We present the initial results from testing of the Low-Cost Optical Terminal (LCOT) at NASA-Goddard Space Flight Center (GSFC). LCOT is designed to be a single modular design that can be quickly reconfigured to support different laser communication missions. LCOT is built around a 70 cm commercially available telescope designed with optical and quantum communications in mind. We have installed a state-of-the-art adaptive optics system, novel high-power laser amplifiers, and other innovative subsystems developed by our team to facilitate laser communications. We have conducted tests of our LCOT system against operational space terminals, demonstrating our ability to receive a downlink and transmit an uplink. We show the results of these tests and give an analysis of the results.

The Black Hole Explorer (BHEX) mission will enable the study of the fine photon ring structure, aiming to reveal the clear universal signatures of multiple photon orbits and true tests of general relativity, while also giving astronomers access to a much greater population of black hole shadows. Spacecraft orbits can sample interferometric Fourier spacings that are inaccessible from the ground, providing unparalleled angular resolution for the most detailed spatial studies of accretion and photon orbits and better time resolution. The BHEX mission concept provides space Very Long Baseline Interferometry (VLBI) at submillimeter wavelengths measurements to study black holes in coordination with the Event Horizon Telescope and other radio telescopes. This report presents the BHEX engineering goals, objectives and TRL analysis for a selection of the BHEX subsystems. This work aims to lay some of the groundwork for a near-term Explorers class mission proposal.

In free-space optical communications in which the signal is coupled into a single-mode fiber, atmospheric distortion leads to loss of signal and reduced receiver sensitivity. We demonstrate a coherent receiver system in which a dual polarization quadrature phase shift-keying (DP-QPSK) signal is coupled into a photonic lantern, which efficiently separates the light from a large multimode core into single mode fibers. Outputs from the lantern are passed to off-the-shelf integrated coherent receivers and digitized, and the resulting signals are coherently combined with optimal weight coefficients. The reconstructed signal exhibits reduced sensitivity to atmospheric distortion and demonstrates improved performance.

We present the Black Hole Explorer (BHEX), a mission that will produce the sharpest images in the history of astronomy by extending submillimeter Very-Long-Baseline Interferometry (VLBI) to space. BHEX will discover and measure the bright and narrow "photon ring" that is predicted to exist in images of black holes, produced from light that has orbited the black hole before escaping. This discovery will expose universal features of a black hole's spacetime that are distinct from the complex astrophysics of the emitting plasma, allowing the first direct measurements of a supermassive black hole's spin. In addition to studying the properties of the nearby supermassive black holes M87* and Sgr A*, BHEX will measure the properties of dozens of additional supermassive black holes, providing crucial insights into the processes that drive their creation and growth. BHEX will also connect these supermassive black holes to their relativistic jets, elucidating the power source for the brightest and most efficient engines in the universe. BHEX will address fundamental open questions in the physics and astrophysics of black holes that cannot be answered without submillimeter space VLBI. The mission is enabled by recent technological breakthroughs, including the development of ultra-high-speed downlink using laser communications, and it leverages billions of dollars of existing ground infrastructure. We present the motivation for BHEX, its science goals and associated requirements, and the pathway to launch within the next decade.

We present a baseline science operations plan for the Black Hole Explorer (BHEX), a space mission concept aiming to confirm the existence of the predicted sharp “photon ring" resulting from strongly lensed photon trajectories around black holes, as predicted by general relativity, and to measure its size and shape to determine the black hole's spin. BHEX will co-observe with a ground-based very long baseline interferometric (VLBI) array at high-frequency radio wavelengths, providing unprecedented high resolution with the extension to space that will enable photon ring detection and studies of active galactic nuclei. Science operations require a simultaneous coordination between BHEX and a ground array of large and small radio apertures to provide opportunities for surveys and imaging of radio sources, while coordination with a growing network of optical downlink terminals provides the data rates necessary to build sensitivity on long baselines to space. Here we outline the concept of operations for the hybrid observatory, the available observing modes, the observation planning process, and data delivery to achieve the mission goals and meet mission requirements.

We present the initial laboratory test results of the adaptive optics (AO) subassembly for the Low-Cost Optical Terminal (LCOT), a flexible communications ground terminal developed by Goddard Space Flight Center. LCOT will receive first light in 2023 testing. This terminal includes a 700mm commercial telescope, 1550nm receive instruments, and uplink transmit systems. Demodulating coherent formats requires AO to correct turbulence effects and allow coupling into single-mode fiber. General Atomics delivered the system to Goddard in September 2021, where engineers have evaluated performance. We describe laboratory testing, turbulence phase plate design, results, and AO field testing plans when installed on LCOT.

