ALIGN: 1 Gbps Optical Inter-Satellite Link Demonstration on 6U CubeSats with the FOCUS Terminal

Free-space optical (FSO) inter-satellite links (OISLs) offer dramatically higher data throughput than traditional radio-frequency (RF) systems for satellite constellations. This paper presents the Free-space Optical Communications Unit for Satellites (FOCUS), a compact 2U payload for 6U CubeSats - the 6U form factor referring to a ∼10×10×30cm nanosatellite platform - demonstrating autonomous >1Gbps optical links at ranges up to 1000km in Low Earth Orbit (LEO). The terminal employs non-return-to-zero on-off keying (NRZ-OOK) modulation at 1.25Gbps. Component-level characterisation using an arbitrary waveform generator (AWG) test setup demonstrates receiver sensitivity below -37dBm, measured bit error rate (BER) as low as 10^-8 under laboratory conditions, closed-loop pointing accuracy below 50μrad using a fast-steering mirror (FSM), and AWG-based end-to-end throughput characterisation at 1.25Gbps. Compared with prior CubeSat-class OISL demonstrations, FOCUS achieves the highest reported ISL data rate at equivalent or better receiver sensitivity within a 2U payload volume. Two 6U CubeSats are planned to launch, subject to funding confirmation, as part of the Autonomous Laser InterSatellite Gigabit Network (ALIGN) mission, funded by the UK Space Agency, to validate optical ISL performance at orbital ranges from 100km to 1000km.