Research On The Acoustic-Frequency Vibration Sensor Based On Single Mode-multimode Optical Fiber Structure

Under the conditions of strong electromagnetic interference, large humidity, high temperature and high pressure, many traditional vibration sensors can’t work effectively in this environment, however, the optical fiber vibration sensing detection technology has become an important part of vibration sensing detection technology due to the advantage of strong electromagnetic interference resistance ability, wide measuring range, high sensitivity advantages etc. Recently, most of traditional optical fiber vibration sensors, by contacting style to obtain vibration information, are complex structure, and hard to be encapsulated. In this work, two kinds of simple structure, low-cost acoustic-frequency vibration sensors are proposed and fabricated, and the characteristics of two kinds of sensors in the acoustic-frequency vibration has carried on the detailed research within a certain frequency range. The encapsulation of the second structure sensitization solution have been studied in detail. The main research contents of this paper are as follows:Based on multimode interference principle, the single –mode – multimode – gap- single-mode(SMGS) fiber acoustic-frequency vibration sensor is proposed and fabricated, which has the advantage of novel structure and high sensitivity. The beam propagation method(BPM) has been adapted to simulate the light propagation in this composted structure of fiber. The finite el ement model of the cantilever is established in order to analyze the vibration characteristics of the cantilever, and it provides a effective way to optimal design of this kind of fiber vibration sensors. In the Experiments, the fiber cantilevers are fabri cated with the different length of multimode fiber(four types), and the length of total fiber cantilever is always 22 mm. The characteristics of vibration of cantilevers with different length of multimode fiber are analyzed about that how the length of th e multimode fiber effects on acoustic-frequency vibration in details. Experimental results agree well with the theoretical simulation results. It shows that our proposed SMGS sensor has the maximum sensitivity of 4 m V/m Pa at the frequency point of 130 k Hz and the linear correlation coefficient is 0.9962. These results of experiment indicates that this kind of sensor has a linear response and good repeatability within given sound pressure levels.The melting method and chemical corrosion method have been ad apted to fabricate a single-mode – multimode- single-mode(SMS) tapered fiber acousticfrequency vibration sensor. The fiber vibration sensor has the advantage of simple structure, easy-encapsulate and high-sensitivity. For the multimode fiber of SMS structure, the HF acid corrosion method is used to change the mode field distribution in multimode fibers and when the intensity of light coupled into the output single mode fiber is maximum, the HF acid corrosion will be stopped. After the corrosion, the fiber becomes tapered producing advantages of high sensitivity and easiness to be vibrated, and it also overcomes the problems of experimental deviation by single mode and multimode fiber welding in the self-imaging point. For the z=2cm-L=3.2cm-SMS tapered sensing structure, the sensor has the maximum sensitivity of 1.49 m V/m Pa at the frequency point of 120 Hz and the linear correlation coefficient is 0.998 in the vibration response range from 20 Hz to 500 Hz.For the SMS tapered fiber acoustic-frequency vibration sensors, cylinder Helmholtz resonant cavity embedded metal encapsulation sensitization structure has been proposed and fabricated. The encapsulation sensitization structure has the advantage of simple process, low-cost and wide response range. Direct-coupled multi-physics analysis software is adapted to design three cylinder Helmholtz resonator sensitization structure, combining with the metal encapsulation structure. The acoustic-frequency vibration sensing properties in details for the SMS-z=2cm-L=3cm,SMS-z=6cm-L=6cm two tapered fiber sensing structures have been studied. Experimental results show that the composite cylinder Helmholtz resonance cavity sensitization structure with horizontally e mbedded metal encapsulation structure can effectively protect the fiber structure from the interference of the external environment, increase responding amplitude values of different frequencies, expand the range of frequency response of the structure. Then these results show that this novel fiber vibration sensor c an satisfy the demand of acoustic-frequency vibration detection in different environment and has very good application prospect.