Generation And Detection Of Multiple OAM Modes Based On Eccentric Elliptical-core Spun Fiber Grating

Since Orbital Angular Momentum(OAM)was proved to exist by Allen in 1992,it has attracted extensive attention from scholars.The vortex light carrying OAM has many excellent characteristics,and has very broad applications in optical manipulation,biomedicine,quantum coding,astronomical measurement,optical imaging,and optical fiber communication and other fields.How to use fiber gratings to generate vortex light carrying OAM is an important topic that many researchers are studying.In this paper,the generation and detection of multiple OAM modes based on eccentric elliptical-core spun fiber grating are studied.The main research work includes the following three aspects:1.Establish coupled-mode analysis model of eccentric elliptical-core spun fiber grating.Based on multiple rotational symmetries of the index profile in the cross section of the spun fiber grating,the mode coupling coefficients in the grating are studied,and the phase and angle matching conditions of the effective mode coupling are identified;the multiple spiral effects caused by multiple rotational symmetries are explored,and the perturbation of the eccentric and elliptical core is expressed as the 1st and 2nd order spiral effects,respectively.As the basis of studying mode coupling,both the numerical and the analytical methods are used to solve the modes,and the HE/EH mode discrimination criterion is determined according to the optical fiber modal intensity diagram,and then realized by the computer program.2.Investigate theoretically on the generation and detection of multiple OAM modes.Based on the matching conditions of mode coupling,the generation mechanism of multiple OAM modes in the eccentric elliptical-core spun fiber grating is analyzed.And based on the 1st and 2nd order spiral effects owing to the eccentric and elliptical-core perturbation,theoretical simulation and parameter design are explored.Next,the interference of the pure OAM modes and the superimposed OAM modes with the reference beam are simulated,the effects of plane wave beam tilt,spherical wave beam divergence,operating wavelength,and OAM mode purity on the interferogram are explored.The simulation results are analyzed,which provides theoretical basis for the experiments.3.Investigate experimentally on the generation and detection of multiple OAM modes.An experimental system is set up for the generation and detection of multiple OAM modes.According to the experimental requirements,the experimental scheme and the required devices are clarified.The OAM modes are generated by the eccentric elliptical-core spun fiber grating with the designed parameters including the length of grating;and then detected by using the interferometry.Combined with transmission spectrum of the grating,the OAM interference patterns are observed and collected at different wavelengths.In this way,the generation and detection of multiple OAM modes are identified in experiments.Then the experimental results are summarized,and the reasons for the differences between experiment and theory are analyzed.