Patent ID: 7809223

Claim:
A polarization-maintaining optical fiber comprising: a core region; and a cladding region formed around the core region, the cladding region having a refractive index lower than a refractive index of the core region, wherein a refractive index profile of the core region is either one of a step shaped or a concave shaped, and the cladding region includes two holes formed in such a manner that a shortest distance W between the core region and a hole represented by W=L−(a+b)/2 is −0.1 μm to 0.2 μm, where L is a distance between centers of the core region and the hole, a is a core diameter of the core region, and b is a diameter of the hole, at locations in opposite to each other across the core region, the cladding region, except the two holes, is formed of a uniform material, a relation a ≧−24.221×Δ 3 +51.718×Δ 2 −40.786×Δ+16.878 is satisfied and Δ is equal to or larger than 0.32%, where Δ is a relative refractive index difference in percent of the core region with respect to the cladding region, and a is a core diameter in μm of the core region, noncircularity of the core region is smaller than 10%, and a leakage loss at a wavelength of 1550 nm is 0.01 dB/km or less.