Mode Selective Coupling And Signal Decoupling Techniques

As a communication method with huge capacity,high privacy,and good signal transmission quality,optical fiber communication is playing an increasingly important role in the information society.However,the existing optical fiber communication system has been difficult to meet people's demands for high-bandwidth and large-capacity communication networks,and it is urgent to promote the innovation of optical fiber communication technology to increase the system capacity.The mode division multiplexing technology based on the few-mode fiber is considered to be the most promising solution.The orthogonal modes in the few-mode fiber are used as independent transmission channels,which can increase the transmission capacity of the system multiple times.In an MDM communication system,it is necessary to use a mode multiplexer/demultiplexer to couple each mode to a few-mode fiber,so a well-designed mode multiplexer/demultiplexer is essential.At the same time,due to impairments such as random coupling between modes and differential group delay in the actual mode division multiplexing system,in order to effectively receive signals at the receiving end,an algorithm is needed to demultiplex the received signals.The main research content of this paper is mode coupling techniques and the signal decoupling algorithms at the receiving end.The working principle and design of a mode selective coupler using fiber waveguides and a blind signal equalization algorithm based on independent vector analysis are studied.The main contents of this paper are summarized as follows.First,the basic principles and concepts of mode division multiplexing technology are analyzed.Based on Maxwell's equations,the mode division,mode orthogonality,and coupled mode equations are derived.Finally,the damage mechanism of the signal during the transmission process is analyzed,the mode multiplexing system is modeled,and the channel of the mode multiplexing system is represented by a transmission matrix method.According to the modeling,two coupling models:instantaneous coupling model and convolutional coupling model are analyzed,and the commonly used MIMO equalization algorithms are analyzed and compared with the ICA and IVA algorithms.Secondly,the mode selective coupler is designed.The finite element simulation software COMSOL and the beam propagation software Rsoft are used to simulate the mode selective coupler.The mode electric field distribution and the energy transfer process of the coupler are obtained.For the MIMO equalization algorithm at the receiving end,the cost function and the optimization method of the cost function are analyzed,thus,iterative formulas of ICA and IVA are obtained.Besides,an IVA algorithm using variable Kaiser window based on the IVA is proposed.Through simulation,the performances of the two algorithms in the instantaneous coupling model and the convolutional coupling model are compared,and the improvement in SER performance of the proposed algorithm compared with the traditional algorithm was demonstrated.Finally,a 2×2 mode division multiplexing communication system experimental platform based on a mode selective coupler was built,which supports two modes of LP₀₁and LP₁₁ as independent channels for transmission.The experimental structure diagram is given and experiments are performed.It is proved that the ordinary time-domain ICA algorithm is not suitable for actual signal equalization,and the IVA algorithm that processes signals in the frequency domain works well in actual systems.It is also proved that the IVA algorithm based on the variable Kaiser window proposed in Chapter 3improves the SER performance than the traditional algorithm.Finally,the experimental data were used to explore the factors affecting the performance of the IVA algorithm,laying a foundation for future algorithm improvements.