Modelling of long-period fibre grating response to refractive index higherthan that of cladding

In this paper we introduce a new mathematical model, which can be used to i nvestigate the response of a long-period fibre grating to changes in ambien t refractive indices of values both greater and less than that of the cladd ing. A numerical method to calculate the HE core and cladding modes is pres ented. The results show that even when the ambient refractive index is high er than that of the cladding, the coupling (resonance) wavelengths experien ce a measurable shift (a few nm). The result obtained suggests that LPGs co ated with a material of higher refractive index than the cladding may be us ed as index sensors.