Optical alignment for CubeSat crosslink laser communication terminals

Optical communication can revolutionize satellite networks by enabling high-data-rate, power-efficient links. The CubeSat Laser Infrared CrosslinK (CLICK) mission demonstrates optical crosslinks between small spacecraft using commercially available components in a low size, weight, and power (SWaP) 3U CubeSat platform. The mission consists of CLICK-A, which demonstrated an optical downlink from low-Earth orbit in 2022, and CLICKB/C, which will establish full-duplex optical crosslink communication at 1550 nm, and data rates of up to 20 Mbps over 25–450 km. CLICK-B/C's fine-pointing control combines spacecraft body pointing with a fine steering mirror using MEMS technology. This work focuses on the alignment of the CLICK-B/C full-duplex crosslink optical terminals. The payloads use two beams, a beacon beam at 980 nm and the data transmit beam at 1537 and 1563 nm, each transmit wavelength corresponding to a specific payload. In order to meet the mission data rate and range requirements, the alignment objective for the optical terminal is to achieve a 0.121 mrad transmit beam divergence and 39 µrad pointing accuracy. Optical link budgets for CLICK are used to assess the impact of optical alignment on mission performance, modeling as-built conditions including optical power, beam divergence, pointing accuracy, and receiver sensitivity. Methods for alignment and divergence testing have been developed, with measured flight telescope divergences of 0.125 ± 0.007 mrad. Optical alignment of the full terminals and their integration with the spacecraft are also underway. In this work, we discuss the alignment and evaluation of the as-built CLICK optical terminal, and use a Python optical link budget model with updated parameters such as divergence, alignment, pointing losses, and inter-satellite distance and assess effects on the mission signal-to-noise ratio (SNR) and bit error rate (BER). Initial investigation into the link budget shows that, with the achieved divergence the operational link range exceeds 350 km and has potential to close beyond 450 km. These results will inform final payload integration and advance high-data-rate optical communication for future small satellite constellations.