Patent ID: 7590323

Claim:
A cladding pumped optical fibre having a longitudinal direction and a cross-section perpendicular thereto, the cladding pumped optical fibre comprising: (a) at least one core having an effective refractive index and comprising at least one rare earth element; (b) a cladding region, said cladding region comprising: an inner cladding region surrounding said core and having an effective refractive index with a lower value than the effective refractive index of said core, said inner cladding region being configured to guide pump light for transfer of energy from cladding modes in said inner cladding to mode(s) of said core; an outer cladding surrounding said inner cladding region, said outer cladding having: (i) at least one first outer cladding region, said first outer cladding region comprising a first outer cladding background material and a plurality of first outer cladding features, said first outer cladding features having a lower refractive index than said first outer cladding background material, said first outer cladding region having an effective refractive index with a lower value than the effective refractive index of the inner cladding; and (ii) at least one second outer cladding region, comprising a second outer cladding background material, and surrounding said first outer cladding region, wherein for a plurality of said first outer cladding features, two nearest neighbouring first outer cladding features have a minimum distance from each other that is smaller than 0.6 μm.