Responses to temperature and thermal gradients of non-identical cascaded LPGs taking into account dispersion

In this paper we present a detailed theoretical and experimental study of the responses of non-uniform cascades of LPGs (C-LPG) to thermal gradients which cause changes in the depth of the interference minima. Simulations have been performed for cascades without and with dispersion of identical and non-identical effective refractive index differences. It has been shown that in case of non-identical LPGs the response to temperature differences shifts in a way to permit the detection of the magnitude and the sign of the thermal gradient. In the case of dispersion the side minima respond to gradients in counter phase and permit differential measurements. Sensitivities to temperature difference variations of 0.13 dB/°C and to temperature of 294 pm/°C were obtained for cascaded LPGs from photosensitive optical fibers.