Research On The High-efficiency Measurement Of The OAM Beams Based On Geometric Optical Transformation And Its Application Technology

With the rapid developments of the information industry,high-speed,large-capacity communication is an inevitable trend.However,the existing communication technology has been unable to meet such requirements.Recently,orbital angular momentum(OAM)beam has been proposed for enhancing the communication capacity and increasing information transmission rate due to its orthogonal characteristics.For the OAM based free-space optical communication,the generation,transmission and measurement of the OAM beams are three most important aspects.This thesis mainly studies the two key issues of the high-efficiency measurement of the OAM beam and its application in optical communication networks.The main work is as follows:(1)High-density Orbital Angular Momentum mode analyzer.Here,we propose a high-density orbital angular momentum analyzer consisting of two OAM mode analyzers and a modified Mach-Zehnder interferometer(MZI),which can effectively avoid the inherent overlap of two adjacent OAM modes in the OAM mode analyzer.In this scheme,the MZI sorts and separates the even and odd OAM modes into different ports respectively.Next,two consecutive OAM mode analyzers can efficiently measure the sorted even and odd OAM modes,respectively.By this method,the topological charge interval between two neighboring OAM beams would be greater than or equal to 2 at both MZI output ports,and then the distance of two adjacent light spots converted from the output OAM modes will be greater than the width of the light spot.Therefore,our proposed scheme can avoid the overlapping effect,from which the analysis of the high-density OAM modes can be realized.(2)Larger detecting range and higher resolution for the mode analyzer.We propose an OAM mode analyzer that can realize a wider detection range and a higher resolution.First,we achieve a wider detection range of the mode analyzer by introducing an amplifying device before the conventional mode analyzer.Next,we introduce a new parameter,which can determine the position of the transformed rectangular beam in the vertical direction,into the geometrical optical transformation.Utilizing the characteristics of the modified geometrical optical transformation,we have designed two special composite phases to achieve arbitrary compression of the detected light spot without any phase distortion.Finally,we combine these two methods to achieve a wider detecting range and a higher resolution for the mode analyzer.(3)The add/drop multiplexer for orbital angular momentum beams.We demonstrate a reconfigurable add/drop multiplexer for orbital angular momentum beams based on geometric transformation to realize the add/drop of OAM beams simultaneously.Firstly,the input multiplexing OAM beams or the added multiplexing OAM beams are converted to light spots with different transverse positions in the focus plane.Then,a spatial light modulator(SLM)placed in the focal plane controls the transmission or reflection of the light spot on each area.Next,the transmitted or reflected light spots are inversely transformed by geometric transformation and restored to the original OAM beams.In the scheme,the addition or drop of multiple OAM beams can be simultaneously achieved by reasonably controlling the reflection and transmission of the corresponding light spots.