涡旋光束TC检测与OAM控制研究：理论与实验

Vortex beams, which possess orbital angular momentum (OAM), are commonly referred to as beams with OAM. In the field of optical communication, wandering perturbations from environment can regulate the OAM of these beams, and opens up a new field with possible applications. The topological charge (TC) is a crucial factor when applications of vortex beams are encountered. Accurate detection of TC is a prerequisite for expanding the application of vortex beams in non-ideal settings. This paper covers the following topics:

(1) A theoretical model of complex wandering perturbations for the purpose of control OAM in a vortex beam is proposed. The analytical formula for the correlation function of Laguerre-Gaussian beams under independent wander perturbations is derived, revealing the periodic variations in the intensity distribution, average OAM, and OAM spectrum in the direction of wander perturbations. These findings are supported by consistent experimental results. To further investigate the effect of complex wander perturbations on this type of beam, this section also presents experimental research on vortex beams that have been impacted by cross-correlation wander perturbations. Experimental results validate the consistency of the findings. Additionally, an experimental study investigates the impact of cross-correlation wandering perturbations on vortex beams. Results indicate that beams exposed to such perturbations exhibit a different periodicity as compared to those subjected to independent wandering perturbations. The theoretical and experimental outcomes of this study can serve as a reference for transforming destructive wandering perturbations into constructive wandering perturbations in turbulent environments.

(2) A novel method is proposed for TC detection of vortex beams, which is capable of accurately detecting the TC value with wandering perturbations and obstacles. The proposed technique utilizes a single cylindrical lens to detect the average OAM value and obtain the TC value of vortex beams. The effectiveness of the proposed method is compared with the methods of light intensity, Fourier transform, and phase distribution in extreme conditions where sector-shaped opaque obstacle blocking is more than 180°. It is found that only the detection methods proposed in this work can accurately recover the TC magnitude and sign of the input beam. This technology addresses the detection problem of vortex beams TC under the dual non-ideal interference of obstacle occlusion and environmental wandering perturbations. The TC detection method is robust in beam wanders and obstruction and hence might have significant applications for optical communication and optical encryption based on vortex beams.

(3) A method for detecting the fractional topological charge (FTC) of a vortex beam is proposed by using a holographic phase plate. Changing the phase difference between two halves of an artificial phase plane which is actually a holographic phase plate, intensity minimum visibilities of the interacting beam can be observed in a regular order that the hollow center intensity shape changes from left opening, right opening, to left opening. These three intensity minimum visibilities continuously appear at the cross-section plane on both sides of the vortex beam when tuning the phase difference between the phase plate. We recorded the values of the phase differences at the three minimum visibilities, and calculate the FTC using a theoretical formula. This method resolves the problem of introducing experimental errors due to the complexity of the FTC device when detecting the entity phase plate. It provides a robust detection scheme for FTC in wandering perturbations environment.