Fibre Bragg gratings (FBGs) of type I and IIA were fabricated in Ge-doped and B-Ge co-doped fibres using a 248 nm excimer laser and their performance characteristics were tested and compared with those of a chemical composition grating (CCG), written in a fluorine-germanium doped fibre, over a wide range of temperatures. Long-term testing (more than 600 h) involving a series of step-wise incremental temperature changes shows for the first time the potential of FBGs for high temperature measurement applications (up to and beyond 1100 • C), this depending on the type of FBG involved and the material and composition of the substrate fibre (the CCG was observed to be the most durable at very high temperatures). These gratings are likely to be useful for the simultaneous measurement of strain and temperature over these higher temperature ranges.

The thermal decay of a type I fiber Bragg grating written at 248 nm in boron-germanium codoped silica fiber was examined in terms of its reflectivity and Bragg wavelength change. In addition to the decay in reflectivity, which was observed, a shift in Bragg wavelength over the temperature range considered was seen. A mechanism for the decay in the reflectivity was developed and modeled according to a power law, and the results were compared with those from the aging curve approach. The wavelength shift was simulated by modification of the power law, which was also found to fit well to the experimental data. Temperature-induced reversible and irreversible change in the grating characteristic were observed and considered to be a means to predict the working lifetime of the grating at comparatively low temperatures. Accelerated aging was also reviewed and compared in terms of reflectivity and Bragg wavelength shift. It was shown that the temperature-induced irreversible shift in the Bragg wavelengths could not be predicted by use of the isothermal decay of the refractive-index modulation. The results were discussed within the framework of the current theoretical approaches for predicting the stability of gratings of this type.

Abstract-A novel Bragg grating-based fiber-optic laser probe for temperature sensing using erbium-doped fiber as the active gain medium is reported. The combination of a chirped grating and a normal grating was used to form the laser cavity to achieve temperature-tunable laser action over a wide measurement range. The laser probe used a metal sheath to enhance its mechanical strength and contain the normal grating at the sensing point. The temperature dependence of the wavelength of the laser probe gives a sensitivity of 12.01 pm/ C and a repeatability of 1 7 C from room temperature to 300 C.

No abstract

No abstract