Research On Dual Parameter Distributed Fiber Optic Sensing Technique Based On Laser Interferometry And Raman Scattering

As the distributed fibr optic sensing technique develops and widely used,the requirement of multi-parameter and multi-function integrated sensing has been emerged.In order to meet the need of vibration and temperature sensing simultaneously for the industry safety and the construction health,a dual parameter distributed fiber optic sensing technique based on laser interference and Raman scattering is proposed.The proposed sensing technique employs wavelength division multiplexing to construct the sensing system based on the structure of dual Mach-Zehnder interferometry and achieve the demodulation of the interferometry signals and Raman backscattering light signal simultaneously,which meet the need of vibration and temperature sensing simultaneously.The paper's innovation includes:1.The integrated demodulation method of distributed fiber optic vibration and temperature sensing technique is proposed based on laser interference and Raman scattering.By employing the structure of dual Mach Zehnder interferometry and also with the help of wavelength division multiplexing and different center wavelength of the lasers,the simultaneous detection of interference and Raman scattering light signals has been achieved.The two lasers sources was combined by inserting an optical coupler on one arm of the dual Mach Zehnder interferometry,then the Raman scattering light was filtered by WDM from the other one in the sensing arm.2.A noise assisted denoised analysis method employing ensemble empirical decomposition based on the mechanism of Raman scattering and the components of the noise is proposed.The method added white noise to Raman scattering signal,then decomposited the signal by empirical mode decomposition.Based on the difference of the spreads between Raman scattering light signal and noise in decomposited signals,which were known as intrinsic mode functions,noise filtering and temperature signal reconstruction can be achieved.The method we proposed can effectively increase the signal to noise ratio and improve the temperature resolution with long sensing distance.3.In order to eliminate the affect of polarization fading and polarization phase shift on the position error of the vibration in distributed fiber optic dual parameter sensing,a fast polarization compensation method is proposed.The polarization compensation method achieved fast directional polarization compensaton by employing a feedback mechanism with the help of polarization controllers and polarization beam spliters.This method eliminated the polarization fading and polarization phase shift by sychonizing the polarization shift of two interfence signal and maximizing the interference visibility,which decreased the positioning error of the vibration event in distributed fiber optic dual parameter sensing.4.The system of distributed fiber optic dual parameter sensing technique based on laser interference and Raman scattering was constructed.For detail,the hardware and software were built by modules with temperature auto-correct mechanism and fast polarization compensation mechanism.The temperature auto-correct module compensated the device temperature shift of the sensing system by detecting the temperature of the fiber coil.The fast polarization compensation module achieved the polarizaiton synchronization by collecting and analysing the outcome of the PBS to adjust the polarization controllers.