Investigation of nonlinear phase noise in differential phase-shift-keying systems

In this thesis, the optical ultra-high bit rate transmission system with the DPSK format has been analysed. One focus was to set on the modelling and characterization of DPSK key competent, including transmitter for phase modulation, and receiver for direct detection. Another focus was to set on some important factors which influence the transmission performance, for example, the transmission issues and the noise. More specifically, the investigation of nonlinear interactions of the signal pulses and between signal and noise during transmission has been emphasized. Beside, the estimation of the system performance was also discussed. among the several useful methods we should choose an appropriate calculation technique to assess performance of a certain system more exactly.

From the simulation results above, we can understand the characteristics of DPSK transmission system by monitoring how the system performance is affected by system parameters and transmission effects. Generally speaking, the DPSK format is advantageous compared with the conventional OOK format. DPSK offers a 3-DB reduction in receiver sensitivity and required OSNR, and has been demonstrated more robust to narrow optical filtering, the effects of some fibre nonlinearities, and fluctuations in signal level at the receiver decision circuit. In addition, we have also investigated the nonlinear transmission of the DPSK format by a certain long-link scheme, which is post-compensation with some residual dispersion and additional compensation at the transmitter and receiver. For the long-haul transmission, the Gordon-Mollenauer effect induced nonlinear phase noise and IFWM effects are the major sources for degrading DPSK systems. The former dominates at lower bit rate around 10 Gbps while the latter mainly influences at 40 Gbps and higher. The results from 10 Gbps to 160 Gbps have been shown above. As the bit rate changes, the dominating transmission effect could switch to another or have different influence on the system performance. From those results we can get deeper impression of how to distinguish these different degradation sources and their impacts on DPSK systems with higher bit rates.

Furthermore, fiber types as well as dispersion schemes also have significant influence on the electrical Q-penalty caused by the nonlinear phase noise. In general, pre-compensation schemes are more robust to nonlinear phase noise than post compensation schemes. The tolerance is around 2dB of launch power. For the influence of different fiber types, SSMF shows more tolerance to the nonlinear phase noise than NZDSF. Furthermore, we observed that optical noise can be washed out from the bit centre during propagation.

 

This complete picture for the simple DPSK system built up in this thesis can provide useful information for systems designs, indicate the problems which need to be mitigated and give some ideas for compensation technique. However, this analysis only focused on the single channel system and only several compensation schemes. In order to follow the current status and solve the transmission problems encountered by the stat-of-the-art techniques, this research work should be extended to more complicated systems, such as WDM system, polarization MUX systems and other modulation formats. Some investigations for different link designs have to be considered as well for system optimizations.