Shack-Hartmann Wavefront Sensing of Optical Vortices in a Turbulent Field
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An optical vortex is a point on an optical wavefront where there is a null of intensity and the phase is undefined. When the phase is defined over the range 2 pi radians, as is commonly done, the phase surrounding the optical vortex takes on all possible values in an incremental manner, resulting in a spiral phase structure. Optical vortices are of interest to fields as diverse as optical manipulation and quantum communications. Here the case of optical communications through the turbulent atmosphere is concentrated on. A Shack-Hartmann wavefront sensor is a common component of many adaptive optics systems and is used to determine the phase of an optical wavefront. In this thesis it is employed to detect the presence and location of optical vortices in the field. A matched filter technique is explained and applied in practice to give an accurate estimate of the Shack-Hartmann spot positions and, thus, accurate wavefront slope values. Two methods of determining the presence and location of optical vortices in the field using these slope values are presented; the branch point potential method and the contour sum method. The reconstruction of wavefronts with optical vortices is not a trivial task and most of the standard reconstructors fail to do so correctly. Two methods of recovering the wavefronts accurately are given, along with associated numerical work. An optical system has been built to test these methods in the laboratory and is described here schematically and in detail. A spatial light modulator is utilised to mimic atmospheric turbulence by applying atmospherically aberrated wavefronts to it. The method used to create these wavefronts is outlined and tested. The approaches employed in the experimental work are specified. Results are obtained comparing the performance of the branch point potential method to that of the contour sum method for optical vortex detection under various atmospheric conditions. These include a range of phase distortions, intensity scintillations, Shack-Hartmann lenslet samplings and for differing spatial separations of optical vortices in the field.
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