The Research On The Fading-suppression Broadband ?-OTDR

In recent years,the scale of civil structures in China are very large.But in the long-term use,the civil structures will have a variety of safety problems,resulting in huge personal and property losses.When civil structures are damaged or have problems,abnormal vibration will occur.Distributed Optical Fiber Sensor(DOFS)can contribute to the safety monitoring of structures because of their advantage of long-distance distributed sensing.Phase-sensitive optical time-domain reflectometer(?-OTDR),belongs to DOFS,has been widely applied in many fields,especially in the field of civil structure safety.It can realize continuous distributed monitoring,fast response and high sensitivity to disturbances.Real-time monitoring of vibration signals can detect abnormalities in time,which means it can respond to safety problems early and prevent them in the future.A ?-OTDR system realizes real-time monitoring of vibration signals by monitoring the intensity and phase of Back Rayleigh Scattering(RBS)signal.However,when civil structures are damaged,the frequency of vibration will have a wide range from 10-1 Hz to hundreds kHz,and the detection frequency range of?-OTDR system should cover the frequency of vibrations.In addition,when RBS signal superposition,coherent fading effect is produced due to the randomness of intensity and phase,which eventually leads to fading.However,the upper limit of frequency response range of the traditional O-OTDR system is limited by the sensing length.Without the integration of other technologies,it is impossible to have both long sensing distance and wide frequency detection range at the same time.Moreover,due to the influence of the optical background noise in the traditional ?-OTDR system,the disturbance with low frequency will be affected,which leads to the difficulty of the detection of disturbance with low frequency.These disadvantages will limit the application of ?-OTDR.To reduce the effect of fading and expand the frequency response range of the system,many scholars are still making unremitting efforts.In this paper,the ?-OTDR system with wide frequency detection range and fading free detection is proposed.The frequency division multiplexing technology is used to expand the frequency detection range of the system.By using fiber embed with artificial microstructure,reflective light with controllable intensity and position can be used as sensing signal.Using reflective light instead of RBS with random intensity and phase can reduce fading position and improve signal-to-noise ratio of the system.Therefore,the ?-OTDR system has better high-frequency response ability and avoids the influence of dead zone at the same time.Secondly,in order to solve the problem of detecting vibration with low frequency,a ?-OTDR system with self-coherent structure is proposed in this paper.By setting the self-coherent structure,the effect of phase change caused by frequency drift of the light source is avoided.By installing vibration isolation device in the self-coherent structure of ?-OTDR,the low-frequency noise from environment is isolated.The frequency range detected in the experiment is from 0.1Hz to 1Hz,in which accurate quantitative measurement can be achieved.Therefore,this structure of?-OTDR can realize the quantitative measurement of low-frequency vibration,and expand the applicable frequency range of ?-OTDR quantitative measurement.The performance of the O-OTDR system has been improved in the field of optical structure,and the fading-free broadband ?-OTDR system is realized.The application of ?-OTDR system in monitoring of civil structures has been further promoted.