Non-contact Vibration Measurement Based On Low Coherent Fiber Differential Interference And It’s Application

With the development of modern industry and technology, vibration measurement technology is increasingly demanded in the field of gas leakage detection, vibration modal testing of aircraft engines, large building security monitoring such as bridges and MEMS precision manufacturing, etc. Common vibration measurements include three categories:mechanical, electrical and optical type. Optical vibration measurement technology has become an important developed direction of vibration measurements with high accuracy, electrical insulation, strong anti-interference, wide frequency range, etc. With the advent of low-loss optical fiber and the improvement of new optical devices, a high-performance fiber optic vibration sensor has been developed. Low coherence optic fiber differential interferometer has the characteristics of absolute amplitude measurement, anti- interference ability, without mechanical adjustment and automatic optical path difference matching, etc., it can be used to measure the absolute magnitude of the speed, vibration, and other parameters.This paper firstly outlines the research background and the significance of optical fiber vibration sensor, and introduces research progress of low coherence differential interferometer at domestic and abroad. Compared with other vibration measurement methods, low coherence vibration measurement technique is widely used in the field of optical measurement own to its advantages of high sensitivity and precision.Secondly, we study the principles and features of low coherence differential interferometry which based on white light interference theory; We introduce the common interference signal demodulation methods which include:classical heterodyne phase demodulation method, passive homodyne quadrature demodulation and phase generated carrier homodyne, and study on their working conditions and their characteristics, and focus on the passive demodulation algorithm based on 3 x3 coupler. We propose a non-contact vibration measurement method based on low-coherence fiber differential interferometry, and we use a Mach-Zehnder and Sagnac mixing interferometer structure and combine three-way detection passive demodulation method to demodulate interference signal. Experimental results show that the low-coherence optical fiber differential interference can measure absolute magnitude remotely, and the measurement results not only have good linearity and frequency response, but also have high measurement accuracy and 4.34pm～2.43mm measuring range.Finally, we measure vibration caused by gas leakage and use ANSYS FLUENT finite element analysis software to simulate gas leakage. The results show that the pressure variables influence the exit velocity of leakage hole mostly. In the paper, we propose a non-contact detection of gas leakage based on low-coherence fiber differential interferometry. Experimental results show that leakage can be qualitatively determined through the collecting signal spectrum; under the same detecting position, the high-frequency component of the spectrum and the amplitude of the signal energy have a upward trend obviously as the increasement of the internal pressure; when the propagation distance increases at the same pressure, the signal amplitude and energy decreases. The measurement results are consistent with theoretical simulations.