Optimal design of silica-based temperature-insensitive long-period waveguide gratings for realization of athermal refractive-index sensor

Based on silica-on-silicon planar technology, a theoretical analysis for the optimal design of a temperature-insensitive long-period waveguide grating (LPWG) is presented in order to achieve an athermal condition for sensing the refractive index of an external medium. Effects of the variation of the core and over-cladding thickness on the temperature sensitivity of the LPWG formed in the above material are discussed. Both of these parameters are found to be important to control the temperature sensitivity when the thermo-optic coefficients of core and cladding materials are of same order. Other grating parameters are also optimized in order to achieve a good contrast of the notch filter having a resonance wavelength near 1.5 μm region and to sense the external medium refractive index over a wide range. Variation of external refractive-index value from 1.00 to 1.45 causes a blue-shift in the LPWG resonance wavelength by 99 nm with its temperature sensitivity as low as 0.004 nm/°C over a temperature range of 0–50 °C, for this optimally designed LPWG.