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Pressure sensors based on fiber Bragg gratings in side-hole optical fiber enable remote monitoring of pressure at multiple points within many otherwise inaccessible environments. However, sensors fabricated from side-hole fiber drawn from a preform have limitations in their design and material composition. The design of such sensors is a compromise between achieving good sensitivity, while also minimizing splice losses due to mode-mismatch.
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.
We demonstrate the fabrication and optimization of waveguide Bragg gratings on single-crystal sapphire substrates using femtosecond laser direct writing. The gratings are fabricated using modulated bursts and are embedded inside single-mode depressed cladding waveguides. Through design optimization, and fabrication parameter tuning, a depressed cladding waveguide with a loss of ~0.8 dB/cm and a Bragg grating with a reflectivity of higher than 90% in the telecommunications wavelength band are demonstrated.