Eight-connected contour method for accurate position detection of optical vortices using Shack–Hartmann wavefront sensor

We propose a simple method of realizing an accurate position detection of phase singularities in an optical vortex (OV) beam using a Shack-Hartmann wavefront sensor (SH-WFS). The method calculates circu- lations which are the discrete contour integrals of phase slope vectors measured by the SH-WFS and then determines the accurate positions of the singular points by calculating the centers-of-gravity with a fixed window size around the local peak of the circulation distribution. We use closed paths that connect the centers of eight- connected, instead of 2 × 2-neighboring lenslet apertures for calculating the circulations. Both the numerical analysis and proof-of-principle experiment were performed to confirm the measurement accuracy. In experi- ments, the positions of singular points in OV beams generated by a liquid-crystal-on-silicon spatial light modu- lator were measured. The root-mean-square error of the position measurement was approximately 0.09 in units of the lens size of the lenslet array used in the SH-WFS. We also estimated the topological charges of the singular points being detected based on the peak circulations, and the results agreed well with theoretical ones. The method achieves both rapid implementation and sublens-size spatial resolution detection and is suit- able for applications that require real-time control of OV beams. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its