Ytterbium-doped photonic crystal fibers have been the major player in the extremely rapid increase of the output power of high power femtosecond fiber laser systems. However, rare earth doping of silica leads to an increased refractive index of the core, which is, in turn, compensated by other codopands. In the case of Very Large Mode Area (VLMA) Large Pitch photonic crystal Fibers (LPFs) for high power laser systems it is vital to obtain an ideal index-matching between the actively doped core region and the passive surrounding air hole structure. In [1] it was demonstrated that index-matching is most important for mode area scaling of core-pumped LPFs with effective single-mode operation through mode-discrimination. However, surrounding the core structure by a pump cladding leads to a completely new structure, where core and cladding modes can interact through anticrossings. In [2] it was shown that fundamental mode (FM) anticrossings can be avoided and higher order mode (HOM) anticrossings can be exploited by the proper choice of the airclad diameter. With growing airclad diameter, the effective index of a core guided mode basically remains constant while cladding modes posses growing effective indices and, thus, will cross the effective index of the core guided FM. However, some HOMs deform when their effective index approaches the FM index, the shape of both modes converge. Finally the former HOM takes over the role of the FM while the former FM is deformed.

© 2011 Optical Society of America

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