Research On Performance Improvement Of ?-OTDR-Based Vibration Detecting System

Distributed fiber sensing technology uses optical fiber as both sensing material and transmission material,and can measure physical parameters along the optical path,such as temperature,strain,vibration,curve of fiber,et al.Optical fiber sensor has advantages of high sensitivity,anti-electromagnetic,wide frequency band,large dynamic range,soft and portability.It can be applied in some special application,such as electrical industry,chemical industry,spacecraft,et al.Phase-sensitive Optical Time Domain Reflectometer(?-OTDR)is more suitable for the application with commond of long range,large area,high sensitivity and multipul locating.However,limited by low singal to noise ratio,fake alarm and singal distortion in long range situation,application of ?-OTDR is limited.Aimed at the problems in ?-OTDR sensing system,this thesis proposes a serial of methods to improve the performance of ?-OTDR,including:1.Established the simplified mathematical model of ?-OTDR,explained the mechanism and limitation of coherent Rayleigh backscattering within and wihtout probe pulse,analyzed limitation of sensing range,frequency reponsed range and detecting sensitivity of?-OTDR,made the foundation for the following performce improvement techniques.2.Analyszed the noise feature and noise source,proposed a self-adaption threshold denoise method based on wavelet.Through evaluating the noise level in the front part of sensing fiber,obtained the correction parameter and then let the denoise threshold to be captable of following the noise change in whole length.The method overcame the affection of non-unmiform amplitude of detector ouput.Tests showed that the method can improve the signal to noise ratio by 10 dB.3.Proposed a long range ?-OTDR and locationg method to improve sensing range of ?-OTDR.Specified the parameter limitation of?-OTDR to avoid nonlinear effect.Increased sensing range by applying a long probe pulse.At the meantime,proposed a response segment determination method based on correlation dimension and time-space image to realize accurate locating and overcame the locating accuracy lost caused by long pulse.Tests result showed that it can improve the sensing range to 60 km,with 6.8m locating accuracy and single-end structure,which can meet the command of pre-warning system for oil and gas pipeline.4.Proposed a structure combined ?-OTDR with M-Z interferometer.Through applying narrow-band FBG to reflect the backscattering light and transmiss the MZI light,optical output from ?-OTDR and MZI can be separated.In the meanwhile,through adding the local light to the backscattering light,forming coherent detection.Through demodulating the sepicific frequency component in the coherent single,removed the leakage of MZI,which improved the SNR of ?-OTDR a lot.Experiment result showed that the new structure can detect both 1Hz~4.95 MHz vibration with 20 m locating accuracy,which means being captable of wide frequency response and accuarate locating.5.Proposed a multi-wavelength ?-OTDR to resist coherent fading.The coherent fading of a specific wavelength light is compensated by other light with different wavelength,which improves the consistency of detecting sesntivity.Experiment showed that when the difference between two wavelength is larger than 0.012 nm,the system can improve the effective sensing points in the whole lenghth by 30%.