一种利用布里渊增益谱边带解调提高布里渊光时域反射系统测温精度的方法∗

摘要

测温精度是衡量分布式光纤温度传感系统的一项重要性能指标.本文提出一种通过解调布里渊增益谱边带,以提高布里渊光时域反射仪测温精度的方法.在此基础上,进一步分析并验证了探测光脉冲峰值功率对测温精度的影响.理论分析表明,利用声光调制器的频移特性可产生布里渊增益谱边带,相比于中心峰解调方法,采用布里渊增益谱边带解调法可获得更高的系统信噪比,进而提高系统测温精度.实验结果表明,在相同测量条件下,布里渊增益谱左边带峰值功率较其中心峰峰值功率高3.27 dB,且其−1 dB谱宽比中心峰窄14.5 MHz.对布里渊增益谱左边带进行频率扫描,由于相干探测时参考光的作用以及消除了相干瑞利噪声的影响,系统信噪比提高了4.35 dB,并在10.2 km的传感距离上实现了±0.5◦C的测温精度.%A novel method by demodulating the sideband of Brillouin gain spectrum (BGS) is proposed and demonstrated in order to enhance temperature measurement accuracy in a Brillouin optical time domain reflectometry (BOTDR) sensing system in this paper. Firstly, the characteristic of frequency shift of an acoustic optical modulator (AOM) is utilized to generate the sideband of BGS, and the influence of the peak power of the probe optical pulse on the temperature measurement accuracy is also investigated. Moreover, the theoretical analysis shows that benefiting from the reference continuous light from the source laser by the coherent detection, the intensity of the sideband is higher than that of the central peak, which indicates that the higher signal-to-noise ratio (SNR) can be expected by demodulating the sideband of BGS instead of the central peak. Thus the demodulating the sideband of BGS can further improve temperature measurement accuracy in the BOTDR sensing system theoretically. Secondly, the experimental setup of the distributed temperature sensing system based on BOTDR is built. The AOM is selected as the optical pulse modulator to produce high-extinction-ratio probe pulse light, following the frequency upshift of the injection light. The beat signal generated by coherently detecting the backscattering light from the fiber under test (FUT) and the reference light from the source laser is acquired. Furthermore, the central peak and the left sideband of BGS are respectively scanned by using microwave heterodyne frequency shift technique. The time domain waveforms at each frequency point are then obtained and Lorentzian curve fitting is performed at each sampling position, thus Brillouin frequency shift (BFS) along the FUT is plotted and the temperature is demodulated along the FUT based on the linear dependence of the BFS on the temperature in the optical fiber. Finally, the experimental results show that the peak power of the left sideband of Brillouin gain spectrum is about 3.27 dB stronger than that of the central peak. Meanwhile, the linewidth of left sideband of BGS is about 14.7 MHz narrower than that of the central peak at−1 dB point in the same conditions. When the left sideband of BGS is scanned, the SNR of the BOTDR system is improved by 4.35 dB due to the contribution of the reference light by coherently detecting and eliminating the effect of the coherent Rayleigh noise, and then the temperature measurement accuracy of ±0.5 ◦C is achieved over a 10.2 km sensing fiber.