Thermal gradients sensing using LPGs with a spatially varying effective refractive index difference

In this work we present the theoretical modeling and experimental results from thermal gradient measurements using long-period gratings (LPG) with a spatially varying effective refractive index difference along the structure. It has been shown theoretically and confirmed experimentally that by introducing a spatial dependence of the effective refractive index difference Δn eff along the LPG, it can sense not only the magnitude but the direction of the thermal gradient. Thermal gradients in LPG fabricated in boron doped photsensitive fibers are shown to cause depth changes of the minimum ΔI m and sensitivities as high as 0.33 dB/°C were observed for a fiber with temperature sensitivity of 2.3 nm/°C. Since average temperature causes wavelength shifts, thermal gradients and average temperature can thus be simultaneously measured by a single LPG.