Peculiarities of thermo-optic coefficient under different temperature regimes in optical fibers containing fiber Bragg gratings

It was found that the first-order thermo-optic coefficient changes between 1.29 and 1.85 × 10− 5 K− 1 for the linear fit and at T = 0 °C its value was found to be close to 2.37 × 10− 5 K− 1 for the polynomial fit of experimental data. The average thermo-optic coefficient undergoes a minimum in the vicinity of 440 °C. Additional observation indicates a negative sign of the second-order thermo-optic coefficient. The value of thermal expansion coefficient was much less (0.5 × 10− 6 K− 1) than that for the average thermo-optic coefficient. Based on the energy dispersive spectroscopy it was determined that thermal erasing of the FBGs at a temperature around 780 °C corresponds to germanium monoxide diffusion out of core in silica-based fibers.