An intrinsic sensitivity calibration scheme for high temperature measurements using femtosecond point-by-point written fiber Bragg gratings

A calibration method is presented for temperature measurements using fiber Bragg grating transducers written with the point-by-point technique and femtosecond laser impulsions in silica optical fibers. This method is based on the determination of the wavelength-independent thermal sensitivity coefficients of the waveguide over the temperature range of interest. The calibration law is calculated using the wavelength comb-like spectrum of high order fiber Bragg gratings, allowing measurement accuracy comparable to type N thermocouples up to 900 degrees C. The effects of thermal cycling and annealing on measurement errors are also discussed.