Research On OAM Beam Size Transformation And Detection Technology In Free Space Optical Communication System

In addition to the traditional modulated demodulation methods such as amplitude phase and ortho-phase shift keying,the Orbital Angular Momentum(OAM)of photons,as a new degree of freedom,has attracted extensive attention in improving the information transmission rate of optical communication systems.OAM can theoretically provide infinite number of orthogonal bases with different quantum states,which can be used to carry information and improve the data capacity of a communication system,creating the widespread attention in the optical communication domain.However,the inherent divergence property of the OAM beam will result in the aperture mismatch at the receiver,which poses a challenge in detecting the purity of the target state.This paper studied how to overcome the phenomenon of aperture mismatch,and the distribution of detection probability in the case of aperture mismatch when using the technical means to minimize the power loss of the communication system.The main works and achievements are as follows:1.Aiming at the defect of receiving end aperture mismatch caused by Laguerre-Gaussian(LG)beam divergence,the working mechanism of beam size transformation based on defocusing dual lens system was studied.Then,the expression for equivalent radius of LG beams through defocusing double-lens-systems after a certain propagation distance was theoretically derived using Collins' diffraction integral formula.The expression of defocus distance with a determined transformation beam size was also derived and the detailed numerical analysis and simulation verification were carried out.2.The power loss of a non-zero radial index LG beam propagated by aperture mismatching was studied in relation to the transmission distance z and the aperture size R0 of the receiver.The expression of intensity distribution after spiral phase plate(SPP)detection was derived by using two-dimensional Fourier transformation and Hankel transformation,therefore the influences of aperture diameter on the normalized intensity distribution of vortex beam were obtained.Meanwhile,the expression of target state detection efficiency ? was derived by using conjugate pattern classification,and the variations of ? with radial index p and angular index l were analyzed,so as to find the parameter conditions needed to reach the optimal ?,which was of great significance for the optimization of detection probability of target state.3.The power loss of the multi-aperture partially receiving system varies with the transmission distance z,the receiver aperture radius of R0 and the transmitted beam size ?0,and the detection probability of different states is obtained by simulation.The recognition method of high order radial index LG light intensity distribution in the condition of aperture mismatch based on VGG16 deep neural network structure and the influence of lateral deviation on the recognition accuracy are studied.The results of the study have certain reference values for the realization of OAM optical communication system.