Fabry-Perot Transducer for Monitoring of Displacement at Low Temperature (05-0469)
Marco Quirion, Roctest Ltd., Canada

This paper presents the testing procedure and results of low temperature testing made on a specially design fiber optic displacement transducer. This transducer may be useful in geotechnical and structural applications where temperature as low as –72°C are encountered; the operating temperature range of the different conventional technologies is generally limited to –55°C or less. Also, the fact that the transducer contains no electronics is an advantage in hazardous environment or in presence of electromagnetic fields. The fiber optic transducer consists of a multimode fiber optic fixed perpendicular and in close proximity to a thin film Fizeau interferometer, which is mounted on a movable shaft. The Fizeau interferometer moves relatively to the fiber optic and the resulting optical signal is converted into displacement. For testing purpose, the transducer was mounted in a cylindrical support sleeve inserted in an environmental chamber and submitted to low temperature varying from 0 to –72°C. The experimental testing set-up for low temperature was first compared with the standard calibration bench and the results obtained were identical. The results of low temperature calibration at -72°C, performed on four transducers, showed a maximum error of linearity of 0.06% of the Full Scale compared to 0.004% at ambient temperature. To assess the performance of the transducer used with fiber optic extensions and connections, from which signal losses may appear, calibration tests were performed with 100 m and 1000 m fiber optic extensions between the sensors and the readout unit. These testing results of fiber optic displacement transducer are very encouraging and tests at cryogenic temperature, as low as –170°C, are to be performed.