Measurement of Thermal Gradients Using Long Period Gratings at Different Levels of Ambient Refractive Indices

In this paper, we present the results for thermal gradient measurements using constant period UV written long-period gratings (LPGs) in photosensitive and SMF-28 fibers. We have studied theoretically and experimentally the effect of thermal gradients on the spectrum of LPGs. It has been shown theoretically and confirmed experimentally that thermal gradients lead to changes of the depth of the grating spectrum while changes of the mean temperature cause shifts of the resonance wavelength. Thus, thermal gradients and average temperature can be independently measured using a single LPG. Sensitivities to temperature difference and to average temperature for LPGs in air have been found to be higher than in water.