Research On Mode Demultiplexing Algorithm In Few Mode Fiber Mode Division Multiplexing System

With the rapid development of the information age, spurred by high-definitionvideo streaming, cloud storage, cloud computing, mobile networking and otherinformation technologies, worldwide data traffic is growing at a rate estimated toexceed50%annually. It brings higher capacity requirements to long-haul optical fibertransmission network, which forms the backbone of the Internet. However, with thechannel number and signal modulation order increasing in the existing WDM system,the coupling power within one single mode fiber is scaling, and as a result, thetransmission capacity per fiber is now approaching fundamental information-theoreticlimits imposed by optical amplifier noise and by the nonlinear response of the silicafiber medium. Therefore, finding an effective way to expand the system capacity hasbecome a urgent problem to be solved in optical fiber communication field.In this context, as a kind of capacity expanding method with great potential forapplication, mode division multiplexing technology has attracted the attention ofresearchers both inland and abroad. Utilizing the spacial modes as new degree offreedom, combined with few mode fiber suitable for long distance signal transmission,mode division multiplexing technology can multiply the capacity of long-haulcommunication systems. However, due to the joint effects of chromatic dispersion,mode coupling and mode group delay, the channel condition in mode divisionmultiplexing system is far more complex, which makes mode demultiplexing one ofthe most critical problems in mode division multiplexing technology. Therefore, themain objective of this paper is to develop an effective digital signal processingalgorithm with low computational complexity to implement mode demultiplexingthrough theoretical analysis on few-mode fiber channel characteristics.This paper briefly introduces the concept of few-mode fiber, analyzes the primarycrosstalk source in few mode fiber: mode coupling and mode of group delay’sconcrete impact on mode multiplexing signals, and establishes the mathematicalmodel of few-mode fiber link based on the above theory. Then, the time-domain andfrequency-domain LMS algorithm used in mode demultiplexing are studiedrespectively, their computational complexity and demultiplexing performance arecompared as well. At last, the frequency-domain ICA algorithm is proposed asdemultiplexing algorithm in mode division multiplexing system. The principle ofF-ICA algorithm is described in detail. At the same time, the symbol error rateperformance of F-ICA is estimated under different optical signal noise ratio and mode coupling factor, and the proposed algorithm’s computational complexity is comparedwith frequency-domain LMS algorithm. The details of this paper are as follows:Firstly, the mode characters of few mode fiber, which is the transmission mediumof long distance mode division multiplexing system, are introduced. Theorthogonality of modes in few mode fiber is demonstrated from the perspective ofoptical waveguide theory to confirm the reasonableness and realizableness of modedivision multiplexing. The two primary impairments in few-mode fiber transmission:mode coupling and mode group delay are discussed and their effects on modemultiplexing signals are studied.Secondly, establish the theoretical model of the two non-degeneracy modes(LP01andLP11mode) in dual-mode fiber with the effect of chromatic dispersion, modecoupling and mode group delay. Calculate the parameters of few mode fiber withMode Solver and build the dual-mode fiber simulation model with VPI transmissionMaker and Matlab programming on the basis of few mode fiber channel modelingtheory. Introduce the basic principles and structure of mode division multiplexingsystem and construct a22few mode fiber mode division multiplexing simulationsystem on VPI platform using the built up few mode fiber transmission model.Thirdly, the principle of typical model demultiplexing algorithms: time-domainLMS algorithm and frequency-domain LMS algorithm are introduced separately, therelationship between the two algorithms’ computational complexity and mode numberor differential mode group delay is analyzed. The two algorithms are used toimplement mode demultiplexing in simulation system and their performance areobserved through the constellation and eye diagram of output signals. Compare theirsymbol error rate performance under different optical signal noise ratio.Finally, the frequency-domain ICA algorithm is proposed as mode demultiplexingalgorithm. The separation model and principle of F-ICA algorithm are described indetail, and F-ICA algorithm is successfully applied in the mode division multiplexingsimulation system. The demultiplexing performance of F-ICA is confirmed throughconstellation diagram and eye diagram. Furthermore, the symbol error rate of F-ICAalgorithm is observed in case of different coupling factors and data length, and thestability of the proposed algorithm is verified. F-ICA’s symbol error rate performanceand computational complexity is compared with F-LMS algorithm in the previouschapter. The results show that under the same symbol error rate circumstance, theF-ICAalgorithm has certain advantage of lower computational complexity.