Research On Demodulation Methods Of Optical Fiber MEMS Fabry-perot Sensor

Optical fiber MEMS Fabry-Perot(FP) sensor, posseses both of the advantagesof optical fiber sensing system, such as high precision, large bandwidth, stronganti-interference capacity of electromagnetic disturbance, etc., and such of MEMStechnology as extremely small size and ease of mass production. Thus, it plays animportant role in industry, agriculture, military and other fields for measurements ofseveral key parameters such as temperature, strain and stress. The main subject ofthis paper is to study demodulation methods for the diaphragm-based FP acousticpressure sensor, which is highly sensitive.In this paper, the structure of the diaphragm-based FP cavity was discussed, forits different spectral characteristics from standard FP. The coupling loss of EFPIwhich is caused by the mode characteristics of single-mode optical fiber wasanalyzed in detail. The transmitting loss of the air cavity and the tilted loss of themembrane to the fiber have been studied. The cavity length-related loss coefficientswere simulated numerically, which indicated that if the cavity length is smallenough, the negative effect of these two losses on the fringe visibilities becomenoticable as the cavity length decreases, thus suppressed the signal-noise ratio of thesensor. The relation between the reflectivities of two faces also determines thevisibilities. The practical spectrum results corresponds the anaylysis well, whichwill have practical value for producing the FP cavity and designing thedemodulation system.Research on intensity-based demodulation methods based on optical fiberbragg grating was conducted. The grating was used as a wavelength filter to formthe compensating optical channel and to counteract the noises such as from the lightsources.A demodulation system was designed and produced including the opticalsystem design, electronic hardware and software debugging. A experiment systembased on PZT micro-displacement platform was built to test the performance of thesystem. The results show a improvement of signal to noise ratio. When the cavitylength changes within a range of300nm，the system has a sensitivity of2mv/nm forcavity length measuremen, with a good output linearity.A acoustic pressure test system was bulit. We used the above system todemodulate a laboratory-made Fabry-Perot acoustic sensor, and compared resultswith the output of a high-precision standarded microphone. The power spetralanalysis was conducted, which showed that the demodulation results can be stableand accurate in a frequency range of between100Hz up to2000Hz for measuringthe acoustic signal. The acoustic pressure sensitivity is about-23.89dB(Re.1V/Pa), taking the sensitivity of microphone(-29.0dB) as the standard. The signal to noiseratio can reach43dB when a acoustic pressure of90mPa@1kHz is applied on thesensor.