The Integrated LCRD Low-Earth Orbit User Modem and Amplifier Terminal (ILLUMA-T) payload will be launched to the International Space Station (ISS) in 2023. ILLUMA-T is an optical communications payload that will make the ISS the first space-based user to communicate with NASA’s Laser Communications Relay Demonstration (LCRD). The system will support all-optical forward links up to 150 Mbps and return links up to 1 Gbps. The payload recently underwent system level Thermal VACuum (TVAC) functional testing at MIT Lincoln Laboratory. We present an overview of the payload’s TVAC functional tests and results.

The Orion Artemis II Optical Communications (O2O) system will demonstrate the operational utility of laser communications for the first crewed Artemis mission scheduled to launch next year. O2O will provide an optical link with data rates up to 260 Mbps return from the moon and up to 20 Mbps forward to the moon. The optical link employs a Serially Concatenated Pulse Position Modulation (SCPPM) communications signal, compliant with the Consultative Committee for Space Data Systems (CCSDS) standard, and a modulated uplink beacon for acquisition and collaborative tracking. O2O employs optical ground stations located at the White Sands Complex (WSC) and Table Mountain Facility (TMF) to support the Earth end of the link. We describe interface testing performed between the space and ground terminals to verify the physical layer communication and beacon signals.

Free-space laser communication systems are increasingly implemented on state of the art satellites for their high-speed connectivity. This work outlines a demonstration of the Modular, Agile, Scalable Optical Terminal (MAScOT) we have developed to support Low-Earth Orbit (LEO) to deep-space communication links. In LEO, the MAScOT will be implemented on the International Space Station to support the Integrated Laser Communications Relay Demonstration (LCRD) LEO User Modem and Amplifier Terminal (ILLUMA-T) program. ILLUMA-T's overarching objective is to demonstrate high bandwidth data transfer between LEO and a ground station via a geosynchronous (GEO) relay satellite. Outside of LEO, the MAScOT will be implemented on the Artemis-II mission to demonstrate high data rate optical communications to and from the moon as part of the Optical to Orion (O2O) program. Both missions leverage the same modular architecture despite varying structural, thermal, and optical requirements. To achieve sufficient performance, the terminal relies on a nested tracking loop to realize sub-arcsecond pointing across a ±120 ° elevation and ±175° azimuth field of regard.

We deployed a narrow-beam optical measurement and communication experiment over several days in the shallow, turbid water of Narragansett Bay, Rhode Island (USA). The experiment consisted primarily of a transmitter module and a receiver module mounted on a metal framework that could be lengthened or shortened. The communication wavelength was 515 nm. The experiment characterized light propagation characteristics, including images of the received beam over time. The experiment included manual beam steering. Images obtained during the steering process provided insight into future development of an automated steering procedure. Water transmissivity was also measured. Over time and tides, the optical extinction length varied between 0.66 m and 1.07 m. The transmitter’s optical power was kept low at 0.25 mW. The receiver included a high-sensitivity photon-counting photomultiplier tube (PMT) and a high-speed linear avalanche photodiode (APD). Both links processed data continuously in real time. The PMT supported multiple channel rates, from 1.302 Mbaud to 10.416 Mbaud. It also included strong forward error correction (FEC) capable of operating at multiple code rates. The PMT link demonstrated near-theoretical channel performance at all data rates, error-free output after FEC, and robust operation during day and night. This link efficiently traded data rate for link loss. It demonstrated error-free performance for input powers as low as -84.1 dBm, or 18 extinction lengths. The APD receiver demonstrated a channel error rate of 1e-9 at 125 Mbaud. Furthermore, it demonstrated a channel error rate correctable by FEC at a link loss equivalent to 9 extinction lengths.

