A core region is doped with an impurity. A first cladding region is formed in a layered structure around the core region, including a microstructure. A second cladding region is formed in a layered structure around the first cladding region, including a homogeneous material. A relative refractive-index difference Δ1 between the core region and the second cladding region is equal to or more than 0.4% and equal to or less than 1.0%.