Research Of Distributed Optical Fiber Sensors Based On Brillouin Scattering

Brillouin optical time domain analysis (BOTDA) has been a favorite of long-range and distributed optical fiber sensors as a result of the key advantages that it posses the features of small in size, immune to electromagnetic interfer-ence, capable of remote measurement and higher signal-to-noise ratio (SNR).And it has been one of the hot research topics in the field of optical fiber sensors for the decades. However, the performances of traditional BOTDA technique greatly suffer from the specific non-local effects, which constrain the allowable probe power below the threshold of stimulated Brillouin scattering (about +5 dBm). Moreover, the Brillouin gain spectrum obtained by scanning the pump-probe frequency is actually the result of convolution between the natural Bril-louin gain spectrum and the pulse spectrum. This means that even in the case of very long width pump pulse, the Brillouin gain spectrum cannot be narrower than the natural Brillouin gain spectrum (about 30 MHz), which limits the per-formance of the sensor. Therefore, it will become very significant for BOTDA technique to narrow Brillouin gain spectrum in terms of high frequency resolu-tion and long-range sensing. In this paper, the main innovative research efforts and achievements are summarized as follows:(1) Research of BOTDA technique based on the orthogonally-polarized four-tone probe waveThis method has been investigated in this paper with an eyes to a novel phenomenon reported recently by the foreign researchers that, in the process-ing of BOTDA scanning, the pulse spectrum will be distorted in frequency do-main owing to the asymmetric feature of the Brillouin gain/loss process when the probe power is higher than -6 dBm/sideband. This distortion in pump pulse will have a fatal impact on the estimated Brillouin frequency shift at the end of sensing fiber. Therefore, a novel BOTDA method based on the orthogonally-polarized four-tone probe wave is proposed in this paper. By introducing an-other double-sideband probe in the orthogonally-polarized direction, the pulse distortion induced by the asymmetric feature of the Brillouin gain/loss process in traditional BOTDA method can be greatly eliminated in the paper. Finally,the distortion of pump pulse can be avoided in case of high probe power and the sensing range is greatly extended.(2) Research of the mathematical model in pulse-based BOTDA sensors and its applicationsIn this section, the general mathematical model of several pulse-based BOTDA sensors are presented for better theoretical investigation. By adjusting the parameter settings in the model, a differential Brillouin fiber sensor based on phase difference is proposed in this paper. The final Brillouin response ex-perienced by the probe in the proposal is coming from the subtraction response induced by the ? and ? section in the three-section pulse configuration. There-fore, the final Brillouin response will become zero when the Brillouin frequency shift of sensing fiber is uniform. In addition, the proposal is hopeful to be very sensitive to small strain/temperture variation and achieve higher spatial or fre-quency resolution in the field of Brillouin sensors. Furthermore, theoretical analysis indicates that the proposal is similar with the traditional ?-phase shift method, in which the final Brillouin response experienced by the probe is com-ing from the accumulation response induced by the ? and ? section. Mean-while, we evaluate the impact that the secondary echo effect has on the hotspot detection. Theoretical results demonstrate that the systematic error will become very serious when the temperature difference between the hot spot and uniform section of sensing fiber is smaller than 17 ?.(3) Research of Brillouin gain bandwidth reduction in Brillouin optical time-domain analysersTo the best of our knowledge, in the field of Brillouin-based distributed sensing, there are no solutions to break through the limitation on the linewidth of Brillouin gain spectrum yet. However, in this paper, the goal to narrow Brillouin gain spectrum for BOTDA sensor can be achieved by using a pair of pulses with same pulse width but different phase configurations. By tak-ing the differential gain of the time-domain traces retrieved by using these two pump pulses, 2.5 m spatial resolution and the BGS with 17 MHz FWHM can be obtained, which leads to (?) times frequency accuracy improvement compared with the conventional single-pulse based BOTDA sensor. Moreover, the more precise BFS estimation in case of complex temperature/strain distribution can be achieved benefiting from the narrower BGS. Therefore, we believe that these results have positive impact on achieving high frequency resolution but with-out the compromise of spatial resolution simultaneously in case of long-range Brillouin sensors. And the narrower BGS can also promote the application in the field of Brillouin-assisted high spectral resolution optical spectrum analyzer and so on.