This PDF file contains the front matter associated with SPIE Proceedings Volume 10910, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

The Cassini spacecraft ended its 20-year voyage of discovery with a fiery plunge into Saturn's atmosphere on Sept. 15, 2017. We will discuss the engineering and scientific rationale for the mission's final scenario and some of the complexities of an entirely new mission for an aging spacecraft. We will also present highlights from the many amazing findings from the spacecraft's 13 years of exploration in the Saturn system.

The goal of a realistic interstellar mission to the nearest star recently announced by NASA as a new space exploration objective targeted for circa 2069, presents a daunting but exciting prospect for creating a pragmatic but visionary mission design. For such a long-range mission goal, mission enabling technological developments need to be developed and evaluated in precursor missions beyond the edge of the solar system. NASA is stimulating such a pathfinder mission starting this year to promote efforts that could build upon what has been accomplished by New Horizons, and for which APL can leverage heat shield technology used on the Parker Solar Probe. An academically-inspired graduate-student supported study at Johns Hopkins University reported on our progress presented at the 2018 Committee on Space Research meeting (COSPAR 2018) in assessing and evaluating several critical subsystem issues and fruitful lines of innovation to improve dual-band downlink performance for a 1000 AU (1.5 × 10¹¹ km) mission. This work recapitulates and extends some work done much earlier at APL. We propose to quantify several critical subsystem trades essential to interstellar spacecraft communications, command and data handling, and critical guidance, navigation, and control (GNC) functions. These include the following: optimizing trajectories during transit and arrival in the targeted system, acquiring a quality mix of relevant highly-compressed scientific data, providing an accurate navigation capability at sub-relativistic speeds, insuring a robust communication system over extraordinary distances, and maintaining an effective command and data handling subsystem, while acknowledging the criticality of a survivable spacecraft bus in an illdefined, harsh and very long duration environment.

Mobile and embedded applications are emerging in the growing field of free space optical links (FSOL). Some mobile applications for FSOL include spacecraft, aircraft, and automotive. These applications by nature require low size weight and power (SWaP) solutions. The main challenge with any FSOL system is the strict pointing requirements. Common solutions to pointing and alignment of FSOL include gimbals, fast steering mirrors, and adaptive optics. All of which provide viable solutions at the cost of increased SWaP. Previously, we presented the use of both large core fibers and double clad fibers (DCF) to interface FSOL transmit and receive optics with small form factor pluggable optical transceivers (SFP). Double clad fibers have been shown to enable a common optical path by transmitting through a single mode core and receiving through a large inner cladding. This enables a single set of symmetric transmit and receive optics, which decreases the SWaP. In addition, using DCF increases the received power stability of the link relative to a multimode fiber (MMF) transmitting. To determine the viability of the system, bit error rate performance needs to be investigated. The results of this paper show that at a bit rate of 10 Gbps, double clad fibers offer similar bit error rate performance to single mode fibers when transmitting and multi-mode fibers when receiving enabling a symmetric duplex FSOL reducing SWaP.

We describe Fibertek’s progress toward commercializing space laser communications and new features of our secondgeneration compact laser communication terminal (LCT). The LCT design is modular, flexible and can accommodate a variety of waveforms and data formats. Fibertek has a unit deployed in space for initial testing followed by additional units for more broad-based market applications. Our first-generation optical telescope assembly was originally designed for NASA Deep Space CubeSat laser communications. It was customized as a complete commercial LEO LCT system which is 2U in size, 2 kg in mass, and provides Gbps data rates. The optical transceiver has a shared transmit/receive optical path that uses a laser beacon to ensure high pointing accuracy, active control of the pointing stability, and ensures a strong optical signal-to-noise ratio (SNR) during link operation. The terminal has been manufactured and tested, providing high accuracy pointing and low jitter. Our second generation LCT system features bidirectional operation and support for an eyesafe beacon for uplink applications. Bi-directional operation is attractive for inter-satellite links (ISL), uplinks of data, pointing acquisition and tracking (PAT), position, navigation and timing (PNT), and for telemetry, tracking, and command (TTandC). The eyesafe uplink beacon makes it easier to get FCC authorization for operation. The LCT includes a 64 mm telescope and a 1.5-μm fiber-amplifier with >2 W optical power that enables future updates to allow operation up to GEO orbit with the addition of SCPPM and 10-100 Gbit/sec.

Clouds are a key driver in the performance of free-space optical communication (FSOC) systems. Clouds are composed of liquid water and ice crystals, and frequently produce atmospheric fades easily exceeding 10 decibel (dB). In these cases, impacts on FSOC systems may be severe even with efficient codes. On the other hand, there are times when transmission loss due to clouds is less than three dB as a result of thin, ice crystal based cirrus. In these cases, optical transmissions may still be possible, resulting in a high performing network. The ability to characterize the distribution of clouds and their transmission effects are critical in order to quantify the performance of FSOC ground networks. A high-end, state-of-the-art, cloud retrieval system has been developed using geostationary visible and infrared imagery. The database contains global retrievals for a 23-year period (1997 to present) at high spatial (4km) and temporal resolution (15 minute). The Lasercom Network Optimization Tool (LNOT) is used along with a space architecture and the cloud database to optimize the configuration of sites including placement that maximizes performance. LNOT factors in the impacts of mission data collection, onboard storage, data rates and link margin for clouds. Thousands of simulations have been performed that indicate that with just a few sites, FSOC performance, as measured by the percent data transferred metric, meets and exceeds that of radio frequency transmissions. Various studies will be presented at the conference.

In order to stably achieve high throughputs in the satellite-to-ground free space optical communication systems, suppression of the influence of the fading due to atmospheric turbulence is an important technical subject to be overcome. Our group has proposed the use of Adaptive Distributed Frame Repetition (ADFR) as a countermeasure of this problem, which enables the seamless switching between the features of high-speed transmission and high tolerant link connection in response to the transmission link conditions. We have developed the test equipment of the functions whose interface was 10GbE, and demonstrated the improvements of the packet error ratio and the throughput against the varieties of the fading patterns. The settings of the ADFR should be carefully determined with the considerations of several trade-offs. The efficient suppression of PER enabled by the excess repetition and interval sacrifice the transmission bandwidth and the system latency, respectively. In this time, we develop a function that extracts the fading characteristics of the moment from the decimated signals after transmission, and, by using this information, realize the continuous optimizations of the ADFR settings to the time-changing fading patterns. The details of the estimation method and the demonstrations with this technique will be presented at the conference.

The continuing need to miniaturize mechanisms with wide range of motion for use in free-space optical communications has motivated the design of a low size, weight, and power (SWaP), two-axis gimbal with an optical fiber wrap as the key enabling feature. Our efforts to design a small gimbal with 100 micro-radian pointing accuracy for free-space optical communications have resulted in an unconventional optical fiber wrap design in order to achieve the low optical noise needed to meet system performance goals. Traditionally, fiber optic leads are installed in a stationary configuration to ensure maximum life and performance for the component. The fiber wrap design employed by Applied Technology Associates utilizes a combination of supplier design specifications and “mechanical spring” design techniques to construct a dynamic, innovative fiber mechanism, with life expectancy scaled to expected on-orbit operations and with negligible performance degradation. An engineering mockup was created to test both life expectancy and polarization performance at accelerated lifetime rates to verify the design. Presented in this paper is the design approach, test configuration approach, resulting lifetime testing (from cyclical stress testing), and polarization performance test outcomes. The polarization performance test outcomes show that the design results exceed planned lifetime goals, and maintain optical performance throughout the testing process. These test results confirm that fiber wrapping is a viable and available tool for miniature mechanisms in compact optical communications gimbals.

