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

Wang Wang,Liu Liu,Toyoda Toyoda,Huang Huang

doi:10.3390/app9193956

Wang Wang, Liu Liu + Show 2 more

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https://doi.org/10.3390/app9193956

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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.

Highlights

Optical vortex (OV) beams, owing to their unique properties, have attracted more and more interest and have been utilized in a wide range of fields—from scientific research to advanced technology applications, such as optical communications [1,2,3,4], optical metrology [5,6,7], and optical trapping and manipulation [8,9,10]
We introduce three methods that conform to the ridge-extracting strategy, which are watershed transformation (WT), maximum average-intensity circle (MAIC) extraction, and the combination of watershed transformation and MAIC extraction (WT-MAIC), as well as two methods that conform to the perfectly round circle generation strategy, which are the fixed-center perfectly round circle method (FC-PRCM) and the shifting-center perfectly round circle method (SC-PRCM)
We displayed various computer-generated holograms (CGHs) with a spiral phase structure on the LCOS-SLM to generate an optical vortex (OV) beam with topological charge (TC) values ranging from ± 1 to ± 20

Summary

Introduction

Optical vortex (OV) beams, owing to their unique properties, have attracted more and more interest and have been utilized in a wide range of fields—from scientific research to advanced technology applications, such as optical communications [1,2,3,4], optical metrology [5,6,7], and optical trapping and manipulation [8,9,10]. The phase along a closed path enclosing the singularity point varies from 0 to 2nπ, where n is an integer known as the topological charge (TC) or the orbital angular momentum (OAM). Sci. 2019, 9, 3956 manipulate the molecules and so on. To meet the requirements of these applications, the determination of how to precisely measure the TC value of an OV beam is an important issue, and many methods, such as interferometry-based methods [13], diffraction-based methods [14,15,16], and model decomposition-based methods [17,18] have been proposed and comprehensively studied

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