Distributed Optical Fiber Vibration Sensor Based On Coherent Detection

Since the state of polarization (SOP) of light propagating down the fiber is subjected to the modulation of the external physical parameters (magnetic field, electrical field, vibration and temperature et al), these parameters can be detected by polarization coherent optical time domain reflectometry (PCOTDR). This method, with high sensitivity, fast response and wide dynamic range, can be used in many fields.In this work, the distributed optical fiber vibration sensor scheme, combining the OTDR with the coherent detection and SOP modulation, is first proposed. Meanwhile, the feasibilities of the proposed schemes are confirmed by experiments. PCOTDR can quickly detect and locate the distributed weak vibration on optical fiber due to the sensitivity of SOP to external changes, the high sensitivity of coherent detection and the single-ended measurement of OTDR. On the basis of the combination of the SOP modulation with the coherent detection, the feasibilities of the proposed schemes are analyzed in detail and the vibration sensor system is constructed. The results show the feasibilities of the schemes.The contents of this thesis are as follows:1. The theory model of PCOTDR, including simulation model, is developed and analyzed. The effects of the SOP, the phase difference and the frequency difference on the system performance are analyzed using computer simulation base on the proposed simulation model. The result shows that compared to the effect of frequency difference, that of phase difference is stronger.2. Two PCOTDR schemes based on the coherent detection are performed to detect vibration on the fiber. The two schemes have the same operation principle. The light beam from laser is divided into two beams. One serves as reference light and the other signal light that is coupled into the testing optical fiber. When the two beams interfere, the external vibration leads to the changes in the SOP in testing fiber. The information on the SOP modulation can be detected by the coherent power. The difference between the two schemes lies in the different optical polarization location. The polarization occurs in primary optical fiber for the first scheme while in branch optical fiber for the second. The measurement show that the both schemes are viable but the first is better.3. The PCOTDR measurement software is designed and programmed, which includes the signal collection module, the human-computer interaction module, the signal processing module and the display module. The basic fundamental guideline during our software program is to average the detected data and then to enhance the signal-to-noise ratio. The result shows that the sensor system based on this software with different schemes can locate the vibration site on the fiber of5km, which is the indicator of the experimental feasibility and the valuable application.