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• Chinese Optics Letters
• Vol. 20, Issue 2, 020601 (2022)
Lin Zhao1、2, Yuan Hao1, Li Chen1, Wenyi Liu1, Meng Jin1, Yi Wu1, Jiamin Tao1, Kaiqian Jie1, and Hongzhan Liu1、*
Author Affiliations
• 1Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, Guangzhou 510006, China
• 2School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, China
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Abstract

Vortex optical communication has been a hot research field in recent years. A key step is mode recognition in the orbital angular momentum (OAM) free-space optical (FSO) communication system. In this article, we propose an OAM mode recognition method based on image recognition technology, which uses the interferogram between the vortex beam and the Gaussian beam to identify the OAM mode. In order to resist the influence of atmospheric turbulence on the recognition accuracy, we added a Gaussian smoothing filter into the recognition process. Moreover, we used random phase screens to generate interferogram sets at distances of 1 km and 2 km. The verification result shows that the proposed scheme produces high identification accuracy for the distorted optical field. The average accuracy can reach 100% and 87.78% under the conditions of medium- and strong-turbulence levels, respectively. It is anticipated that these results might be helpful for improving the reliability of the OAM-FSO communication system in the future.

1. Introduction

Nowadays, vortex beams (VBs), carrying orbital angular momentum (OAM), have been known as the hot spots and shown great potential in various fields such as optical micro-manipulation, optical sensors, optical transmission, optical lasers, and amplifiers[1]. In 1992, Allen et al. indicated that for beams with helical phase fronts of $exp(ilϕ)$ the OAM in the propagation direction has the discrete values of $lh$ per photon, where $h$ is the reduced Planck constant, and $l$ is usually an integer that represents the topological charge (TC), whose sign indicates the direction of the phase spiral[2]. In theory, the VBs with different OAM modes are mutually orthogonal, and their intensity distribution presents unique “doughnut” shapes due to phase uncertainty at the beam cross section[3]. The discovery of optical vortices carrying OAM has accelerated progress in many research areas[4]. Especially in optical communications, VBs are of great potential in increasing capacity and modulation ability by providing additional physical dimension[5]. By multiplexing VBs, the transmission rate of Tbit/s is realized in free-space optical (FSO) communication[6,7]. Also, further studies have shown that hybrid multiplexing of OAM and other dimensions can achieve communication capacity close to Pbit/s[8]. However, the OAM shift keying communication, which encodes digital signals to OAM modes, is severely hampered because the locking effective detection methods can hardly adapt to the fast switching modes. Moreover, the VBs are susceptible to atmospheric turbulence (AT), which may cause wavefront distortion and lead to mode diffusion[9]. Hence, fast and high-accuracy OAM mode recognition is a challenge in the practical application of VBs.

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Lin Zhao, Yuan Hao, Li Chen, Wenyi Liu, Meng Jin, Yi Wu, Jiamin Tao, Kaiqian Jie, Hongzhan Liu. High-accuracy mode recognition method in orbital angular momentum optical communication system[J]. Chinese Optics Letters, 2022, 20(2): 020601