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Thermal characterization of etched FBG for applications in oil and gas sector

In this work, we experimentally study the thermal sensitivity of etched fibre Bragg grating immersed in four different commercial petroleum hydrocarbon samples with different refractive index and negative thermo-optic coefficient. The results show that when the surrounding refractive index increases a nonlinear behaviour in the grating wavelength shift becomes noticeable. This nonlinear behaviour is owing to a non constant thermal sensitivity of the device. For the sample with higher refractive index the increase in the temperature causes an increase in the sensitivity from 0.1 pm/degC to 6.2 pm/degC.