending from 0.1 to farther than 2 astronomical units (AU). The capabilities validated though this demonstration, we expect, could spur the use of optical communications infrastructure around Mars in the next few decades. In this paper we summarize ongoing activities underway at the Jet Propulsion Laboratory in preparation for the DSOC technology demonstration and go on to present discussions on the drivers for developing a robust deep space laser communications operational capability.

Delivery of large volumes of data from low-Earth orbit to ground is challenging due to the short link durations associated with direct-to-Earth links. The short ranges that are typical for such links enable high data rates with small terminals. While the data rate for radio-frequency links is typically limited by available spectrum, optical links do not have such limitations. However, to date, demonstrations of optical links from low-Earth orbit to ground have been limited to ~10 to ~1000 Mbps. We describe plans for NASA’s TeraByte InfraRed Delivery (TBIRD) system, which will demonstrate a direct-to-Earth optical communication link from a CubeSat in low-Earth orbit at burst rates up to 200 Gbps. Such a link is capable of delivering >50 Terabytes per day from a small spacecraft to a single small ground terminal.

Interest in free space optical communications has increased in the last decades as it has many advantages over RF communications, especially for space-borne applications. However, high power, good spectral quality and beam quality are needed for efficient data transmission over long distances. To meet the need of having lightweight and compact laser sources with such qualities for FSO, semiconductor based MOPA systems (Master Oscillator Power Amplifier) have been developed. In this paper we present the experimental results and compare them to simulation results for a threesection monolithically integrated semiconductor Master Oscillator Power Amplifier emitting at 1.5 μm wavelengths, designed for LIDAR applications that can also be used for free space telecommunications. The MOPA includes a distributed feedback laser section for single mode light emission, an intermediate section for data modulation and a flared semiconductor optical amplifier section for power amplification, which allows us to generate a high power beam with good spectral characteristics. The impact of bias conditions of the different device sections and device design on performances have been studied. Single mode operation at 4 different wavelengths near 1550 nm is achieved for optical output power up to 400 mW in continuous-wave (CW) regime for a SOA current of 3 A and 800 mW for SOA pulsed operation for currents up to 5 A. Near-field profile is also analyzed for different modulator current. Small-signal dynamic response is measured and analyzed.

Bidirectional, high data rate, low size, weight, and power (SWaP), and low cost free space optical links are needed for space and aeronautic communication applications to send and receive large volumes of data. We are exploring design strategies for optical transceivers to reduce SWaP and cost through increased misalignment tolerance (pointing requirement reduction) and sharing the optical transmit and receive paths (imposing optical symmetry). In applications where the detector is fiber coupled, the receive fiber numerical aperture is the main driver of the pointing accuracy requirement. Increasing the numerical aperture of the receive fiber reduces the pointing requirement. In an optically symmetric design, the fibers both transmit and receive the light. Hence, increasing the receive fiber numerical aperture requires a similar increase of the transmit fiber. Unfortunately, increasing the transmit fiber numerical aperture causes instability in received power over small misalignments. Double clad fibers offer a solution. These fibers transmit from a single mode core and receive light in a larger numerical aperture. Results show that as transceiver fibers, double clad fibers have an improved misalignment tolerance and a higher stability for small changes in misalignment when compared to single mode fibers and multimode fibers. Also, double clad fibers are shown to match the performance of an asymmetrical link design with a single mode transmit fiber and a multimode receive fiber.

This work demonstrates the operation of a photonic integrated circuit transmitter for space optical communication utilizing an RZ-DPSK modulation format realized on an indium phosphide monolithic integration platform. It includes a widely tunable sampled grating distributed Bragg reflector laser, a semiconductor optical amplifier for amplification and burst mode operation, a dual drive Mach-Zehnder modulator (MZM) that efficiently encodes phase information, and an electro-absorption modulator RZ pulse carver.

The laser tuning range is approximately 35 nm across the telecommunications C-band. The MZM DC extinction ratio exceeds 15 dB for a differential drive voltage of 6 V peak-to-peak. Clear eye diagrams were demonstrated at 3 Gbps for OOK modulation and 1 Gbps for RZ-DPSK modulation.

