Gregory Allan, Massachusetts Institute of Technology; Lulu Liu, Massachusetts Institute of Technology Lincoln Laboratory; Kerri Cahoy, Massachusetts Institute of Technology
Keywords: speckle, shack, hartmann, laser, deorbit
Abstract:
Adaptive optical compensation for imaging or illumination of a remote object relies on accurate sensing of atmospheric aberrations. In the case of a laser system for engagement of space debris, the reflected light from the target can be used as an artificial guide-star for wavefront sensing (WFS). This WFS modality is desirable since, unlike a laser guide-star, it is sensitive to tip-tilt aberrations and is not subject to focal anisoplanatism. However, the coherent illumination of a target object with a rough surface results in random interference in the reflected wave, and ultimately in speckling of phase and amplitude in the far field. In a Shack-Hartmann Wavefront Sensor (SHWFS) measurement, these speckles cause aberration in the intensity of focal spots and random errors in the position of their centroids relative to those expected from purely atmospheric effects. The resulting wavefront measurements therefore cannot be used for proper atmospheric phase conjugation. This paper discusses the effect of laser speckle on the accuracy of SHWFS measurements. Mitigation of other sources of residual wavefront error, such as anisoplanatic effects due to target motion and the finite speed of light, will not be discussed here.
The statistics of the random speckle pattern are determined by the properties of the target and imaging geometry. It is commonly considered that if the target is rotating or translating at a high rate, the errors due to individual speckle pattern realizations are averaged, and acceptable WFS performance can be achieved. Conditions for complete speckle averaging are not always met, but operation of a wavefront sensing system in this case is not precluded. In order to assess the degradation of wavefront sensing performance due to speckle noise prior to system implementation, we perform a rigorous analysis of the effects of incomplete speckle averaging in a SHWFS.
In particular, we focus on a hypothetical system for engagement of space debris using a phased array of CW lasers. Wavefront sensing in this system is performed using a distributed SHWFS made up of discrete apertures. Using a simulation of speckle propagation in homogeneous media we model the dependence of wavefront sensing performance on optical system parameters as well as the properties of the target and its motion. The simulation is validated by laboratory experiments and an analytical approach.
Date of Conference: September 15-18, 2020
Track: Optical Systems Instrumentation