Steering Mirror System with Closed-Loop Feedback for Free-Space Optical Communication Terminals

Graham, Chris; Bramall, David; Younus, Othman; Riaz, Amna; Binns, Richard; Scullion, Eamon; Wicks, Robert T.; Bourgenot, Cyril

Abstract

Precision beam pointing plays a critical role in free-space optical communications terminals in uplink, downlink and inter-satellite link scenarios. Among the various methods of beam steering, the use of fast steering mirrors (FSM) is widely adopted, with many commercial solutions employing diverse technologies, particularly focusing on small, high-bandwidth mirrors. This paper introduces a method using lightweight, commercial off-the-shelf components to construct a custom closed-loop steering mirror platform, suitable for mirror apertures exceeding 100 mm. The approach involves integrating optical encoders into two off-the-shelf open-loop actuators. These encoders read the signal reflected on purposefully diamond-machined knurled screw knobs, providing maximum contrast between light and dark lines. The resulting steering mirror has the potential to complement or replace FSM in applications requiring a larger stroke, at the expense of motion speed. In the presented setup, the mirror tilt resolution achieved based on the encoder closed-loop signal feedback is 45 μrad, with a mean slew rate of 1.5 mrad/s. Importantly, the steering assembly is self-locking, requiring no power to maintain a steady pointing angle. Using the mirror to actively correct for a constantly moving incoming beam, a 5-fold increase in concentration of the beam spot on the center of the detector was obtained compared to a fixed position mirror, demonstrating the mirrors ability to correct for satellite platform jitter and drift.

Citation

Graham, C., Bramall, D., Younus, O., Riaz, A., Binns, R., Scullion, E., Wicks, R. T., & Bourgenot, C. (2024). Steering Mirror System with Closed-Loop Feedback for Free-Space Optical Communication Terminals. Aerospace, 11(5), Article 330. https://doi.org/10.3390/aerospace11050330

• Journal Article Type: Article
• Acceptance Date: Apr 18, 2024
• Online Publication Date: Apr 23, 2024
• Publication Date: Apr 23, 2024
• Deposit Date: May 22, 2024
• Publicly Available Date: May 22, 2024
• Journal: Aerospace
• Electronic ISSN: 2226-4310
• Publisher: MDPI
• Peer Reviewed: Peer Reviewed
• Volume: 11
• Issue: 5
• Article Number: 330
• DOI: https://doi.org/10.3390/aerospace11050330
• Keywords: pointing acquisition tracking, CubeSat, acquisition, free-space optical communication, tracking, fast steering mirror, position sensing detector, low-earth orbit
• Public URL: https://durham-repository.worktribe.com/output/2437611