A temperature-compensated fibre optic extrinsic Fabry-Perot interferometric displacement sensor for fault measurement in geomechanics

A fibre optic extrinsic Fabry-Perot interferometric (EFPI) displacement sensor for fault measurement in geomechanics is experimentally demonstrated. By using a metal tube and metal bars the EFPI displacement sensor can demonstrate high strength and high stability, as the effects of the harsh environment are mitigated. The sensor is fixed to a long stainless steel pipe which is inserted into a drilled hole and placed across the fault zone. In this way, the sensor can be placed easily and accurately in the fault zone. In order to eliminate the influence of temperature two metal bars of different materials, and therefore with different thermal expansion coefficients, are used. Thus, the sensor itself is capable of compensating for temperature. The experimental results show that the ratio of the cavity length to the temperature reduces from 0.142 to -0.045 m °C^-1. In a continuous test, the measurement results vary over a range of 4.954 m and the standard deviation is 1.196 m, when the temperature is changed from 17 to 80 °C.