Double-clad Yb3+-doped fiber lasers with virtually non-circular cladding geometry

We report on cladding pumped Yb3+-doped fiber lasers with novel virtual-non-circular cladding geometries. The geometry of the inner-cladding is in fact circular with D-shaped holes, that form virtual-D (one hole), and virtual-square (four holes) structures for the inner-cladding. The laser performance of this structure is assessed at several wavelengths and lengths, showing efficiencies as high as those obtained with other geometries for the inner-cladding.