Adaptive filter techniques for optical beam jitter control

Abstract

The objective of this research is to develop control methods to attenuate laser beam jitter using a fast-steering mirror. Adaptive filter controllers using Filtered-X least mean square and Filtered-X recursive least square algorithms are explored. The disturbances that cause beam jitter include mechanical vibrations on the optical platform (narrowband) and atmospheric turbulence (broadband). Both feedforward filters (with the use of auxiliary reference sensor(s)) and feedback filters (with only output feedback) are investigated. Hybrid adaptive filters, which are a combination of feedback and feedforward, are also examined. For situations when obtaining a coherent feedforward reference signal is not possible, methods for incorporating multiple semi-coherent reference signals into the control law are developed. The controllers are tested on a jitter control testbed to prove their functionality. The testbed is equipped with shakers mounted to the optical platform and a disturbance fast-steering mirror to simulate the effects of atmospheric propagation. Experimental results showed that the feedback adaptive filter controller was superior to the feedforward technique, and the hybrid method achieved the best overall results.