Study On Distributed Optical Fiber Sensors Based On Forward/backward Stimulated Brillouin Scattering

Distributed fiber sensing based on backward stimulated Brillouin scattering(BSBS)has been paid special attentions due to its superior sensing performance.Currently,sensing distance with>100 km and spatial resolution up to centimeters magnitude has been achieved.At present,the main development direction is to realize rapid measurement and dynamic response.As a new category of fiber sensors,distributed acoustic impedance sensing with forward stimulated Brillouin scattering(FSBS)can not only detect the temperature and strain,but also detect the liquid acoustic impedance outside fiber.This is meaningful for chemical sensing,oil and gas exploration,water pollution monitoring,liquid detection,etc.This paper summarized its theoretical basis;proposed a new signal enhancement approach for Brillouin optical time-domain analysis(BOTDA)with FSBS-based distributed Brillouin amplification(BOTDA);performed an optimized study for FSBS-based distributed acoustic impedance sensing.In addition,we designed and implemented the ultra-long BOTDA without frequency sweeping by combining dual-bandwidth pumping and optical chirp chain(OCC).The main contents of this paper are summarized as follows:(1)A novel configuration for signal enhancement of distributed fiber sensors based on FSBS-DBA was proposed.Distributed Brillouin amplification(DBA)based on BSBS is well known for its ability to effectively extend the sensor distance and signal-to-noise ratio,but its amplification region is mainly limited to the back end of fiber.In this paper,FSBS was used for long-distance pulse amplification of BOTDA.The Stokes pulse is effectively amplified in front end of fiber,and its gain distribution is flatter.As a proof-of-concept,sensing distance of?74.2 km and spatial resolution of?10 m have been realized for BOTDA based on FSBS-DBA.The standard deviation of Brillouin frequency shift(BFS)was less than?2 MHz along the whole fiber.The pumping efficiency is higher than that of traditional distributed Raman amplification(DRA).(2)An optimization study on the distributed acoustic impedance sensing using FSBS was performed.With the previously reported separation technology for activation and reading pulses,the activation pulse stimulates the transverse acoustic wave by FSBS.Then a reading pulse was used to detect the cross-phase-modulation(XPM)from FSBS.The amplitude of probe light can be stabilized using injection locking.Using BOTDA technology,the detuning relative to BFS is reasonably selected in frequency sweeping to avoid the polarization noise of 0^th sideband.The FSBS local spectrum of uncoated fiber in air,water and ethanol was obtained by demodulating the sideband of the reading pulse.The sensing distance of?635 m and the spatial resolution of?15 m were realized based on 1^st-order local spectrum,which is meaningful for distance extension of FSBS-based acoustic impedance sensors.(3)Fast sensing is a key direction in the development of distributed fiber sensing.DBA-BOTDA can realize the temperature/strain sensing with>100 km distance using only milli-watt level of pump power.Our research group has reported a DBA-BOTDA scheme based on direct demodulation of injection locked dual-bandwidth pump,which achieved an ultra-long sensing distance of?148.3 km and spatial resolution of?5 m with only 100 times averages for a single scanning point.However,the time-consumed frequency sweeping makes rapid measurement difficult.To solve this issue,this paper combined this scheme with optical chirp chain(OCC)to realize the ultra-long BOTDA without frequency sweeping.The dual-bandwidth pump can adaptively compensate the loss of pulse;the use of injection locking greatly reduces the parasite amplitude noise;the probe input power can be enhanced to?9.7d Bm due to the suppressed non-local effect(NLE).It takes only?0.3 s to complete a measure without sweeping and average times of 200.