Temperature insensitivity of a fiber optic Bragg grating sensor

摘要

Recent findings strongly suggest that the glass in the core of a fiber optic Bragg grating has been periodically compacted resulting not only in residual tension but also in a structural transverse isotropy within the core of the fiber. These effects result in a difference of strain- temperature dependence within and outside the Bragg grating that provides a tool for separation of measurands. Hence temperature compensation can be achieved for a Fabry-Perot interferometer with chirped Bragg gratings as end reflectors when used in conjunction with a pulsed light source. The analysis presents in this paper show that if carefully chirped gratings are used as reflectors in a Fabry-Perot interferometer it is possible to control the optical path imbalance of the sensor such that one measurand can be fully compensated for, e.g. strain or temperature. Mainly temperature compensation is treated since it is considered the primary objective, even though the concept applies for any arbitrary measurand. Several relationships are derived for temperature compensation of Fabry-Perot interferometers with chirped Bragg grating reflectors including both free and embedded sensors. The boundary conditions of an embedded sensor are significantly altered since the surrounding host material superimposes temperature induced and transverse strains that will contribute substantially to the strain field inside the sensor. Simulations were carried out according to a geometric grating model assuming continuous chirp and a strict geometrical reflection criterion. These simulations are illustrating the compensation effect for various load cases to both free sensors and sensors embedded in composite laminates.