Detection of topological charge for composite perfect vortex beams in atmospheric turbulence

Aiming at the problem of detecting the orbital angular momentum (OAM) states of the composite perfect vortex beams (CPVBs) under atmospheric turbulence (AT) channels, we propose and investigate a joint scheme of combing an optical detection method and the Gerchberg-Saxton (GS) algorithm. We firstly used the GS algorithm to pre-compensate the distorted CPVBs, and then the optical intensity distribution, together with the far-field diffraction patterns of the CPVBs after they passing through an elliptical aperture which shields part of the azimuth angle (EA-shielded), can tell us the topological charges (TCs) carried by each sub beam. The results show that the tailing direction of the bright stripes in the CPVBs intensity distribution denotes the sign of the higerh-order TC, counterclockwise is positive, clockwise is negative. The difference between the TCs of the two beams is indicated by the number of stripes, that is, the number of stripes is equal to the absolute values of differences between the two beams. After passing through the EA-shielded, the far-field diffraction intensity patterns is related to the magnitude and sign of the lower-order TC. If the bright stripes are distributed above, the TC is positive, otherwise, the TC is negative, and the number of dark stripes is equal to the lower-order TC-1. The results provide a new method for detecting the TC information of the CPVBs after phase distortion.