We present the space qualification of a multi-channel mid-power booster optical fiber amplifier (OFA) suitable for 1550nm LEO satellite to ground laser communication downlinks. The end-to-end OFA development from conceptual design all the way through qualification testing followed ECSS-level Product Assurance guidelines for deployed materials, components and processes. The environmental qualification test programme relied on ECSS-E-10-03C which is the ESA standard for qualification testing of space segment hardware. The qualification results show robust functional and structural performance following stress at all 3 possible excitation axes with high level sine vibration, random vibration and mechanical shock as well as thermal cycling between survival and operating temperatures in vacuum condition. In addition to thermo-mechanical tests, proton and gamma radiation tests performed on component and sub-assembly level suggest that the OFA is capable to deliver its performance under ionizing and non-ionizing radiation levels found in the LEO orbit. The OFA has been delivered for system integration into the Optel-μ terminal, applicable to small satellite platforms.

The actual challenge for space researchers is to increase the free space telecommunications data speed transfer. One of the most promising solutions is the optical communication systems. This technology can be used for the inter-satellite and/or satellite-ground links, reaching the TB/s speed for data transfer in the case of Dense Wavelengths Division Multiplexing (DWDM) based technologies. However, to achieve such systems, two main issues need to be overcome: the first one is to validate that no unexpected radiation effect appears when the optical amplifier working in the DWDM configuration and the second one is to estimate the degradation of the Erbium/Ytterbium co-doped boost (High Power - HP) amplifier performances during the space mission lifetime. In this last case, the used high powers will result in a complex response of the amplifier due to photobleaching, photodarkening and thermal effects. In this work, we estimate the radiation effects on an Er/Yb co-doped boost amplifier operating in a Dense WDM configuration. Both radiation hardened and a conventional versions of EYDFA have been considered. The obtained results allow estimating the performances of our fibers under exposure in such amplification setup and also to validate its potential for use in an actual space mission. We demonstrate the good radiation resistance of Er/Yb co-doped 12 μm core diameter fibers reaching 20 W of output power for telecommunication applications. This core diameter provides a fewmode optical output signal (with low dispersion) and with enough power to ensure the signal propagation trough the atmosphere. This study is fundamental as several phenomena such as Photo/Thermal bleaching, photo-darkening… are in competition due to the high-power light density in the fiber core and the system radiation response cannot yet be predicted by actual simulation tools.

HgCdTe avalanche photodiodes offers a new horizon for observing spatial or temporal signals containing only a few infrared (IR) photons, enabling new science, telecommunication and defence applications. The use of such detectors for free space optical communications is particularly interesting for both deep space and high data rate links as it enables wide field of view free space optical coupling to the detector at high sensitivity, down to single photon level and with a close to negligible loss of the information contained in the strongly attenuated photon flux. Measurement of the response time and dark current shows that such devices can be operated at room temperature with bandwidths up to 10 GHz in a back-side illuminated configuration. This configuration allows to use micro-lenses fabricated directly into the APD substrate and enables to use a large photosensitive area while maintaining a high bandwidth, low dark current and /or high operating temperature. We report on the expected performance 4-quadrant APD detector demonstrator with single photon sensitivity, which is currently developed to be used in deep space telecommunications by ESA and present the potential use for high data rates links of 10 Gbits/s.

We report investigations on the mode diversity receiver applied in optical links from low-earth-orbital satellites to a ground terminal. We numerically simulated temporal power variations coupled to each mode of a FMF under atmospheric turbulence at several elevation angles. For weak influence of turbulence, the optical beam mainly coupled to the fundamental mode of the FMF; for strong influence of turbulence, it coupled significantly to higher order modes of the FMF and mode diversity reception was more effective. Emulating the fast-changing coupled power to each mode of the FMF with variable optical attenuators and arbitrary waveform generators, we experimentally evaluated the performance of three-mode diversity reception of 10 Gb/s QPSK. Digital signal processing was performed on 20-ms-long successively acquired data bursts, which were long enough to contain typical features of fading. We confirmed that digital combining could track the severe power variations regardless of elevation angles. We extended our investigation to the possibility of more severe conditions and we showed through simulation further improvement using multiple mode diversity receivers combined with aperture diversity.

