Optical fibers; Optical fiber devices; Optical fiber sensors; Optical fiber testing; Reflectometry

Sapphire fiber can withstand temperatures of around 2000°C, but it is multimoded, giving poor precision sensors. We have demonstrated a single-mode sapphire fiber Bragg grating temperature sensor operating up to 1200°C. A single-mode sapphire fiber was formed by writing a depressed cladding waveguide along the length of a 100-µm diameter sapphire fiber using a femtosecond laser. A second-order Bragg grating sensor was inscribed within the waveguide, achieving a reflectivity of >90% and a narrow bandwidth of <0.5 nm.