Design And Research Of All Optic Fiber Mode Division Multiplexer Based On Selective Coupling Principle

The mode division multiplexing（MDM）based on the optical fiber system is a technolo-gy that introduces the mode as a new degree of freedom to expand the transmission channel,mode conversion and multiplexing are the core links in the MDM technology,and it is also the primary problem to be solved in the realization of mode division multiplexing.Starting from the actual needs of the project“Research on multi-core/few-mode microstructure opti-cal fiber（MOF）and its passive optical devices”,this paper focuses on the mode conversion and multiplexing of few-mode fiber MDM systems,and conducts an in-depth theoretical analysis and simulation application of the mode selective coupling principle,a three-core three-mode multiplexer and a five-core five-mode multiplexer on the basis of MOF are pro-posed to meet the requirements of the device specifications in the project,such as operating bandwidth,insertion loss,inter-mode crosstalk.The specific research content of the article is as follows:First,a 2×2 mode converter based on the selective coupling principle is designed and simulated using the finite element method-based simulation software COMSOL.Then,ac-cording to the simulation parameters,the converter was fabricated experimentally with single-mode fiber and few-mode fiber by taper-drawing technology and a novel side-polishing tech-nology.The test results show that the converter can realize the conversion from LP₀₁mode to LP₀₂mode,and the conversion efficiency is 13%.Secondly,a scheme of designing a mode division multiplexer on the basis of MOF is proposed.By adjusting the material refractive index of the doped rod and the arrangement and diameter of the air holes,three cores for localized light transmission were designed in the MOF.And according to the principle of selective coupling,each side core and the central core can meet the broadband phase matching conditions,so that the device can realize the conversion and multiplexing of the three modes of LP₀₁,LP₂₁and LP₃₁in the bandwidth range of more than 140 nm.In addition,the smaller core spacing of the fiber results in lower substrate loss when light energy is coupled,higher coupling efficiency,effectively shortening the length of the fiber,and improving the integration of the device design.At the same time,a scheme of combining a multi-core fiber space division multiplexer with the mode division multiplexer is proposed,which can reduce the coupling complexity and expand the communication capacity.Finally,a MOF-based MDM that can realize five-mode multiplexing is designed.The fiber structure of the multiplexer is simple,only consisting of five high-refractive-index doped rods and a quartz substrate.The doping concentration of the refractive index rod is designed reasonably,and the core spacing is controlled in the range of 16.5～29μm,which makes the device easier to fabricate.And through the regulation of the core spacing,the device can realize the synchronous conversion of the five modes of LP₀₁,LP₁₁,LP₀₂,LP₃₁and LP₁₂,which greatly reduces the insertion loss caused by the asynchronous mode con-version under the same fiber length.Further,the mode cut-off condition is used to limit the core diameter,which can improve mode purity and conversion efficiency.The input port of the multiplexer can be used with a multi-core space division multiplexer,and the output port is similar in size to the few-mode fiber used for long-distance transmission,which is convenient for the fusion between fibers.