Dpsk And Dqpsk Optical Fiber Communication System And Comparison

In recent years, optical fiber communication system began to use the DPSK (Differential Phase Shift Keying) format to replace the traditional OOK format. The benefit of DPSK format is the 3dB OSNR (Optical Signal Noise Ratio) advantage, and DQPSK (Differential Quadrature Phase Shift Keying) format is also getting more attention based on theoretical analysis with better tolerant of non-linear effect. In the third chapter, we simulate the 40Gb/s DPSK and DQPSK fiber communication system, investigate the degradation factors in balanced receiver.It turns out the frequency offset between laser and Mach-Zehnder interferometer can lead to the most serious receiver sensitivity penalty. In real transmission system, such a frequency offset can be induced by wavelength drift of optical source or aging of MZI itself. For proper operation of balanced detection DPSK or DQPSK system, it is essential to minimize the frequency offset , since any such offset results in an imperfect interference within the interferometer, and consequently , in a receiver performance degradation. Since a DPSK and DQPSK receiver relies on the interference of two optical fields, it is fundamentally sensitive to such a mismatch. After the simulation, we find out we have to limit the frequency offset below 10% for 40Gb/s DPSK fiber transmission system, and 5% for 40Gb/s DQPSK fiber transmission system to maintain good performance.In the fourth chapter, we present the fiber communication system combined with DPSK or DQPSK with Amplitude-Shift-Keying (ASK) to encode multiple bits per symbol. While the basic functionality of DPSK-ASK and DQPSK-ASK receivers has been covered, little can be found on the influence of receiver parameters on system performance. We identify and discuss important design criteria in 80Gb/s DPSK-ASK and 120Gb/s DQPSK-ASK fiber communication systems by only using 40Gb/s equipment.For RZ-DPSK-ASK and RZ-DQPSK-ASK, the extinction ratio of the ASK signal is a tradeoff between good eye opening for the amplitude signal and good eye opening for the demodulated phase signal, so we fist find the best extinction ratio that can result in equal receiver sensitivities for both amplitude and phase modulated signals, hence resulting in the optimum performance for the system as a whole.After the investigation of the performance of 80Gb/s DPSK-ASK and 120Gb/s DQPSK-ASK modulated optical fiber communication system, we find the frequency offset between laser and MZI is still the most serious degradation factor in receiver, but due to the ASK receiver is insensitive to the frequency offset, the tolerance ability for 80Gb/s DPSK-ASK system is almost the same for 40Gb/s DPSK, even if the bit rate is doubled. For 120Gb/s DQPSK-ASK system, due to the bit rate is tripled, the frequency offset should be limited to 2.5%.We investigate the effect when multiple factors are present at the same time in 120Gb/s DQPSK-ASK fiber communication system. According to the simulation result, the degradation of the receiver is not the linear combine, it can lead to much worse degradation even the individual factor itself can only induce little, there is also a chance one factor can suppress another factor's effect. This conclusion is also valid in 40Gb/s DPSK fiber communication system.