Design Of Three-mode Multi/Demultiplexer Based On 2D Photonic Crystals For Mode-Division Multiplexing Transmission

In recent years,with the rapid development of mobile internet and multimedia video information technology,human society has entered a new era of information technology.In the new information age,the bandwidth requirements have been greatly exceeded the people's expectations.However,with the capacity of optical fiber approaching the non-linear Shannon limit,the bandwidth of optical network will not satisfy the needs.Mode division multiplexing is a key technology to solve the contradiction between supply and demand,which utilizing few/multi-mode fiber to transmit information in different modes.It greatly improves transmission capacity and utilization efficiency of frequency.The mode division multiplexer based on photonic crystals has a combination of the advantages of mode division multiplexing technology and photonic crystals.The mode division multiplexing/demultiplexing device based on photonic crystal has not only the technical advantage of mode division multiplexing,but also the advantages of photonic crystal providing a way for the design of integrated optical mode division multiplexers.The main contents of this paper are as follows.Firstly,the coupled mode theory is analyzed,and the mode coupling of the normal type transverse coupling structure is studied in particular.Secondly,based on the finite difference time domain method,the coupling characteristics of the photonic crystal waveguides is studied.Then the coupling characteristics is verified by the finite difference time domain method.Finally,two kinds of three mode multiplexing/demultiplexer based on photonic crystal are designed according to the principle of phase matching.One is that the bus waveguide contains dielectric cylinder,and the other one does not contain dielectric cylinder.The bus waveguide with dielectric three-mode mode multiplexing/demultiplexer can be achieved the single-mode waveguide and multi-mode waveguide by adjusting the size of the dielectric cylinder in the waveguide.And the other one can be achieved the single-mode waveguide and multi-mode waveguide by adjusting the waveguide width.At the same time,the waveguide width gradient structure is adopted between the above waveguides,which effectively suppresses the mode mismatch between different waveguides.In the end the both multiplexing/demultiplexer can realize the multiplexing and demultiplexing of TE₀,TE₁ and TE₂ at 1550 nm wavelength.The transmission characteristics of the device are analyzed by time domain finite difference method.The simulation results show that,the insertion loss is less than 0.27 dB,channel crosstalk is less than-25.4dB for the bus waveguide with dielectric cylinder three-mode mode multiplexing/ demultiplexer.And for the other one,the insertion loss is less than 0.14 dB,the channel crosstalk is less than-20 dB.In this paper,we study the effect of manufacture error on the insertion loss and channel crosstalk of the photonic crystal mode multiplexing/demultiplexer.The effect of waveguide width,dielectric column size and dielectric column displacement on the device performance were analyzed by finite difference method.The results show that the mode multiplexing/demultiplexer can maintain high performance level within a certain manufacture error range.The mode multiplexing/demultiplexing based on photonic crystal proposed in this paper has a great value of the mode division multiplexing optical communication system,which has the characteristics of excellent transmission performance,simple structure and easy integration.