Distributed Fiber-optic Dynamic Strain Sensing Techniques Based On Backscattering In Optical Fiber

Optical fiber sensing is a powerful sensing technique which can precisely measure the external physical quantities such as temperature,strain,magnetic field and refractive index by monitoring the lightguide-property variation of the optical fiber.Compared with traditional sensors,optical fiber sensors draw much attentions since they have various advantages such as small size,light-in-weight,anti-corrosion and anti-electromagnetic interference.The distributed optical fiber sensors employ the existing traditional optical fiber as the sensing medium,which can compose large-scale sensing network thanks to the extremely low cost of each single sensing point.In recent years,the intelligent city becomes a new trend along with the rapidly developing of information and internet-of-thing technologies.Distributed optical fiber dynamic strain sensing techniques are able to simultaneously measuring the dynamic strain information of thousands of points in a structure with desirable characteristics such as low cost,installation easiness and high reliability.Distributed optical fiber dynamic strain sensors are realized by employing the Rayleigh backscattering(RBS)and stimulated Brillouin scattering(SBS)effects.The RBS is usually used to measure the weak-amplitude high-frequency strain signals,which remains to be insufficient on the measuring reliability and system cost.On the other hand,the SBS is suitable for largeamplitude low-frequency strain measurement,while the measurement bandwidth and dynamic range still need to be improved.Focusing on these points,in this thesis,we studied the phasesensitive OTDR(?-OTDR)with phase extraction and slope-assisted Brillouin optical time domain analysis(SA-BOTDA)to improve the measurement bandwidth,dynamic range and reliability of dynamic strain sensors.The detailed work is as follows:(1)We realized a long-range high-sensitivity distributed dynamic strain sensor by employing Hilbert transform into the coherent detection ?-OTDR to extract the phase signal through I/Q demodulation.The noise theory of phase extraction based on coherent detection was built up for the first time,and the generation mechanism and influence factors of phase noise in phase extraction were investigated in detail.Besides,we analyzed the deadzone effect due to Rayleigh coherent fading and its influence on strain signal locating,and proposed an anti-fading-noise strain signal locating method based on analyzing the RBS phase variance trace.The long-range distributed dynamic sensor was experimentally demonstrated,which has a measurement distance of 31 km with a 10-m spatial resolution and a 3-k Hz sampling rate to the strain signal.The dynamic strain signal at a distance of 30 km with a peak-to-peak value of 300 n? was located and measured without the influence of fading noise.A 20-d B measurement SNR can be achieved.(2)For the first time we proposed a frequency division multiplexing(FDM)direct-detection?-OTDR system to realize a distributed dynamic strain sensor which exhibits high bandwidth and low cost while is able to break through the tradeoff between the measurement distance and strain sampling rate.The influence of FDM technique on the system performance was investigated,while the noise and harmonics due to the crosstalk and sampling error were analyzed thoroughly.The windowed probing pulse is employed to eliminate the spectral leakage and suppress the noise introduced by crosstalk.With a 4-time frequency multiplexing,we experimentally demonstrated the distributed dynamic strain sensing over a 10-km fiber with a40-k Hz strain sampling rate,while the receiving bandwidth is only 75 MHz.By employing the pre-calibration for the responses of different interrogation frequencies to eliminate the harmonics while using raised-cosine pulse to suppress the crosstalk,a 0.5-d B response flatness and a 40-d B SNR are achieved.Besides,an anti-fading-noise strain locate method which suitable for FDM system is proposed with an accuracy of 27 m.(3)We studied the SA-BOTDA system to improve its strain dynamic range and measurement performance.Firstly,we measured Brillouin phase shift and Brillouin gain respectively,and using the Brillouin phase-gain ratio(BPGR)as the sensing quantity to eliminate the theoretical limitation on the strain dynamic range of the SA-BOTDA system and the influence of pump power variation as well as polarization fading.The different influence factors of the BPGR spectrum are also analyzed thoroughly.In experiment,we successfully measured the dynamic strain signal over a 2-km fiber with a 2.5-m spatial resolution and a 1-k Hz strain sampling rate.The strain dynamic range is enlarged to be 2000??,which is around 4-times of that in traditional SA-BOTDA system.Since the SA-BOTDA using BPGR requires a high receiver bandwidth and a complex demodulation process,we proposed a novel SA-BOTDA configuration which manipulate the shape of Brillouin gain spectrum(BGS)and enlarge the strain dynamic range by modulating the pump pulses.By modulating the pump pulse with a dual-frequency signal,the dynamic range of SA-BOTDA system is also enlarged to 2000??.Based on the BGS shape manipulation,we realized distributed dynamic strain sensing over a 400-m distance with a 2.5-m spatial resolution.The sampling rate to strain is 1 k Hz while the strain accuracy is ??.As a conclusion,this thesis focuses on the distributed optical fiber strain sensor based on RBS and SBS.The proposed dynamic strain sensor based on ?-OTDR is able to achieve desirable sensitivity,long measurement distance and high bandwidth,while the reliability and system cost are improved.Furthermore,we employ the BPGR or modulated pump pulse to significantly enlarge the strain dynamic range of SA-BOTDA system.The strain measurement accuracy and reliability are also improved.