The National Aeronautics and Space Administration (NASA) Glenn Research Center (GRC) has developed a laboratory transmitter and receiver prototype of a space-to-ground optical communications link. The system is meant to emulate future deep space optical communication links, such as the first crewed flight of Orion, in which the transmitted laser is modulated using pulse position modulation and the receiver is capable of detecting single photons. The transmitter prototype consists of a software defined radio, a high extinction ratio electro-optic modulator system, and a 1550 nm laser. The receiver is a scalable concept and utilizes a single-pixel array of fiber coupled superconducting nanowire single photon detectors. The transmit and receive waveforms follow the Consultative Committee for Space Data Systems (CCSDS) Optical Communications Coding and Synchronization Standard. A software model of the optical transmitter and receiver has also been implemented to predict performance of the optical test bed. This paper describes the transmitter and receiver prototypes as well as the system test configuration. System level tests results are presented and shown to align with predictions from software simulations. The validated software model can be used to in the future to reduce the design cycle of optical communications systems.

The information capacity of an optical channel under power constraints is ultimately limited by the quantum nature of transmitted signals. We discuss currently available and emerging photonic technologies whose combination can be shown theoretically to enable nearly quantum-limited operation of a noisy optical communication link in the photon-starved regime, with the information rate scaling linearly in the detected signal power. The key ingredients are quantum pulse gating to facilitate mode selectivity, photon-number-resolved direct detection, and a photon-efficient high-order modulation format such as pulse position modulation, frequency shift keying, or binary phase shift keyed Hadamard words decoded optically using structured receivers.

Sustainable photonic communication systems can resolve the global digital divide. Free-space optical (FSO) systems offer the ability to distribute high speed digital links into remote and rural communities where terrain, installation cost or infrastructure security pose critical hurdles to deployment [1]. We will discuss the development of a low-cost FSO system prototype that could allow for out of the box self aligning optical systems that requires no specialist engineer for installation. Our prototype system is based on commercial telescope mount, which is controlled by Raspberry Pi 3 (RPi) compact computer and incorporates 4 spatially multiplexed channels, each with an integrated 1-Gbps small form factor pluggable (SFP) transceiver. Using the global positioning system (GPS), the location of the transceiver is determined and communicated to near by transceivers through low-speed radio link operating at 446-Mhz. The low-speed radio link supports the communication of automated alignment instructions between the remote transceivers. To perform the alignment, we adopt a spiral path alignment method widely for used for inter-satellite and ground-to-space optical communication systems [2, 3]. To facilitate this alignment method both the Transmitter and Receiver systems are equipped with a laser beacon, which is detected by CCD camera located on the external casing of each transceiver. The system automatically completes three stages of alignment to fully align a duplex spatially multiplexed FSO link. In our experimental test of the system over both 15m and 200m, we measured the total link loss to be 10dB and 15dB respectively and demonstrate error free-transmission at 1 Gbps per channel. References: 1. Lavery et al., “Tackling Africa’s digital divide,” Nature Photonics 12, 249–252 (2018). 2. G. Baister et al., “Pointing, acquisition and tracking for optical space communications," Electronics & Communication Engineering Journal, vol. 6, pp. 271-280 (1994). 3. T. Nguyen at al., "Development of a pointing, acquisition, and tracking system for a CubeSat optical communication module," Proc. SPIE 9354, Free-Space Laser Communication and Atmospheric Propagation XXVII, 93540O (2015).

The NASA Glenn Research Center's development of a high-photon efficiency real-time optical communications ground receiver has added superconducting nanowire single-photon detectors (SNSPDs) coupled with few-mode fibers (FMF). High data rate space-to-ground optical communication links require enhanced ground receiver sensitivity to reduce spacecraft transmitter constraints, and therefore require highly efficient coupling from fiber to detector. In the presence of atmospheric turbulence the received optical wavefront can be severely distorted introducing higher-order spatial mode components to the received signal. To reduce mode filtering and mismatch loss and the resulting degradations to detector coupling efficiency, we explore the use of few-mode fiber coupling to commercial single-pixel SNSPDs. Graded index 20-μm few-mode fibers allow the commercial single pixel SNSPD's active area to couple with equal efficiency as single mode fibers. Here we determine detector characteristics such as count rate, detection efficiency, dark counts, and jitter, as well as detection efficiencies for higher-order fiber spatial modes. Additionally, we assess the laboratory performance of the detectors in an optical system which emulates future deep space optical communications links.

Free space optical links offer secure and mobile high-data-rate communication. However, the availability and reliability of long-range links are restricted by turbulence-induced fading. One way of mitigating turbulence is to use spatial diversity. However, classically-used multi-aperture systems often present a large footprint that is detrimental to their implementation. In this work, we show that using a spatial mode demultiplexer enables a significant increase in signal collection at the receiver in the presence of strong atmospheric turbulence while requiring only one telescope and being compatible with standard single-mode fiber (SMF) based telecom components. A Gaussian beam propagating through the atmosphere suffers from perturbations which modify the spatial mode of the beam and therefore causes severe fading of the signal when coupling into an SMF. These beam perturbations can be decomposed on a limited number of orthogonal spatial modes: we show that 3 to 15 Hermite-Gaussian modes are sufficient to cover most high order turbulence effects. The Multi-Plane Light Conversion technique enables to efficiently implement a spatial mode demultiplexer designed to take the selected perturbation modes in input (coupled in free-space or in a multi-mode fiber) and convert them into SMFs at the output. By detecting then combining these output signals, one can passively collect the full energy of the perturbed beam, therefore strongly increasing the coupling efficiency under turbulent conditions. We demonstrate that this approach offers a similar level of fading mitigation compared to a multi-aperture approach, using uncorrelated spatial mode channels instead of uncorrelated single-mode paths.

For free-space optical (FSO) communications through the turbulent atmospheric channel, multi-spatial-mode photoncounting detectors can provide an attractive high-sensitivity receiver solution. However, multi-mode detection also increases optical background noise, which can degrade the overall system performance. Narrow band optical filtering becomes an important background rejection tool that can enable good performance in background-limited conditions. Here, we report the development of low-insertion-loss high-contrast-ratio optical filters at 1.55μm that are well suited for FSO communications. The filter has less than 0.7 dB insertion loss, 46 dB contrast ratio, 2.9 GHz 3-dB bandwidth, and 3.5 GHz noise-equivalent bandwidth, which is well matched to 1 GHz return-to-zero (RZ) pulsed waveforms. Other factors that influence filter performance and stability such as angular and temperature dependencies are characterized. Additional theoretical analysis and experimental results highlighting the trade-offs between spectral bandwidth, spatial bandwidth, filter insertion loss, and temperature-induced performance changes are presented. When coupled with photon-counting detection, these filtering techniques can enable sensitive FSO receiver operation, even in the presence of significant background.

Photonic lanterns provide an efficient way of coupling light from a single large-core fiber to multiple small-core fibers. This capability is of interest for space to ground communication applications. In these applications, the optical ground receivers require high-efficiency coupling from an atmospherically distorted focus spot to multiple fiber coupled single pixel super-conducting nanowire detectors. This paper will explore the use of photonic lanterns in a real-time ground receiver that is scalable and constructed with commercial parts. The number of small-core fibers (i.e. an array of single or few-mode cores) that make a photonic lantern determines the number of spatial modes that they couple at the larger multimode fiber core end. For instance, lanterns made with n number of single-mode fibers can couple n number of spatial modes. Although the laser transmitted from a spacecraft originates as a Gaussian shape, the atmosphere distorts the beam profile by scrambling the phase and scattering energy into higher-order spatial modes. Therefore, if a ground receiver is sized for a target data rate with n number of detectors, the corresponding lantern made with single-mode fibers will couple n number of spatial modes. Most of the energy of the transmitted beam scattered into spatial modes higher than n will be lost. This paper shows this loss may be reduced by making lanterns with few-mode fibers instead of single-mode fibers, increasing the number of spatial modes that can be coupled and therefore increasing the coupling efficiency to single pixel, single photon detectors. The free space to fiber coupling efficiency of these two types of photonic lanterns are compared over a range of the free-space coupling numerical apertures and mode field diameters. Results indicate the few mode fiber lantern has higher coupling efficiency for telescopes with longer focal lengths under higher turbulent conditions. Also presented is analysis of the jitter added to the system by the lanterns, showing the few-mode fiber photonic lantern adds more jitter than the single-mode fiber lantern, but less than a multimode fiber.

