High-Speed Optical Communications for CubeSats: End-to-End Terminal Demonstration and BER Performance Characterisation

Free-space optical communication (FSOC) is increasingly considered for small satellites because it can provide significantly higher data rates than RF while remaining compatible with strict size, weight, and power (SWaP) constraints. This capability is attractive for Earth-observation and science missions where downlink volume is a limiting factor. Despite this potential, CubeSat-class optical terminals are challenging to operate reliably in practice. Link performance is highly sensitive to pointing and alignment, onboard power and thermal margins are limited, and repeatable assembly, integration, and test (AIT) procedures are required before flight integration.

In this work, we present a laboratory end-to-end demonstration and performance characterisation of a CubeSat-scale optical communication terminal operating at 1550 nm. The system is developed within the ALIGN mission, a UK university-led multi-satellite optical communications programme, and is designed as a repeatable testbed for terminal-level verification. The platform integrates a compact transmitter/receiver optical chain with alignment monitoring instrumentation to support controlled indoor link testing and repeatable AIT workflows. The objective is to establish baseline system-level performance metrics and identify integration considerations that influence achievable link margin.

Terminal performance is characterised over an indoor free-space optical link by measuring received signal quality and bit error rate (BER) as a function of received optical power and operating conditions. The terminal supports 1.25 Gb/s data transmission and achieves measured BER down to 10^-9 under controlled laboratory conditions. Operational stability is assessed during extended runs, and sensitivity to misalignment is discussed, including its impact on received power and link quality. While outdoor propagation effects are outside the scope of this indoor campaign, the results provide a flight-relevant baseline for CubeSat FSOC terminal benchmarking and ongoing development toward payload-level integration and future flight demonstration.