Inscription and characterization of Bragg gratings in single-crystal sapphire optical fibres for high-temperature sensor applications

We report the fabrication and high-temperature characteristics of Bragg gratings in single-crystal sapphire fibres inscribed by infrared femtosecond laser pulses. For stable read-out of the multimode Bragg reflection signal, the sapphire fibre was illuminated by broadband light from a superluminescent diode applying a long section of a step-index multimode silica fibre. This evenly distributed excitation of many guided modes in the sapphire fibre resulted in the polychromator measurement of stable reflection peak shapes and allowed for multiplexed temperature sensing up to 1745 °C at remote read-out with repeatability better than 1 °C. During the temperature tests an annealing process of the sapphire grating was observed at temperatures above 1400 °C. Possibilities for further improvement of sensors performance are discussed with a view to avoiding instabilities from the lack of a fibre cladding that shields the sapphire waveguide from the environment.