FSO-based communication systems experience difficulty with receiving and separating signals arising from multiple transmitters, a capability that would facilitate implementation of MIMO systems. Current implementations require multiple, distinct optical antennas, each tracking a single user, which proves bulky and costly, especially if the transmitters are moving and must be tracked. A fiber-bundle receiver has the potential to use multiple pathways, corresponding to the individual fibers within the receiver, to capture different combinations of the incoming optical signals. If the bundle provides linear combining of the optical signals from both the individual fibers in the bundle and amongst the incoming optical signals, signal processing could extract the individual signals from the combinations. In this paper, we experimentally investigate whether the fiber-bundle receiver possesses sufficient linearity of operation to allow the separation of two signals by simple processing algorithms, for both turbulent and non-turbulent conditions. Data from two distinct sources enters a single-bundle, single field of view receiver, and a basis signal from one transmitter provides the reference for performing simple subtraction-based extraction of the unknown signal from the other transmitter. The experimental results show that the receiver does operate linearly, and that the linear operation remains sufficiently intact in the presence of turbulence to extract a recognizable copy of one signal from the other. The ability of the fiber bundle receiver to mitigate turbulence effects appears to assist in maintaining this sufficient level of linearity.

Adaptive Optics (AO) is a key technology for ground-based astronomical telescopes, allowing to overcome the limits imposed by atmospheric turbulence and obtain high resolution images. This technique however, has not been developed for small size telescopes, because of its high cost and complexity. We realized an AO system based on a Multi-actuator Adaptive Lens and a Shack-Hartmann wavefront sensor (WFS), allowing for a great compactness and simplification of the optical design. The system was integrated on a 11” telescope and controlled by a consumer-grade laptop allowing to perform Closed-Loop AO correction up to 400 Hz.

A 20-meter free-space optical link (FSOL) is proposed for data transmission between external ISS payload sites and the main cabin at a target rate of 10 Gbps (gigabits per second). Motion between a payload site and the main cabin is predicted to cause up to 5 cm in lateral misalignment and 0.2 degrees of angular misalignment. Due to the harsh environment of space it is advantageous to locate the optical transceivers inside the spacecraft or in a controlled environment. With the optical components and transceivers in separate locations, a fiber optic cable will be required to carry light between the two. In our past work we found that the use of large-core fibers provide an increased misalignment tolerance for such systems and could eliminate the need for active control of the optics. In that work, it was shown that a 105 μm core diameter fiber optic cable offered a viable low SWaP (size, weight, and power) solution for the ISS application; however, the effects of modal dispersion were not investigated. This paper will present bit error rate performance of the FSOL using these large-core fibers.

Deep space optical communication is a highly efficient alternative to radio frequency (RF) technology offering higher data bandwidths. The challenge is that deep space optical communication is photon limited. Rejection of extraneous light is critical to maximizing signal quality. High transmitting, ultra-narrow bandpass filters with high out of band optical density (OD) can meet this requirement while improving signal throughput. Design trade-offs and fabrication results are presented for ultra-narrow bandpass filters with bandwidths as narrow as 0.2 nm full width half maximum (FWHM) with on-band transmission greater than 95% and off band rejection of greater that OD 5. Filters are designed to match laser wavelengths in the region of 1550 nm.

In free-space optical communication systems, narrow beam diameters necessitate precision pointing over a relatively wide field of regard. Control systems with such large dynamic range requirements sometimes employ multi-stage architectures, where a "coarse" mechanism provides low-bandwidth control over a large range of travel, and a "fine" mechanism provides high-bandwidth disturbance rejection over a limited range of travel. In such systems, the two stages must be coordinated in their dynamic response, so as to avoid undesirable coupling and even interference with each other. This topic has been studied in various literature - especially in the field of hard disk drives, but also for optical pointing systems. However, most of this literature considers systems where the output of the two stages combines by simple linear sum. Dual-stage control becomes even more challenging when the two stages combine via nonlinear kinematic relationships, as they would in an optical pointing system employing multiple gimbaled steering mirrors. This paper presents an approach for handling these nonlinear kinematic effects, and demonstrates the viability of this approach via simulation.

