Effects of High Temperature and Pressure on Silica Optical Fibre Sensors

Abstract

Many optical fibre sensors rely on the fact that the measurand modulates the optical path length and/or refractive index of the “stable” silica waveguide. If the long-term stability of silica is questionable, the accuracy of such sensors is dubious. In this paper, we present recent results which demonstrate some of the problems of silica optical fibres as sensors in high temperature and pressure environments and report on the need for suitable “hermetic “ coatings for such devices. By monitoring the pressure-induced birefringence of a side-hole optical fibre [1] under conditions of high temperature and pressure, we have found that the birefiingence drifts considerably. This drift increases non-linearly with increasing temperature and the effect is permanent. The high temperature instabilities have been further investigated using Hi-Bi fibre pressure sensors, fibre Bragg gratings and standard telecommunications single-mode fibre. Similar results for the drift in all cases show that the observed effects are inherent to silica and are not due to geometry. It is therefore expected that similar effects will be observed for any silica interferometric or polarimetric sensor in high temperature fluid environments.