| Nowadays the optical fiber transmission system is developing towards high capacity and long distance. In order to bearer more and more services, higher transmission rate is required. So how to guarantee the performance of system while improving the capacity is the hottest topic now. In the optical fiber communication system, how to load the baseband signal to optical wave and use which parameter to modulate the signal are material differences to optical signal generated. Different modulation format and different code shape of optical signal has their own relative merits. Moreover, one of most important part in fiber transmission system is fiber channel. Since there are many effects in the fiber, such as dispersion and nonlinear effect, these uncertain factors will have complex influence on the signal. Not only the transmission distance is related to performance, the modulation format is also an effect. How to design the novel modulation format which is suitable to fiber transmission system and how to effectively decrease the influence of fiber channel to signal is the research emphasis of next generation optical communication technology. The paper proceeds from multiple optical modulation formats and fiver nonlinear effect, the major research contains the following issue:1. Study the optical modulation and demodulation mechanism. Through the analysis of the optical modulation device theory, the paper research the generation principle based on different modulation parameters, and the realize scheme of multiple code shape. In addition, the paper also studies the resistance on dispersion and nonlinearity of fiber transmission system based on different modulation format and code shape.2. On the basis of producing and demodulation method of several optical modulation formats, the paper research multiple or orthogonal modulation and the system performance based on them. During the research, the paper proposes a novel DPSK/FSK orthogonal modulation. By the combination the Mach-Zehnder modulators and setting the parameter appropriately, the process of modulation and demodulation is researched.3. The process of optical signal transmitting in fiber is studied. The paper start from the basic equations of signal transmission process, do the research on the linear effect, attenuation and dispersion, and nonlinear effect, self-phase modulation, cross phase modulation and four wave mixing effect. Analyze from the theory about these effect on pulse and spectrum of optical signal when transmitting, and the evolution process as transmission distance increase. Moreover, the simulation by matlab is implementing to prove these influences on signal.4. Through the generation principle of nonlinear effect, the author designs a pre-compensation module to resist the nonlinear effect. By the theoretical analysis, modeling and simulation, the result of this pre-compensation module is proved by the comparison of the constellation diagram and the performance curve of system receiving. The result proves that it has great improvement to system performance.5. A super orthogonal modulation combining intensity, phase and frequency is studied and the demodulation scheme is design. By the simulation of super orthogonal modulated signal, the performance of each branch signal and the comparison to other modulation format is analyzed. In addition, a WDM system based on super orthogonal modulation is researched. Based on the previous theoretical analysis, the simulation of different channel number and different interval is consistent with the theory. And some idea about improving the performance of WDM system. |
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