Quasi-Distributed Temperature Sensing Using Type-II Fiber Bragg Gratings in Sapphire Optical Fiber to Temperatures up to 1300°C

A sapphire optical fiber, with an internal cladding and an array of type-II Bragg gratings inscribed into it, was tested to temperatures up to 1300°C. The sapphire fiber was interrogated with an optical frequency domain reflectometer, using the Bragg gratings in the fiber as simply “enhanced Rayleigh scatter.” The sapphire fiber was able to produce consistent temperature measurements up to 1300°C. Above 1000°C, small oscillations appeared in the measured temperature, which we believe were a consequence of vibrational strains on the fiber that were due to air currents within the furnace. A calibration was added to the optical frequency domain reflectometer that assumed a linear relationship between the fiber temperature and fiber length. We used this calibration to correlate phase shifts that were measured using the optical frequency domain reflectometer to temperatures that were measured using a B-type thermocouple. This calibration could be improved to increase the accuracy of the temperature measurements.