Resolution of Closely-Positioned Optical Vortices by Phase-Shifting Interferometry

Optical vortices have been usually identified by using the characteristic fork-like structure in the interference pattern formed between a reference beam and the optical vortex field. It might, however, lead to misinterpretation of the property of vortices when the reference beam's phase difference between vortices is less than pi. We confirmed this experimentally and propose that closely positioned optical vortices can be resolved better by analyzing the phase map obtained through phase-shifting interferometry.