High gain Erbium Doped Fiber Amplifiers (EDFAs), while revolutionizing optical communications, remain vulnerable to optical damage when unseeded, e.g. due to nonlinear effects that, due to spontaneous Q switching, produce random pulses with high peak power, i.e. giant pulses. Giant pulses can damage either the components within a high gain EDFA or the external components and systems coupled to the EDFA. We explore the conditions under which a reflective, polarization-maintaining (PM), core-pumped high gain EDFA generates giant pulses, provide details on conditions under which normal pulses evolve into giant pulses, and provide results on the transient effects of giant pulses on an amplifier’s fused-fiber couplers, such as pump-signal combining wavelength division multiplexers (WDMs), an effect which we call Fused-coupler Overload Induced Leakage (FOIL). While FOIL’s effect on fused-fiber couplers is temporary, its damage to forward pump lasers in a high gain EDFA can be permanent, e.g. damage to the pump’s front facet.

Space-to-ground photon-counting optical communication links supporting high data rates over large distances require enhanced ground receiver sensitivity in order to reduce the mass and power burden on the spacecraft transmitter. Superconducting nanowire single-photon detectors (SNSPDs) have been demonstrated to offer superior performance in detection efficiency, timing resolution, and count rates over semiconductor photodetectors, and are a suitable technology for high photon efficiency links. Recently photon detectors based on superconducting nanowires have become commercially available, and we have assessed the characteristics and performance of one such commercial system as a candidate for potential utilization in ground receiver designs. The SNSPD system features independent channels which can be added modularly. We analyze the scalability of the system to support different data rates, as well as consider coupling concepts and issues as the number of channels increases.

The promise of Free Space Optical Communications (FSOC) is the potential for very high data rate links over long distances without issues of radio frequency (RF) allocation and interference issues. The desired very high data rates implies a combination of high laser power and large transmit and receive aperture sizes. Increasing the size of the receiving ground terminal will be significantly less expensive than increasing the satellite laser power and satellite transmitter telescope size. Unfortunately, telescope costs scale at roughly D2.6 implying a very high increase in cost for a moderate increase in FSOC Ground Terminal (FGT) energy capture area. One way to mitigate this is through a combination of smaller telescopes. There is the potential for additional savings if adaptive optics is used to enable coupling the received signal into optical fiber followed by a fiber laser pre-amplifier. The use of a fiber-laser preamplifier enables very high detection efficiency for very small optical signals at a low cost compared to other detectors with similar performance that require cryogenic cooling. Adaptive Optics (AO) systems can be very expensive as the size of the telescope increases. However, AO components for small telescopes are readily available and much less expensive.

A key component in the Integrated Radio and Optical Communications project at the National Aeronautics and Space Administration’s (NASA) Glenn Research Center (GRC) is the radio frequency (RF) and optical software defined radio (SDR). A NASA RF SDR might consist of a general purpose processor to run the Space Telecommunications Radio System (STRS) Architecture for radio command and control, a reconfigurable signal processing device such as a field programmable gate array (FPGA) which houses the waveform, and a digital to analog converter for (DAC) transmitting data. Prior to development, SDR architecture trades on how to combine the RF and optical elements were studied. A modular architecture with physically separate RF and optical hardware slices was chosen and the optical slice of an SDR was designed and developed. The Harris AppSTAR^TM platform, which consists of an FPGA processing platform with a mezzanine card targeted for RF communications, was used as the base platform in prototyping the optical slice. A serially concatenated pulse position modulation (SCPPM) optical waveform was developed. The waveform follows the standard described in the Consultative Committee for Space Data Systems (CCSDS) Optical Communions Coding and Synchronization Red Book. A custom optical mezzanine printed circuit board card was developed at NASA GRC for optical transmission. The optical mezzanine card replaces the DAC, which is used in the transmission of RF signals. This paper describes RF and optical SDR architecture trades, the Harris AppSTAR^TM platform, the design of the SCPPM waveform, and the development of the optical mezzanine card.

