Research On Distributed Optical Fiber Vibration Sensing Technology Based On Interferometry And φ-OTDR Combination

Distributed optical fiber sensing is a technology using optical fiber as the sensingmedium, and carries on remote and real-time monitoring of the target along the sensingfiber. It is widely used in long-range safety monitoring (i.e. pipeline monitoring ofnatural gas and oil) or health monitoring of civil structures (i.e. bridge and highbuilding). Up to now, distributed optical fiber vibration sensing based on interferometryand backscattering technology (Rayleigh or Brillouin scattering) are deeply researchedby domestic and foreign scholars. The frequency response of interferometries is veryhigh, but the signal demodulation method needs to be further simplified and the spatialresolution should be improved，more importantly, interferometries based distributedvibration sensing systems have no capability of multiple vibration points location anddetection simultaneously. On the other hand, the frequency response of vibrationsensing based on Brillouin scattering is relatively low. Rayleigh scattering based OTDRtechnology owns the following merits: high spatial resolution, multi-point detection andsimple signal processing. However, due to the weak intensity of Rayleigh backscatteringlight, averaging method is adopted in signal processing which results in the frequencyresponse sharply decreasing. Therefore, we propose a composite sensing schemecombining the interferometry and Rayleigh backscattering technology to achievedistributed vibration measurement with high spatial resolution and high frequencyresponse simultaneously.The main contents of this thesis are as following:①Theories on interferometries and Rayleigh backscattering are described briefly,and the feasibility of combining two technologies is discussed. Firstly, according to thegeneration mechanism of Rayleigh backscattering light in optical fiber, the operatingprinciple of phase-sensitive OTDR (φ-OTDR) system is analyzed by building thediscrete model, and the technical feature of distributed vibration sensing based onOTDR technology is proposed. Meanwhile, the principle of interferometries measuringexternal vibration is briefly analyzed, and the advantages and disadvantages ofdistributed vibration sensing based on interferometry technology are also discussed.Finally, the light path feature of interferometry and OTDR technology is analyzedrespectively, and the combination can be achieved by optical multiplexing technology. ②To achieve high spatial resolution and high frequency response measurement,we propose a merged distributed vibration sensing system of interferometry and OTDRtechnology. In the thesis, a system merged Michelson interferometer and φ-OTDRtechnology based on wavelength division multiplexing (WDM) technique isdemonstrated. High frequency component of vibration signal is measured by Michelsoninterferometer, and location information and low frequency component is detected byφ-OTDR technology. Experimental results show, the frequency response of vibrationsystem is up to~8MHz and the spatial resolution is~2m when20ns pulse is launchedinto the sensing fiber with1km length.③φ-OTDR system is highly sensitive to external vibration. The intensity ofRayleigh backscattering light changes abruptly with vibration point, and the intensity ofnon-vibration point remains constant. Based on this fact, we propose a two-dimensionaledge detection method to extract location information of vibration point. Due to theaveraging effect of edge detection operator within the neighborhood, SNR of locationinformation is obviously enhanced, and the spatial resolution of system is also improvedby optimizing the matrix size of operator. Experimental results show that, when50nsprobe pulses are launched into the single mode fiber with1Km length, the spatialresolution and SNR is~3m and8.4db respectively, which proved to be a powerfulmethod to enhance the performance of φ-OTDR system.④For dynamic vibration measurement, frequency information is an importantparameter. Fast Fourier Transform (FFT) is only satisfied to analyze the frequencyspectrum of stationary and periodic vibration signals. However, for non-stationary andinstantaneous signals, such as the crack of materials and vibration of engine, FFT is notable to reveal the inherent feature within the signal. Hence, we propose empirical modedecomposition method (EMD) to complete the time-frequency analysis ofnon-stationary vibration signals. In the experiment, a sweep signal whose frequencyranges from500Hz to1kHz is applied on a piezoelectric tube (PZT) to simulatenon-stationary vibration, and experimental results processed by EMD method matcheswell with the frequency information of sweep signal, which is proved an effectivemethod to evaluate non-stationary vibration events in φ-OTDR system.