Abstract
This thesis presents a distributed optical fiber vibration sensor. The purpose of this sensing system is to monitor, in real time, the status of railcars by burying an optical fiber underground beside the rails. Using a coherent homodyne technique, the system measures Rayleigh scattering whose phase is modulated by vibrations. In order to obtain a large sensing area, an external cavity ring laser is used as the light source for a high degree of coherence. The linewidth of this laser light can be as small as 30 kHz. The optical fiber used as the sensor is a single-mode fiber. A wavelet technique is used at the backend of this sensing system for signal processing to provide multi-resolution information of the signal. For high resolution, the system uses a data acquisition board with 50 MHz A/D sampling. A Labview CVI based program is used for data collection. Finally the signal collected is decomposed into different resolution levels by discrete wavelet transform (DWT). Other techniques, such as fast Fourier transform (FFT), can be used to obtain signatures of the signal at these levels. This process can be implemented in real time. Using optical time domain reflectometry (OTDR), a sensitivity test is conducted to measure reflected light by fiber ends. It is shown that the sensor system has very high sensitivity and can successfully detect the vibration added to the fiber by a speaker. The distributed sensor setup improves noise performance over the OTDR configuration. The experiment using a distributed sensor setup is not successful. Rayleigh scattering in the system is not detectable because of the high noise level and lower power of the light. In order to study the noise problem of this system, several experiments are made and show that noise level in the system tremendously increases with increase of fiber length. A theoretical analysis is also performed for system noise due to intrinsic parameter fluctuations such as pressure and temperature. To solve the above problems, a suggestion is presented to use Brillouin scattering measurement, which is a local effect, and an erbium-dope fiber amplifier (EDFA) for increasing the power of the light source.
Yu, Hui (2001). Distributed optical fiber vibration sensing. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -2001 -THESIS -Y9.