Silicon Micromachined Fiber-Optic Accelerometer for Downhole Seismic Measurement

The fabrication and characterization of a fiber-optic micro accelerometer for downhole seismic measurement is described. This sensor consists of a Fabry-Perot interferometer using a half-mirror at the end of an optical fiber and a total reflection mirror on a silicon mass suspended with thin beams. When acceleration is applied to the sensor, the intensity of the reflected light is modulated interferometrically according to the displacement of the silicon mass. The sensor is fabricated by using silicon micromachining and hence is small sized (6.6 mm in diameter and 11 mm in height) and cost effective. By reason of the small size and the low cost, it is possible to install many sensors in a micro borehole. Since the sensor has not active devices like transistors, it can stand in harsh environment i.e. high temperature. The fabricated sensor has the frequency range from DC to 300 Hz, the maximum detectable acceleration of 0.27g and the dynamic range of 36 dB.