Study On Generation And Measurement Of The Higher Order Helical Beams Carrying Orbital Angular Momentum

Light beam contains both spin angular momentum(SAM) and orbital angular momentum(OAM) in paraxial approximation. The spin angular momentum relates to the electirc vector field. On the other hand, the orbital angular momentum is determinded by transverse field distribution normal to the propagation direction. The spiral beam with helical wavefront is typical beam carrying OAM and has been paid great attention in recent years. Laguerre Gaussian beams(LGB) is a kind of spiral beam usually used. It has important applications in space optical communication, image processing, optical tweezers techonlogy, and etc. This work mainly studies the generation and detection of Laguerre Gaussian beams. We also demosntrate the possibility of communication using helical beams as information carriers experimentaly and numerically.The main items of this work are list below:1. The computer-generated hologram(CGH) method used to generat and detect the Laguerre Gaussian beam is verified by simulation and experiment from the diffractive and interfere patterms. We derived the diffraction expression of two Laguerre Gaussian beams with tiny angle between their propagation axis and certified the correct of it by simulation and experiment. Theoretical analysis and experiments show that the coaxial interference is sensitive to the angle, to get the obvious interference results it must be less than 10-3 radian. We also discussed the generation of Laguerre Gaussian beam using phase fork-like grating. We derived the expression of arbitrary diffraction orders generated by fork-like grating in terms of Laguerre Gaussian beams and confirm it numerically and experimentally.2. We analyzed the principle of the generation of Laguerre Gaussian beam using spatial variable retardant plate(SVRP) and its mode components. Simulation is also given to show the expected results.3. Principle of binary amplitude fork-like grating generation and detection of Laguerre Gauss beams has been derived. We build a communication system using the binary amplitude fork-like grating as core components in the modulation and demodulation. We using 8 Laguerre Gauss beams with different orbital angular momentum as orthogonal communication channels. This is so called orbital angular momentum division multiplexing(OAMDM). The communication system transmitedinformation about 4m in free space and illustrated the possibility of orbital angular momentum division multiplexing in free space optical communication. Theory and experiment show that to get all information of a channel we just need to detect bright spot of single one specific position.