We have successfully developed a new design of an extrinsic fibre Fabry-Perot interferometer (EFPI) sensor dedicated to the characterization of vibration and displacement of a target. This device, based on a low finesse Fabry-Perot cavity formed by the end of a 'sensing' optical fibre and the target, gives information on the direction of the motion without the use of an additional reference arm. The incoming light, emitted by a 1310 nm laser diode, is decomposed according to two orthogonal polarization orientation inside the cavity. The two resulting interference signals are then carried back by the same optical fibre and sent to two photodiodes via a coupler and a polarizing beam splitter. With a relatively
simple signal processing, a precision of λ/4 is achieved for the measurement of the displacement, for which the direction is also extracted. In addition, one can determine the velocity of the motion, that have been successfully compared with a reference sensor. The use of a polarization maintaining fibre as sensing arm, not mandatory for
monitored laboratory set-up, allows the use of this sensor principle even with external perturbation (temperature changes, mechanical stress...).
An extrinsic fibre Fabry-Pérot interferometric (EFFPI) sensor has been successfully demonstrated for the measurement of displacement and/or velocity of a vibrating target. The target employed consists of a combination ofa strip ofretro-reflective microprisms and a linear polariser. It is then bolted onto an oscillating piezo-electric transducer (PZT). Hence, two dissimilarly polarised lightwaves of similar frequency are reflected from each ofthe two surfaces and interfere with light reflected from the end ofthe "sensing" fibre arm, resulting in a double cavity within the arm ofthe interferometer. This provides a relatively simple and practical technique of determining the direction or sense of vibration of the target. The sensing system was found to be relatively immune to minor perturbations, such as small temperature changes (~±5 °C), since the two sets of interference signals propagate along the same optical path. Further, the evolution of the sensor for velocity measurements with angular inclination of the target has been found to agree well with predicted values while changes in velocities measured by the EFFPI sensor with frequency variation of the target at 0° incidence was found to contain slight discrepancies compared to those from the PZT. Typical PZT frequencies investigated ranged from ~14 — 136 Hz. Polarisation effects on the reflected beam from the retro-reflective microprisms have been observed and induced birefringence in the sensing arm due to bends and twists has been attributed as the main reason for measurement errors. The potential resolution attainable from the EFFPI sensor is predicted at better than ?8.