This paper provides the status of ongoing work at NASA-Goddard Space Flight Center (GSFC) to build a low-cost flexible ground terminal for optical communication. For laser communication to be cost-effective for future missions, a global network of flexible optical terminals must be put in place. There is a need for a single ground terminal design capable of supporting multiple missions ranging from LEO to lunar distances. NASA's Low-Cost Optical Terminal (LCOT) has a single modular design that can be quickly reconfigured to support different laser communications missions. The LCOT prototype uses a 70cm commercially available telescope designed with optical and quantum communications in mind. This telescope is currently being integrated with a state-of-the-art adaptive optics system, and novel high-power laser amplifier demonstrate its utility as an optical communications receiver by receiving a downlink from the recently launched Laser Communication Relay Demonstration (LCRD). LCOT uses commercially available components wherever possible, and where commercial options are not available, the LCOT team works with vendors to create commercial options. This paper discusses the development progress for the blueprint of NASA's future global ground terminal network.

We present the status of ongoing work at NASA’s Goddard Space Flight Center (GSFC) to build a prototype, low-costof- production, flexibly-configured ground terminal for space optical communication. For laser telecommunication to be cost effective for future missions, a wide-spread global network of operationally responsive optical terminals should be established. There has been a decades-old need for a single modular open systems approach (MOSA) ground terminal architecture capable of supporting multiple space missions ranging from LEO to Lunar distances with 2-way laser communications. At the heart of LCOT’s design concept is the Free-Space Optical Subsytem (FSOS). The major subassemblies of LCOT/FSOS that address most optical comms configurations are : (1) Single 700mm F/12 Nasmyth folded Rx R-C Telescope, (2) Four independent 150mm diameter high-power all-reflective Tx beam directors (XOA), (3) Non-coherent direct detection Rx bench on starboard side of telescope (SOB), and (4) Coherent (possibly Quantum) optical communications bench on port side (POB). The Low-Cost Optical Terminal (LCOT) research and development (R&D) prototype is designed to be a generalized system that can be quickly field-reconfigured to support a wide variety of laser communications missions past, present, & future.

The Event Horizon Explorer (EHE) is a mission concept to extend the Event Horizon Telescope via an additional space-based node. We provide highlights and overview of a concept study to explore the feasibility of such a mission. We present science goals and objectives, which include studying the immediate environment around supermassive black holes, and focus on critical enabling technologies and engineering challenges. We provide an assessment of their technological readiness and overall suitability for a NASA Medium Explorer (MIDEX) class mission.

At NASA Goddard Space Flight Center, we have been developing spaceborne lidar instruments for space sciences. We have successfully flown several missions in the past based on mature diode pumped solid-state laser transmitters. In recent years, we have been developing advanced laser technologies for applications such as laser spectroscopy, laser communications, and interferometry. In this article, we will discuss recent experimental progress on these systems and instrument prototypes for ongoing development.

We establish a viable laser payload design for the Orbiting Configurable Artificial Star (ORCAS) mission. We share observational considerations and derive the engineering requirements for the laser payload. Developed by General Atomics Electromagnetic Systems, the dual-wavelength laser will operate at 1064 nm and can be frequency-doubled to 532 nm, with two possible beam divergence modes and tunable power. The laser payload can be operated at pulse repetition rates greater than 10 kHz to enable compatibility with Adaptive Optics systems and to maintain pointing requirements. We show that such a laser payload can be constructed based upon a high-TRL amplified fiber Laser Communication Terminal modified to meet the mission requirements.

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

The use of epoxies in space-based instruments is often unavoidable in situations where the bonding of dissimilar materials such as glass and metal is required. While there are epoxies that exhibit low total mass loss (TML) and collected volatile condensable materials (CVCM) in vacuum, in some applications they can still be a source of problematic contamination. Epoxies can also be incompatible with exposure to chemical environments some space instrumentation may be exposed to. In high power laser instruments such as LIDAR systems where optical components must be securely bonded to metal mounts, the impact of epoxy outgassing can be especially acute. Even with very low outgassing levels, the intense laser can break down the outgassed material and preferentially deposit it on optics that handle high optical power. This laser induced contamination in turn leads to laser induced damage, leading to degradation of optical components and reducing the reliability and operational lifetime of laser instruments [1-6]. Alternative bonding methods that avoid introducing additional contaminants could greatly improve reliability and operational lifetime of space instruments.

