Research On Optical Fiber Vibration Sensor Based On Multimode Interference

Vibration detection and monitoring is an important means to do nondestructive testing of buildings, bridges, roads and dams and other infrastructure. The vibration sensor not only can detect the structure health of engines, ships, spacecraft structural, but also can monitor the environmental of seismic activity and volcanic eruptions. With small size, light weight, and can be distributed over long distances tested, optical fiber vibration sensor can well avoid the disadvantages of conventional piezoelectric sensors which were easy affected by electromagnetic interference, and has been widely studied and developed rapidly.In this paper, we present a optical fiber vibration sensor based on the multimode interference. Light transmission in the sensing structure, vibration characteristics of the sensor structure, and sound vibration testing of optical fiber sensing structure were researched.(1) we briefly introduce the background and significance of the subject, and discuss the theory of multimode interference and bending vibration beam, give the theoretical foundation for the multi-mode optical fiber vibration sensor based on interference.Then introduce the vibration detection principle and select the detection method of this paper as hole intercept detection method, which is low-cost and easy to implement.(2) A optical fiber vibration sensor based on single molde-multimolde-single molde(SMS) cantilever structure was studied. First we simulate the cantilever beam of light transmission, then discuss the vibration of the cantilever beam from theoretical calculations and simulations. Finally, we test the sound vibration character of the cantilever beam. The relationships between sound pressure level and vibration amplitude of the cantilever, the impact of cantilever length to vibration response and the influence of the length of multimode fiber on the structure sensitivity were discussed. Experimental results show that the cantilever mixed with multi-mode fiber splice can realize the measurement of sound and the larger of the decibel,the greater of the vibration amplitude of the cantilever. Cantilever length has a effect on the natural frequency of the beam, and also determines the frequency of the strongest response.The shorter multimode fiber length can make cantilever more sensitive.(3) An optical fiber vibration sensor based on SMS structure was studied. First we simulate the optical transmission of SMS structures, then analysis the vibration of the beam with both fixed ends from theoretical calculations and simulations. Finally, we test the sound vibration character of the SMS structure, and proposed several solutions to improve the structure sensitivity. The test result shows SMS structures has a better identify to the frequency within 50Hz-1000 Hz range. Three solutions of structural sensitizing was presented. One is using sensitivity film, this solution does not change the structure of SMS itself, but attach SMS structure to the film, which will enhance sensitivity of the part of the frequency range. It is more apparent when using of Helmholtz resonators. The second solution is using femtosecond laser to etch the multimode fiber in the middle portion of SMS structure, and the etch depth will affect the vibrations response. The third solution is tapering the SMS stucture, which can significantly improve the sensitivity of the structure. The elongated cone with less loss of the light will be better.