Application Of Mode-Division Technology Of Optical Orbital Angular Momentum In Optical Communication Field

The existence of light with orbital angular momentum（OAM） was confirmed through experiment by Allen and others in the 1990 s. Henceforth, applications ranging from optical communication,optical manipulations to quantum information processing are being benefited.It has a bright future of using optical vortices with different values of topological charge l（different values of the orbital angular momentum） for mode division multiplexing in free space optical communication. In free space optical communication field, the principle of mode division multiplexing of light with orbital angular momentum is the light with same frequency but different orbital angular momentum plays the role of information carrier. Mode division multiplexing is the same as time division multiplexing, frequency division multiplexing, wavelength division multiplexing and space division multiplexing has great value in communications technology, which can increase the transmission capacity of information. Spatial light modulator is an important module in optical communication. Now most of the spatial light modulator’s principle using in experiments and applications is similar with the principle of generating orbital angular momentum mode by phase plate. Compared with the previous mechanical modulation,it has made a lot of progress. However, the spatial light modulator’s refresh rate is not enough to meet high modulated speed in application.Besides, most of the equipment is too expensive and complex. In this article spatial light modulator based on attenuated total reflection principle is studied on. In order to meet the requirement of experiment,the optimization of factors such as metallic materials, crystal materials and parallelism were finished by the software of MATLAB.The modulator was made up of a total of three layers. The top layer is gold film whose thickness is 40 nm. The middle layer is LiNbO3 whose thickness is 5mm. The bottom layer is gold film whose thickness is 200 nm. Its working principle is that when the wave vector components along the interface is equal to propagation coefficient of the waveguide, mode resonance could happen. As a result of mode resonance,the intensity of reflected light will decrease. Because the refractive index of LiNbO3 is under the influnce of electric field, propagation coefficient will change with applied electric field on LiNbO3. The end result is that the intensity of reflected light will be determined by the voltage.In our experiment, the beams are modulated by modulator based on attenuated total reflection（ATR） and then generated helical beams with orbital angular momentum by fork-like grating. The helical beams are then superposed coaxially into one beam. After transfer in the free space, the beam is processed by the receiver.Main work in this article is as follows:1 Introducing the research of optical communication using light with orbital angular momentum and the existing problems briefly. Introducing the affecting of the atmospheric disturbances on communication quality of long distance transmission and the development of novel optical fiber in detail. Introducing characteristics of spatial light modulators based on different physical principles in detail.2 The principle of generating light with orbital angular momentum by spiral phase plate was deduce in detail and the simulation has been done. Introducing the generation,detection method and application of light with orbital angular momentum briefly. The simulation on Laguerre-Gaussian beam has been done. Introducing the application of light with orbital angular momentum in detail, especially used as information carrier and encoded unit in free space optical communication field.3 Theory of planar optical waveguides which is relevant to the principle of spatial light modulator was deduced. An effect on the characteristics of waveguide produced by the variation of waveguide layer’s refractive index has been simulated. The electro-optic effect of LiNbO₃, the theory of attenuated total reflection were deduced in detail. The optimized results of influence factors by software of MATLAB was presented.4 The performance, processing, test result and possible improvement of spatial light modulator were analyzed in detail.