Research On Key Technology Of C-Band Microwave Signal Coherent Optical Transmission

The optical fiber transmission of microwave signals is characterized by large message capacity, low fiber loss, high bandwidth, easy maintenance, and high safety. In addition, the optical coherent detection of microwave signals solved a lot of problems caused by direct detection, such as low sensitivity and limited power in direct optical coherent detection. Therefore, it is widely applied in wireless communication system, radar system, and satellite communication system etc. The thesis focused on the study of the coherent detection technology of C-band(3.7-4.2GHz) analog signals, and theoretically analyzed its detection methods, as well as its non-linear features. The thesis designed the transportation system of intensity modulation and heterodyne detection.The thesis theoretically analyzed AM, PM, FM, and homodyne detection and heterodyne detection. Then the thesis deduced the basic mathematics model of these three modulation methods, and observed changes of frequency spectrum. After that the thesis analyzed the relative coherent demodulation theory.The thesis studied non-linear features generated in the modulation process of MZ modulator of 3LiNbO substrate. According to the modulation property curves picture, the standard point of MZ modulator is V2?. Through the analysis of nonlinear error values, the bigger the deviation to standard point, the worse its linear effects are. And then analyzed intermodulation distortion produced when using multifrequency signals as modulation signals, we got the conclusion that non-linear features are related with modulation depth. Therefore, we can control non-linear situation by changing its modulation depth.The thesis also studied the noise composition of the fiber-optic link, and simulated the microwave signals intensity modulation-direct detection(IM-DD) process to analyze the relationship between the receiving sensitivity and the carrier-to-noise ratio, as well as the relationship among local-oscillator power, signal power, and carrier-to-noise ratio during coherent detection, and the result indicated that the sensitivity in homodyne detection is 3 dB higher than that in heterodyne detection. When carrier-to-noise ratio is 20 dB, we get the optimum value. Concretely, 0.3LP ?mW, 45.86S?-BP d m in heterodyne detection and 0.3LP ?mW, 48.87S?-BP d m in homodyne detection. Which means that the carrier-to-noise ratio declined while the intensity of noise coefficient increased, and it finally becomes stable.The results show that MZ-3LiNbO strength modulation and heterodyne detection is one of the best schemes for long-distance fiber analog signals transmission. The thesis used C band microwave signals to transmit a distance of 200 km communication system. At the best value of local-oscillator power and signal light power, the transmitter power is 3.14 dBm. In addition, the study used heterodyne asynchronous detection to construct the link on the Optisystem plat and then analyzed the mixed signal waveform picture and the spectrogram of demodulated signals in link simulation.