Abstract

Yb-doped Double Cladding Photonic Crystal Fibers (DC-PCFs) with low numerical aperture have been successfully exploited to realize high-power lasers and amplifiers. Recently, in order to obtain the high effective area values required to avoid detrimental nonlinear effects, PCFs with core obtained by removing 19 cells from the cladding lattice have been proposed . A critical issue dealing with fibers with such large core is the possibility to obtain single- mode guiding, which is a mandatory requirement for most high -power applications. So far, a thorough analysis of the single - mode regime of this kind of fibers has not been presented. The cut-off properties of PCFs with 1 - cell or 7- cell core have been studied extensively in literature, for example in [1], by considering the effective index of the Fundamental Space - filling Mode (FSM) as the refractive index of the fiber cladding. As a consequence, cut - off condition has been set where the effective index of the guided mode equals the one of the FSM. It has been shown that the hypothesis of an infinite cladding, which is assumed in this approach, no longer holds when 19 -cell core DC PCFs are considered [2].

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