Research On Brillouin Optical Time Domain Analysis System(BOTDA) Based On Double Sidebands Interference

In the distributed optical fiber sensing technology,the Brillouin stimulated scattering based optical time domain analysis(BOTDA)technology can achieve high-precision temperature or stress sensing in a large range,so it has a good application prospect in the structural health monitoring of large buildings.BOTDA uses the linear relationship between temperature / stress and Brillouin frequency shift for sensing.Due to the limitation of phonon lifetime,the spatial resolution of traditional Brillouin sensor is at meter level.BOTDA based on the pulse optical pre-pump(ppp)technology is a kind of sensing technology that can break through the relaxation time limit to achieve the spatial resolution of centimeter scale.However,due to the nonlocal effect in the sensing process,the Brillouin spectrum collected at the hot spot position is the superposition of the Brillouin spectrum of the whole pulse covered fiber section,so there will be a multi peak structure at the hot spot position.The wider the Brillouin spectrum is,the larger the fitting error will be,which will lead to the degradation of frequency resolution.In order to reduce the fitting error caused by spectrum superposition,this paper proposes to use the double-sideband continuous light as the detection signal and use its interference spectrum for sensing.By using the narrow linewidth of the spectrum,the error of bimodal fitting can be reduced,and the frequency resolution of the system can be improved,so as to realize the distributed temperature sensing with high spatial resolution and high frequency resolution.This paper includes the following contents:1.The detection method of Brillouin interference spectrum with two sides is proposed.We theoretically explain the principle of interference spectrum formation,and carry out simulation and experimental verification.The results show that the FWHM of the interference spectrum is twice as wide as the Brillouin eigenspectrum.It is beneficial to improve the frequency resolution of the sensor.2.The influence of the induced refractive index change caused by the SBS effect on the spatial resolution of the system and the influence of the interference effect on the frequency resolution of the system are studied experimentally.The results show that the bodda system based on two-sided continuous light can alleviate the deterioration of spatial resolution caused by narrow pulse distortion,and the interference effect of two-sided continuous light can improve the spatial resolution of the system.3.The distributed temperature measurement of the pulse pre pumping system based on the interference effect of double-sideband is realized.Using the advantage of narrow linewidth of interference spectrum of double-sideband,a high spatial resolution of 50 cm and the frequency resolution of 0.37 Mhz is achieved in the pulse pre pumped BOTDA system,and compared with the traditional pre-pumped BOTDA system with the same spatial resolution,the frequency resolution is increased by two times.This sensing scheme provides a new technical direction for the realization of Brillouin distributed sensing with high spatial resolution and high frequency resolution.