Dissertation Defense: Jeff Richey, "Astronomical Wavefront Sensing with Partially Coherent Beacons"

    Monday, June 27, 2022 - 2:00pm

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    Astronomical adaptive optics (AO) systems and wavefront sensors (WFS) have advanced significantly over the past three decades. However, WFS development has typically focused on applications with reference sources that can be approximated by the coherent or incoherent limit. This work seeks to extend the design and optimization of astronomical WFS to sources that cannot be adequately approximated by either extreme of spatial coherence. These partially coherent sources are not numerous but include the important case of laser guide star (LGS) systems using AO to pre-compensate the laser uplink, achieving a beacon size below the limit imposed by atmospheric turbulence. This work develops a theoretical treatment for partially coherent wavefront sensing and derives a numerical approximation of the partially coherent Fisher information matrix (FIM) for a common class of astronomical WFS. The derivation provides insight into the degradation in the sensitivity of a wavefront measurement with a partially coherent source and suggests an approach for mitigating the deleterious effects by limiting interference to coherent point pairs in the pupil. The derived numerical approximation is further developed into a new approach for simulating wavefront sensors with a partially coherent beacon using the coherent impulse response of the wavefront sensor and the spatial coherence magnitude at the entrance pupil to directly simulate the effects of spatial coherence on the wavefront sensor. Using this new approach, two novel methods for optimizing WFS sensitivity with partially coherent sources are explored: pupil segmentation and impulse response engineering. The former method takes the form of a hybrid WFS which uses a lenslet or other method to subdivide the WFS entrance pupil into subapertures and then applies a high sensitivity coherent wavefront sensing method to measure the wavefront for each subaperture. The latter method modifies the coherent impulse response to limit the interference between points in the pupil to a coherent area defined by the spatial coherence of the source. Simulation results, using both the coherent response method and the more traditional Fresnel propagation method, indicates a significant decrease in the photon noise gain can be achieved with each approach. The efficacy of the pupil segmentation approach was demonstrated using a novel hybrid Shack-Hartmann Pyramid WFS setup while the impulse response engineering approach was explored using modifications to a Zernike WFS. The simulation results were further confirmed using a novel software-defined WFS installed on a bench top AO system to directly measure the sensitivity of various WFS configurations with an extended beacon simulating a partially coherent source. The bench-level results showed good agreement with the simulation results confirming the validity of the proposed approaches for optimizing WFS sensitivity with a partially coherent source.