Optimal beam steering using time-delayed wavefront measurements in airborne laser target engagements

Document Type

Conference Proceeding

Publication Date

12-1-1998

Abstract

In Airborne Laser (ABL) target engagements, beam steering should account for the effects of turbulence-induced wavefront tilt. Due to the finite temporal response of adaptive-optical systems, steering commands must be based upon time-delayed wavefront measurements. The dynamics of the ABL target engagement scenario and turbulence-induced anisoplanatism cause the time-delayed wavefront measurements to be decorrelated from the required wavefront tilt correction. This decorrelation leads to suboptimal beam steering when time-delayed wavefront measurements are used directly in the control loop. To assess beam steering performance, we calculate the aperture-averaged mean square residual phase error after tilt correction, as a function of measurement time delay, and show that this quantity is minimized by the minimum mean square error (MMSE) estimate of tilt given a set of time-delayed wavefront measurements. Therefore, a temporal MMSE tilt estimator is constructed using time-delayed tilt plus higher-order aberration measurements. We define the tilt correction interval τc as the time delay at which the residual phase error is 1% larger than its value at zero time delay. Our results show that τc using the MMSE tilt estimator with modal measurements through Zernike 15 is over 3 times larger than τc using time-delayed tilt measurements only. Accordingly, the beam steering bandwidth for ABL target engagements is reduced by more than 70% when higher order modal measurements are used in optimal tilt correction.

Publication Title

Proceedings of SPIE - The International Society for Optical Engineering

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