The Theoretical Study Of Differential Quadrature Phase-Shift Key In High-speed Optical Fiber Communications

With the enhancement of optical fiber transmission systems capacity, performance improvement in nonlinear effect, chromatic dispersion tolerance and polarization mode dispersion (PMD) tolerance has drawn researchers' attention of new modulation formats. In this dissertation, the processes of modulation and demodulation of the different formats, especially Differential Quadrature Phase-Shift Key (DQPSK), are investigated in detail, based on the theories of polarization mode dispersion (PMD), PMD compensation and the principles of different modulation formats. 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 and various compensation methods are reviewed and summarized. The PMD compensation technology in feedback optical compensation and the study of using degree of polarization (DOP) as the feedback control signal are discussed in detail.The theories and modulation processes of different duty cycleOn-Off Key (OOK) modulation formats and Differential Phase-Shift Key (DPSK) modulation formats are discussed in detail. The power versus time and optical power spectra figures of these modulated signals are obtained. The demodulation process of DPSK is analyzed in detail. The coherent reception of DPSK is accomplished and the eye diagrams of different duty cycle DPSK formats are obtained through numerical simulation.The modulation and demodulation principles of DQPSK modulation format are analyzed in detail, the modulation and demodulation equations are derived. The pre-coding, modulation and demodulation modules of DQPSK are accomplished, and this simulation system is verified. The figures of power and phase change versus time, eye diagram and the output signals after interferometer of different duty cycle DQPSK formats are obtained by numerical simulation.The relationships of DOP value and phase variation versusdifferential group delay (DGD) are studied in detail. It is the phase modulation format and higher duty cycle which is the most appropriate formats in the PMD compensation system using the DOP as feedback signal.The PMD compensation performances of different modulationformats are simulated. The DOP values, the residual DGD values and eye diagrams before and after PMD compensation are obtained. The results show that CSRZDQPSK has two-fold increase in the bit rate on the basis with maintaining the same performance of DPSK and OOK formats, better PMD compensation performance, higher spectral efficiency and PMD tolerance, so it is the most advantageous formats in the next generation networks.