A microstructured optical fiber comprises a doped core region embedded in a cladding layer, and a plurality of longitudinal tubes, wherein a radial cross-section of the optical fiber comprises a central hexagonal portion comprising a plurality of holes arranged according to a hexagonal grid surrounding a core section. Each hole corresponds to a respective tube, within a hexagonal boundary of the grid, and the plurality of holes comprises holes of first and second types arranged in a biaxial mirror-symmetric configuration. The holes of the first type are arranged in two side holey structures comprising distinct sub-grids of the hexagonal grid, defined by respective outer boundaries corresponding to portions of the hexagonal boundary of the grid and respective inner boundaries. Outer tangential lines to the respective inner boundaries cross each other at the opposed side of the core with respect to the side of the respective side holey structure.