Few-mode Tracking for Free-space Optical (FSO) Communications

A critical function of a lasercom terminal is alignment with a distant companion terminal. Typically, an active Pointing, Acquisition, and Tracking (PAT) system is employed for this purpose. Such systems often require notable Size, Weight, and Power (SWaP) and a pickoff of received power that could otherwise be used by the communication channel. The technical challenges of PAT are further compounded by platform jitter/motion and turbulence. Here, we explore tracking with a few-mode receiver. Few-mode reception allows for greater coupling efficiency and increased tolerance to misalignment compared to single-mode receivers. These factors allow for higher-performance lasercom terminals with relaxed requirements. Additionally, tilted incident wavefronts couple in unique ways to the spatial modes of the few-mode receiver, and modal power distribution can be used to recover the incident tilt angle. Recovery of the incident angle enables precise correction of pointing errors. Use of a few-mode receiver as a fine tracking sensor was analytically characterized and experimentally demonstrated. Analytic and experimental results for coupling efficiency and incident angle recovery from spatial mode power distribution are discussed.