Small inexpensive satellite platforms, such as cubesats, offer opportunities for pathfinder experiments, space qualification of components and systems, and enhancements of larger assets. The Aerospace Corporation has been developing cubesats with lasercom transmitters for downlinking payload data to the ground from orbit. Recently we demonstrated a 200 Mbps link with a 1.5 U cubesat, AeroCube-7B, under NASA’s Optical Communications and Sensors Demonstration program . The nearly error free link was accomplished using on-board star trackers for attitude determination and control and without any forward error correction. The capability developed under this effort has been implemented in follow-on missions for two 3U AeroCube- 11 vehicles where camera recorded data has been downlinked to the ground. While very modest data volumes (~1.5 GB) have been transmitted (to date) in a single pass under non-ideal conditions, the utility of this architecture for certain types of missions has been demonstrated. Additional experiments involving the illumination of the ISARA cubesat with the laser from AeroCube-7B demonstrated the ability to locate and illuminate a remote satellite in orbit, a necessary step towards realizing intersatellite links with this class of vehicles.
In this presentation, we discuss the first demonstration of a lasercom downlink from a LEO 1.5U 2.3 kg CubeSat to our optical ground station at The Aerospace Corporation in El Segundo, CA. Two vehicles, AC7-B&C, built under NASA’s Optical Communications and Sensors Demonstration (OCSD) program and described in previous presentations, 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 manuscript submission) communications links up to 100 Mbps with bit error rates near 10-6 without any forward error correction. Further optimization of the vehicle pointing and detection electronics and operating the transmitter at its full power capacity should enable performance improvements and potential for higher data rates.
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.
A 1064 nm, 1 mJ pulsed fiber MOPA module, housed in 16”x14”x2.5” package for application in a lunar and planetary in-situ surface dating instrument is demonstrated. The module is based on a three-stage MOPA with a 60 μm core tapered fiber terminal amplifier. The master oscillator and first two preamplifier stages, which generate 20 μJ pulses, are all contained on a 13”x11”x1” board. Several improvements to the electronic signal control were instrumental to the laser development, including bipolar drive of the phase modulator for SBS suppression, shaping of the seed pulse to compensate pulse steepening, and pulsed operation of the power amplifier pump to reduce spontaneous emission at low pulse repetition frequency. The packaged laser runs at a repetition rate of 10 kHz and generates 10 ns pulses at 1 mJ with a 40 GHz linewidth, an M2 ~ 1.2 beam quality, and an 18 dB polarization extinction ratio. The modular design enables seven independent lasers to be stacked in a 20”x18”x16.25” enclosure, supporting a path towards a fiber laser based LARIMS for advanced materials characterization and chronological dating in harsh and remote environments.
We demonstrate high power, deep ultraviolet (DUV) conversion to 266 nm through frequency quadrupling of a nanosecond pulse width 1064 nm fiber master oscillator power amplifier (MOPA). The MOPA system uses an Yb-doped double-clad polarization-maintaining large mode area tapered fiber as the final gain stage to generate 0.5-mJ, 10 W, 1.7- ns single mode pulses at a repetition rate of 20 kHz with measured spectral bandwidth of 10.6 GHz (40 pm), and beam qualities of Mx2=1.07 and My2=1.03, respectively. Using LBO and BBO crystals for the second-harmonic generation (SHG) and fourth-harmonic generation (FHG), we have achieved 375 μJ (7.5 W) and 92.5 μJ (1.85 W) at wavelengths of 532 nm and 266 nm, respectively. To the best of our knowledge these are the highest narrowband infrared, green and UV pulse energies obtained to date from a fully spliced fiber amplifier. We also demonstrate high efficiency SHG and FHG with walk-off compensated (WOC) crystal pairs and tightly focused pump beam. An SHG efficiency of 75%, FHG efficiency of 47%, and an overall efficiency of 35% from 1064 nm to 266 nm are obtained.
A pair of 2.2 kg CubeSats using COTS hardware is being developed for a proof-of-principle optical communications demo from a 450-600 km LEO orbit to ground. The 10x10x15 cm platform incorporates a 25% wall-plug efficient 10-W Yb fiber transmitter emitting at 1.06 μm. Since there are no gimbals on board, the entire spacecraft is body-steered toward the ground station. The pointing accuracy of the LEO craft, which governs the data rate capability, is expected to be ~ 0.1-0.2 deg. Two optical ground stations, located at the Mt. Wilson observatory, have receiver apertures of 30 and 80 cm. Launch of the CubeSat pair is anticipated to be mid to late 2015.
The collection efficiency and collimation ability of high numerical aperture circular and cylindrical GaP lenses were evaluated using single index-guided and gain-guided laser diode emitters. Comparisons were made between 200 micrometers focal length cylindrical lenses (NA equals 0.75) fabricated by an accurate repetitive step-wise etching method and 70 micrometers focal length cylindrical lenses (NA > 1) fabricated by a simple resist reflow technique. Lens arrays (200 - 300 micrometers fl, 0.5 - 0.75 NA) fabricated by the repetitive resist-etch method were used to collimate the output of a diode bar consisting of 100 index-guided elements. Refocusing of the collimated light with a macro-optic (for pumping a Nd:YVO4 laser) produced a spot that was on average 2 - 3 times larger than the diffraction limit and contained up to 88% of the total bar output. The deviation from the theoretical limit was examined in terms of lens fabrication accuracy and alignment tolerances between the diode and lens arrays, which were shown to be on the order of 1 - 2 micrometers .
The performance of longitudinally pumped Nd:YAG was evaluated before and after exposure to 60Co gamma radiation. For comparison, other Nd-doped materials, Cr:GSGG and YLF, were also included in this study. The cw unirradiated optical-to-optical slope efficiencies for Nd:YAG and Nd:YLF were 63% and degraded to 48% and 36%, respectively, after 600 kRads of irradiation. Nd:Cr:GSGG performed significantly worse, exhibiting a slope efficiency of 42%, but was not affected by irradiation (a result that is in agreement with previous reports). Electron paramagnetic resonance studies of the Nd:YAG samples indicated that there was no modification of the Nd3+ sites resulting from exposure to the radiation. It is concluded from the performance and spectroscopic analysis that the degradation in Nd:YAG is primarily due to an induced passive optical loss of approximately 0.02 cm-1. Furthermore, this effect was observed to saturate at exposure levels of 50 kRad. The relatively low induced loss indicates that Nd:YAG systems employing pulsed diode pumping in the longitudinal configuration, should be resistant to ambient space environment radiation damage. This point was experimentally verified with respect to the effect of gamma rays on performance.