| High-speed optical transmission systems,especially over 40Gbit/s systems, have larger capacity and wider band compared to conventional systems.But group velocity dispersion(GVD),nonlinear optical effects and polarization mode dispersion(PMD)become the major obstacles in these systems.Especially the polarization mode dispersion which can be neglected in low-speed communication system is the most difficulty problem in high-speed communication systems. Fortunately,optical modulation formats have good characteristics on spectrum, dispersion and nonlinearity tolerance.So,using correct modulation format based on the difference condition is a right choice of improving the performance of the systems.Based on the theories of polarization mode dispersion(PMD)and the principles of different modulation formats,the processes of modulating the different formats are investigated in detail in this dissertation.The PMD mitigation performances of different modulation formats in the transmission link are discussed, especially a dynamic combination of the PMD mitigation technology and PMD compensation technology is also discussed.The research works in the dissertation are summarized as follows:◆The basic concepts of PMD,the main state module of PMD,their numerical models used as emulation,the distribution of the first-and higher-order PMD.◆The theories and modulation processes of different modulation formats (such as return zero format(RZ)with 33%duty cycle and 50%duty cycle respectively,carrier suppressed RZ format(CSRZ),RZ-Differential Phase Shift Keying format(RZ-DPSK)and CSRZ-DPSK et.al)are discussed in detail.The demodulation process of DPSK is analyzed in detail.The DPSK receiver is accomplished and the eye diagrams of NRZ-DPSK,RZ-DPSK,CSRZ-DPSK are obtained through writing the balanced receiver program of DPSK.◆The modulation and demodulation experiments of OOK and DPSK formats are designed and the eye diagrams and spectrum have been obtained.For DPSK formats,the demodulation of DPSK is successfully realized.The eye diagrams and spectrum of "constructive interference" and "destructive one" have been obtained by using two modulators in the experiments.◆Several PMD compensation methods now available are investigated and compared comprehensively.The detailed principle and topological of partial swarm optimization algorithm which is the core control algorithm in PMD compensation system are demonstrated.◆The basic concept of degree of polarization(DOP)is reviewed.The relationship of DOP and DGD of different modulation formats are analyzed in order to use DOP as the feedback control signal.For OOK format,the dynamic range of DOP is small.Contrarily,that of RZ and CSRZ format is larger.However,it can be easier to trap in the local maximum.For DPSK formats,with the increase of the DGD,the DOP drops monotonically which is beneficial to PMD compensation system. It is the DPSK format which is the most appropriate formats in the PMD compensation system using the DOP as feedback signal.◆Using the modulated OOK and DPSK formats as the signals in the PMD compensation system,the dynamic combination of modulation technique and PMD compensation technique is realized via experiments.The compensation performance is very good by comparing the eye diagrams before and after PMD compensation.◆The PMD compensation of different modulation formats are simulated. Through comparing the DOP values,the residual DGD values and eye diagrams before and after PMD compensation,the results show that the PMD compensation performance of DPSK is better than that of OOK format because the more narrow spectrum of DPSK format makes it be little influenced by PMD.In all the OOK and DPSK formats,CSRZ-DPSK has the best PMD compensation performance, so it is the most ideal transmission format.◆On the basis of the modulator principle,the modulation and demodulation equations of multi-level phase modulation formats are derived and the modulation schemes are introduced. |
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