Research and development of a novel method for a secure free-space optical communication system has been done in NICT since 2018, and demonstration experiments between an aircraft and a transportable optical ground station are planned in near future. In order to establish a stable and highly accurate optical communication link, the system must have a fine pointing mechanism in both the aircraft and the ground station. A compact and light-weight tracking system is required to be mounted on the aircraft, and there will be needed to have an adjustment function of the beam divergence control to allow stable communication under various altitude and atmospheric conditions. The transportable optical ground station should maintain vibration resistance when moving, and it must be easily deployed on each site where is the appropriate optical ground station site with respect to atmospheric turbulence condition.

The most important single attribute of a Laser Communications Ground Station is the receiver aperture area to maximize the received signal and hence signal-to-noise ratio (SNR). However, the larger the aperture, the greater the negative effects imparted by the atmosphere on the signal, thus causing signal fading and negatively effecting SNR. To mitigate the atmospheric effects of a large aperture, adaptive optics are needed. It has been previously proposed to use a number of smaller telescopes with only simpler tip/tilt correction with non-coherent power combining as a lower cost way to achieve the benefits of a large signal energy capture area without the significantly higher cost of a single large telescope with adaptive optics. This paper will investigate optimal trades of the number and size of individual telescopes to achieve a desired signal capture area of a single large telescope with adaptive optics. The cost of the telescopes, extra beam combining, and especially the atmospheric effects as a function of the size of a telescope with only tip/tilt correction will be addressed.

Optical communication Technologies are considered to be one of the next major revolutions in satellite communication, bringing unprecedentedly high levels of transmission rates, data security and resilience. However technical developments and early implementations cannot demonstrate its full capabilities, as the optical solution is mainly used in nonoptimized (SatCom) systems. To address the system level aspects ESA and its member states have implemented the operational European Data Relay System (EDRS) providing routine Quasi-Real-Time-Data Services to the European Commission Copernicus satellite fleet. Furthermore, a dedicated programme for Optical Communication was created called "ScyLight" which stands for a "SeCure and Laser communication Technology" Framework Programme. To integrate satellite and terrestrial networks ESA is now preparing its next logical step in optical communication systems by creating the elements for a High Throughput Opticial Network called HydRON. In HydRON optical interconnections in the Tbps (Terabit per second) region will be established including "All-Optical payloads” providing the means for a truly "Fibre in Space" network. Technically speaking HydRon is aiming for Tbps "All-Optical Network” solutions, dividing the satellite payload into a network part and an application part - similar to optical fiber networks on ground. The application is hooked to the network. HydRON will prepare Optical Feeder uplinks into a network of in orbit Technology Demonstrators (called HydRON#1, #2, etc.), which will be interconnected by means of Tbps laser intersatellite links. WDM Laser terminals (ground/space) and optical routing capabilities on-board the network nodes in space will be implemented together with optical payloads to enable a high throughput network connection to the applications. The space network concept will reduce the dependency on single ground stations as all HydRON nodes will 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. HydRON shall not be seen as THE solution for all, but shall give a platform to demonstrate the capabilities of multiple industry players and to prepare for the future: a European/Canadian SHOW CASE on Optical Communications!

This paper describes progress toward a space -based 51 W average power amplifier for deep space PPM and Earth GEO links. We demonstrated a broadband WDM amplification at 50W with flat gain across a 25 nm bandwidth. Similarly, for 5 W amplifier we demonstrated a flat gain across a 32 nm bandwidth. These amplifiers demonstrate the feasibility for multi-channel space optical communications links. To increase the bandwidth GEO links to multi-Tbps and deep space links to > Gbps. The laser supports kW/channel SBS limited peak power for PPM and achieves an optical-to-optical efficiency of > 40%. In a separate but related effort for a deep space uplink beacon, we achieved 500 W average power, 2.6 kW peak power PPM (2,2) for a 1 μm uplink transmitter. Reliable SBS free operation is achieved with phase modulation resulting in 26 GHz transmitter linewidth. Uplink transmitter is optimized for 65 usec (pulsewidth) slot size—achieving fastest possible rise/fall times (<10 usec) and pulse uniformity.

We report on the design, development, and testing of the high-power Laser Transmitter Assembly (LTA) supporting the Deep Space Optical Communications (DSOC) demonstration hosted on the Psyche Discovery class mission, due to launch in 2022. The DSOC project, under development by NASA’s Jet Propulsion Laboratory, will test space-to-ground high-bandwidth laser communications while en route to the Psyche-16 asteroid in the main asteroid belt, in what will be the longest range high rate optical communications link in history. The LTA is based on a master-oscillator power-amplifier optical architecture, using highly-efficient cladding-pumped amplification. The transmitter is designed to deliver average optical output powers <4 W at 1550 nm for low power consumption data links at <100 Mbps. The output signal operates across multiple pulse-position modulation (PPM) orders and pulse-widths to optimize the space-to-ground link. The architecture is designed for high-reliability and radiation hardness, and features hardware interlocks and secondary signal/pumping paths to reduce single points of failure. We also detail the effective management of optical nonlinearities which could damage the LTA or impact the communications link. These include the suppression of stimulated Brillouin scattering, self-phase modulation, and pulseto- pulse energy variation (PEV), which arises from the gain dynamics of the power amplifier, and will manifest when the LTA is configured for large pulse energies and long inter-pulse delays. The LTA also incorporates hardware and software controls to enable autonomous operation, including closed-loop control of intra-stage and output power levels, modulator bias control, and detailed reporting of LTA status through telemetry.

Pumping solid state lasers in LCTs requires the application of highly reliable, low-noise semiconductor lasers. Two design variants of pump lasers have been developed and tested. The first design consists of broad area laser arrays, spectrally stabilized by an external Bragg grating. Those lasers exhibit decent reliability and thus they are utilized in various space missions. The drawback of that design is due to hardly controllable intensity noise at certain operating conditions induced by optical feedback. To overcome this drawback, DBR RW arrays with monolithically integrated Bragg grating have been optimized aimed at low noise performance and high reliability over an extended operating time. Life test results indicate that the reliability goal can be achieved by careful preselection of the devices and eventually by increasing the number of active emitters.

High power optical amplifiers (HPOAs) are utilized in many high reliability applications. The need for greater bandwidth in flight and space missions is driving the development of free space optical transmission systems which require high reliability HPOAs over mission lifetimes which may exceed 10 years. One challenge in developing such models is that typically when designing HPOAs, redundancy and derating approaches are utilized to augment reliability and avoid single points of failure. In these designs, redundant components experience a stress profile which depends on statistical failure probabilities of its sister components. For example, in the case of HPOA gain stages, typically several multimode pumps are combined to pump a gain fiber. These multimode pumps are typically run derated when the mission begins, but in the event of a component failure, the power to each remaining operational pump is increased to maintain constant output. As such it is more important to characterize when the entire ensemble of pumps fails. Accurate models for ensemble reliability require an approach which accounts for time and stress profile dependent failure rates which are hard to access experimentally (due to high component reliability) or analytically. In order to address this problem, we have utilized a Monte Carlo simulation approach which can rapidly simulate ensemble failures for any given stress profile. By running several simulations, we are able to build up the failure function for the ensemble, thus providing a more reliable model for failure rate for the ensemble. This failure model can be used to build a more accurate picture for HPOA reliability.

