Research On Mode Multiplexer/Demultiplexer Based On Multi-Plane Light Conversion

With the rapid development of information networks,the demand for high-bandwidth and large-capacity communication networks is becoming more and more urgent.However,the maximum capacity of conventional single-mode fiber has almost reached its physical limit.As a new dimension,space division multiplexing can greatly improve the transmission capacity of the channel.The mode division multiplexing technology in space division multiplexing can independently transmit information through each mode in the few-mode optical fiber,thereby exponentially increasing the transmission capacity of a single fiber.Mode multiplexer/demultiplexer can realize mode conversion and multiplexing,which is the key component of mode division multiplexing.Among them,the mode multiplexer/demultiplexer based on multi-plane light conversion(MPLC)has the advantages of strong reconfiguration,low insertion loss and low crosstalk,and can realize the conversion and multiplexing of hundreds of modes at the same time.Reduces the overall complexity of the mode division multiplexing system.Facing the diffuse light spot(multiple transverse mode combination)under atmospheric turbulence,the MPLC-based mode demultiplexer can combine multiple transverse modes for diversity reception,thereby effectively improving the coupling efficiency of space light to single-mode fiber and resisting the atmosphere turbulence shaking ability.To this end,for the application of atmospheric laser communication.this paper studies the basic principles of MPLC-based mode multiplexer/demultiplexer,designs simulation models,and conducts theoretical derivation,simulation analysis and experimental research on the coupling performance of MPLC-based spatial light to single-mode fiber.The specific research content is as follows:1.The paper sorts out the current research status of mode multiplexer/demultiplexer at home and abroad,especially the research progress of the device design and application of the mode multiplexer/demultiplexer based on MPLC at home and abroad,and summarizes the design and Application development trend.2.The basic theory of mode conversion and multiplexing are studied,including spatial beam propagation theory,the phase modulation and fourier transform properties of lenses,and the basic principles of multi-plane light conversion;the optical field distribution of Gaussian beams and Hermitian-Gaussian beams and the principle of the wavefront matching algorithm based on MPLC are studied,and the algorithm design and code writing are completed for the above principles.3.Based on the written algorithm,the simulation model of the MPLC three-mode multiplexer/demultiplexer is established.The phase mask on each phase plate and the light field distribution after passing through the phase plate are simulated.At the same time,the insertion loss,mode-dependent loss and crosstalk of MPLC are analyzed.Then the simulation analyzed the effect of wavelength,number of phase plates and spacing on the performance of MPLC.With increasing the number of phase plates,the beam transition is smoother and the energy loss is reduced.4.The simulation model for the coupling efficiency of space light to fiber under ideal conditions is established.Taking single-mode fiber and three-mode fiber as examples,the effect of different spot sizes on the coupling efficiency of space light to fiber is analyzed.The simulation model of atmospheric turbulence based on power spectrum method and Zernike polynomial method is established,simulate atmospheric turbulence with random phase screen.Based on the coupling efficiency model and the atmospheric turbulence model,the simulation study based on MPLC mode demultiplexing under atmospheric turbulence from space light to single-mode fiber diversity reception.The coupling performance of the MPLC space mode diversity conversion is analyzed,which is better than the coupling efficiency of space light to single-mode and three-mode fibers.5.The light field modulation experiment of the spatial light modulator is designed,and the modulation characteristics of the spatial light modulator on the spatial distribution of the input light field is studied;then the coupling performance experiment of the spatial light to the single-mode fiber based on the MPLC three-mode demultiplexing is designed,the atmospheric turbulence is simulated by self-made turbulence plate,The spot profile and optical power jitter of the demultiplexed MPLC using 3 and 5 phase masks are experimentally studied,as well as the coupling efficiency of the demultiplexed beam,Obtained that the profile of the spot using 5 phase masks is relatively complete,and the optical power jitter is lower.Finally,the coupling efficiency experiment of space light directly to single-mode and three-mode fibers is designed.Compared with the coupling of space light directly to singlemode and three-mode fibers,the coupling efficiency of space light to single-mode fiber based on MPLC three-mode demultiplexing is improved by 31.52% and 31.45% respectively.This paper mainly focuses on simulation analysis and experimental research on the coupling performance of space light to single-mode fiber based on MPLC mode demultiplexing,and provides technical options for improving the coupling efficiency of space light to fiber in atmospheric laser communication.