Silicon Photomultipliers Implemented as Free-Space Optical Communication Sensors

Free-space optical communications (FSOC) is an emerging field offering an alternative to the standard radio frequency (RF) communications. Optical communication systems have efficient systems with lower size, weight, and power (SWaP) terminals compared with RF technologies. The narrow beam divergence of FSOC terminals enables power efficiency and security over long-range communication links. FSOC terminals can be used by platforms constrained by size, such as satellites. The use of FSOC terminals for smallsats enables higher data rates but requires precise pointing. NASA's CubeSat Laser Infrared Crosslink (CLICK)-B/C mission is an example that utilizes a beacon-based pointing, acquisition, and tracking (PAT) system to correct for angular misalignment, and an avalanche photodiode (APD) receiver to detect the communication signal. CLICK-B/C's high gain APD allows signal detection over distances ranging from 25 km to 580 km. This work explores the performance of Silicon Photomultipliers (SiPMs) as optical communication sensors. SiPMs, which are arrays of APDs operating in Geiger mode, produce nanosecond output pulses with with gains in the order of 10<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> electrons per photon. This paper presents the experimental results of implementing a SiPM in a 2x2 array configuration as a dual pointing and communication sensor for FSOC terminals in LEO. The proposed SiPM setup directly measures the misalignment between the transmit and receive terminals, eliminating the need for a dedicated beacon laser and quadcell detector for PAT. The pointing performance of the proposed SiPM configuration is characterized by measuring the noise equivalent angle (NEA) experimentally, while the communication performance is assessed by testing the maximum detectable pulsing frequency of a laser. Experimental results indicate an NEA smaller than 70 urad and a maximum detectable pulse rate of 80 MHz.