Patent ID: 7756375

Claim:
An optical fiber, comprising: a core region that is doped with an impurity; a first cladding region that is formed in a layered structure around the core region, the first cladding region including a plurality of microstructures positioned on three concentric circles around the core region, the plurality of microstructures including a first microstructure positioned on one of the three concentric circles closest to the center of the core region and having a first cross-sectional area, and a second microstructure positioned on the one of the three concentric circles closest to the center of the core region and having a second cross-sectional area smaller than the first cross-sectional area, the first microstructure and second microstructure being in line symmetry, the rest of the plurality of the microstructures, those not on the one of the three concentric circles closest to the center of the core region, having a cross-sectional area substantially equal to the second cross-sectional area; and a second cladding region that is formed in a layered structure around the first cladding region, the second cladding region including a homogeneous material, wherein a relative refractive-index difference Δ 1 between the core region and the second cladding region is equal to or more than 0.4% and less than 1.0%.