| Optical fiber sensing technology has been expanding rapidly along with the development of fiber communication technology from 1970’s, which employs the Light wave as carrier and fiber as medium to sense and transmit the measured external signals. Based on the principle of phase modulation, the fiber interferometer sensing system is widely concerned because of its high sensitivity. But there are still some technology should be developed to make the sensor have advantages of stability, flexibility, high sensitivity, long-distance location and low location error in the field application. Therefore, it holds important application prospects to have an in-depth study of the technology of the fiber interferometer sensing system.In the paper, a line-based fiber optic sensor employing time delay estimation to locate time-varying disturbance is proposed. The sensor consists of two line-based interferometers, multiplexed with a broadband light source, interfering unit and sensing fiber by wavelength division multiplexer, and hence detected signals with different wavelengths generated by the same disturbance are obtained. After a unique data processing algorithm, two signals with fixed time delay are achieved and the time delay is determined by the position of the disturbance. By the signal correlation method to extract the time delay, the position of the disturbance can be measured. Analysis has been done on the system’s structure, the demodulation and location method. The performance of the sensor is examined and compared with the conventional sensor through an experimental setup to demonstrate that the sensor proposed have advantages of stability, flexibility, high sensitivity, long-distance location and low location error in the field application.The research contents and results are as follows:1. An overview of the structure of the fiber optic sensor, the demodulation and location method are studied. The features and shortcoming are discussed to lay a foundation for the design of the optical fiber sensor used in field application.2. A line-based fiber optic sensor employing time delay estimation to locate time-varying disturbance is proposed. The sensor consists of two line-based interferometers, multiplexed with a broadband light source, interfering unit and sensing fiber by wavelength division multiplexer, and hence detected signals with different wavelengths generated by the same disturbance are obtained. After a unique data processing algorithm, two signals with fixed time delay are achieved and the time delay is determined by the position of the disturbance. By the signal correlation method to extract the time delay, the position of the disturbance can be measured.3. Problems associated with the polarization-induced signal fading produced by the random fluctuation of the polarization states of the two interfering optical waves are established based on the Jones matrix model of the optical polarization. It is shown that only employing Faraday rotating mirrors (FRM) cannot eliminate the polarization-induced signal fading problem. An elimination method of polarization-induced signal fading in fiber-optic sensor is proposed. The sensor employs polarization-maintaining fiber, polarization-maintaining fiber components to form a polarization-immune interfere unit (IU), which includes a key port with the polarization axes offset by π/2 to have the interference happen. Meanwhile, a Faraday rotating mirrors (FRM) is connected at the end of the sensing fiber to completely eliminate the polarization-induced signal fading in the sensing fiber. Theoretical analysis and experimental results in support of this statement are presented.4. Problem associated with the effect of low frequency noise and the parasitic interference caused by the backscattering lights is investigated theoretically and experimentally. It is shown that the low frequency noise and the parasitic interference lead to low optical signal-to-noised ratio and limit the sensitivity and the effective measurement range. In the work, an adaptive noise canceling technology based on the phase generated carrier is proposed to eliminate the low frequency noise and parasitic. The signal-to-noised ratio, sensitivity and measurement range can be improved, which have the advantageous for the weak signal detection and long distance perimeter security application.5. The experimental platform is set up according to the optical sensor system designed. The signal detection, analysis and processing is done by Labview program. The performance of the sensor is examined and compared with the conventional sensor. Experimental results show that the sensor have advantages of stability, flexibility, high sensitivity, long-distance location and low location error in the field application.The main innovations are as follows:1. A line-based fiber optic sensor employing time delay estimation to locate time-varying disturbance is proposed. The sensor consists of two line-based interferometers, multiplexed with a broadband light source, interfering unit and sensing fiber by wavelength division multiplexer, and hence detected signals with different wavelengths generated by the same disturbance are obtained. After a unique data processing algorithm, two signals with fixed time delay are achieved and the time delay is determined by the position of the disturbance. By the signal correlation method to extract the time delay, the position of the disturbance can be measured.2. Problems associated with the demodulation of acoustic signal produced by fiber optic interferometer are investigated. It is shown that existing demodulation method utilizing 3×3 fiber coupler do not work well for that weak signal cannot reach the full amplitude of the sensor to compensate the dc and normalize the amplitude. Meanwhile, the problems arising from the asymmetry of the 3x3 fiber coupler in the demodulation method is analyzed. Modifications to improve the performance of the previously proposed demodulation method are suggested. A modulation signal is introducing to research the full amplitude of the sensor to meet the requirement of the dc compensation and amplitude normalization.3. A method to have two coherent signals is proposed. Two composed signals are separately combined by two sub-signals with time delay. Let the two composed signals subtract to have a combined signal. Subsequently, let one composed signal delayed and subtract with other composed signal to have the other combined signal. The two combined signals have the same waveform shape but are time-delayed. Therefore, two coherent signals are obtained from the composed signals. By extracting the time delay, the position of the disturbance can be measured.4. An elimination method of polarization-induced signal fading in fiber-optic sensor is proposed. The sensor employs polarization-maintaining fiber, polarization-maintaining fiber components to form a polarization-immune interfere unit (IU), which includes a key port with the polarization axes offset by π/2 to have the interference happen. Meanwhile, a Faraday rotating mirrors (FRM) is connected at the end of the sensing fiber to completely eliminate the polarization-induced signal fading in the sensing fiber. Theoretical analysis and experimental results in support of this statement are presented.5. An adaptive noise canceling technology based on the phase generated carrier is proposed to eliminate the low frequency noise and parasitic. The signal-to-noised ratio, sensitivity and measurement range can be improved, which have the...
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