Research On Key Techniques Of Distributed Optical Fiber Sensor Based On Spontaneous Brillouin Scattering

Acoustic surveillance in the littoral consisting of large-scale fiber optic hydrophone array and long-distance transmission fiber cable is one of the important developmental directions of fiber optic hydrophone. However, the transmission cable and the detecting cable of optical fiber hydrophone array are often destroyed by submarine current, halobios, man-made activities and so on. Frequent optical cable troubles not only bring great loss to the economy, but also deduce the surveillance system stop working. Moreover, military communication cables may be distinct targets of wiretap and attacks from the enemy states. Distributed optical fiber sensing based on Brillouin scattering has drawn a lot of interest in the sensing field because of its high precision of temperature and stress test. Only a fiber should be used in the distributed optical fiber sensor system to detect the fault of the submarine optical cable, which can fast locate the fault after the trouble happens, reduce the time of repair, and decrease the economical loss. Whit's more important is that it can inspect the stress distribution of the cable and warn danger previously before the accident.Combining with the research on optical fiber hydrophone in our group, we investigate the key techniques of distributed optical fiber sensing based on Brillouin optical time-domain reflectometer (BOTDR). Making use of the frequency shift characteristics of a LiNbO₃ waveguide electro-optic intensity modulator (EOIM), we obtain the referenced light with Brillouing frequency shift for the heterodyne detection in Brillouin stress sensing. This thesis includes the following work and innovations:(1) The sensing principle of Brillouing scattering is analyzed in detail, including the relation of Brillouin frequency shift and power versus temperature and stress. The practical sensing functions are obtained.(2) The principle of Mach-Zehnder interferometer as a narrow-band optical filter and frequency distinction is investigated theoretically. The free spectral range (FSR) of the interferometer is as two times wide as the Brillouin frequency shift for direct detection of Brillouin sensing.(3) The frequency shift characteristics of LiNbO₃ waveguide electro-optic intensity modulator are studied theoretically and experimentally. The intensity of the output optical sideband of EOIM changes differently versus the DC bias voltage due to different test approach for low frequency (～kHz) and high frequency (～GHz) modulation. It is convenient to change the optical intensity of the sidebands by tuning the DC bias voltage while the input optical intensity is fixed. The relation between the modulation depth C and the microwave modulation frequency and intensity is analyzed in detail and verified to be correct by experiments. This is one of the innovations in the thesis. (4) The EOIM is used in a BOTDR sensing experimental system. The optical characteristics of the system are analyzed and measured in detail. The relation between the Brillouin intensity and stress is achieved through the stress experiments.(5) The Brillouin frequency shift is measured and deduced by a scan F-P interferometer with 1.5GHz free spectral range. A new definition of the threshold of stimulated Brillouin scattering is presented according to theoretical and experimental investigations, which is another innovation in this thesis.