Free-space optical communications in space offer many benefits over established radio frequency based communication links; in particular, high beam directivity results in efficient power usage. Such a reduced power requirement is particularly appealing to small satellites with strict size, weight and power (SWaP) requirements. In the case of free-space optical communication, precise pointing, acquisition and tracking (PAT) of the incoming beam is necessary to close the communication link. Due to the narrow beam of the laser, the critical task of accomplishing PAT becomes increasingly arduous and often requires complex systems of optical and processing hardware to account for relative movement of the terminals. Recent developments in body pointing mecha- nisms have allowed small satellites to point with greater precision. In this work, we consider an approach to a low-complexity PAT system that utilizes a single quad-cell photodetector as an optical spatial sensor, and exploits the body pointing capabilities of the spacecraft to perform the tracking maneuvers, eschewing the need for additional dedicated optical hardware. We look at the PAT performance of this approach from a systems analysis viewpoint and present preliminary experimental results. In particular, we examine the implementation of the system on NASA's TeraByte InfraRed Delivery (TBIRD) demonstration.

This paper reports on a two-color acquisition beam with continuously variable divergence for free-space laser communication links. This approach is useful for terminals using wavelength-separated beacon and communi- cation signals amplified by a common high-power optical amplifier (HPOA). The acquisition beam features a controllable power-split ratio between the two waveforms that varies during the acquisition sequence. With this scheme, the area of the acquisition beam can be expanded by a factor of 100, while maintaining power levels of the two colors within a specified range on an acquisition sensor in a partner terminal. An optical transmitter produces beacon and communication waveforms from distinct master oscillators, and balances these signal levels during a multi-step transition sequence to achieve the desired relative power levels at each wavelength. An HPOA then boosts this transmitter signal to power levels necessary to overcome link losses. The HPOA and transmitter adjust power levels during the sequence to maintain beacon and communication irradiances within specified ranges. A divergence setting assembly (DSA) simultaneously adjusts the beam width from a 10x broadened beacon to a narrow, diffraction-limited beam. We demonstrate control of the on-axis beacon power to within a 10-dB dynamic range while the beacon area varies by a factor of approximately 20 dB. This paper describes the hardware and software control of the various units used to perform acquisition.

The backend optical assembly module for a space-based, laser communication terminal is presented. The backend optical assembly utilizes voice coil-fast steering mirror technology embedded into a control loop that both provides terminal-level pointing capability, and maintains receive channel fiber coupling. The fast steering mirror technology presents a technical solution for operating within the space environment, while simultaneously meeting the bandwidth requirements for characteristic satellite vibration profiles. The system’s architecture design meets the demands of onplatform, jitter-rejection performance to establish and maintain a communication link.

Optical satellite links have gained increasing attention throughout the last years. Especially for the application of optical satellite downlinks. Within the OSIRIS program, DLR's Institute of Communications and Navigation develops optical terminals and systems which are optimized for small satellites. After the successful qualification and launch of two precursor terminals, DLR currently develops OSIRISv3, a 3^rd generation OSIRIS terminal with up to 10 Gbps downlink rate, and OSIRIS4Cubesat, a miniaturized version optimized for Cubesat Applications. The University of Stuttgart's Institute of Space Systems develops small satellites, which are used to demonstrate novel technologies in the Space domain. Together, DLR and University of Stuttgart integrated the first OSIRIS generation onboard the Flying Laptop satellite, which was launched in July 2017 and has been successfully operated since. This paper will give an overview about DLR's OSIRIS program. Furthermore, it will show first results of OSIRISv1 on Flying Laptop. Therefore, the Flying Laptop satellite and OSIRISv1 will be explained. Preliminary results from the validation campaign, where optical downlinks have been demonstrated, will be given.

In this presentation, we discuss the first demonstration of a lasercom downlink from a LEO 1.5U CubeSat to our optical ground station at The Aerospace Corporation in El Segundo, CA. Two vehicles, AC7-B&C, were built under NASA’s Optical Communications and Sensors Demonstration (OCSD) which is a flight validation mission to test commercial-off-the-shelf components and subsystems that will enable new communications and proximity operations capabilities for CubeSats and other small spacecraft. As designed, the 1.5 U CubeSats weigh 2.3 kg and consume ~2 W during most of the mission life. During lasercom engagements, ~3 minutes, the spacecraft consumes an additional 10-20 W power depending on the set point of the laser transmitter, which yields 2-4 W at 1.06 m. The transmitter consists of a directly modulated laser diode followed by a Yb fiber amplifier and exhibits an overall wall-plug efficiency ~20%. The AC-7B&C vehicles were launched in November 2017 and placed in a 450-km circular orbit. Following on-orbit checkouts and preliminary pointing calibration utilizing on-board star trackers, we have demonstrated (at the time of this submission) first time communications downlinks up to 100 Mbps from the 7B vehicle using open loop pointing (beaconless) to our ground terminal, which is near sea level. The preliminary link experiments at 50 and 100 Mbps (OOK/PRBS23) using the AC-7B CubeSat were recorded at 100 ms intervals. At 50 Mbps, error rates near 1E-6 were observed with numerous error free intervals. At 100 Mbps we observed BERs approaching 1E-6. At the time of these collects, however, the B vehicle was still exercising a scan pattern since the final alignment had not been completed. Thus, the optical link was not continuous over the entire pass. Link budget estimates indicate that lower BERs should be achievable and we will continue to assess the link performance as the system is optimized.

With the start of the European Data Relay Service (EDRS) in June 2016, laser communication in space has entered a commercial application. The focus of the EDRS laser communication system is the low latency relay of earth observation (E/O) data from low earth orbiting space crafts. The anchor customers for EDRS in its initial phase are the Sentinel spacecrafts, constantly generating E/O data products in the framework of the European Union´s Copernicus Program. The vast majority of these data are in open access for the general public. Tesat is contracted for trending analysis and in orbit maintenance for the four Sentinel LCTs. The standard data delivery from the Sentinel 1 and Sentinel 2 spacecrafts is via X-band to polar stations and Italy and Spain, however using a LCT-GEO relay creates a “virtual ground station” and enables data transfer were no RF ground station is in the field of view of the space craft.

Broadband internet access has become a vertex for the future development of society and industry in the digital era. Geostationary orbit (GEO) satellite can provide global broadband coverage, becoming a complementary solution to optical fiber network. Low-earth-orbit (LEO) constellations have been proposed in the last years and they may become a reality soon, but still based on radiofrequency for the ground-to-satellite links. Optical technologies offer multiple THz of available spectrum, which can be used in the feeder link. The DLR’s Institute of Communications and Navigation has demonstrated Terabit-per-second throughput in relevant environment for GEO communications, in terms of the turbulent channel. In 2016 DLR set the world-record in freespace communications to 1.72 Tbit/s, and in 2017 to 13.16 Tbit/s. Two terminals, emulating the satellite and the ground station have been developed. Bi-directional communications link with single-mode-fiber coupling at both ends was demonstrated. Adaptive optics for the downlink and uplink (pre-distortion) improved the fiber-coupling in downlink and decreased signal fluctuations in uplink. A 80 Gbit/s QPSK system based on digital homodyne reception was also developed, demonstrating the use of coherent communications under strong turbulence conditions. These activities were performed in the frame of two internal DLR projects, THRUST and Global Connectivity Synergy project. Several measurement campaigns took place in the last years in a valley-to-mountain-top test-link. Turbulence has been monitored at both ends and the point-ahead-angle has been emulated by separating the downlink beacon from the receiving aperture. An overview of the system and the main results will be presented.

