Topological Charge Recognition of Vortex Beams Using PDs

The efficient recognition of topological charge (TC) is an essential function for vortex beams-based applications. However, image acquisition devices and image processing algorithms are always demanded by conventional diffraction- or interference-based methods. In this study, we have proposed a frequency measurement method to recognize the amplitude of TC by using rotating slits and photodetectors (PDs) instead of image acquisition devices. In addition, the sign of TC for the conjugated vortex beams can be identified using a designed optical structure, which is composed of vortex retarders and conjugated vortex beams-based Mach-Zehnder interferometers (CVMZIs). The experimental results show the vortex beams with TC varying from -6 to +6 can been efficiently recognized, indicating the feasibility of the proposed method. To our knowledge, this is the first time that the recognition of amplitude and sign of TC of vortex beams has been realized using PD-based simple structure. Since no additional diffraction devices, image acquisition devices with limited sampling rate and resolution, and complex image processing algorithms are required, the proposed method can recognize TC of vortex beams low-costly, efficiently, precisely, and automatically, which may further promote and develop the TC recognition method for vortex beams.