Polarization maintaining optical fiber with improved polarization maintaining property

A polarization maintaining optical fiber, comprising:

• a core;
• and
• a photonic crystal structure cladding provided in a surrounding of the core and having a diffraction grating with a lattice interval Λ for realizing confinement of light within the core, the photonic crystal structure cladding being divided into four divided portions along a circumferential direction, only lattice holes closest to the core among lattice holes in a first pair of divided portions that are facing each other along one direction having a larger lattice hole diameter d2 than a smaller lattice hole diameter d1 of all other lattice holes among the lattice holes in the first pair of divided portions, all lattice holes in a second pair of divided portions that are facing each other along another direction orthogonal to the one direction having the smaller lattice hole diameter d1, and in a graph in which a vertical axis represents d1/Λ and a horizontal axis represents d2/Λ, a ratio of the larger lattice hole diameter d2 and the smaller lattice hole diameter d1 takes a value within a region enclosed by four straight lines given by four equations:
• d1/Λ=d2/Λ
• d1/Λ=0.2(d2/Λ)
• d1/Λ=d2/Λ−0.20
• d2/Λ=1.0
• as a condition for realizing a single peak in a light intensity distribution in a vicinity of the core.