Polarization maintaining optical fiber and polarization maintaining optical fiber manufacturing method

A method of manufacturing a polarization-maintaining optical fiber including at least one polarization maintaining core including a core made of glass and a pair of low-refractive-index portions each having a refractive index lower than a refractive index of the core, an optical cladding surrounding the at least one polarization maintaining core, and a common physical cladding surrounding the optical cladding, wherein inequalities below are satisfied:

• 0.8≤r40/r10≤2.0.
• 0.2≤(d−r40)/r10≤0.6.
• 0.5% ≤Δ10-Δ40≤2.0%, and
• 0.5% ≤Δ10-Δ21≤2.0%,
• where r10 is a radius of the core, r40 is a radius of each of the low-refractive-index portions, and r21 is a radius of the optical cladding,
• where Δ10 is a relative refractive index difference of the core, A40 is a relative refractive index difference of each of the low-refractive-index portions, and A21 is a relative refractive index difference of the optical cladding, with the refractive index of the common physical cladding used as a reference, and
• where d is a distance between a central axis of the at least one polarization maintaining core and a central axis of each of the low-refractive-index portions, the method comprising:
• preparing an optical preform made of glass and having cylindrical symmetry, the optical preform including a core portion and an optical cladding portion;
• preparing low-refractive-index-portion preforms each made of glass and each having cylindrical symmetry, the low-refractive-index-portion preforms each having a thermal expansion coefficient that differs by 5×10−7/K or less from a thermal expansion coefficient of the core portion;
• forming a pair of holes each having a cylindrical shape into the optical preform such that the pair of holes are point-symmetrically arranged with respect to a central axis of the optical preform in a cross section perpendicular to the central axis of the optical preform and have central axes parallel to the central axis of the optical preform;
• inserting each of the low-refractive-index-portion preforms into a corresponding one of the pair of holes of the optical preform and integrating the low-refractive-index-portion preforms with the optical preform by heating the low-refractive-index-portion preforms and the optical preform in such a manner as to form at least one polarization-maintaining optical preform corresponding to the at least one polarization maintaining core;
• and
• spinning the at least one polarization-maintaining optical preform by drawing the at least one polarization-maintaining optical preform after or while providing a common physical cladding portion around the at least one polarization-maintaining optical preform.

A method of manufacturing a polarization-maintaining optical fiber including at least one polarization maintaining core including a core made of glass and a pair of low-refractive-index portions each having a refractive index lower than a refractive index of the core, an optical cladding surrounding the at least one polarization maintaining core, and a common physical cladding surrounding the optical cladding, wherein inequalities below are satisfied:

• 0.8≤r40/r10≤2.0,
• 0.2≤(d−r40)/r10≤0.6,
• 0.5% ≤Δ10-Δ40<2.0%, and
• 0.5% ≤Δ10-Δ21≤2.0%,
• where r10 is a radius of the core, r40 is a radius of each of the low-refractive-index portions, and r21 is a radius of the optical cladding,
• where Δ10 is a relative refractive index difference of the core, Δ40 is a relative refractive index difference of each of the low-refractive-index portions, and Δ21 is a relative refractive index difference of the optical cladding, with the refractive index of the common physical cladding used as a reference, and
• where d is a distance between a central axis of the at least one polarization maintaining core and a central axis of each of the low-refractive-index portions, the method comprising:
• preparing a basic-optical-fiber preform including a core portion made of glass and having cylindrical symmetry, an optical cladding portion surrounding the core portion and having cylindrical symmetry, the optical cladding portion made of glass and having a thermal expansion coefficient that differs by 5×10−7/K or less from a thermal expansion coefficient of the core portion, and a common physical cladding portion surrounding the optical cladding portion and made of glass, an outer periphery of the common physical cladding portion having translational symmetry along a predetermined axis, and the common physical cladding portion having a thermal expansion coefficient that differs by 5×10−7/K or less from the thermal expansion coefficient of the core portion,
• forming a pair of holes each having a cylindrical shape into the basic-optical-fiber preform, the pair of holes being point-symmetrically arranged with respect to a central axis of the core portion in a cross section perpendicular to the central axis of the core portion and having central axes parallel to the central axis of the core portion, such that at least a portion of each of the pair of holes is formed across the core portion and the optical cladding portion,
• preparing low-refractive-index-portion preforms each made of glass and each having cylindrical symmetry, the low-refractive-index-portion preforms each having a thermal expansion coefficient that differs by 5×10−7/K or less from the thermal expansion coefficient of the core portion and from a thermal expansion coefficient of the common physical cladding portion, and outer diameters of the low-refractive-index-portion preforms being adjusted such that each of the low-refractive-index-portion preforms is insertable into a corresponding one of the pair of holes, and
• inserting each of the low-refractive-index-portion preforms into the corresponding one of the pair of holes of the basic-optical-fiber preform and spinning the low-refractive-index-portion preforms and the basic-optical-fiber preform by drawing the low-refractive-index-portion preforms and the basic-optical-fiber preform after or while integrating the low-refractive-index-portion preforms with the basic-optical-fiber preform.