Single-mode amplification in Yb-doped rod-type photonic crystal fibers for high brilliance lasers

This paper presents the effect of a low refractive index ring in the Yb-doped rod-type photonic crystal fibre core on the guided mode propagation and analyzed through a spatial and spectral amplifier model. The ring provides a higher differential overlap between the fundamental mode (FM) and the higher-order mode (HOM), stressing the difference between their spatial distributions, with respect to the uniform refractive index core. In the present analysis a rod-type PCF with a 19-missing air-hole core, whose radius is 30 mum, has been considered. Initially, a PCF step-index model has been applied to identify a proper ring characteristic that is width, position and refractive index. Then rod-type PCF designs have been optimized with a full-vector modal solver based on the finite-element method. Then, the amplification properties of the Yb-doped rod-type PCFs have been investigated by assuming a forward-pumped configuration.