摘要：

A clad part (12) arranged periodically with a plurality of columnar voids (14) having a diameter d at a central interval Λ is provided around a core part (11) having a geometrical diameter ρ where the central interval Λ of the voids (14) is set not shorter than 1.5 times the wavelength λ of a propagation light and a value V determined according to a following expression is set between 2.4 and 3.3. At least one of the geometry of the voids (14) at the clad part (12) and the optical constant distribution of the clad part (12) or the core part (11) is set less than trigonal-axis symmetry with respect to the central axis of the core part (11), thus releasing polarization degeneracy of the propagation light. V=2πρ(n2core-n2eff)1/2/λ (neff: effective refractive index at the clad part, ncore: refractive index at the core part).

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