Sapphire fiber Bragg grating for high-temperature diagnostics

Sapphire fiber Bragg gratings (SFBG) are a new type of high temperature sensors. SFBG are inscribed by fslaser pulses in single crystalline sapphire fibers. By wavelength multiplexing several gratings can be inscribed in one fiber. SFBG were tested from room temperature (RT) up to 1900 degrees C. A parabolic temperature dependence was observed increasing from 23 p.m./K at RT to 35p. m./K at 1500 degrees C. Fitting an adapted peak function to the reflexion spectra allows determining a Bragg wavelength. Temperature changes of +/- 1 K can be detected. SFBG are not influenced by electromagnetic fields. They can be applied in air as well as in inert or reducing atmosphere. On the other side SFBGs have to be protected against contamination. A short time exposition to 1900 degrees C as well as a four week test at 1400 degrees C did not lead to drift or other ageing-effects in the SFBGs. SFBG may be an alternative to thermocouples or pyrometers.