Study Of PMD Theory And Compensation Control Algorithm In Large-Capacity Optical Fiber Communication System

In high-speed optical fiber systems, polarization mode dispersion (PMD) can spread optical signal, and is acknowledged as one of the serious degradation factors limiting the bit rate and the transmission distance, since it is a statistical quantity and leads to random broadening of optical transmitting signal. As an ideal solution to the severe effect of PMD on system performance, dynamic PMD compensation has an irreplaceable wide technical and application foreground. Accordingly, the relative PMD compensating theories and technologies have been one of the most significant topics in recent researches related to high-speed and long-haul optical communication system. The main work includes:The essential of PMD effect has been analyzed, and from the foundamental equation of optical signal transmission in single mode fiber, the nonlinear Schr?ndinger equation describing PMD can be obtained, from which pulse broadening due to PMD can be deduced. Various discussions have been made in different situations, by which can approximate module be obtained. The relationship between differential group delay (DGD) and degree of polarization (DOP) can be established using self-relative function of input optical signal.In order to solve the problem of online PMD detection, the schieme combing polarization scrambing with perpendicular polarization splitting detection has been proposed, in which the differential design can be used to increase the measurement sensitivity. Through the comparison between the three compensation schemes, electrical domain compensation, optical domain compensation and optoelectronic compensation, the design solution combing polarization controller with variable (or fixed) delay line has been chosen to realize PMD compensation.In the thesis, partical swarm optimization (PSO) has been deeply studied and adopted to design the searching algorithm. An improved PSO algorithm has been proposed. Two main factors influencing the properties of the algorithm, the degree of aggregation and the changing velocity of global optimum for the particles, have been considered to establish the stratedy of changing the inertia weight dynamically. Besides, the immune clone principle has been adopted to maintain the diversity of the swarm. From the algorithm test, the global searching ability is obviously better than the standard PSO algorithm.In order to track the random fluctuation of the global optimum accurately and quickly, an improved simplex method (SM) has been proposed to search accurately in a local domain. Where, in the reflection operation, besides the reflection operation based on the worst vertex point and the center of gravity for the other nodes, the reflection operation based on the best vertex point and the center of gravity for the other nodes has been added. A better direction can be chosen to perform reflection operation and expansion operation, which can reach the optimum rapidly. The mapping operation has been adopted to replace the vertex point substitution operation, which can avoid the degeneration in the procedure of iteration.Based on the searching algorithm and the tracking algorithm, the general design scheme for the feedback control algorithm has been proposed. The whole algorithm includes IC-PSO algorithm, improved SM, and agent migration operation. Through the combination of IC-PSO and improved SM, the balance between global exploration and local deep development can be realized.The adaptive PMD compensation system has been established, in which the searching algorithm and the tracking algorithm proposed above has been used to search and track the optimum of DOP. From the experimental results, the response time is about 71 ms, and the recovering time is less than 12 ms. It has been demonstrated that the feedback algorithm has perfect ability to avoid local optimum and reach the global best compensation point. Compared the eye diagram before and after compensation, the opening of the eye diagram has been improved obviously. And from the statistic results for the bit error rate (BER), the BER of the system has been reduced increasingly, and the minimum can be attained to 3.59e-16. The quality of transmission signal has been improved obviously, which can meet the requirement of dynamic adaptive PMD compensation.