Research Of Mode Conversion And Mode Multiplexing Used In Fiber Mode Division Multiplexing

In the existing long-distance fiber-optic communication systems, the communication capacity of single-mode fiber transmission systems has been greatly enhanced. However, people’s demand for bandwidth is much greater than the increase in the bandwidth of a single mode fiber transmission system. In order to resolve the enormous pressure to the optical transport network caused by the emerging bandwidth consumption business, a variety of methods has been proposed to promote the capacity of optical network. Single mode fiber may reach the foreseen bandwidth exhaust due to the inherent nonlinear effects of the single mode fiber. The mode division multiplexing based on multi-mode fiber come into being in this background. The limited and stable modes are used as independent signal channels for mode division multiplexing technology, which can exponentially increase the system capacity and the spectral efficiency. The paper is based on such background to study the mode division multiplexing technology in multi-mode fiber. As mode multiplexing in multi-mode fiber is an emerging technique, this paper will focus on the most basic configuration and technologies of mode multiplexing.This paper consists of three main parts:The first part is derivation of model theory and BPM algorithm, including the nature of mode, multimode interference effects, few-mode fiber transmission characteristics. Starting from the Maxwell equations, after Pada approximation, Crank-Nicholson difference, and ultimately get the BPM difference equations.The second part is the research on existing models to solve MDM transmission system. We put them into two major groups according to the use of different devices: MDM based on fiber structure, this category excites higher order modes which can be used as independent transmission channels in a multimode fiber based on transmission characteristics of the optical fiber. This kind of MDM has simple structure, high coupling efficiency; MDM based on optics, they achieve the MDM through a number of optical element (such as a lens). They often more complex than the all-fiber mode multiplexing system, but easy to implement.The second part introduces a kind of all-fiber LP02mode converter. The converter is based on multimode interference in a multimode fiber. Period of multimode-fiber(MMF) interconnects a single-mode(SMF) with a few-mode fiber. The proposed LP02converter is reciprocal and polarization independent, and supports a very broad optical bandwidth with an>20dB extinction ratio, as well as an efficient coupling and easy connectivity. It performs mode filtering as well, therefore supporting mode demultiplexing. The dependence if the performance of the converter on the MMF length and core diameter is further investigated. Beam propagation method (BPM, Beam-Propagation-Method) simulation for the LP02mode converter is carried out. We get the light field distribution, analyze some parameters.