Research On The Technology Of Quantitative Measurement In ?-OTDR Based On Its Linear Distribution Of Phase Change

Phase optical time domain reflectometer(?-OTDR)has received extensive attention in the field of distributed monitoring due to its high sensitivity.In the last decade,more and more researchers have devoted themselves to the research of measuring the perturbation event based on the phase information of ?-OTDR.The phase information of ?-OTDR is usually contained in different interference patterns of the Rayleigh backscattering signal.Consequently,in order to retrieve the phase,many interference schemes were developed,including coherence detection with reference light,double pulse interference,interferometer,etc.At the same time,many demodulation methods were also developed such as three-port demodulation,Hilbert demodulation,orthogonal demodulation,etc.These studies have greatly promoted the development of quantitative measurement of ?-OTDR.However,there are still many problems in the technology of quantitative measurement in ?-OTDR.These problems include: the relationship between the statistical phase of ?-OTDR and the external perturbation cannot be perfectfully expressed,and worth still,the spatial distribution of phase is hard to show;the waveform distortion of phase signal caused by noises including laser frequency drift is not fully considered;the initial phase of ?-OTDR falls within the range of [-p p],and phase unwrapping will face the limitation that the absolute difference of adjacent phases should not be more than p. Consequently,in coherent ?-OTDR,the following four aspects are mainly studied:1.The elastic deformation and elastic-optical effect caused by the external perturbation on the optical fiber are equivalent to the change of fiber refractive index,and the mathematical model for quantitative measurement is established based on the one-dimensional impulse response model of OTDR.On this basis,the linear characteristic of phase change along the fiber is deduced.2.The fitting method is introduced to obtain the proportional relationship between the phase change and the fiber length at any arbitrary pulse time,which effectively suppresses the impacts of various noises such as phase noise of reference light,polarization fading,interference fading,acquisition error,etc.on the measurement results.At the same time,it is pointed out that when the slope of the fitted curve is very small,the phase change of the fitted curve at the starting point of the perturbation can well represent the information of the external perturbation.3.By calculating the correlation between the phase change of any position on the optical fiber and the phase change of the back-end,the number of jumps of the correlation coefficient reflects the number of perturbaitons on the optical fiber.The amplitude of phase change increases with the increase of overlap length between optical pulse and event region,but the time of the correlation coefficient jump is earlier than the stable time of phase change.Therefore,two events on the length of half pulse width can be seperated by using this property,which breaks the traditional spatial resolution limit.4.Using the dual-frequency light scheme to solve the limit of the phase unwrapping algorithm.Relative analysis shows that the correctly demodulated phase change varies regularly along the fiber whereas the wrongly unwrapped phase shows a chaotic profile.Based on this feature,the limit of the phase unwrapping algorithm is broken by using an auxiliary pulse light in ?-OTDR.Meanwhile,using the static phase as a reference can also successfully break this limit.