: MIT Lincoln Laboratory conducted a successful field demonstration of narrow-beam laser communications (lasercom) from March 28-April 1, 2016 in the Narragansett Bay, RI, in collaboration with the Naval Undersea Warfare Center (NUWC) Newport. The demonstration achieved real-time communication through turbid harbor waters utilizing a green (515 nm), low-power commercial laser and commercial detectors. Using a single-photon sensitive commercial photo-multiplier tube (PMT), we demonstrated communication rates between 0.7 and 8.7 megabits per second (Mbps) with a sensitivity of better than 1.5 detected photons per bit. In the second communication demonstration, we used a wideband width commercial avalanche photodiode (APD) to achieve 125 Mbps communication rates. In addition to the communication demonstration, high resolution imaging cameras characterized the laserbeam propagation through the harbor waters, providing valuable data to inform lasercom systems design.The field test was a successful proof of concept demonstration providing key milestones toward a high performance lasercom terminal suitable for a mobile undersea platform.

Narrow-beam laser communication (lasercom) can enable high-rate, long-range undersea communication. Light in the beam is absorbed and scattered by suspended particulates, altering the spatial, angular, and temporal properties of the transmitted light. Development of high quality data links is aided by accurate simulations that can be adapted to different water conditions, from turbid harbors, to clear oceans, and even laboratory test beds with artificial scatterers. To this end we have developed a method to create an empirical scattering function from volume scattering function measurements that can be used to generate random scattering angles in an undersea optical propagation simulator. This method is presented, along with cross validation of its predictions during a recent narrow-beam communication demonstration in Narragansett Bay, Middletown, RI, USA. VSF measurements taken during the trials were used to run the simulator and the results are shown to agree with in situ images of the pupil and focal planes of the communication laser beam.

Undersea laser communications represent a promising area of research with a large set of applications. Wide dynamic range receivers are necessary to operate through a range of possible water qualities and link distances. In the signal-starved regime, photon-counting photomultiplier tubes (PMTs) are a key technology for high-sensitivity communications. When more signal is available, linear avalanche photodiodes (APDs) provide an opportunity for higher-rate communication. We have designed a receiver terminal employing both kinds of detectors to show robust operation over nearly two orders of magnitude in power and data rate. An optical link including this receiver terminal was submerged in Narragansett Bay, RI to demonstrate underwater optical communication over several days. The PMT receiver demonstrated robust, error-free performance over channel rates from 1.302 Mbaud to 10.416 Mbaud for received optical power levels ranging from -84.1 dBm to -75.3 dBm. The PMT link demonstrated an error-free user rate of 8.68 Mb/s. This corresponded to nearly-ideal detector efficiency on the order of one detected photon per bit. The PMT receiver was contained entirely within the submerged enclosure and demonstrated full real-time decoding, including strong forward error correction. A low-power transmitter was used to demonstrate a link with loss equivalent to 18 extinction lengths. With moderately-powered transmitters, this distance could be extended to 22.4 extinction lengths. The PMT receiver was capable of operating at near-theoretical limits during the day and night. Its multi-rate operation demonstrated the capability of trading sensitivity for data rate efficiently. With the same low-power transmitter, the APD receiver achieved a bit error rate less than 1×10 -9 at 125 Mbaud. Furthermore, it achieved an error rate correctable by forward error correction for a link with loss equivalent to 9 extinction lengths.

We report a demonstration of narrow-beam laser communication through the waters of Narragansett Bay in Rhode Island, USA. The transmitter and receiver were mounted on an aluminum truss and placed in the water alongside a pier operated by the Naval Undersea Warfare Center. The transmitter consisted of a real-time modulator and encoder, a 515 nm wavelength commercial laser, collimating optics, and a steering mirror. The receiver included a steering mirror, a focal plane camera, a linear-mode avalanche photo-diode (APD), a photo-multiplier tube (PMT) single photon detector, a large area imaging camera, an iris to vary the field of view, optics to split the beam between the various detectors, and field-programmable gate array (FPGA) electronics for real-time demodulation and decoding. The PMT and APD detectors were used for communications demonstrations; the imaging and focal plane cameras were used for channel characterization measurements and system alignment. Communications and characterization data were collected through a variety of conditions over the five day field experiment, including day and night, calm and high winds, and flood and ebb tide. In the experiment, the transmit power, receiver field of view, and link distance were varied. The water transmissivity and volume scattering function were measured throughout the experiment to calibrate the results. Real-time communications demonstrations with the PMT were carried out between 1 megabit-per-second (Mbps) and 8.7 Mbps at 7.8 meters, which represented between 8 and 12 beam extinction lengths. With the APD, 125 Mbps were demonstrated at 4.8 meters, representing approximately 5 extinction lengths.

