Sugathevan S., Agrawal B.

Optical beam pointing and jitter control have become important research topics in recent years due to their growing applications ranging widely from laser communications to space missions. The ever growing demands such as arc-second accuracy and nano-radian jitter make this problem very difficult and challenging. The control of jitter is very crucial to precise pointing and tracking because the presence of jitter reduces the intensity of the laser beam and causes fluctuations in the optical beam pointing. In order to study the effects various disturbances and to develop and test the efficient disturbance attenuation techniques, a unique test bed is developed in the Spacecraft Design and Research Center at Naval Postgraduate School. This test bed is made up of a Disturbance Injection Fast Steering Mirror, a Control Fast Steering Mirror, position sensing devices, beam splitters, folding mirrors, a shaker and an accelerometer. The components are mounted on a Newport Optical Bench. Jitter attenuation problem for this optical test bed is considered in this paper and several feedback and adaptive control methods are tested. After showing analytically that the standard state feedback lacks the required degree of freedom for good broadband disturbance attenuation, the standard setup is modified by introducing an additional output feedback. It is argued that having a lowest sensitivity function gain at low frequencies is very crucial to attenuate the effects of disturbance and this can be better achieved by the proposed approach than the standard state feedback approach. The control laws for broad band jitter are derived using Linear Quadratic Regulator algorithm and to suppress the narrowband jitter, Least Mean Square algorithm is used. The proposed control laws are implemented on the optical laser test bed. The experimental results show that the proposed control laws provide the required broadband as well as narrowband disturbance attenuation.