Beaconless pointing and tracking for bidirectional optical links using MEMS mirror nutation

Abstract

Free-space laser links traditionally utilize an independent spatial tracking channel with a beacon laser and tracking sensors to meet stringent pointing requirements. In this work, we propose a miniaturized monostatic beaconless fiber transceiver that infers fine tracking information using existing receiver optoelectronics and a small injected pointing dither (nutation). A single MEMS steering mirror is used to both fine-point the beams and inject nutation. While this results in some additional link loss due to disturbed fiber coupling and transmit beam pointing, our analysis shows the loss becomes negligible for sufficient SNR. Links without point-ahead correction need an SNR of about 35 dB to minimize the dither loss below 0.1 dB and also maintain the RMS spatial tracking noise below a tenth of the beam divergence. Since the pointing and tracking bandwidth is typically many orders of magnitude slower than the receiver communication bandwidth, such SNR is usually achievable on the receiver with appropriate filtering. If point-ahead correction is needed, we show that depending on the available link margin, a transceiver based on single-mode fiber can reach up to about 1 beamwidth of correction, while a few-mode fiber design can reach up to about 1.75 beamwidths due to improved coupling sensitivity at higher point-ahead offsets. Finally, we propose the use of double-clad fiber with a secondary detector to help further minimize the incurred coupling loss.

MOEMS-based Lens-Assisted Beam Steering for Free-Space Optical Communications

“…Various methods are used to perform these functions, including gimbals [12], fast steering mirrors (FSMs) [13], adaptive optics [14], RF-FSOC hybrids [15], and others. Many technologies for steering the optical beam have been developed, including small micro-electromechanical system (MEMS) mirrors or mirror arrays [16] - [18], liquid lenses or variable focal length lenses [19] - [21], optical phased arrays [22] [23], liquid crystal electro-optic scanners [24], and image-based beam steering [25] - [27].…”