Research On Sbs And Phase Noise Suppression In Remote Interferometric Optical Fiber Sensing Systems

After dozens of year's development, the interferometric fiber sensing technology has been widely applied in the fields of underwater acoustic detection, oil exploration and seismic wave detection. With the development of optical amplification and array multiplexing, the interferometric fiber sensing system is developing in the direction of longer distance and larger scale. However, the increasing transmission length leads to the decrease of the nonlinear effects threshold such as SBS, FWM and MI, which sets a serious effect on the system's performance. SBS is the nonlinear effect with the lowest threshold in remote interferometric fiber sensing system. When SBS occurs, the forward propagating power suffers a serious loss and the phase noise increases dramatically, which reduces the detection sensitivity seriously. SBS suppression technique has been researched extensively in the fields of optical fiber amplification and communication, The power loss induced by SBS is the main problem that concerns in optical fiber amplification while the effect on bit error rate of digital signal in optical communication system. However, not only power loss but also the phase noise which determines the detection sensitivity has to be concerned in interferometric fiber sensing systems. In the past, few researches concern about the SBS suppression in interferometric fiber sensing system, and the results show that the suppression techniques in which still have disadvantages for practical application. So it is necessary to study the SBS suppression techniques in the special case of remote interferometric fiber sensing systems.For the interferometric fiber sensing system which runs in practical environment, phase modulation and optical frequency modulation are effective and convenient methods to suppress SBS. Phase modulation generates multi-frequency laser by phase modulator to suppress SBS. The research before shows that the phase modulation brings excess phase noise, which limits the application of phase modulation in the system. Aimed to the excess phase noise, detailed study was carried out to show the physical mechanism of SBS suppression by phase modulation in the paper. The origin of excess phase noise induced by phase modulation is analyzed. Two phase noise suppression techniques based on phase modulation are proposed to suppress the excess phase noise: the method of modulation and demodulation and the method of matching interferometry. By either of the methods, SBS can be suppressed effectively with a low phase noise in the system. As a result, the maximum input power and transmission length can be increased effectively. These are the most important innovation of the dissertation.Optical frequency modulation method modulates the output frequency of the laser directly to suppress SBS. In the paper, SBS suppression by frequency modulated laser in long-haul fiber was studied in detail. A theoretical model was proposed to show the physical mechanism. An experiment was carried out and the results of which agree well with the theoretical model. With the simulation and experimental result, the frequency modulation parameters were discussed for practical application. The paper also analyzed the combined application of frequency modulation in PGC technique and SBS suppression. The relationship between the optical path difference(OPD) and the frequency modulation index was analyzed to choose the modulation parameters. The results show that shorter OPD and larger frequency modulation index leads to higher SBS threshold and lower phase noise in the system.The SBS suppression methods proposed in the paper were discussed in detail for the practical application. The influence of the methods on the transmission length was discussed. The effects of various nonlinear effects(SBS, FWM and MI) on phase noise were analyzed and the mutual relations were discussed. A 175 km interferometric fiber sensing system was carried out. The measured phase noise level is comparable to that of the short-haul system.In conclusion, the results of the subject are very meaningful to the design of remote interferometric fiber sensing systems.