Modelling of Long Period Gratings with Metallic (Pd) Jacket

Metal coated long period grating based sensors and tuneable filters have attracted considerable interest in recent years. The replacement of guidance of cladding modes by total internal reflection with reflection by a metallic layer has complicated the system with no simple formal model to describe the transmission spectra of these forms of LPG being available. We report the extension of standard long period grating (LPG) modelling techniques to incorporate an infinite thickness (optically opaque) metallic jacket layer. The effect of the inclusion of a metal layer on the available radially symmetric cladding modes is discussed both for low order and higher order, double coupling cladding modes. Particular care is taken to consider the effect on the coupling coefficients between core and cladding modes which result in a significant reduction in the effective strength of the grating in the presence of a complex, absorbing refractive index layer.