Dissertation Defense: Derek Burrell, "Speckle Phenomena in Active Electro-Optical Applications"

When

2:30 to 5:30 p.m., Aug. 11, 2023

Where

Title: Speckle Phenomena in Active Electro-Optical Applications

Abstract:

Active electro-optical systems benefit from stronger signal returns than their passive counterparts by providing scalable illumination power in otherwise limited ambient light. From an imaging standpoint this means higher signal-to-noise ratios for tracking purposes, leading to higher probabilities of detection, classification, recognition and identification of potentially distant objects. By creating an artificial beacon, active illumination also enables wavefront sensing where there is no natural beacon available to act as a reference source. In either application, however, actively illuminating an object of opportunity gives rise to a unique form of multiplicative noise known as speckle. A speckle pattern exhibits spatial variations in both amplitude and phase that result from diffuse scattering off an optically rough surface. Image quality suffers greatly from the presence of fully developed speckle, as the noise in such an image is on the order of the signal level itself. In the case of wavefront sensing, speckle contaminates measurements such that phase aberrations from the object become indistinguishable from those in the atmosphere. Mitigating speckle generally involves increasing the number of degrees of freedom in a speckle field, whether by manipulating coherence or polarization or system dynamics. The latter option allows access to a rich trade space for studying speckle mitigation on a wave-optics simulation basis. With that in mind, this work begins by exploring how different modes of object motion translate to varying degrees of speckle decorrelation in both the pupil and image planes of an optical system. Next, it derives scaling laws that describe the positional uncertainty associated with speckle to quantify active tracking performance. Adapting these scaling laws to the geometry of a Shack–Hartmann wavefront sensor then gives an indication of its open-loop performance limitations, while applying decorrelation theory extends the analysis to cover partially correlated frame-to-frame speckle. Finally, closing the loop on an adaptive-optics control system gauges the ability to compensate for atmospheric turbulence with both mitigated and unmitigated speckle noise. An additional chapter offers a system-level treatment of radiometric noise performance that includes both speckle and scintillation, and each of these theoretical contributions is validated through high-fidelity wave-optics simulations with strong agreement.