障碍物遮挡下漂移涡旋光束拓扑荷数的测量

Abstract

Objective Topological charge (TC) is a key factor to characterize the orbital angular momentum (OAM) of vortex beams. Accurate determination of TC is an essential prerequisite for the applications of OAM beams in optical communication and sensing. In complex environments, wandering perturbation and opaque obstacles destroy the amplitude and phase of the optical field and put challenges to accurate measurements. In this paper, we propose a method to determine the TC of obstructed wandering vortex beams. According to the theoretical results of the propagation of two uniform offaxis multicenter vortex beams and a random offaxis wandering vortex beam through a sectorshaped opaque obstacle (SSOO), it is observed that the averaged OAM of wandering vortex beams can reveal the TC of the input field, and even wandering perturbation and largeangle SSOO are encountered. Experimental measurement of the averaged OAM is carried out based on a single cylindrical lens (CL), and the results agree with the theoretical predictions. Resultsshow that the method proposed here works well when the methods of light intensity, Fourier transform, and phase distribution fail to determine the TC in extremely complex environment such as the angle of SSOO of 270° and large wandering perturbations.Methods In this work, three types of multicenter offaxis vortex beams, namely, the beams of uniform offaxis coherent superposition (UCS), uniform offaxis incoherent superposition (UIS), and random offaxis incoherent superposition (RIS), are studied both in theory and experiment. In the theory part, utilizing an equivalent matrix method (EMA), we obtain the propagation of the three types of vortex beams after the SSOO . Theoretical results of the distribution of the light intensity, Fourier intensity, and phase are presented. According to the ABCD propagation law of a tilted CL, the average OAM of the three types of beams is calculated for various input topological charges. In the experimental part, the intensity distribution is detected, and the average OAM is obtained using a single 45 otilted CL.Results and Discussions Fig. 2 presents the theoretical results for three types of vortex light intensity distributions and average OAM values. It shows that as the area of SSOO increases, the dark hollow structure inside the intensity disappears, which means that in the propagation with a largearea SSOO, the TC of the input beam cannot be recognized according to the distribution of the intensity of the beams. Figs. 3 and 4 are the theoretical results for the Fourier intensity distributions and phase distributions, respectively. It can be seen that when the obstructed area of the SSOO is larger than one half, methods based on the Fourier intensity and the phase distributions lost their efficiency, while the average OAM always keep around the value of the input TC. It means that in the case of a largearea SSOO, the method based on the averaged OAM works well. Fig. 7 presents the experimental results of the Fouriertransformed intensity distribution and the ave...