Long-term thermal stability tests at 1000 °C of silica fibre Bragg gratings made with ultrafast laser radiation

Using an ultrafast Ti:sapphire 800 nm laser and a phase mask, fibre Bragg gratings (FBGs) with high thermal stability were fabricated in Ge-doped SMF-28 fibre for sensor applications and were subjected to long annealing tests at 1000 °C. FBGs that maintained more than 99.95% reflectivity after several hundred hours at this temperature are demonstrated. The gratings perform well in cycling experiments up to 1000 °C, and hysteresis in the wavelength response was not detected. At a temperature of 1050 °C, a permanent drift of the central wavelength is observed which is associated with a reduction of the grating strength. The capability of this new type of FBG to be used for high temperature sensors is discussed.