Research On Phase-modulated Technologies In High-speed Fiber Transmission System

In recent years, optical phase modulation formats have become one of the hot issues in the study of optical transmission at home and abroad. The transmission systems based on phase modulation formats have higher receiver sensitivity and larger nonlinear tolerance than the transmission systems based on OOK modulation format. Therefore, the phase modulation formats are very suitable for the upgrade of the transmission systems. However, the modulation/demodulation methods of the phase modulation formats are immature, and the research of the phase noises in the phase-modulated systems is imperfect, which limit its future application in the optical transmission system. Based on these problems, this paper thoroughly studies the phase modulation formats. Firstly, the paper studies the operating principle of phase modulator and intensity modulator, and the modulation/demodulation methods of DPSK and DQPSK, and then presents a method of deducing the logical expressions of the DQPSK precoder. Secondly, it simulates and compares the dispersion tolerance and nonlinear tolerance of several modulation formats. The results show that the phase modulation formats based on CRZ have the optimal performance. Finally, this paper analyzes in detail the nonlinear phase noise in phase-modulated optical transmission system. The joint probability density function of nonlinear phase noise and received intensity is obtained from the statistic model of phase noise, on the base of which, the exact expression of nonlinear phase noise's bit-error ratio is deduced.