Study On Fiber Acoustic Sensor Based On MEMS

The combination of Microelectromechanical System (MEMS) and Fiber optic sensors introduces the design as well as the investigation of the acoustic transducer into a brand new direction-MEMS fiber acoustic sensor. Compared with the traditional acoustic sensors, the fiber acoustic sensor based on the MEMS technology presents notable advantages of high sensitivity, electrical insulation in detection end, immunity to electromagnetic interference, small size, light mass, being suitable for long-distant transmission and multiplexing etc, and can have good applied prospect in many special fields. In this thesis, the fiber optic acoustic sensor and fiber optic ultrasonic sensor with different structures based on the MEMS technology were presented, and the relative fabrication processes were explored.First of all, a Fabry-Perot interferometric type silicon micro Fiber acoustic sensor was presented, whose design of structure strived for improving performance of current audible-frequency acoustic sensors. The operation principal and structure advantages were presented in this article as well. Besides, the optimal design parameters as well as the ideal maximal sensitivity have been successfully achieved through detailed explanation for the optical and mechanical function design optimization of F-P cavity, with an accessorial analysis of Finite Element Method. We not only studied the key processes such as thin diaphragm etching with low surface roughness, the shaping of optical dielectric films, but also discussed the influences of F·P cavity performances brought by the surface roughness and successfully realized the low roughness etching which satisfies the basic requirement of device's performance and accomplished the Pyrex glass deep etching with high quality surface. A complete fabrication process based on large amount of experimental results was presented, in which the F-P cavity unit has been fabricated and static optical signals of the sensor have been tested.Due to the low sensitivity, the traditional sensitive thin membrane structure of uniform thickness faces certain limits when it is applied to high frequency ultrasonic transduction. An innovative ultrasonic sensor based on a double-side diffraction grating structure was introduced. for the first time in this paper. This device attempted to achieve the ultrasonic detection utilizing exact phase modulation produced by the inflection of silicon grating sensitive diaphragm. Via MEMS process, the working point can be shifted to the highly sensitive linear area, and the packaging was achieved using dual fiber collimator by coupling. Thus not only the devices can be guaranteed to maintain the high sensitivity, but also the frequency limitation of uniform silicon membranes can be avoided. The frequency was enhanced by means of optimizing the thin silicon membrane's size, periodic structure parameters as well as the thickness of the ridge area. Therefore, the resonance frequency (MHz upwards) was achieved via simulation calculation. Moreover, the mechanic sensitivity level of the thin diaphragm was also maintaining in nm/Pa scale. This device was successfully fabricated on SOI material, an elementary test has been carried out and the results has been analyzed.Finally, in this thesis, the testing method, result of fabrication of two acoustic sensors and the analysis were investigated. The future work was also discussed.