We demonstrate a multi-rate burst-mode photon-counting receiver for undersea communication at data rates up to 10.416 Mb/s over a 30-foot water channel. To the best of our knowledge, this is the first demonstration of burst-mode photon-counting communication. With added attenuation, the maximum link loss is 97.1 dB at λ=517 nm. In clear ocean water, this equates to link distances up to 148 meters. For λ=470 nm, the achievable link distance in clear ocean water is 450 meters. The receiver incorporates soft-decision forward error correction (FEC) based on a product code of an inner LDPC code and an outer BCH code. The FEC supports multiple code rates to achieve error-free performance. We have selected a burst-mode receiver architecture to provide robust performance with respect to unpredictable channel obstructions. The receiver is capable of on-the-fly data rate detection and adapts to changing levels of signal and background light. The receiver updates its phase alignment and channel estimates every 1.6 ms, allowing for rapid changes in water quality as well as motion between transmitter and receiver. We demonstrate on-the-fly rate detection, channel BER within 0.2 dB of theory across all data rates, and error-free performance within 1.82 dB of soft-decision capacity across all tested code rates. All signal processing is done in FPGAs and runs continuously in real time.

The Lunar Laser Communication Demonstration (LLCD) program will demonstrate the first high-bandwidth optical communication payload on a NASA space mission. The inertially stabilized 108 mm aperture telescope will fly on NASA's LADEE spacecraft and is fabricated nearly entirely of beryllium, providing a high stiffness-to-weight ratio. The telescope consists of a two-axis fine positioning stage using inertial sensors and coarse and fine optical tracking. The stabilized telescope uses a two-axis coarse positioning gimbal to provide a large field-of-regard. Inertial stabilization provides local disturbance rejection while allowing modest optical uplink power to provide an absolute pointing reference. The telescope is a three-wavelength design providing separate uplink acquisition and communication wavelengths, and a downlink communication wavelength. Acquisition and coarse tracking of the uplink beacon is via a photodiode quadrant detector, while fine tracking is via nutation tracking and piezoelectric actuation of the receive fiber. Control of the downlink point-ahead angle is via piezoelectric actuation of the transmit fiber. The telescope is thermally stabilized during normal operations. The transmit and receive beams are fiber-coupled to a separate optoelectronic module and the telescope line-of-sight will be stabilized to better than 2.5 microradians during normal operations. Provision for self-test and boresighting during on-orbit operations is provided.

The potential of lasercom could often be much more attractive to system designers if the terminals could be made very small. In particular, in systems where one end of the link is allowed to be somewhat more capable than the other, the lesser of the two terminals could take advantage of the asymmetry and shrink as much as possible. We have investigated how such asymmetry factors into the requirements for a small terminal and have designed a terminal with a very small aperture (35-75 mm) and an inertial stabilization scheme. The space-worthy terminal has applicability to Moon-to-Earth as well as near-Earth lasercom missions.

Acquisition and tracking performance of a space-based optical communications system engineering model has been quantitatively measured using a dedicated optical test set. The test set includes a beacon with simulated angle jitter for tracking, a heterodyne optical receiver, and an accurate, calibrated line- of-sight monitor. The system has the capability of measuring each component of the pointing and tracking budget with angles down to approximately 1% of a diffraction-limited bandwidth. Acquisition and handover-to-tracking probabilities exceeding 99.9% are achieved at power levels 9-dB below expected on-orbit levels, and tracking rejection with jitter amplitudes comparable to expected spacecraft levels has been measured optically at -70 dB. Point- ahead angle repeatability over a POM 15 bandwidth range is found to be better than 0.01 beamwidths.

The components and performance of a prototype high angle sensor with a measured angular resolution of 1 nanoradian at an update rate of 1 KHz are described and experimental results are presented. Calibration methods are discussed and the performance of a differential gas refractometer is presented.