Research On The Improvement Of Phase-sensitive Optical Time Domain Reflectometry

The phase-sensitive optical time domain reflectometry(?-OTDR)is one kind of distributed optical fiber sensors(DOFS).In the ?-OTDR,the backward Rayleigh scattering curve exhibits a jagged appearance due to the coherent superposition of the signal generated from massive scattering centers within the optical pulse duration,because of the long coherent length of the laser source.The perturbations such as vibration,temperature and strain would change the local refractive index and length of the sensing fiber,thus it will induce the change of the jagged appearance at the corresponding position.Therefore the perturbation along the sensing fiber can be detected and located spatially by distinguishing the change of scattering curve.?-OTDR has the advantage of high sensitive to perturbations,distributed measurement,fast response,long sensing range,and so on.It has shown a great potential in many sensing applications,such as health monitoring of building structures,intruder detection of important places,safety monitoring of communication link or live wire.However,traditional ?-OTDR still has some shortcomings.One of these shortcomings is disability of strain/temperature measurement.Due to the randomness of the coherent superposition for the scattering signal,there is no constant relationship between the perturbation and the change of the jagged appearing.Therefore,?-OTDR has always been used to measure vibration,but detecting the change of strain and temperature quantitatively is almost impossible.Nevertheless,vibration,temperature and strain all are important parameters in health monitoring.The existing OTDR technologies including ?-OTDR,BOTDR and POTDR can't monitor both the vibration and strain/temperature simultaneously.In addition,the Rayleigh backscattering coefficient in fiber is very small-the power of the RBS for a probe pulse with 1 ?s pulse width is about-53 dB lower than the pulse's peak power.So normally one has to average many ?-OTDR curves obtained in different probe pulses to improve the signal to noise ratio(SNR).This procedure,however,will decrease the maximum detectable frequency of ?-OTDR.When the length of sensing fiber is L,the maximum detectable frequency would be c/4nfL when using the Fast Fourier Transform(FFT)algorithm to derive the vibration frequency,where nf n is the effective refractive index of fiber and c is the velocity of light in vacuum.And it will be decreased to c/4NnfL with N averages.Two improved schemes of traditional ?-OTDR have been made to overcome the above two shortcomings.Firstly,a system based on ?-OTDR and laser frequency sweep is proposed for simultaneously strain and vibration sensing.The strain of fiber is detected by comparing the patterns of signal for different laser frequencies,and the vibration of fiber is detected simultaneously from the signals for any certain laser frequency.During the measurement,frequencies of the probe optical pulses are modulated sequentially in ascending or descending order.Using the signals generated by optical pulses with the same frequency,the vibration of fiber is detected with fast response speed;using that with different frequencies,the strain of fiber is detected with high resolution.In our experiment,a sensing system with 2 m spatial resolution,up to 1 kHz frequency measurement range and 10 n? strain resolution is realized for a 9 km sensing fiber length.Secondly,an vibration sensing technique using double optical pulses based on?-OTDR and ultra-weak FBG(UWFBG)array is proposed.The single mode sensing fiber is integrated with UWFBG array which has uniform spatial interval and ultra-weak reflectivity.The high reflectivity of the UWFBG,compared with the Rayleigh scattering,gains a high SNR for the signal,which can make the system achieve the maximum detectable frequency limited by the round trip time of the probe pulse in fiber.A corresponding experimental ?-OTDR system with a 4.5 km sensing fiber integrated with UWFBG array is setup for the first time.There are about 88 UWFBGs and the identical reflectivity is about-40dB.Distributed vibration sensing is successfully realized with spatial resolution of 50 m.The sensing range of the vibration frequency can cover from 3 Hz to 9 kHz.