Branch-point detection using a Shack-Hartmann wavefront sensor

Abstract

In this paper, two methods for identifying branch points from Shack–Hartmann wavefront sensor (SHWFS) measurements were studied; the circulation of phase gradients approach and the beam-spread approach. These approaches were tested using a simple optical-vortex model, wave-optics simulations, and with experimental data. It was found that these two approaches are synergistic regarding their abilities to detect branch points. Specifically, the beam-spread approach works best when the branch point is located towards the center of the SHWFS’s lenslet pupil, while the circulation of phase gradients approach works best when the branch point is located towards the edge of the SHWFS’s lenslet pupil. These behavior were observed studying the simple optical-vortex model; however, they were further corroborated with the wave-optics and experimental results. The developments presented within support researchers looking to study high scintillation optical-turbulence environments as well as will inform efforts looking to develop branch-point tolerant reconstruction algorithms.

Experimental and computational investigation of Shack–Hartmann wavefront sensor measurements through a free shear layer

“…It has recently been shown that when shock-wave-induced phase discontinuities 38,41 as well as branch-point-induced phase vortices 34,42,43 are located within the SHWFS subaperture lenslet pupils, there is appreciable beam spreading in the resultant image-plane irradiance patterns. Therefore, it is also expected that if sharp shear-layer-induced phase gradients intersected the SHWFS subaperture pupils, and the SHWFS lenslet resolution is coarse, similar localized beam spreading would be observed.…”

Scaled-laboratory demonstrations of deep-turbulence conditions

“…Similar to computational wave-optic simulations [37,Chapter 9], we can also command repeatable high-resolution phase screens to the reflective LcPMs with the proper path-integrated Kolmogorov statistics (both spatial and temporal). Since deep-turbulence conditions can provide Greenwood frequencies on the order of 1 kHz for some beam-control applications [1-4], this scaled-laboratory setup does not allow for realtime demonstrations since the max framerates for the commercial-off-the-shelf LcPMs are on the order of 100 Hz for visible light; however, it does provide a flexible scaled-laboratory environment in which to test novel beam-control solutions, such as those which use branchpoint-tolerant phase reconstructors [38][39][40][41][42][43][44][45][46][47].…”

Comparison of branch-point detection approaches using a Shack–Hartmann wavefront sensor

“…5 Conversely, a new approach that utilizes a subaperture irradiance pattern beam spread to identify branch points has recently been introduced. 32,33 Although other approaches exist, [34][35][36][37][38] these two approaches are the ones emphasized in this paper. Here, the circulation of phase gradients approach and beam-spread approaches are compared using a simple optical-vortex model, wave-optics simulations, and experimental data.…”