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

Direct experimental measurements of the thermo-optic for fixed temperature intervals (20-200 degrees C, 200-500 degrees C C, 500-660 degrees C, 660-780 degrees C) in fused silica fiber containing fiber Bragg gratings (FBGs) were conducted. The diffraction efficiency of a FBG fluctuated with temperature between 2.01 x 10(-4) and 0.17 x 10(-4) while the temperature shift of the Bragg's peak was monitored between 1300 and 1311 nm with sub-Angstrom precision. Numerical simulations were focused on FBG's diffraction efficiency calculations accounting for the temperature drift of the gratings, and found to be in excellent agreement with obtained experimental data. It was found that the first-order thermo-optic coefficient changes between 1.29 and 1.85 x 10(-5) K-1 for the linear fit and at T=0 degrees C its value was found to be close to 2.37 x 10(-5) K-1 for the polynomial fit of experimental data. The average thermo-optic coefficient undergoes a minimum in the vicinity of 440 degrees 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 x 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 degrees C corresponds to germanium monoxide diffusion out of core in silica-based fibers. (C) 2011 Elsevier B.V. All rights reserved.