The Experimental Study Of Free-Space Transmission Characteristics Of Orbital Angular Momentum Beam

Orbital angular momentum(OAM)of light as a potential degree of freedom,theoretically,can provide an infinite number of orthogonality states with different topological charges.The application of OAM beam multiplexing technology in free space optical communication and optical fiber communication has become one of the most important research directions in the field of optical communications.A huge challenge for OAM beam free space communication system is that the intensity and phase distribution of OAM beam will be affected by external factors,such as atmospheric turbulence,and change during transmission.In addition to the effects of beam jitter,intensity fluctuation,and phase shift,these factors can also damage the OAM beam pattern characteristics and cause crosstalk between different OAM modes.Mode purity of OAM beam is very important for free space multiplexing communication system.Therefore,the key work of this paper is to design and build the OAM beam free space communication system based on Digital Micromirror Array(DMD)and to study its transmission characteristics.Firstly,based on the principle of the generation of OAM beam by the computational holographic grating method,the transmittance function of the corresponding diffraction grating is theoretically calculated,and a corresponding grating diagram is generated.Further,the DMD digital micromirror array is loaded fork raster images with different topological charge number to generate light fields of different OAM mode.Second,the intensity and phase distribution of the generated OAM beam are detected.The beam intensity distribution of the OAM beam is obtained by using a beam analyzer.The phase distribution of the OAM beam and the OAM mode are then detected by using interferometry.And,the purity of the OAM beam mode is detected base on a single-channel Sagnac interferometer which constituted DOVE prisms.Finally;we set up an OAM beam free-space optical communication system based on DMD and carried out 50m indoor OAM space optical communication experiment.We studied the transmission characteristics of OAM beam deeply,including the broadening effect of OAM beam,interference characteristics and mode purity of OAM beams.In addition,the intensity center and power of the OAM beam were monitored.The effect of pointing error on the OAM free space optical communication system was studied.The experimental results show that the purity of OAM beam mode decreases with the increase of pointing error.