Fiber Optic Pressure Sensor Using White Light Interferometry

Abstract

One of the main potentiel markets for fiber optic sensors is the absolute static pressure measurement field. However, in most transducing principles that have been proposed a pressure variation causes a variation in attenuation of the propagating light; this leads to intensity modulated sensors which are characterised by low dynamic range because of sensitivity to external sources of attenuation such as bending, variable connector loss, source instability ... Consequently, high dynamic range pressure sensor will be obtained using optical principles unsensitive to uncontroled attenuation variations of the optical path. Spectral encoding is one of the most promising solutions to this problem. In the device we present, the light of a wide spectrum LED is fed into a multimode fiber, at the end of a Fizeau interferometer creates fringes in the spectrum of the light. The two reflectors of this interferometer are the tip of the fiber, and a mobile membrane placed close to the fiber; the spectrum of the light returning from the interferometer is analysed by a spectrophotometer in order to compute fringe number and position in a given spectral interval. This gives an absolute measurement of the path difference of the interferometer, which is directly related in our case to the pressure applied onto the membrane. As the measured quantity is not the intensity of the returning light, this method leads to high precision measurements and preliminary measurements have demonstrated precisions of 10-3 of the pressure range. Theoretical possibilities and experimental results will be presented.