Research On Mode Conversion Based On Free Space Optical Path In Mode Division Multiplexing System

Mode Division Multiplexing(MDM)could exponentially improve the transmission capacity of optical fiber communication system and effectively resolve the current increasingly serious crisis of transmission capacity.The first problem to be solved is the technology of mode excitation and mode conversion.They are widely used in the sender,receiver and network node of mode division multiplexing system.Mutual conversion between arbitrary mode with high efficiency,high precision and low complexity,is the prerequisite for widely used in mode division multiplexing system.The current research on the mode excitation and conversion mainly divided into two directions.One is the method based on the free space optical path,such as using spatial light modulator(SLM)or phase plate to change the phase and amplitude of field distribution of incident light.Another is the method based on optical waveguide structure,such as using the long period fiber grating,photon lantern,and fiber coupler to realize the mode conversion.These optical waveguide structures have the advantages of low loss,low crosstalk and can be easily integrated.But the conversion bandwidth is limited,cannot be refactoring and manipulating with high complexity.The method based on free space optical path could use a relatively simple process and some devices to realize the efficient mode conversion.It is reconfigurable and easy to operate.So this paper mainly discusses the solution for mode conversion based on free space optical path,and analyzes the principle of mode conversion with high efficiency by using SLM or phase plate.In the dissertation,the mode theory of optical fiber and the diffraction theory in free space are studied theoretically,and then the principle of mode conversion based on free space optical path has been theoretically analyzed.The comparison between simulation results and experimental results verifies the correctness of conversion theory.In the experiment,the blazed grating is introduced to solve the problem of fuzzy field distribution.The technique of combined phase and amplitude modulation is proposed to improve the accuracy of mode conversion.Finally,four channels of Gaussian beams achieving mode conversion and multiplexing simultaneously have been demonstrated by displaying special designed composite phase pattern on one single SLM.The experiment results tally with the simulation results and verify the correctness and feasibility of this scheme.The main research work and innovations are listed as follows:1.Combined phase and amplitude modulation method for mode conversion with high accuracy.In the simulation results and experimental results,there exists high order diffraction component while using the phase-only modulation method.Then the combined phase and amplitude modulation method is proposed to make full use of the amplitude information.In the dissertation the pixels on the SLM have been divided in a certain strategy.The adjacent four pixels are regarded as a big pixel.The average phase embodies the phase modulation ability of the big pixel.The differences between these pixels embody the amplitude modulation ability of the big pixel and cooperate with the 4f system to achieve the amplitude modulation.2.Four channels of Gaussian beams achieved mode conversion and multiplexing simultaneously by displaying special designed composite phase pattern on one single SLM.In order to improve the efficiency of the devices and improve the compactness of the system,this dissertation puts forward that the single SLM is divided into multiple regions,and each region acts as an independent SLM for mode conversion.Simultaneously,the blazed grating is introduced to deflect these beams.So the mode conversion and multiplexing can be realized simultaneously and the cost of multiple beam splitters removed.At the same time this method has certain extensibility.The number of multiplexing mode can be increased with a finer division of the SLM.