Higher-order mode suppression in rod-type photonic crystal fibers with sectioned doping and enlarged core

Yb-doped rod-type photonic crystal fibers are double-cladding large-mode-area fibers with the outer dimension of few millimeters. The higher-order mode suppression through gain filtering has been demonstrated in these fibers, by enlarging the core radius from 30 μm to 40 μm, while keeping fixed the doped-area dimension. Sectioned core doping, obtained by adding a low refractive index ring in the fiber core, has been taken into account, in order to design fibers with an effective single-mode behaviour. Moreover, the gain competition among the guided modes in the enlarged-core rod-type PCFs has been analyzed with a spatial and spectral amplifier model, showing the positive effect of the gain filtering in improving the fundamental mode amplification, to the detriment of the higher-order mode one. Comparisons have been made with the properties of rod-type fibers with 30 μm core radius, both with uniform and sectioned doping, in order to show the effectiveness of the down-doped ring in the enlarged core for the higher-order mode suppression.