Fading Noise Reduction In Phase-interrogation Based ?-OTDR For Vibration Sensing

Phase sensitive optical time domain reflectometry(?-OTDR)can be classified as intensity-interrogation based and phase-interrogation based?-OTDRs according to the parameters of Rayleigh signal to be interrogated for vibration measurement.The intensity-interrogation based?-OTDR detects vibration perturbations via sensing the variations of the intensity of Rayleigh signal.Due to the nonlinear-ship between the magnitudes of the vibration and induced intensity variation,the intensity-interrogation based?-OTDR fails to quantify the vibration perturbation in terms of magnitude.The phase-interrogation based?-OTDR implements vibration measurement by interrogating the phase variation of Rayleigh signal.Considering the linear-ship between the magnitude of phase variation and that of a vibration perturbation,the phase-interrogation based?-OTDR is preferable for quantification of the magnitude of a vibration perturbation.Equipped with narrow line-width light source,phase-interrogation based?-OTDR not only is sensitive to a vibration perturbation but also is vulnerable to fading noise.Fading noise deteriorates the signal-to-noise(SNR)of measurement of intensity vibration and leads to detection bugs.In this study,the approaches of fading noise reduction in phase-interrogation based?-OTDRs are studied.Firstly,a phase-interrogation based?-OTDR is built up,and the distributed vibration sensing is realized.Secondly,wavelength dependence of the intensity of Rayleigh signal is analyzed based on the modified one-dimension model that describe the intensity of Rayleigh signals in a?-OTDR,and the fading noise reduction methods utilizing multi-wavelength based and intensity-modulation based?-OTDR are proposed.In addition,the?-OTDR with orthogonally polarized pulses is proposed to reduce the fading noise,which utilizes polarization dependence of the intensity of Rayleigh signals.The main innovations and achievements of this study are as follows:1.Wavelength dependence of the intensity of the Rayleigh signal based on modified one-dimension model is analyzed,which reveals that the intensity of the Rayleigh signal is wavelength dependent while the phase of the Rayleigh signal is wavelength independent.Based on the theoretical analysis,the fading noise reduction method utilizing multi-wavelength pulses is proposed.In 20 times repetition test,the number of fading noise channels accounts for 30% of the total sensing channels in single-wavelength based?-OTDR,while the number of fading noise channels accounts for 16% of the total sensing channels in multi-wavelength based?-OTDR.The proposed approach reduces the fading noise by a decrease of a 14% in number of fading channels compared with the number in single-wavelength based?-OTDR.The?-OTDR with fading noise reduction utilizing intensity-modulation is proposed,and a decrease of 15.3% in number of fading channels is achieved compared with the number in non-intensity modulated based?-OTDR in 10 times repetition test.2.The polarization dependence of the intensity of Rayleigh signal is analyzed,and the fading noise reduction method utilizing orthogonally polarized pulses is proposed.The?-OTDR with orthogonally polarized pulses is built up.The proposed approach reduces the fading noise,which is experimentally confirmed in 10 times repetition test by a decrease of 9% in number of fading channels compared with the number in single-polarized pulses based?-OTDR.3.A new method is presented to evaluate the fading noise level of the phase-interrogation based?-OTDR.The magnitudes of 100 Hz frequency components in phase noise spectra of all sensing channels are statistically analyzed.The analysis reveals that the magnitudes distribute in two distribution region,[-80,-30](dB re 1rad/sqrt(Hz))and [-30,0](dB re 1rad/sqrt(Hz))respectively.In this study,a sensing channel is regarded as fading channel if the magnitude of 100 Hz frequency component in the phase noise spectrum is higher than-30(dB re 1rad/sqrt(Hz)),and the fading noise level of the sensing system is evaluated by the number of fading channels.This method is simple and effective to evaluate the fading noise level of a distributed sensing system.