Pyramid wavefront sensors are widely used in adaptive optics system, and in particular in extreme adaptive optics applications, due to their advantages in terms of sensitivity with respect to other wavefront sensors such as Shack Hartmann or curvature wavefront sensors. In particular, as the adaptive optics loop is closing, the reference star image size gets smaller on the pyramid tip, providing increased sensitivity. However, non common path aberrations are usually present due to light splitting between wavefront sensor and scientific optical beams, because of the presence of different optical elements. This causes image distortions that can hinder the scientific throughput and are then usually corrected by introducing offsets on the pyramid closed loop control systems. This increases the size of the star image at the pyramid tip, leading to a decrease in sensitivity that compromises the aforementioned advantages. In this work we aim at correcting the non common path aberrations by inserting a multi actuator deformable lens in the sensing path to recover the optimal working conditions and the ideal gain. In particular, we aim at demonstrating that it is possible to insert the multi actuator deformable lens in the sensing arm and drive it while the main adaptive optics loop is working, maximizing the scientific image sharpness. The use of this lens avoids changes in the optical configuration, providing a simple, yet effective way to correct for non common path aberrations in existing setups.

To date, undersea optical communication has been driven by wide-beam LED systems. Directional laser systems have several advantages | increased range, increased data rate, and better performance in solar background | but require a precise tracking system to maintain laser pointing through vehicle motion. We have demonstrated an underwater laser communication system with a bi-direction, all-optical pointing, acquisition, and tracking system. Laser communication terminals were mounted on two remotely operated vehicles that were piloted to the ends of a pool (a separation of 20 m), coarsely aligned to within about 10 degrees, and then set to autonomously acquire and track each other. Acquisitions occurred within a few seconds, and the link never broke during maneuvers. To our knowledge this is the first demonstration of a functional undersea laser tracking system between mobile vehicles. The demonstrated precision and robustness can enable 1+ Gbps data links between independent, moving vehicles, over several 100 meters in clear ocean water. Additionally, this approach provides precise (cm- class) relative positioning between the communicating parties, enabling relative position, navigation, and timing (PNT) distribution between independent vehicles. This technology is a crucial enabler of undersea wireless optical networking for manned and unmanned vehicles.

Flexible multi-rate capabilities are attractive for communications over the dynamic free-space optical (FSO) channel since it enables links to be efficiently established over a variety of distances, channel conditions, aperture sizes, and transmitter power levels. Recently, multi-rate differential phase shift keying (DPSK) modulation has been shown to provide good performance with modest complexity over a wide range of data rates. For these reasons, multi-rate DPSK has become the high-rate baseline format for NASA’s Laser Communication Relay Demonstration (LCRD) and other space-based FSO systems currently in development, operating at channel rates from 72 Mbit/s to 2.88 Gbit/s. Here, we report WDM-scalable power-efficient optical-transceiver advances that can extend single-channel rates by a factor of four to 11.52 Gbit/s using LCRD-compatible waveforms. Transmitter waveforms are implemented using time-frequency-windowed directly modulated lasers (DMLs) that can generate burstmode 2-DPSK and 4-DPSK waveforms at the standard LCRD 2.88 GHz symbol rate and twice the rate at 5.76 GHz. At the receiver, standard 2.88 GHz delay-line-interferometer (DLI) based demodulation is implemented for 2-DPSK. For 4-DPSK, a second 2.88 GHz DLI is required to demodulate the 2-bit-persymbol format, with the pair of DLIs biased at ±/4. With proper precoding of the transmitter waveforms, the double-rate 5.76 GHz symbols may be demodulated using the same 2.88 GHz DLIs using non-adjacent DPSK demodulation. In this manner, a common DPSK receiver platform may be used to demodulate standard multi-rate LCRD waveforms as well as 4-DSPK waveforms at 1x and 2x the standard symbol rate.

We assess the viability of a state-of-the-art 100G/200G commercial optical coherent DSP ASIC (16 nm FinFET CMOS technology) for space applications through heavy ion testing to (1) screen for destructive SELs and (2) observe for nondestructive heavy ion SEEs on the ASIC. The ASIC was exposed to heavy ion radiation while operating both optically noise-loaded uplink and downlink to an optical “ground” modem. There were no destructive SEEs, such as SELs, observed from the heavy ion radiation test campaign.

We present the transponder design and verification to obtain over 10 Gbit/s capability for an optical feeder link. The transponder employing a transmitter and a receiver supports differential phase shift keying (DPSK) optical communication. In order to provide receiver stability against optical power fluctuations, we utilize the terrestrial technology and devices such as clock data recovery (CDR), a hard-decision forward error correction (FEC) and temperature control of delay line interferometer (DLI) to realize the enhancement of dynamic range of received optical power. We have also investigated reliability of terrestrial devices including photonics integration circuit (PIC) type DLI and CDR for satellites.

Free-space optical communication technology has always been touted as a solution for high data-rate communication needs where RF and Microwave systems reach their capacity limits. In many respects, optical communication technology meets the challenge but has its own limitations. RF, Microwave and millimeter-wave researchers have advanced the state-of-the-art and achieved the types of data-rates that until this decade were available only from laser communications (lasercom). The aim of this paper is a tutorial for the lasercom community to become better aware of the competing technologies that are either available or are at the verge of becoming commercially available.

The security of sensitive information exchange has become a major topic in recent years. Quantum Key Distribution (QKD) provides a highly secure approach to share random encryption keys between two communication terminals. In contrast with traditional public cryptography methods, QKD security relies on the foundations of quantum mechanics and not on computational capabilities. This makes QKD unconditionally secure (if properly implemented) and it is envisaged as a main component in the next–generation cryptographic technology. QKD has already been successfully demonstrated in different contexts such as fibre-to- fibre, and free-space ground-toground as well as ground-to-air communications. However, Size, Weight and Power (SWaP) constraints have prevented previous implementations to be demonstrated on small form airborne platforms such as Unmanned Aircraft Systems (UAS) and High Altitude Pseudo-Satellites (HAPS). Project Q-DOS aims to deliver a QKD module using compact, cutting-edge photonic waveguide technology, which will allow low-SWaP aerospace requirements to be met. This module uses 1550 nm single photons to implement a BB84 protocol, and will enable the demonstration of a secure, high-speed optical communication data link (~0.5 Gbps) between a drone and a ground station. The targeted link range is 1 km. The airborne communications module, including the QKD terminal, tracking modules, traditional communications systems, optics and control electronics, must not exceed a mass of 5 kg and a power consumption of 20 W.

Time-energy entangled photon pairs are created by a system consisting of a 1064 nm pump diode laser that is fiber coupled to a high generation rate photon pair source. The source is a dual element periodically poled Magnesium Oxide doped Lithium Niobate (MgO:LN) waveguide that upconverts 1064 nm photons to single 532 nm photons in the first stage. In the second stage, the green photons are down converted to time-energy entangled photon pairs at 794 nm and 1614 nm. The output photon pairs are guided by fiber to sorting optics where they are separated and sent into high-efficiency photon detectors. In particular, the 1614 nm photons are detected by a superconducting nanowire with efficiency near 85% and dead time less than 30 ns. Detector output electrical signals are sent to a time tagger with bin resolution as narrow as 25 ps for coincidence counting. The ultimate goal of this setup is to demonstrate a single-source, high efficiency, high data rate, low noise, quantum communication system to enable Earth-space quantum networks. Test results that characterize the time-energy entangled photon pair creation rates of our source will be presented, via measures of accidental and true coincidence rates versus pump current. To reduce noise (accidentals) as much as possible, and for better understanding of our overall quantum system path-efficiency, studies of fluorescence caused by our pump’s 1064 nm and 532 nm photons will be investigated and discussed. Finally, characteristic measurements of our superconducting nanowire detector, such as dead time and detection efficiency versus electrical bias, will be offered. Please verify that (1) all pages are present, (2) all figures are correct, (3) all fonts and special characters are correct, and (4) all text and figures fit within the red margin lines shown on this review document. Complete formatting information is available at http://SPIE.org/manuscripts Return to the Manage Active Submissions page at http://spie.org/submissions/tasks.aspx and approve or disapprove this submission. Your manuscript will not be published without this approval. Please contact author_help@spie.org with any questions or concerns.