Fiber amplifiers have been used in many laser communication applications due to their compactness and high efficiency. However, fiber amplifiers for free space laser communications have been limited to low peak power communication formats due to nonlinear effects such as Self Phase Modulation (SPM). At high peak powers, SPM can broaden the spectrum of free space laser communication signals to an unacceptable degree, moving much of the transmitted power outside the receiver’s designed bandwidth. The Laser Communication Relay Demonstration (LCRD) needed a fiber amplifier that could amplify a 3.5 GHz wide signal to peak powers of nearly a kilo-Watt with little to no spectral broadening due to SPM. We tested several different commercial amplifiers and found an Er-doped, Very Large Mode Area (VLMA) fiber amplifier, pumped by a 1480 nm Raman fiber laser, met the needs for both high peak power and minimal spectral broadening. The high peak power performance of the VLMA amplifier is enabled by the large effective area of ~ 1100 μm² . We will present a detailed analysis of the effects of SPM on the amplified signal spectrum for both pulse position modulation (PPM) and differential phase shift keyed (DPSK) communication formats at peak powers up to 1 kW. Additionally, Bit Error Ratio (BER) performance data taken with the LCRD modem showed the signal did not suffer a measureable penalty from amplification with the VLMA amplifier. The ability to reach the required high peak power with this fiber amplifier makes it possible to consider its use for deep space laser communication, e.g. with high order M-ary PPM formats where such high peak powers are needed.

Free Space Optical (FSO) communication is widely recognized for its powerful features, especially when compared to other wireless technologies utilized in point-to-point communication links. Although current literature focuses primarily on point-to-point transmission, multi-user FSO systems are beginning to draw significant attention. The primary objective in a multi-user communication system is to estimate individually transmitted signals from received signals, namely Blind Source Separation (BSS). A solution to the BSS problem in an FSO multi-user communication link is proposed. A multi-point FSO system composed of two independent transmitters operating at different wavelengths and a dual path fiber bundle receiver was used. The FastICA algorithm was exploited for multi-user detection. Experimental results demonstrate that this method can separate original transmitted signals from their received mixtures. Effects of signal power, data rate, misalignment error, and turbulence severity on signal separation are also explored to define the working range for achieving best performance.

Given the rapid demand for higher and higher bandwidth and the contention for radio frequency (RF) spectrum allocations, the need for space-based Free Space Optical Communications (FSOC) systems is ever increasing. We have previously presented design concepts for a lightweight, small, and high performance two-axis gimbal focusing on the use of commercial off-the-shelf (COTS) subsystems. Our efforts to design a small gimbal with 100 micro-radian pointing accuracy for FSOC have resulted in an unconventional optical fiber wrap design in order to achieve the low optical noise needed to meet the design goals. However, the design raised concerns about stress induced in the optical fibers. To verify our design assumptions and the actual gimbal performance, a mockup of the fiber wrap configuration was subjected to a Cyclical Stress Test conceived to mimic the desired on-orbit lifetime. At regular intervals throughout the test duration, optical power measurements were collected to characterize the degradation of the fiber wrap fiber. The results of the Cyclic Stress Test validated assumptions regarding the design performance, and provided insights used to modify the design prior to hardware implementation into compact free space optical communications gimbals.

Within the scope of the FALCON project Mynaric Lasercom GmbH, in collaboration with Facebook Inc., has built two laser terminals for optical communications: One airborne terminal MLT-70-ATG and one optical ground station GS-200. Both terminals are designed to establish communications between the stratosphere and ground. The athermalized design of the MLT-70-ATG, its efficient temperature management system and an optimized dynamic behavior for high-altitude platforms qualify Mynaric’s system to be easily integrated into carriers that fly up to tens of kilometers. The GS-200 achieves and sustains fast and reliable free-space data transfer links between the airborne segment and ground. It is designed for outdoor operations and is mounted on a stationary stable platform. A flight campaign executed by both companies has demonstrated a 10 Gbps bidirectional error-free link between the airborne laser communication terminal and the optical ground station in a representative scenario. The optical link was acquired successfully in a few seconds and both terminals maintained a steady link. Limitations in the line of sight between the communication partners, due to the flight patterns followed by the aircraft, triggered reacquisitions that were handled by the terminals autonomously. Bidirectional data transmission with maximum data throughput has been achieved. Even for strong fluctuation conditions, which were experienced during ground-to-ground tests, the link was error-free thanks to the coding in the Laser Ethernet Transceiver (LET). The LET converts the user data to a proprietary format. The systems could recover successfully outages up to 10 milliseconds. The coding and synchronization schemes have been optimized for overcoming the inherent spurious effects of the free-space optical communication channel.