1.2.4 - First-order sapphire fiber Bragg gratings for high temperature sensing

Sapphire fiber Bragg gratings (SFBG) are presented as a new kind of high temperature sensor that is not sensitive to electromagnetic fields and can be operated in air as well as inert atmosphere. For inscription of first order gratings femtosecond laser pulses with a wavelength of 400 nm are applied. Multiplexing of several gratings within one single crystal fiber is demonstrated. The gratings were tested for short term up to 1,900°C and for a period of 28 days at 1,400°C. An adapted signal processing of the reflexion spectra of the highly multimode air clad sapphire fibers with a diameter of 100μm allows detecting relative temperature changes better than ±1K. Tubes can protect the air clad sapphire fiber from reactions with the environment. Applying a bare fiber allows to detect temperature changes with several Hz speed. Strain sensor designs and strain measurements based on SFBG for applications up to 600°C were tested, too. The sensitivity of multimode SFBG strain sensors is ∆l / l = 10 (10μstrain). Overall, fiber Bragg grating inside Sapphire fibers provide a new base for precise high-temperature sensors with key advantages such as signal multiplexing, large temperature bandwidth and insensitivity to electromagnetic fields.