Quantum Key Distribution (QKD) will be the first applications for quantum technologies, addressing information security needs by providing a tool for quantum encryption. An Entangled Photon Source EPS, the key building block for QKD, is presented that is based on a non-linear, Spontaneous Parametric Down Conversion SPDC process in a temperature controlled ppKTP crystal that is double-side pumped at 405 nm in a sagnac interferometer setup. The polarization entangled photons are emitted at 810 nm and at a rate of up to ca. 300k per second at 8 mW pumping power. Visibilities in the HV and the DA polarization base in the range of 97-99% have been measured. The EPS integration was realized in a ruggedized, minimized setup to prove space-suitability during thermal and mechanical environmental testing.

Quantum Key Distribution (QKD) is a communication method which exchanges secret keys using cryptographic protocols involving elements from quantum science. Continuous Variable (CV) QKD is a method to implement key exchange using sampling of Gaussian signals. Reconciliation in CV-QKD is fundamentally realized via coding of the Alice or Bob binary labels of the Gaussian samples using either one-way or interactive communications between the parties, Alice and Bob, over a public authenticated channel. We assume that the communication is performed through an Optical Wireless (OW) or Free Space Optics (FSO) channel. In that case the received signal suffers from stochastic fading due to pointing jitter or atmospheric turbulence. As a result of the channel fading and noise Alice and Bob Gaussian samples will not match. Information Reconciliation (IR) is the phase of the CV-QKD protocol that makes sure that Alice and Bob agree on a common and identical labeling of their samples, i.e. agree on a common stream of bits that we denote as “reconciled key.” The information reconciliation in CV-QKD could be done by one-way or two-way channel coding to correct the actual labels so that they do match. To do so Bob generates a sequence of parity bits using a systematic code. These bits are sent to Alice over an authenticated public channel. Alice then uses her own sequence of labels she obtains after quantization with the redundancy provided by Bob to recover Bob’s binary sequence. In this work we analyze the problem of information reconciliation for continuous variable quantum key distribution over a free space optics channel.

Current satellite communication systems based on RF technology are often limited by the available spectrum. Free-Space Optical Communication Links are a promising solution to overcome this bottleneck. The available spectrum enables Terabit-per-Second data rates, and the point-to-point nature of FSO links makes spectrum regulation unnecessary. Therefore, the use of optical links is investigated in a number of scenarios, as e.g. for GEO feeder links serving multimedia applications. Transmitter Diversity might be a solution to overcome atmospheric impacts on the ground-to-satellite transmission in an optical GEO feeder link application. Transmitter Diversity can e.g. be implemented by using various spectral channels. Despite the virtually unlimited bandwidth available in atmospheric transmission windows, practical implementations of an optical GEO feeder link need to use available hardware, namely fiber amplifiers in the C- and L-band – Therefore, a more efficient use of the available spectrum will enable in a higher system throughput. In this paper, we present a bandwidth efficient transmitter diversity scheme, called "phase-division in bit-time". It is based on Intensity Modulation with Direction Detection, and makes use of phase modulation for the transmitters to mitigate the impact of atmospheric phase distortions on the quality of the received signal.

Optical link from ground to space is susceptible to turbulence induced fades that can last from a fraction of millisecond to longer than 10ms. Unmitigated, the fade can lead to frequent drop outs and unacceptable link performance. In this paper, we describe the system design tradeoff for various fading mitigation approaches, and concluded that channel coding is the most viable approach to provide reliable data transmission. The paper then discusses the design trades for a channel coder/interleaver for an optical ground-space feeder link.

Optical communications will complement radio frequency (RF) communications in the coming decades to enhance throughput, power efficiency and link security of satellite communication links. To enable optical communications technology for intersatellite links and (bi-directional) ground to satellite links, TNO develops a suite of technologies in collaboration with industry. Throughout these developments there is a particular aim for high levels of system integration, compactness and low recurring cost in order to meet the overall requirements related to market viability. TNO develops terminals with aperture sizes of 70 and 17 mm, coarse pointing assemblies and fast steering mirrors. This paper discusses the state of development of these different technologies and provides and outlook towards the future.

The European Data Relay System (EDRS) is operational, optically transferring data from currently four LEO Earth observation satellites to the geo-stationary EDRS-A spacecraft at 1.8 Gbps. The demand has increased to extend these point-to-point optical links towards a full optical network in space and enable high data rate links between space assets and between ground and space. This article presents the ESA developments towards high data rate optical free space feeder links. The performance of an optical link from a ground station to a geostationary relay spacecraft experiences major limitations by atmospheric turbulence. To overcome this limitation, a free-space optical link experiment over 13 km is being set up. It shall assess the gain in irradiance and corresponding reduction of the scintillation index by pre-distortion of the optical “uplink” beam based on the measured wave-front disturbances of the “downlink” beam using an adaptive optics system. A second experiment will answer the question if the isoplanatic angle covers the point ahead angle in a ground to GEO link. This was/will be done by correlation measurements on double stars separated between 3.6 and 4.1 arcsec in varying elevation angles and atmospheric turbulence conditions. A third experiment shall address the potential gain and limitations of the implementation of Wavelength Division Multiplexing (WDM) into optical inter-satellite links. WDM being a standard technique to increase the data handling capacity of fibre networks by injecting multiple data streams into one single fibre using only one set of transmit and receive optics.

A new method is described for optical data transmissions from satellites using laser arrays for laser beam pointing. It combines a lens system and a vertical-cavity surface-emitting laser (VCSEL)/Photodetector Array, both mature technologies, in a novel way. This system is applied to satellites in low-Earth orbit, (LEO). It can replace current architectures which use dynamical systems, (i.e., moving parts) to point the laser, and which may use vibration isolation platforms. Results of computer simulations show diffraction limited beam propagation. Possible additional applications are to planetary distances (deep space optical communications, (DSOC)), to optical multiple access, (OMA), to communication between a constellation of close satellites, and to satellites that use modulating retro-reflectors.

Atmospheric turbulences limit the achievable performance of free-space optical (FSO) satellite communication systems. Particularly in retro-reflective FSO satellite communication, tip-tilt disturbances are a dominant source of performance degradation and thus prevent the exploitation of the full potential of this communication system. This publication investigates the total tip-tilt error of a terrestrial optical communication platform for reflective optical communication using a 14-inch telescope with tip-tilt compensation. The compensation system consists of a fast steering mirror (FSM), a quad photo diode (QPD) and a controller. Dynamic error budgeting is used to systematically analyze the system components’ interplay and their contribution to the total error. Based on the results of the system analysis, a feedback controller for the compensation system is designed and tuned for disturbance rejection. The system’s performance is evaluated with a reflective FSO communication link over a distance of 600 m in urban environment. The atmospheric aberration statistic is put into relation with comparable measurements using satellite to earth communication links. Measurement results successfully demonstrate the system’s performance, effectively reducing the tip-tilt error up to a factor of 10.

