Distributed Fiber Sensing System Based On Stimulated Brillouin Scattering And Its Applications

Distributed optical fiber sensors allow long-haul, large-dynamic andmulti-parameter measurement. This kind of sensor has found technical superiority andwide applications in the fields which relate to national defense and people life andwealth safety, such as safety defence, oil and gas pipeline, electric power network,structure health monitoring of ships, airplanes, and civil structures. Among all thesesensors, distributed optical fiber sensors based on Brillouin scattering have gainedglobal interest for their ability to allow long-haul, large dynamic, and high precisiontemperature and strain measurement along the fiber. At present, this kind of sensor hasbeen an advanced technology that the developed countries hold. Nevertheless, there stillhas a lot of work to do for the domestic research.Firstly, Brillouin scattering in fibers and its sensing principle are analyzed in detail;power estimation of the Brillouin distributed fiber sensing systems is implemented.Then, a distributed sensing system based on stimulated Brillouin scattering (SBS)(i.e.,a BOTDA system) is designed and constructed. This system is applied to monitor thestrain distribution of the optical transmission cable of some fiber optic hydrophone(FOH) system; consecutive and in-line strain monitoring of the cable is realized.Polarization beam splitters (PBSs) are used to upgrade the BOTDA system, which isused to measure the uniformity of polarization-maintaining (PM) fibers. In particular,this upgraded system is applied to inspect the winding uniformity of PM fiber opticgyroscopes (FOG), which makes winding state inside the FOG intuitionisticlyvisualized. Because the electro-optic modulator with microwave frequency modulationin the system brings problems of multi-sideband and power fluctuation, DFB laserinjection locking technology is adopted to generate stable single-sideband frequencywith Brillouin frequency shift. Brillouin-erbium fiber laser (BEFL) is deeplyinvestigated to cut the system cost. Ultra-short ring-cavity (~10m) BEFLs are realized.A BOTDA system with BEFL is constructed, which confirms the feasibility of applyingBEFL to Brillouin sensing systems.The main results are summarized as follows:1. Power estimation of Brillouin sensing systems is implemented. The SBSthreshold is investigated theoretically and experimentally. The upper input powerlimitation of the BOTDR system, i.e., the SBS threshold suitable for optical fibersensors, is achieved. This threshold is defined as the pump power whose1%is equal tothe backscattered power. The distribution of pump and Stokes powers as well asBrillouin gain along the fiber is simulated and analyzed under various injected powers,which is achieved by solving the SBS coupled wave equations numerically. Thesimulation indicates that an appropriate pump power value is needed. Once the pump power is chosen, the probe power should be low enough. Thus, the BOTDA system canachieve uniform Brillouin gain along the fiber at a certain signal-to-noise ratio (SNR).The power estimation is significant for constructing Brillouin sensing systems.2. A BOTDA system based on microwave electro-optic frequency shift is designedand constructed. The Brillouin frequency shift temperature and strain coefficients of thesensing single-mode fiber are1.01MHz/℃and0.57MHz/g (in weight), respectively.The spatial resolution is1m and the temperature resolution is better than0.5℃with asensing fiber length of25km.3. DFB laser injection locking technique is used to extract stable single-sidebandfrequency with Brillouin frequency shift. Because the electro-optic modulator withmicrowave frequency modulation in the system brings problems of multi-sideband andpower fluctuation, DFB semiconductor laser injection locking technique is investigated.The results indicate that the locked slave laser under weak injection acts as anarrow-bandwidth optical filter. On account of thin point, the frequency-shifted lightafter the EOIM is injected into the slave laser and stable single side-band frequencywith Brillouin frequency shift is obtained, which can improve the BOTDA systemperformance.4. The BOTDA system is for the first time applied to realize consecutive andin-line strain monitoring of the optical transmission cable of some FOH system. Twoidle fibers inside the cable are connected to the BOTDA system. The strain distributionof the cable before and after hanging hydrophone is analyzed through Brillouinfrequency shift measurement, which is beneficial to FOH design. The BOTDA systemcan also be used to monitor cable state during FOH test over lakes and seas.5. Winding uniformity inspection of PM-FOG coils is realized. PBSs are used toupgrade the BOTDA system, which guarantee that the pump and probe light are bothlinearly polarized and injected into the same axis of PM fibers. Thus, the Brillouinfrequency shift distribution of the two axes can be measured, respectively. Theupgraded BOTDA system is applied to measure the Brillouin frequency shiftdistribution of some axis of PM-FOG coils. Because the temperature is constant, thestrain distribution inside the coils is exclusively and accurately decided by the measuredBrillouin frequency shift. Consequently, the winding uniformity of FOG coils isintuitionisticly visualized. The primary FOG coil screening is thus made, which issignificant for the FOG precision control.6. Ultra-short ring-cavity (~10m) BEFLs are for the first time designed andconstructed. There are two ways to realize this kind of lasers. The first is based onBrillouin pump pre-amplification. Only5m single-mode fiber and4m erbium-dopedfiber are used to provide the nonlinear Brillouin gain and linear EDFA gain,respectively. The second one only uses4m erbium-doped fiber which provides both nonlinear Brillouin gain and linear EDFA gain. Because the BEFL frequency is nativelyBrillouin frequency shift lower than its pump frequency, it can be used to constructnovel low-cost Brillouin distributed fiber sensing systems.