Sensing Performance Φ-OTDR System Based On Phase-Modulated Optical Frequency Comb

Phase-sensitive Optical Time Domain Reflectometer(Φ-OTDR)is a distributed fiber optic sensor based on Rayleigh scattering effect,with fast response speed and high sensitivity,usually used for structural health monitoring of large projects such as tunnels,bridges and oil pipelines.Due to the Rayleigh scattering effect of light waves in optical fiber,the backscattering(Rayleigh Backscattering,RBS)signal generated by the detection pulse light can be monitored,and the quantitative measurement of vibration events can be achieved by doing post-differential demodulation of the RBS intensity at two undisturbed locations before and after the vibration location.Due to the existence of the coherent fading effect,the RBS signal strength is very low position so that the phase information of the vibration event is distorted,can not restore the complete external vibration information,resulting in false alarms in engineering monitoring,so suppressing the coherent fading effect in the Ф-OTDR system is very important to achieve high fidelity extraction of vibration information.To address the above problems,this paper focuses on the suppression of coherent fading effect in the Ф-OTDR system,combined with the different frequency detection light intensity distribution characteristics,the use of phase-modulated optical frequency comb form of detection light to suppress the coherent fading effect;and on this basis,combined with the distributed amplification technology to improve the system sensing distance.The specific research of this paper is as follows.First,the recent development of distributed fiber optic sensing technology at home and abroad is described,and the development history of the four main research aspects of the Ф-OTDR system,namely,expanding the sensing distance,improving the response bandwidth,suppressing the coherent fading effect,and improving the signal-to-noise ratio of the system,is elaborated.Secondly,the Φ-OTDR system based on the phase-modulated optical frequency comb is built,in which a single-phase modulator is used to generate an equalamplitude optical frequency comb containing multiple frequency components,which fully suppresses the coherent fading effect of the system and achieves the high-fidelity extraction of the phase information of the vibration signal.The system has the advantages of simple structure,flexible and controllable number of modulable frequencies,accurate phase delay,and larger modulation bandwidth.The seven frequency shifts of the experimentally modulated optical frequency comb form detecting light are: 80 MHz,120MHz,160 MHz,200MHz,240 MHz,280MHz,320 MHz,respectively.Combined with the amplitude evaluation method,the coherent fading probability of the system is significantly reduced to 0.08% at a detection pulse width of 100 ns and a repetition frequency of 10 k Hz,and the corrected phase fidelity rate increases from 90.17%-94.51% to 99.92% for a single frequency,indicating that the system fully suppresses the coherent fading effect.Finally,the combination of the distributed forward-pumped fiber Raman amplification technique with the phase-modulated optical frequency comb-based Φ-OTDR system allows distributed amplification of the detection optical power to improve the system signal-to-noise ratio and significantly extends the sensing distance of the system.With a high degree of coherent fading suppression,the maximum sensing distance in the forward Raman amplified Φ-OTDR system increases from 65 km to about 82 km without amplification when the pump optical power is increased to500 m W;when the Raman pump light source is added to the system with a delay of20 km and its power is 500 m W,the sensing distance can be further increased to 98 km.