A Compact, Low Power Pulsed Optical Communication System for Spacecraft

The goal of this project is to develop a miniature optical communications transmitter (MOCT) designed for small spacecraft or any spacecraft where volume mass and power are very constrained. The MOCT is a compact pulsed laser transmitter with a target mass, volume and power consumption of 2 kg, 2 L, and ~5 W while transmitting. This Early Career Faculty grant supported the development of a prototype implementation of this technology to TRL 3 in the University of Florida’s Precision Space Systems Lab (PSSL).
Spacecraft communications using optical frequencies is widely recognized as a potentially more power efficient scheme compared to radio frequency communications. This is because the fundamental divergence angle limits of electromagnetic waves are proportional to their wavelength, and therefore, the fraction of transmitted power that is captured by the receiver is inversely proportional to the wavelength squared. Optical frequencies have wavelengths that are many orders of magnitude less than that of radio frequencies used for communications. Pulsed optical communications systems utilizing pulsed fiber lasers or Q-switched lasers allow high optical energies per pulse, while exhibiting only modest average power levels. This is important for long distance communications, because accurate detection of these pulses depends on the number of photons (or energy) received.
The most recent demonstration of long haul laser communications from space was the LLCD on board LADEE. This system demonstrated a record breaking 622 Mbps downlink and 20 Mpbs uplink over lunar distances (~400 Mm). There are several similarities between LLCD and the MOCT, including the use of Pulse Position Modulation (PPM) and a fiber amplified 1550 nm laser diode system. However, the primary technical driver for MOCT is to minimum size, weight, and power (SWaP), while maintaining similar data rates as LLCD. To achieve a reduction in SWaP, the MOCT sacrifices some bandwidth relative to LLCD and takes advantage of several efficiency maximizing techniques. One such technique is using fewer average laser pulses per second to lower the average power consumed.
The MOCT system is composed of two main technologies: the software-defined pulse modulator (SDPM) and the master oscillator power fiber amplifier (MOPFA) laser system. Both subsystems take advantage of precision timing components and electronics developed in the Precision Space Systems Lab for the CHOMPTT laser time-transfer CubeSat mission, which will demonstrate ground-to-space clock synchronization to 200 ps. The CHOMPTT CubeSat is manifested for launch in mid-December 2018, through NASA’s CubeSat Launch Initiative program.
The MOPFA system for MOCT is a 1550 nm fiber laser with target pulse widths of ~100 ps and pulse energies of up to 10 µJ. To keep the average power low, the minimum period between pulses, or guard time Tg, is chosen to be longer than traditional pulsed laser communications systems. This allows lower power pump lasers to more slowly store the needed energy in the erbium fiber. The MOPFA uses a ~50 mW 1550 nm seed laser diode with a custom gain-switched pulse driver. The light pulses emitted by this laser are fed into a single stage erbium-doped fiber amplifier, reversed-pumped by a ~4 W maximum, 980 nm laser diode.
The SDPM generates rising-edge triggers for the seed laser diode driver with 50 ps precision. The modulator is entirely implemented in a Field Programmable Gate Array (FPGA) to keep power, volume and complexity under control. A compact, low power cesium oscillator, the Chip-scale Atomic Clock (CSAC), is used as reference to generate the delays. The signal of the CSAC is fed to a delay-locked loop to account for environmental effects on the FPGA modulator. 
MOCT is designed to be a versatile instrument useful for a fairly wide array of mission concepts. Given its low size, weight and power requirements, and its software-defined modulation rate, it could be incorporated in a CubeSat of at least 3U or on larger deep space spacecraft that are power-constrained.
To create a design space for MOCT, three optical link scenarios were investigated: Moon-to-Earth, Low Earth Orbit (LEO) crosslink, and LEO-to-Earth. These links could be incorporated into a nanosatellite optical communication network that could relay information around Earth and as far as the Moon. It could also be adapted for other astronomical bodies (i.e. Mars) so lasercom could be utilized throughout the solar system, where power efficiency is desirable and communication windows are short. Analysis of these three optical link scenarios led to a required range of optical peak powers of 1 W to 1 kW, for a photoreceiver capable of reliably detecting 200 nW of peak power. 
During the first two years of this ECF project, we Developed a hardware prototype of the SDPM and the MOPFA. The SDPM consisted of a commercial FPGA, a precision oscillator and associated software. We used this system to demonstrate that the SDPM using pairs of delay chains could generate electrical pulses used to trigger the laser system with a timing accuracy of ~10 ps, well below our timing accuracy goal of 100 ps. We also selected an FPGA softcore and designed a timing calibration scheme accurate to 10 fs level using the softcore’s limited resources.
We assembled and calibrated a hardware testbed for the Master Oscillator Power Fiber Amplifier (MOPFA) laser system. A 1550 nm seed laser was mounted on a custom PSSL pulse driver board called the Fast Laser Advanced Switching High-frequency Emitter (FLASHE). FLASHE is capable of generating subnanosecond pulses at a repetition rate greater than 20 MHz. The seed laser is fed into an erbium-doped fiber amplifier that is pumped by a 980 nm laser diode. To facilitate the design of flight laser systems for MOCT in the future, we created a numerical model describing the energy level populations inside the erbium doped fiber amplifier. This simulation was compared with measurements made with the MOPFA testbed to validate laser amplifier designs. Using the experimentally validated numerical model, a flight version of the MOPFA was designed to meet the requirements for the candidate mission scenarios discussed above. In the last year of the ECF project, we integrated and performed end-to-end testing of the Miniature Optical Communications Transceiver prototype. These tests included end-to-end transmission of pseudorandom strings of test data at rates in the range of ~50 Mbps. We demonstrated error-free communications using pulse energies higher than expected for the link scenarios analyzed. We are currently performing tests that will measure the bit error ratio of the system for realistic pulse energies at the photoreceiver. We also used this testbed to measure the pulse jitter of the entire system, which was 50 ps, and showed that the MOPFA did not adversely impact the timing jitter of the SDPM and seed laser driver.
The MOCT project at the University of Florida received additional support from the Space Technology Mission Directorate’s Small Satellite Technology Partnerships (SSTP) program. This follow-on project is advancing technical readiness of MOCT from TRL 3 to TRL 5 by mid 2019. The SSTP grant also set up a partnership between UF and the NASA Ames Research Center (ARC) with the goal of developing a CubeSat demonstration mission for MOCT.
In 2017, the UF and NASA ARC teams partnered with the STAR lab at MIT to develop the CubeSat Laser Infrared CrosslinK (CLICK) mission, which subsequently became a NASA-led mission with the laser crosslink payload provided by an MIT/UF collaboration. The CLICK mission, planned for launch in ~2020 will demonstrate a >20 Mbps optical cross link between two 3U CubeSats separated by ~100-600 km. The mission will also demonstrate time-transfer and intersatellite ranging at the 10 cm-level. The CLICK payload incorporates several elements of MOCT, including the SDPM modulator/demodulator that references a chip-scale atomic clock (CSAC), as well as MOCT’s avalanche photodetector-based photoreceiver system utilizing a time-to-digital converter for precision time-stamping.
In the future, we envision using this optical cross-linking technology to provide communications and precision orbit determination for nanosatellites flying beyond the GPS sphere or to enable small satellites to provide navigation solutions and data relays to human or robotic explorers operating near the Moon or Mars. It will allow constellations of Earth orbiting nanosatellites to network with one another, to precisely correlate astronomical or Earth observations or to coordinate spacecraft flying in formation to produce a distributed aperture telescope.