OPTICAL FIBER

An optical fiber for guiding an optical signal, said optical fiber having a longitudinal, optical axis (A) and a cross section perpendicular thereto, said optical fiber comprises:
a core region (2) being capable of guiding an optical signal in a fundamental core mode with an effective refractive index, n_c, at an optical signal wavelength, λ₁;
a cladding region surrounding the core region, the cladding region comprising an inner cladding region (3) and an outer cladding region (6), said inner cladding region comprising a background material having a refractive index, n_b, and a plurality of inner cladding features arranged in said background material, wherein at least a plurality of said plurality of inner cladding features are of a first type of feature (4), said first type of feature comprising an air hole surrounded by a high-index region comprising a high-index material having a refractive index, n_r, that is larger than the refractive index of the inner cladding background material, said first type of feature supports an optical mode with an effective refractive index, n₁, which is lower than or equal to the effective refractive index of the fundamental core mode, n_c, at said optical signal wavelength, λ₁,
wherein at least a plurality of said plurality of inner cladding features are of a second type of feature (5) comprising an air hole in direct contact with the background material having a refractive index, n_b, and
wherein said inner cladding features are arranged in a substantially hexagonal lattice and the six nearest neighbors surrounding a first type of feature are of said second type of feature, and
wherein no first type of feature is surrounded by less than six nearest neighbors of the second type of feature.