Enhanced Phase-Sensitive Optical Time Domain Reflectometry Based On Ultra-Weak Fiber Bragg Grating Array

Phase-sensitive optical time-domain reflectometry(?-OTDR)has been applied in many fields such as intruder detection due to its ability in fully distributed vibration sensing along the entire length of sensing fiber with fast response and high sensitivity.However,there is no definite relationship between strain value and amplitude variation of the measured backscattering power,therefore,ordinary ?-OTDR system cannot be used to get quantitative fiber length variation.In previous research,ultraweak fiber Bragg grating(UWFBG)array based ?-OTDR system has been proposed to realize quantitative strain measurement.In this system,UWFBG array are uniformly distributed in the sensor fiber,the absolute strain value can be obtained by analyzing the interference between adjacent UWFBG reflections.However,UWFBG array based ?-OTDR system has two disadvantages.First,the existence of UWFBG will no doubt induce additional attenuation to the optical signal.If UWFBG array is distributed too closely,the transmission loss will be too high and the measuring distance will be limited.Therefore the interval between adjacent UWFBGs cannot be too small.Spatial resolution of this system is equal to the UWFBGs interval.Therefore,this system is unsuitable for situations that require both quantitative measurement ability and high spatial resolution monitoring.Second,wide pulse has to be used in this system so that the pulse can cover three adjacent UWFBGs,which increases the noise caused by Rayleigh scattering,so the SNR is decreased.To deal with this problem,the reflection of UWFBG has to be increased with the Rayleigh scattering gets stronger to maintain sufficient SNR.However,the increase of UWFBG reflectivity brings about the quicker intensity decline,therefore,the detectable fiber length is limited.To solve these two problems,UWFBG array enhanced ?-OTDR system has been proposed.To the first problem,this enhanced system combine both the ordinary and the UWFBG array based ?-OTDR system together.Quantitative measurement on the external vibration is still achieved by the UWFBG array.But Raleigh backscattering is also recorded and analyzed to obtain high spatial resolution monitoring.To deal with the second problem,we propose using narrow pulses in UWFBG based ?-OTDR system to replace wide pulses.To make it work,we combine the UWFBG array based ?-OTDR system and MZ interferometer.By adjusting the optical path difference of the two beams,we can make the adjunct UWFBG reflections of the two beams overlap.Experimental results showed that the UWFBG array enhanced ?-OTDR system succeed in improve these two problems.Firstly,this system realize quantitative strain measurement and high spatial resolution monitoring by combining ordinary ?-OTDR system with the UWFBG array based ?-OTDR system.In this experiment,the interval between adjacent UWFBGs was 50m and we realized a 4m spatial resolution monitoring,which was more than 10 times better than that in UWFBG array based(DOTDR system.Secondly,the interval between adjacent UWFBGs was 50m and we utilized 50ns pulse,which was more than 10 times narrower than the pulse used in the UWFBG array based ?-OTDR system.