Yb-doped double-cladding large mode area rod-type photonic crystal fibers are a key component for power scaling in fiber laser systems. Recently, designs with 19-cell core defect, that is with 19 missing air-holes in the center of the photonic crystal cladding, have been proposed, with reported core diameter up to 100 mm. In this paper an analysis of the cut-off wavelength of the first high-order mode in such low-NA fibers is reported, accounting for different approaches for the definition of the cladding effective index. Results have shown that taking into account the finite fiber cross-section and considering the first cladding mode of the actual fiber is mandatory to obtain a correct estimate of the cut-off wavelength.
Cut-off analysis of 19-cell Yb-doped double-cladding rod-type photonic crystal fibers / F. Poli; E. Coscelli; T. T. Alkeskjold; D. Passaro; A. Cucinotta; L. Leick; J. Broeng; S. Selleri. - In: OPTICS EXPRESS. - ISSN 1094-4087. - 19:10(2011), pp. 9896-9907. [10.1364/OE.19.009896]