Optical Fiber Vibration Sensor Based On Phase Generated Carrier Modulation Techniques

Fiber optical sensor use fiber as sensing and transmission element, and it will not introduce electronic signal to the detecting zone, so this technology can be applied in the field that traditional sensor can't reach. For example, easy-get- fired area, or under-water area. So this technology expands the field of the sensor application. It has been widely used in environmental monitoring and area safety defending field. Fiber sensor that based on phase modulation has a higher sensitivity than other sensors, and it has the solution of nanometer which is the laser wavelength. This type of fiber sensor is mainly used in long range distributed environment sensing. In all of the application of the fiber sensor, the safety defending system that based on the external vibration detecting becomes popular, for the reason that, it has higher sensitivity, as well as the feature of vibration locating, no electronic element, and the easy way of setting.Phase generated carrier technology is introduced to distributed fiber sensors to make sure the work point do not shift, in order to lower the noise of the system. By this means, we can improve the sensitivity of the detected vibration and improve the locating solution of vibration as well. On the other hand, with the application of this technology, we can achieve complex of the interferometer, which means use one interferometer to monitor several areas simultaneously. By this means, the cost of equipment will be saved. Since the phase generated carrier technology has so many advantages, it has been widely used in phase modulation based fiber optic sensor systems.In this paper, the application of phase generated carrier technology in all fiber distributed vibration sensing system is the research target, and the main research work and results are as below:1. Research the phase generated carrier technology, and introduce it to the existing vibration detecting system. Achieve the interferometer output formula through theoretical derivation. Meanwhile, get the demodulation formula from the interferometer output signal through derivation.2. Describe the principle of the frequency and amplitude selection of the carrier signal being used in the modulation.3. Through a lot of experiments to compare the locating performance between fiber optic vibration sensor with and without phase generated carrier technology. The results show that, the locating solution and locating range both improved with the introduction of phase generated carrier technology.4. Theoretical derivation show that, if carrier signal used in the demodulation process has different frequency or phase with the signal applied on PZT, the demodulated signal will be less stable and will have lower amplitude. In order to get carrier signal applied on PZT, the output interferometer signal has been analyzed, and a signal component with same frequency and phase with carrier signal is found in the output interferometer output signal. The signal component can be extracted by a narrow-band band pass filter and can be used to the demodulation process. By means of carrier extracting method, we can achieve the demodulation algorithm with computer software, in order to reduce the hardware complexity.5. Simulated the algorithm of carrier extraction with software, and found that, the amplitude of the extracted carrier signal will drift if the vibration signal contains low frequency components. And the influence of low frequency signal will get weak as the signal frequency rise up. In order to reduce the demodulation error, we introduce a normalized algorithm to get an amplitude-stable carrier signal.6. Introduce the synchronous carrier extracting method to the actual system, and successfully extract the carrier signal, validate the feasibility of this method.