The use of epoxies in space-based instruments is often unavoidable in situations where the bonding of dissimilar materials such as glass and metal is required. While there are epoxies that exhibit low total mass loss (TML) and collected volatile condensable materials (CVCM) in vacuum, in some applications they can still be a source of problematic contamination. Epoxies can also be incompatible with exposure to chemical environments some space instrumentation may be exposed to. In high power laser instruments such as LIDAR systems where optical components must be securely bonded to metal mounts, the impact of epoxy outgassing can be especially acute. Even with very low outgassing levels, the intense laser can break down the outgassed material and preferentially deposit it on optics that handle high optical power. This laser induced contamination in turn leads to laser induced damage, leading to degradation of optical components and reducing the reliability and operational lifetime of laser instruments [1-6]. Alternative bonding methods that avoid introducing additional contaminants could greatly improve reliability and operational lifetime of space instruments. We present the results of our investigations of the use of femtosecond laser welding as an alternative to epoxies in the construction of spaceflight optical instruments. Unlike conventional laser welding, femtosecond laser welding uses extremely short (~100fs) low energy (10’s of micro-Joules) pulses- as a result there is negligible heating of the materials being welded. The bond is not created by melting of materials, instead it is created by the extremely rapid formation of a plasma in which the two materials being bonded are mixed and rapidly recombine. The process we used to create these bonds will be describes as well as analysis of the bond strength. Electron microscope images take of the resulting bonds show significant mixing between the metal and crystalline materials being welded. We have demonstrated that the bonds created via femtosecond welding can have strengths comparable to epoxy and can be hermetic. Furthermore, we demonstrate that the femtosecond laser welding process allows bonds that are not possible using traditional methods. We have bonded Calcium Fluoride to stainless steel, diamond to Invar, sapphire to titanium, sapphire to Invar, Zerodur to Invar and fused silica to copper to name just a few combinations. We plan to pursue this technique and demonstrate its utility in the fabrication of space-flight instrumentation.

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.

Fiber amplifiers have been used in many laser communication applications due to their compactness and high efficiency. However, fiber amplifiers for free space laser communications have been limited to low peak power communication formats due to nonlinear effects such as Self Phase Modulation (SPM). At high peak powers, SPM can broaden the spectrum of free space laser communication signals to an unacceptable degree, moving much of the transmitted power outside the receiver’s designed bandwidth. The Laser Communication Relay Demonstration (LCRD) needed a fiber amplifier that could amplify a 3.5 GHz wide signal to peak powers of nearly a kilo-Watt with little to no spectral broadening due to SPM. We tested several different commercial amplifiers and found an Er-doped, Very Large Mode Area (VLMA) fiber amplifier, pumped by a 1480 nm Raman fiber laser, met the needs for both high peak power and minimal spectral broadening. The high peak power performance of the VLMA amplifier is enabled by the large effective area of ~ 1100 μm 2 . We will present a detailed analysis of the effects of SPM on the amplified signal spectrum for both pulse position modulation (PPM) and differential phase shift keyed (DPSK) communication formats at peak powers up to 1 kW. Additionally, Bit Error Ratio (BER) performance data taken with the LCRD modem showed the signal did not suffer a measureable penalty from amplification with the VLMA amplifier. The ability to reach the required high peak power with this fiber amplifier makes it possible to consider its use for deep space laser communication, e.g. with high order M-ary PPM formats where such high peak powers are needed.

NASA is planning to launch the next generation of a space-based Earth relay satellite sometime in the middle of the next decade to join the current Space Network, consisting of Tracking and Data Relay Satellites in space and the corresponding infrastructure on Earth. While the requirements and architecture for that relay satellite are unknown at this time, NASA is investing in communications technologies that could be deployed to provide new communications services. One of those new technologies is optical communications. The Laser Communications Relay Demonstration (LCRD) project, scheduled for launch in 2019 as a hosted payload, is a critical pathfinder towards NASA providing optical communications services on the next generation space-based relay. This paper will describe the concept of operations and the impacts of the U.S. Federal Aviation Administration and the U.S. Laser Clearinghouse on the Laser Communications Relay Demonstration. It will provide a high level overview of the link budgets and discuss the analysis done on both space to ground links and geostationary (GEO) to low Earth orbit (LEO) links. The U.S. Laser Clearinghouse is a United States Air Force Strategic Command organization that provides predictive avoidance analysis and deconfliction with U.S. and allies satellites and operations. NASA's policy is to be in compliant with the U.S. Laser Clearinghouse. Having a valid concept of operations that is compliant with the U.S. Federal Aviation Administration and the U.S. Laser Clearinghouse is critical to making optical communications a reality on future NASA science and exploration missions.

The Lunar Lasercom Ground Terminal (LLGT) is the primary ground terminal for NASA's Lunar Laser Communication Demonstration (LLCD), which demonstrated for the first time high-rate duplex laser communication between Earth and satellite in orbit around the Moon. The LLGT employed a novel architecture featuring an array of telescopes and employed several novel technologies including a custom PM multimode fiber and high-performance cryogenic photon-counting detector arrays. An overview of the LLGT is presented along with selected results from the recently concluded LLCD.

We present an implementation overview and demonstrated error-free coded performance over the 400,000-km link between an Earth-based laser communication terminal and the LADEE satellite orbiting the moon at 9.72-Mbps and 19.44-Mbps uplink rates.

Improvements to a ground-based 40W 1.55 micron uplink transmitter for the Lunar Laser Communications Demonstration (LLCD) are described. The transmitter utilizes four 10 W spatial-diversity channels to broadcast 19.4 - 38.9 Mbit/s rates using a variable-duty cycle 4-ary pulse position modulation. At the lowest rate, with a 32-to-1 duty cycle, this leads to 320 W peak power per transmitter channel. This paper discusses a simplification of the transmitter that uses super-large-area single mode fiber and polarization control to mitigate high peak power nonlinear impairments.