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

• Published in: Optics and laser technology Vol. 122; p. 105836
• Main Authors: Eftimov, T.A., Koffi, N'G., Lesage, F.J., Mikulic, P., Bock, W.J.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.02.2020
• Subjects: Long period gratings (LPG); Photosensitive fibers; Temperature sensors; Thermal gradients; Temperature sensors; Long period gratings (LPG); Photosensitive fibers; Thermal gradients
• ISSN: 0030-3992; 1879-2545
• DOI: 10.1016/j.optlastec.2019.105836

•A matrix model of LPGs with asymmetric Δneff along the structure is presented.•Spectral responses of LPGs with temperature differences ΔT are simulated.•Experiments have been performed to corroborate the simulation results.•It has been shown that uniform LPGs sense only the absolute value |ΔT|.•It has been shown that non-uniform LPGs can sense the value and the sign of ΔT. 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 Δneff 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 ΔIm 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.