Modeling of long-period fiber gratings with both refractive index and diameter modulation

We present a mathematical model of long-period fiber gratings (LPFGs) which accounts for simultaneous perturbations of various parameters of the optical fiber: its refractive index and radius. We also present the results of the calculation of the cross-coupling between the fundamental mode of the fiber core and symmetric LP modes of the cladding as a function of the mode order, for the step-index optical fiber with harmonic modulation of the refractive index and radius both of the core and cladding.