The Study On Fiber-Optic Michelson Displacement Sensor For Slope Monitoring

The landslide is one of the geologic hazards that seriously threaten people's lives and property.China,as a country with complex geology,is one of the countries that have the most serious landslide hazards in the world.Landslides can destroy road and bridge causing traffic interruption,damage infrastructures leading to huge economic losses,and even cause heavy casualties.A variety of methods for slope monitoring have been present since its birth 100 years ago,from the traditional methods such as macroscopic geological monitoring method and the station observation method to later advanced technologies like time domain reflectometry(TDR),GPS technology,optical fiber sensing technology and so on.Optical fiber sensing technology has been developed rapidly since the late 1970 s.With many advantages,optical fiber sensing technology has been widely used for side slope monitoring.In this thesis,a fiber-optic Michelson displacement sensor system based on spectral demodulation was designed for side slope monitoring.The system includes sm125-500 scanning laser demodulator,fiber-optic coupler,fiber-optic Michelson interferometer,information processing device,etc.The Labview software on computer was employed to realize the demodulation of the displacement signal:The cubic spline interpolation method was used to transfer the interference data acquired from wavelength domain to frequency domain with equal sampling interval;and then the Hanning filter was used for noise reduction while the Hilbert transform method was used to eliminate the influence of light source spectrum;finally the fast Fourier transform algorithm was designed as the demodulation method of the signal for high efficiency and large dynamic range.The polarization fading problem of the experimental system was analyzed comprehensively,and the Faraday rotation mirror was applied to eliminate polarization fading of the system.The effect of strain on the change in fiber refractive index had been studied in detail.The effective refractive index was obtained through theoretical deduction.Then experiments were carried out to calibrate the sensor system to ensure the high measurement performance of the sensor.The optical fiber stretching device was used to stretch the optical fiber to simulate the actual slope sliding.Experimental results demonstrated that the accuracy of this low coherence Michelson interference system can be 19?m,the repeatability can be 17.7?m,and the highest resolution can be 30 nm with a dynamic range of 10 cm.At the end of this paper,the site selection and installation process of this displacement monitoring system in the actual slope monitoring were briefly introduced,and a stable method of fiber-optic fixation with no macro-bending loss was employed.The effective data obtained from the actual slope monitoring showed that the measured displacement changed steadily with temperature within a day.Moreover,the whole slope had a slight slip trend during the measuring period.In actual monitoring of the slope,the displacement measurement system operates well and its good performance and measurement results were in agreement with the ideal expectation,which proved it is reliable in monitoring side slope displacement.