Study On The Performance Of DPSK Optical Fiber Communication System

Along with the continual development of the economy, the requirement information is increasing rapidly. In order to satisfied the requirement of the society, the capacity and the rate of the communication system must be improved. In recent years, the wavelength division multiplexing(WDM) technology has developed swiftly, but the impact of the chromatic dispersion and the fiber nonlinearity become seriously in the high transmission rate and long-haul communication system. In order to compensate these impacts, some advanced technologies must be used in the optical fiber communication system. The differential phase-shift-keying (DPSK) modulation format is one of these new technologies.The DPSK modulation format not only can improve the receiver sensitivity but also can enhance immunity to the effect of fiber nonlinearities. But the theoretical analysis of DPSK optical fiber system is not easy, in the presence of transmission impairments, such as chromatic dispersion and the fiber nonlinearities, the conventional Q-factor cannot be used to predict the bit-error-rate(BER) of DPSK transmission. There are two main reasons: one is that the transmission impairments and receiver imperfections make the pulse broaden and distorted, it is difficulty to characterize the statistic fluctuation of the received signal; The other reason is that the fiber nonlinearities leads to a complex interaction between the signal and the optical noise, The noise spectrum does not remain white during the fiber propagation, the noise distribution at the receiver deviates from Gaussian and becomes unknown.In order to solve the problems encountered in the theoretical analysis of DPSK, and on the basic of the study on DPSK modulation format, the Karhunen-Loeve Series Expansion (KLSE)method is used to simulate the performances of the DPSK system. Including the comparison between the single-ended detection and the balanced detection, the determination of the optimal bandwidth of the filters, the power penalties caused by chromatic dispersion (CD), polarization-mode dispersion (PMD) and receiver imperfections. And then, the influence of the fiber nonlinearities on the performance of DPSK system was analyzed. The simulation results show that the method used in this thesis can solve the problems encountered in the theoretical analysis of DPSK, and also can give good instruction to the practical work.The details of this thesis is given as follows:1. Introduce the structure and essential components of the DPSK optical fiber communication system. According to the characters of the DPSK modulation, the optical LiNbO3 phase modulators are used at the transmitter to produce the different formats, and then the spectrum characters of the formats are analyzed. The signals are influenced by CDand PMD when propagating in the fiber, so the fiber transfer function which taking account of both CD and PMD can be obtained by analyzed the linear transmission equation. Then the principles of the components at the receiver are introduced, and the transfer functions of them are obtained. Finally, the schematic of DPSK system is introduced.2. The performance of the single-ended and balanced detection are analyzed. When the fiber nonlinearities can be neglected, the Karhunen-Loeve Series Expansion can be used to describe the optical noise, and the probability distribution of decision variables can be obtained from mathematical calculation. Then the BER of the system can be calculated through the saddle-point integration approximation. Monte-Carlo simulation verified that the KLSE method can yield highly accurate BER for DPSK system. With this method, the BERs of the single-ended and balanced detection are simulated. In this condition, the balanced detection has better performance than the single-ended detection. In the presence of fiber nonlinearities, the output signals of this two receivers are compared, the analysis results show that in this condition, the threshold of the single-ended detection receiver is also easy to be obtained, so the the advantage of the balanced detection diminished, the single-ended and balanced detection have a similar performance.3. The impact of CD, PMD, receiver imperfections and fiber nonlinearities on DPSK optical fiber communication system are analyzed. The optimal bandwidths of the filters are obtained by comparing the BERs of different bandwidths. In the optimal bandwidth, the 2DPSK over perform the 4DPSK when the transmission impairments are neglected. The simulation results indicate that the power penalties become larger when the transmission impairments become larger; 4DPSK demonstrates higher CD and PMD tolerance than 2DPSK, but demonstrates much lower interferometer phase error (IPH) tolerance than 2DPSK;RZ-4DPSK demonstrates higher CD, higher PMD tolerance than NRZ-4DPSK, but the RZ-2DPSK(50% duty cycle) is more sensitive to CD and more immune to PMD than NRZ-2DPSK. Finally, the impact of fiber nonlinearities to the performance of DPSK system was analyzed.