Topological Charge Detection Using Generalized Contour-Sum Method from Distorted Donut-Shaped Optical Vortex Beams: Experimental Comparison of Closed Path Determination Methods

Abstract

A generalized contour-sum method has been proposed to measure the topological charge (TC) of an optical vortex (OV) beam using a Shack–Hartmann wavefront sensor (SH-WFS). Moreover, a recent study extended it to be workable for measuring an aberrated OV beam. However, when the OV beam suffers from severe distortion, the closed path for circulation calculation becomes crucial. In this paper, we evaluate the performance of five closed path determination methods, including watershed transformation, maximum average-intensity circle extraction, a combination of watershed transformation and maximum average-intensity circle extraction, and perfectly round circles assignation. In the experiments, we used a phase-only spatial light modulator to generate OV beams and aberrations, while an SH-WFS was used to measure the intensity profile and phase slopes. The results show that when determining the TC values of distorted donut-shaped OV beams, the watershed-transformed maximum average-intensity circle method performed the best, and the maximum average-intensity circle method and the watershed transformation method came second and third, while the worst was the perfect circles assignation method. The discussions that explain our experimental results are also given.

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“…In terms of Optical Coherence Tomography (OCT), Yi et al [11] report a mesh-based Monte Carlo model in order to study OCT signals reflecting the structural and functional activities of brain tissue as well as to improve the quantitative accuracy of chromophores in tissue. Furthermore, Wang et al [12] evaluate the performance of different closed path determination methods in order to measure the topological charge (TC) of an optical vortex (OV) beam and Fricke et al [13] present a non-contact dermatoscope with ultra-bright light source and liquid lens-based autofocus function. Moreover, Fricke et al [13] could demonstrate, i.e., with their prototype, feature resolution of up to 30 µm and feature size scaling fulfilling the requirements to apply the device in regular skin cancer screening.…”

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“…For example, an optimal contour for the experimental measurement of TC by using a Shack–Hartmann sensor was studied in Ref. [ 9 ].…”