Research On Distributed Vibration Sensing Technology Based On Optical Pulse Modulation

Distributed optical fiber sensing(DOFS)has attracted considerable attention in the last decades due to its properties of high integration,multi-parameter sensitivity and stability in harsh environment.In particular,phase sensitive optical time domain reflectometer(?-OTDR),which enables quantitative measurements of vibration,acoustic wave,and strain/temperature changes,is playing an increasingly significant role in the fields of perimeter security and large-scale structure fault detection.In terms of sensitivity,response speed,stability,etc.,the performance of ?-OTDR can be compared with the quasi-distributed sensing system which is represented by Bragg grating.However,there are still many problems and challenges in ?-OTDR based distributed vibration sensing(DVS),which limit the industrial application of ?-OTDR system.For example,the ?-OTDR systems with broadband vibration response range require large modulation and demodulation bandwidths,increasing the complexity and system cost.The ?-OTDR systems with long sensing range need double-ended pump and distributed amplification in sensing arm,which brings noise and is difficult to phase demodulation.Furthermore,the multi-parameter distributed sensing systems barely measure the dynamic vibration and static strain/temperature simultaneously.To overcome the above-mentioned problems,various pulse modulation methods,such as additive random pulse interval modulation,multi-carrier non-linear frequency modulation and Gaussian pulse modulation,are proposed to further improve the frequency response range,sensing length,spatial resolution and signal-to-noise ratio of ?-OTDR systems.Multi-parameter sensing of vibration,temperature and strain are realized.The main contents are as follows:(1).The research progresses of DVS and distributed multi-parameter sensing are reviewed.The basic theories of Rayleigh scattering-based DVS and spontaneous Brillouin based temperature/strain sensing are presented.The nonlinear effects and light detection methods of DOFS are analyzed,which provides theoretical supports for improving the sensing performances of DOFS through modulation methods.(2).A pulse modulation method based on additive random sampling is proposed to extend the frequency response range of DVS.With the characteristics of low redundancy and anti-spectral aliasing,the additive random sampling eliminates the limitation of the sensing distance to the frequency response range in the DVS system.The theory of additive random sampling is established,and the key parameters of additive random sampling for DVS are optimized.Based on matching pursuit,a signal reconstruction method immune to aliasing noise is proposed.High-frequency sparse vibration signals up to 150 k Hz are analyzed in ?-OTDR with 10 km sensing range,and based on the matching tracking idea,the signal is reconstructed in the time domain with a high dynamic range of 30 d B.The proposed method does not increase the bandwidth of optical detection and signal acquisition,which provides a new solution to the DVS with wide vibration frequency response range.(3).A pulse modulation method based on multicarrier non-linear frequency modulation is proposed to simultaneously improve the sensing distance,spatial resolution,phase demodulation and suppress the interference fading in non-distributed-amplified ?-OTDR.In order to reduce the interaction between Rayleigh signals at different positions,a non-linear frequency modulated(NLFM)pulse having a higher sidelobe suppression ratio is used.Three-carrier modulation is achieved by an electro-optic modulator,which eliminates the interference fading in ?-OTDR.In order to solve the problem of pulse frequency shift caused by the self-phase modulation effect of the distortion optical pulse,an iterative pre-distortion method is used to suppress the influence of nonlinear effects on the pulse compression.The ?-OTDR with phase demodulation is achieved in 80 km sensing fiber without distributed amplification,in which a 17 d B intensity signal-to-noise ratio at fiber rear end and as high as 45 d B signal-to-noise of phase demodulation are achieved.(4).A Gaussian pulse modulation method is proposed to achieve vibration,temperature,and strain distributed sensing with ?-OTDR and Brillouin Optical Time-Domain Reflectometer(B-OTDR)hydride system.The system detects the fast vibration signal and the static temperature and strain signals simultaneously through one detection channel,which reduces the system complexity and cost.According to the characteristic of Brillouin gain spectrum,Gaussian optical pulses are used for B-OTDR sensing,and the beat frequency signal is analyzed by Gaussian window short-time Fourier transform,by which a sub-meter spatial resolution is realized over a 10 km sensing range.To meet the needs of practical engineering applications,a prototype of the system was developed.