Research On Broadband Mode Multi/Demultiplexer Based On Cascaded Vertical Waveguide Directional Couplers

As a technology that can effectively solve the capacity bottleneck of single-mode optical fiber,mode-devision multiplexing technology has received extensive attention in recent years.The realization of mode-devision multiplexing technology depends on various multi/demultiplexers.This dissertation focuses on the design of a broadband mode multi/demultiplexer based on cascaded vertical waveguide directional couplers?DCs?,which can realize the multi/demultiplexing of six modes(E₃₁,E₁₃,E₂₂,E₂₁,E₁₂and E₁₁ mode)in a few-mode waveguide.The core and cladding materials of the waveguide are polymer materials EpoCore and EpoClad,respectively.The vertical DC can solve the problem that the coupling between the E₁₁ and E₂₁ modes based on horizontal DC cannot be realized because the coupling coefficient is zero.Reasonably designing their relative position can realize the coupling of five higher-order modes and the fundamental mode,respectively,which is the basis for achieving multi/demultiplexing.The introduction of the tapered waveguide in the coupling region is the sufficient condition to achieve a wide bandwidth.The device adopts a three-layer waveguide structure,which can provide greater design flexibility,and it can shorten the total length of the device originally increased by the tapered waveguide compared to a two-layer waveguide structure.The main contents of this dissertation are as follows:First of all,the development history of mode-devision multiplexing technology is introduced,and the domestic and foreign research achievements based on the vertical DC structure to realize a variety of optical waveguide devices and based on the asymmetric Y branch to achieve mode-devision multiplexing are presented.Secondly,the related theoretical basis is introduced,focusing on the analysis of rectangular waveguides by the effective refractive index method.And the working principles of the horizontal symmetric/asymmetric and vertical DCs are discussed based on the coupled mode theory.Thirdly,the individual coupler is studied first,and then the overall device is studied.The device parameters are designed through Matlab,COMSOL and Rsoft.The effective refractive index method,the finite element method,and the finite difference beam propagation method are involved.Finally,the simulation of Rsoft verifies the function of the device and analyzes its characteristics.The results show that at the operating wavelength of 1530 nm-1630 nm?covering the C+L band?,the coupling efficiencies of the E₃₁,E₁₃,E₂₂,E₂₁ and E₁₂modes are higher than 97.5%,85.0%,88.5%,99.2%and 99.1%,respectively,the mode crosstalk is lower than-11.3 dB,the total length of the device is 32 mm,and its polarization is weakly correlated.In addition,in order to demonstrate the flexibility of the vertical DC and explore its potential for integration with other waveguide structures to achieve mode-division multiplexing,a second wideband mode multi/demultiplexer was designed based on the same materials as the first device.The device integrates two asymmetric Y branches and three vertical DCs to achieve the same six-mode multi/demultiplexing.The device is a double-layer waveguide structure,two asymmetric Y branches realize the conversion of three modes at the same position,which makes the device highly integrated.The results show that at the operating wavelength of 1530 nm-1630 nm,the conversion efficiencies of the E₃₁,E₁₃,E₂₂,E₂₁ and E₁₂ modes are higher than 92.7%,85.3%,85.8%,98.4%and97.75%,respectively,the mode crosstalk is lower than-10.9 dB,the total length of the device is 22.65 mm,and its polarization is weakly correlated.The two mode multi/demultiplexers designed in this paper are clever in structural design.The main characteristics of the two devices are high efficiency and wide bandwidth,which have important research value for the development of integrated broadband waveguide mode-devision multiplexing devices.