Temperature cross-sensitivity for highly refractive index sensitive nanocoated long-period gratings

Abstract

There is a demand for highly refractive index sensitive devices simultaneously insensitive to temperature (T) changes. We investigate both T and refractive index sensitivity of long-period gratings coated with 100 nm thin high refractive index (n ≈ 1:9 at λ = 1550 nm) silicon nitride (SiNx) film. The long-period gratings with period of 500 m were induced in standard Corning SMF28 fibre using electric arc and nanocoated with SiNx by radio frequency plasma enhanced chemical vapour deposition method. As a sensorial effect we investigated resonance shift with temperature variations (from 30 to 70°C) for measurement in deionized water (nD = 1:33) and glycerine (nD = 1:46). In case of measurements in water for resonance observed at λ = 1510 nm, refractive index sensitivity of -85 nm/RIU and temperature sensitivity of 0.09 nm/°C (H2O) were obtained which gives refractive index-T sensitivity ratio (RI/T) of -944 °C/RIU. Refractive index and T sensitivity of sensor covered with SiNx film increased to -618 nm/RIU and 0.22 nm/°C, respectively. Thanks to nanocoating deposition, refractive index-T sensitivity ratio was improved by factor of 3. In case of measurements in glycerine significant influence of glycerine's thermo-optic coeficient on obtained results was observed. It is worth noticing that SiNx coating stabilizes sensing.