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, KT, was found to be quadratic in the −50–200 °C range, while it may be considered linear for the −20–100 °C range; KT 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; KT does not depend on the hydrogen-loading process or the gratings coupling strength; KT 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 KT = 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 KT. The previous achievements allow, by having knowledge of KT 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 KT for gratings inscribed in several fibers are given, as well as an expression for KT as a function of the effective refractive index. We have also fully analyzed the potential sources of error in KT determination.