For the next generation of very high throughput communication satellites, free-space optical (FSO) communication between ground stations and geostationary telecommunication satellites is likely to replace conventional RF links. To mitigate atmospheric turbulence, TNO and DLR propose Adaptive Optics (AO) to apply uplink pre-correction. In order to demonstrate the feasibility of AO pre-correction an FSO link has been tested over a 10 km range. This paper shows that AO pre-correction is most advantageous for low point ahead angles (PAAs), as expected. In addition, an optimum AO precorrection performance is found at 16 AO modes for the experimental conditions. For the specific test site, tip-tilt precorrection accounted for 4.5 dB improvement in the link budget. Higher order AO modes accounted for another 1.5 dB improvement in the link budget. From these results it is concluded that AO pre-correction can effectively improve high-throughput optical feeder links.

Laser communications onboard CubeSats is an emerging technology for enabling high-speed space-based communication links. In this paper we present the development of a 25 cm³ and second iteration 0.3 U CubeSat-class laser transmitter operating at data rates of up to 500 Mbps using OOK modulation and an output power of up to 300 mW over the entire C-band. We present results of the development and characterization of the transmitter. From this testing the design will be demonstrated up to TRL 4/5 with the view for future qualification work and electronics integration.

Modern time-of-flight (TOF) cameras have improved to the point that with their current level of performance they are already being highly sought after in industrial, scientific, and commercial settings. Despite recent advances, TOF cameras still suffer from relatively low depth resolution, poor outdoor performance, low lateral resolution and a high cost of development. To date the most ubiquitous TOF systems have been on chip solutions such as photon mixing devices (PMD). Currently these devices only operate in the visible band and face significant challenges in improving illumination power, and modulation frequency. Here we propose a new approach to TOF imaging using an open architecture (OA) design where the demodulation and signal collection elements of the system have been operationally and spatially isolated. The design relies heavily on a novel stepped quantum well (SQW) large area modulator. The SQW is placed in the beam path between the collection optics and the imager (camera sensor). An open architecture approach allows for a modular TOF system where the imager can be freely chosen to match any application specific needs. Decoupling of the imager and demodulation stage of the system allows for a significantly higher modulation frequency and lateral resolution than what can be found in standard PMD devices. Additionally, the development cost of such a TOF system is significantly reduced. By analyzing the energy expenditure per bit, we show that our approach is fundamentally very efficient. We compute the energy per bit of our current short wavelength IR OA-TOF system to be 28 nJ up to 4 meters with a depth uncertainty below 1 percent of the imaging distance. We also show that a currently in development near IR version of the OA-TOF system can yield energy per bit values below 2 nJ, which is 10 times lower than the Kinect 2.

Atmospheric turbulence plays an important role in the performance of ground-to-space laser communication systems. The probability distribution of atmospheric fading is of particular importance in the design of a laser communication system, as fade depth and frequency are key drivers in the design of a free-space optical communication system, stressing average power requirements as well as the design of forward error correction, interleaving, and data retransmission processing systems. Atmospheric turbulence in traditional visible band, large aperture, space-imaging systems results in severely speckled far-field patterns with exponential intensity statistics. In contrast, laser communication systems typically use longer wavelengths and moderate aperture sizes, only a few r0 in diameter. When propagated through turbulence, these moderate aperture systems create a far-field pattern with a single central lobe that can be characterized by its overall tilt, and a peak intensity within the limits of the Strehl approximation. With these constraints, a closed-form statistical model of atmospheric fading is developed by decomposing atmospheric phase aberrations into a number of independent modes, including tilt and higher order terms, each with a normally distributed amplitude. These predicted fade probability distributions are then assessed and shown to agree with Monte-Carlo analyses of wave optics propagations.

Atmospheric turbulence plays an important role in the performance of ground-to-space laser communication systems. Many of the effects of turbulence in the downlink path can be compensated using high-bandwidth precision tracking and adaptive optics techniques. The point-ahead angle required by the relative satellite velocity results in an uplink data beacon which traverses a different atmospheric path than the downlink beam, causing anisoplanatic differences between the downlink and uplink wavefront phase. In his foundational paper on anisoplanitism in adaptive optics, David Fried noted that the magnitude of the anisoplanatic effect can be characterized by an isoplanatic patch size, and is less severe in systems with smaller transmit diameters. In contrast to traditional space-imaging systems, laser communication systems use moderate aperture sizes, a few r₀ in diameter. The isoplanatic patch size also scales with wavelength, resulting in a larger isoplanatic patch and lower losses at typical communication system wavelengths. The anisoplanatic loss data as a function of aperture diameter and offset angle presented by Fried is fit to provide a direct calculation of the residual phase error of an uplink data beacon in ground receiver systems that provide high bandwidth tilt correction or full adaptive optics system correction of the downlink beam path. The effects of anisoplanitism on both full aperture and sub-aperture uplink beams is considered. Significant correction of the uplink beacon is shown for moderate aperture laser communication systems. The analysis results are shown to be in good agreement with Monte-Carlo analyses of wave optics propagations.

Adoption of terrestrial optical communication devices are effective to realize a free-space optical communication technology for satellite use earlier. However, the quality assurance level of terrestrial optical communication devices has not generally been reached for satellite use. In this paper, we described a screening test method to update the quality assurance level of optical communication devices for satellite use.

We introduce RF frequency dissemination and synchronization system through an outdoor 880-m-long beam path with sub-picosecond timing jitter. The 880-m-long link is configured as a round-trip beam path between rooftops of two independent multi-story buildings. The reference RF frequency of 1.25 GHz is extracted from the fifth harmonic component of the 250 MHz optical frequency comb, which is propagated through the turbulent 880-m beam path. Excess phase noise during the propagation is measured by frequency mixing between the reference and transferred RF signals. After the suppression of the excess phase noise by using a voltage controlled phase shifter, we achieved the instability of 4.5×10^-13 (1.3×10^-16) at 1 (1000) s averaging time in terms of modified Allan deviation. The timing drift measured over 5000 s is 397 fs (rms).

Free Space Optical (FSO) multi-user communication provides high aggregate bandwidth and link robustness attributable to spatial diversity. The primary challenge for this technology is interference among multiple users at the receiver. A non-orthogonal multiple access (NOMA) technique multiplexes numerous users in the power domain at the same time and frequency resource. Thus, different users simultaneously transmit their signals with various power levels. The receiver then decodes user data individually from the overlapped signal using successive interference cancellation (SIC). This paper reports the use of NOMA in an FSO link with two users and details an investigation of the effects of power allocation and channel estimation on the user’s signals demodulation accuracy. Analysis of varying data rate and system capacity gain are explored. Experimental results indicate that accurate channel estimation and optimum power allocation ratio can improve the accuracy of signal reconstruction. Difference in data rate tested proved negligible to signal demodulation quality.

We present the preliminary design and experimental results of a 1550 nm solid-state beam pointing system based on an optical phased array (OPA) architecture. OPAs manipulate the distribution of optical power in the far-field by controlling the phase of individual emitters in an array. This allows OPAs to steer the beam in the far field without any mechanical components (e.g., steering mirrors). The beam-steering system presented here uses waveguide electro-optic modulators to actuate the phase of each element in a 7-emitter OPA, enabling kHz bandwidth steering with sub-milliradian pointing precision. The control system used to stabilize and control the phase of each emitter in the OPA exploits a technique called digitally enhanced heterodyne interferometry, allowing the phase of each emitter to be measured simultaneously at a single photodetector, dramatically simplifying the optical system. All digital signal processing is performed using a field-programmable gate-array. Applications of this technology include free-space link acquisition and tracking for satellite-to-satellite laser communications and light detection and ranging (LiDAR).

