Research On The Suppression Method Of Fading In Φ-OTDR

Phase-sensitive optical time domain reflectometer(Φ-OTDR)has shown great potential in many applications,such as structure health monitoring,intrusion detection and seismic monitoring owing to the advantages of distributed sensing,fast response and high sensitivity.The quantitative measurement of external vibration can be achieved by extracting the phase difference of Rayleigh backscattering(RBS)between two sections before and after the vibration.Since narrow linewidth laser with long coherence length is always used in Φ-OTDR,the RBS produced by the probe pulse interferes within the pulse,resulting in random fluctuations of the obtained RBS intensity.The intensity fluctuation,called coherent fading,is closely related with the random distribution of scattering.Coherent fading can cause the RBS intensity close to zero in some sections,and phase demodulation is easy to make mistakes in these sections.Even if the RBS intensity is high at some point,it remains possible that the RBS intensity tend to zero due to the frequency drift as time goes on,leading to distortion of the reconstructed signal and frequent false alarms,which brings many inconveniences.To solve the above problems,two coherent fading suppression methods are proposed in this paper.One is based on frequency division multiplexing Φ-OTDR,which utilizes the characteristics that detection pulses of different optical frequencies have different RBS intensity distributions.At the same time,phase reconstruction of RBS generated by detection pulses of different optical frequencies is carried out,and the other is based on optimal tracking.The amplitude of RBS obtained by Φ-OTDR shows random fluctuation.At the same time,phase reconstruction is carried out for multiple reference regions which are not affected by external vibration before and after the vibration region.Then,according to the magnitude of the signal,the most accurate reconstructed signal is always selected for continuation at any time.The proposed two methods realize high fidelity reconstruction of external vibration signals without sacrificing spatial resolution and without changing the structure of traditionalΦ-OTDR system,and reduce false alarm rate.The experimental results show that the proposed two coherent fading suppression methods can effectively improve the fidelity of reconstructed signals,which plays a very good role in breaking the limitation of coherent fading in engineering applications,and is expected to further expand the application of O-OTDR in engineering.