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.

Qualification testing of fiber based laser transmitters is required for NASA’s Deep Space Optical Communications program to mature the technology for space applications. In the absence of fully space qualified systems, commercial systems have been investigated in order to demonstrate the robustness of the technology. To this end, a 2.5 W fiber based laser source was developed as the transmitter for an optical communications experiment flown aboard the ISS as a part of a technology demonstration mission. The low cost system leveraged Mil Standard design principles and Telcordia certified components to the extent possible and was operated in a pressure vessel with active cooling. The laser was capable of high rate modulation but was limited by the mission requirements to 50 Mbps for downlinking stored video from the OPALS payload, externally mounted on the ISS. Environmental testing and space qualification of this unit will be discussed along with plans for a fully space qualified laser transmitter.

High-speed interband cascade lasers have been fabricated and the. first experimental evidence that these devices can be directly modulated at a frequency of 3.2 GHz and above is reported.

To meet the future laser transmitter requirements of deep-space optical communications, a breadboard fiber-based master oscillator power amplifier (MOPA) laser was developed and tested. A polarized output with improved efficiency and higher data rates as compared with previous designs was demonstrated in a modular system. Several source laser configurations were tested and compared along with a programmable pulse-position modulator data encoder. Average powers of up to 10 W at 100 MHz for sub-nanosecond pulses at a 1.06-µm wavelength were measured, extending the previously developed system capability to higher-order pulse-position modulation data formats.

Deep-space optical communications requires high-power laser transmitters with good beam quality that can be modulated at fairly high data rates. We report here on the performance of a fiber-based master oscillator power amplifier (MOPA) laser, commercially developed to JPL requirements with specifications derived from a Mars mission optical downlink scenario. The laser demonstrates up to 1.6-kW peak power and an average power of around 10 W at 1060 nm with pulse repetition rates from 3 to 30 MHz and diffraction-limited beam quality. A programmable highrate pulse-position modulator (PPM) also was developed as a data source, and the performance of the laser under variable pulse delays, as required for transmitting data streams, was measured.

Observations made during a mountain-top-to-mountain-top horizontal optical link demonstration are described. The optical link spans a range of 46 Km at an average altitude of 2 Km above sea level. A multi-beam beacon comprised of eight laser beams emerging from four multi-mode fiber coupled lasers (780 nm) is launched through a 0.6 m diameter telescope located at the JPL Table Mountain Facility (TMF) in Wrightwood, California. The multi-beam beacon is received at Strawberry Peak located in the San Bernardino Mountains of California. The NASA, JPL developed optical communications demonstrator (OCD) receives the beacon, senses the atmospheric turbulence induced motion and using an upgraded fine steering loop actively points a communications laser beam (852 nn, 400 Mbps on-off key modulated, PN7 pseudo random bit sequence) to TMF. The eight-beam beacon allowed a four-fold reduction in normalized irradiance or scintillation index. This in turn was sufficient to eliminate beacon fades sensed by the OCD and enabled performance evaluation of the fine steering loop. The residual tracking error was determined to be +/-1.1 to +/-1.7 mu rad compared to a model prediction of +/-3.4 mu rad. The best link performance observed showed average bit error rates (BER) of 1E-5 over long durations (30 seconds), however, instantaneous BER's of at least 0.8E-6 over durations of 2 ms were observed. The paper also discusses results pertaining to atmospheric effects, link analysis, and overall performance.