Wide-range fine pointing mechanism for free-space laser communications

A breadboard model of a Wide-range Fine Pointing Mechanism (WFPM) designed for free-space laser communications and composed of electromagnetic actuators and a flexible support system has been developed. The WFPM is compact (45 x 45 x 39 mm3) and light in weight (160 grams). Its actuators, which consist of four moving-coil-type motors with long strokes, ensure a wide optical scan range, +/-4 degrees. The support system, which consists of a center torsion bar and four preformed thin springs, allows the mechanism's 20 mm-diameter mirror to be rotated freely about a vertically constant point; the mirror can be rotated about its x- and y-axes simultaneously. The breadboard model has passed a launch vibration test under conditions of 20 G rms overall. With its coarse and fine quadrant detectors, the WFPM is able to provide stable acquisition and tracking without any need for built-in sensors. The digital control system of the WFPM has a 1 kHz control bandwidth and -90-dB disturbance suppression at 1 Hz. Use of the WFPM by an acquisition and tracking terminal significantly shortens acquisition time and ensures accurate tracking. Acquisition and tracking simulations have shown fast acquisition time (58.7 ms for 3.5-milli-radian initial error) and high tracking precision (+/-1 micro-radian, 3 sigma). To maintain high tracking precision, we employ a tradeoff between noise reduction and disturbance suppression. The WFPM has the potential to play an essential role in helping to provide high-data-rate free-space laser communications.