Researches On Feed-Forward System Of Compensation For Porlarization Mode Dispersion

Polarization Mode Dispersion (PMD) is caused by bi-refringence of optical fiber, which is caused by the asymmetry of the structure of the optical fiber, outside stress and temperature changes of environment, etc. PMD limits the capacity and span of relay distance of the optical fiber communication system, as it induces pulse broadening and distortion. Thus it is one of the key problems to solve the PMD problem in the high-speed optical fiber communication system. PMD changes randomly as external condition of fiber and communication speed change. This characteristic of PMD makes the PMD compensation extremely difficult. The main work of my thesis is discussing the methods of PMD compensation and the realization of adaptive PMD compensation. What have been done are listed as followed:Firstly, many methods of PMD compensation are discussed and compared. A new feed-forward PMD compensation method which has not been talked about at home is brought forward. By utilizing polarization scrambler at the transmitter side and finding and calculating the principal state of polarization (PSP) and differential group delay (DGD) at the side of receiver, this method could complete first-order dynamic PMD compensation successfully. For the control signals are acquired by calculation other than searching, this method could overcome the disadvantages of searching algorithm—it provides fast compensation speed and does not cause instant worsening comparing with searching method. Our compensation system can complete compensation within 50ms.Secondly, the two key equipments--polarization controller (PC) and differential delay line (DDL) are studied by both theory and experiment. The impacts between different wave plates on SOP are found. Validity of these influences is proved by both Matlab emulation and a great deal of experiments. This validity lays the foundation of realization of polarization stabilizer. Furthermore, the formula to calculate the time delay to control DDL has been derived. This formula makes the relationship between DOP and the delay time clearer. And the delay time is easier to calculate in this formula.Thirdly, polarization stabilizer experiment is completed successfully based on the syudies of PC. This polarization stabilizer makes sure that the output SOP is stable no matter what is the input SOP. Stabilizers with the same functions as ours are not found by us until now. Its precision can reach 5 degree at best and its average response time is 50ms. The realization of polarization stabilizer not only facilitates the PMD compensation systems but also other systemts where polarization stabilization is needed.