Accurate Determination of the Temperature Sensitivity of UV-Induced Fiber Bragg Gratings

Over the past 18 months, we have performed hundreds of temperature characterizations of fiber Bragg gratings inscribed in different germanium-doped silica glass fibers. Under experimental conditions, the main conclusions are as follows: the temperature dependence of the "temperature gauge factor" or the normalized temperature sensitivity, K<i>_T</i>, was found to be quadratic in the -50-200 °C range, while it may be considered linear for the -20-100 °C range; K<i>_T</i> values at 20 °C vary from 5.176 × 10^-6 K^-1, for a B/Ge co-doped fiber up to 6.724 × 10^-6 K^-1, for a highly Ge-doped fiber; K<i>_T</i> does not depend on the hydrogen-loading process or the gratings coupling strength; K<i>_T</i> is essentially independent of wavelength in the 1500-1600 nm range, its value being accurately determined with a relative error ~0.2%; based on the accurate value of K<i>_T</i> = 6.165 × 10^-6 K^-1, at 20 °C, obtained for gratings inscribed in the SMF-28 fiber, we calculated a value of 19.4 × 10^-6 K^-1 for the thermo-optic coefficient of bulk germanium glass; and gratings produced by femtosecond-laser radiation and UV-laser radiation exhibit comparable values of K<i>_T</i>. The previous achievements allow, by having knowledge of K<i>_T</i> for a single grating, the accurate determination of the temperature dependence of the Bragg wavelength for any other grating inscribed in the same fiber; the presented methodology enables one to determine the "unknown" gratings' temperature sensitivity, typically with an error of 0.01 pm/°C, being, therefore, very useful in research labs and computer simulations. Thus, expressions for the temperature dependence of K<i>_T</i> for gratings inscribed in several fibers are given, as well as an expression for K<i>_T</i> as a function of the effective refractive index. We have also fully analyzed the potential sources of error in K<i>_T</i> determination.