Temperature dependence of temperature sensitivity of resonant mode of a long period fiber grating

This paper presents experiment and analysis on variation of temperature sensitivity of resonant modes of a long period grating (LPG) with change in temperature. The theoretical analysis explains the corresponding experimental data on temperature sensitivity of LPGs written in house in Ge-B codoped and H₂-loaded telecom fiber. The LPGs were first annealed for thermal stability. The average temperature sensitivity of annealed gratings investigated in range 50–300 °C for LPGs in Ge-B codoped fiber and in the range 50–500 °C for LPGs in hydrogenated SMF-28 fiber. The temperature sensitivity increased with temperature. However, the absolute value of temperature sensitivity decreased for LPGs in Ge-B fiber. The analysis shows that the temperature dependence of nonlinear evolution of resonant wavelength is due to second order partial derivatives of differential effective refractive index of a resonant mode with respect to temperature and wavelength. The linearity of temperature sensitivity depends on first and second order partial derivatives of effective refractive indices of core and cladding modes coupled. The study will help in design and development of LPG based devices and sensors operating at elevated temperature.