The purpose of this study is to investigate a new approach to the modulation of an optical signal which requires high extinction ratio (ER). A deep space, optical, pulse position modulated (PPM) link, may require an extinction ratio approaching 33 dB1 which is not easily achieved through the use of a single optical modulator. In a system where the slot clock is equal to the slot width, it is often not possible to meet ER requirements due to Inter Slot Interference (ISI). Furthermore, the high frequency ER of state of the art optical amplitude modulators is not large enough to allow for implementation losses. By using a second optical modulator in series with the first, it is possible to address both issues. A phase delay placed between each modulator allows for precision control of the pulse width, reducing ISI. While the attenuation in the off slots combines linearly, increasing attenuation in the off slots. Using this approach a series of 1 ns pulses was measured at a series of phase delays to approximate PPM pulses. These measurements were used to extrapolate the ER of a PPM signal at various PPM orders. An ER above 33 dB was observed for all PPM orders of 16 and above. At PPM 256 an ER of 48.2 dB was achieved.

The solution of the infinitesimal propagation equation for atmospheric propagation of single-photon and entangled quantum states, represented in terms of Laguerre-Gauss modes, which is a discrete orbital angular momentum (OAM) basis, is compared with numerical simulations for the propagation of optical fields that carry OAM in atmospheric turbulence. The numerical simulations are performed using the multi-phase screen model based on the Kolmogorov theory of turbulence. The comparison was done under various turbulence conditions and propagation distances to allow comparison under both weak and strong scintillation conditions. The results show that there is an agreement between the infinitesimal propagation equation and the numerical simulations. Also, we note that in the limit of weak scintillation both methods, the infinitesimal propagation equation and numerical simulations, agree with the predictions of single-phase screen model.

Motivated by the requirements of the gravitational wave observatory mission, a diode laser module based on resonant optical feedback from a monolithic LiNbO₃-cavity has been realized. This approach enables the diode laser to reach a phase noise level comparable to that of an NPRO. In order to allow for a compact setup, a hybrid micro-integration technology was utilized to omit any movable parts. Frequency tuning is realized thermally and electro-optically. Results of a prototype module emitting at 1064 nm will be presented. In addition to its intended purpose, this type of laser is ideally suited for coherent optical communication applications.

The development of advanced power grids is necessary to expand mankind’s habitat into previously uninhabitable regions, such as deep space or sea. Wired electrical or gas pipeline networks have been conventionally used to construct effective power grids. However, the use of metal wires or pipes in extreme environments is considered problematic. Therefore, wireless power grids are considered to be useful and technologically advanced alternatives for applications in deep space or sea. In this report, we discuss an optical wireless power grid for an artificial space satellite network that uses a high-power laser and high-efficiency optoelectric (OE) converter technologies. We estimate a high-energy transport efficiency of 8.5 % between the power units in two artificial satellites that utilize a free-space optical connection based on a high-power laser and an OE converter. Based on these considerations, we expect that optical wireless power grids that utilize free-space optical networks will emerge as a cutting-edge technology to achieve power interchange between a large number of small space satellites.

Performance of free-space optical communication links can be affected by spatial and temporal fields of the refractive index created by atmospheric turbulence. They result in wave front distortions and lead to performance degradation, which can be expressed quantitatively by reduction of the signal power and increase of the bit-error-rate (BER). Under severe turbulence conditions, these effects can be profound even in short-range links. While it is impossible to obtain closed-form solutions for instantaneous realizations of wave front distortions, several spectral models of atmospheric turbulence can be used to study statistical properties of the refractive index fluctuations and to find the scintillation index. This analysis can be further expanded to include communication performance of free-space optical links. From a practical stand point, it would be very beneficial to find a relationship between the expected system performance and specific variables responsible for wave front distortions, such as air temperature and pressure, temperature gradient, wind speed, cross wind, net radiation, soil-heat flux, etc. In this paper, we present the results of an experimental study and subsequent analysis that mathematically justifies some of these relationships after conducting a series of tests over an extended time period. This measurement data was obtained for a free-space laser communication link established between two fixed-point terminals and designed for transmitting a data stream using 1550 nm as the operating wavelength.

This work presents the initial activation of the Mobile Ultrafast High-Energy Laser Facility (MU-HELF) located on a 1 km test range at the Townes Institute Science and Technology Experimentation Facility (TISTEF). The MU-HELF was designed to study nonlinear laser propagation effects including filamentation and produces pulses at 800 nm with current peak powers as high as 5 TW. The pulse width, energy, size, and focusing conditions of the launched beams are all readily adjustable. Several data collection techniques have been implemented that enable high-resolution, single-shot beam profiles, spectra, and energy measurements at any point along the range. Atmospheric conditions are also continuously measured during laser propagation using the array of monitoring equipment available at TISTEF. The newly active test facilities and data collection procedures demonstrated here will drive future in-depth high-intensity laser propagation studies and development of field-deployable applications.

Free-space optical (FSO) communication has gained popularity for wireless applications over legacy radiofrequency for advantages like unlicensed band, spatial reuse, and security. Even though FSO can achieve high bit-rate, range limitation due to attenuation and weather dependency has always restricted its practical applications. Building-to-building communication, smart cars, and air-subsea links are potential futuristic applications for mobile and secure ad-hoc FSO networks, where in-band full-duplex FSO (IBFD-FSO) transceivers will potentially increase network capacity significantly to improve performance and reliability. In this work, we model an IBFD-FSO transceiver consisting of a VCSEL and a photodiode to determine the range and weather dependent performance of the link.

We studied and demonstrated a wavelength discriminant structure that consists of one circulator, one or more Fiber Bragg Gratings and two photodiodes. The discriminants are built in NASA’s LCRD (Laser Communication Relay Demonstration) flight modems to measure the transmitter and pilot laser wavelengths on orbit. The performance of the discriminants is evaluated in ambient and thermal vacuum chamber environment. The paper reports on results of a few discriminants working at different wavelengths and power levels. The trending of the discriminant performance under ambient and TVAC cycles is discussed. The discriminant can achieve sub-picometer wavelength accuracies if calibrated properly.

A combination of scattering and scintillation effects is considered to compare the resilience of mid-IR and near-IR free space communication links. Beam-wander r_w(t) under different scintillation conditions for mid-IR and near- IR beams are recorded simultaneously in real-time by a unique broad-band camera. The correlation coefficients show that both beams undergo the same pathway and experience the same scattering and scintillation effect which allows to identify and distill wavelength dependent effects. Using the frequency dependence in the observed beam wander allows for classification of our simulated atmospheric turbulence. Under these lab conditions, perceived refractive index structure parameter C_n^2 is extracted for each wavelength and a long-term radius is estimated for a long-distance link. In addition, the transmissivity and broadening of beams are calculated using discrete ordinate method to account for the scattering by fog. Considering both scattering and scintillation effects, the experimental and analytical results show that mid-IR (4.328 μm) beam has higher resilience in low visibility conditions compared to a near-IR (1.550 μm) beam.

We present results of an indium phosphide (InP) monolithic photonic integrated circuit (PIC) transmitter suitable for space-optical communications up to 10 Gbps for NRZ-OOK and DPSK modulation and up to 5 Gbps RZ-DPSK modulation. The PIC includes an SG-DBR laser tunable across the entire telecommunications C-Band, a semi-conductor optical amplifier (SOA), a Mach-Zehnder modulator (MZM) for efficient encoding of phase information, and an electroabsorption modulator (EAM) which serves as an RZ pulse carver. The transmitter PIC is integrated in a testbed with a custom board that provides biasing and driving electronics. The commercial-off-the-shelf (COTS) differential driver generates an estimated 4.5-5 Vpp differential modulation voltage for the dual drive MZM. An identical driver was used for the EAM 50% RZ pulse carver but only a single output was used with the other output terminated with a 50Ω load. The SOA and laser gain sections were biased at 90 mA each. Clear eye openings were achieved for all modulation formats.