A method for generating a diameter-enlarged end on an optical fiber, includes placing a longitudinal subsection of a longitudinal section of the fiber into a heating zone and heating the longitudinal subsection, wherein first and second sides of the longitudinal section on either side of the longitudinal subsection are situated outside the heating zone; compressing the heated longitudinal subsection in a longitudinal direction of the optical fiber; pushing the first side of the longitudinal section toward the heating zone in the longitudinal direction and pulling the second side of the longitudinal section away from the heating zone in the longitudinal direction, wherein the first side of the longitudinal section is pushed to a greater degree than the second side of the longitudinal section is pulled, and generating an optical entry surface of the fiber by cutting the enlarged longitudinal subsection transversely to the longitudinal direction of the fiber.

A polarization maintaining fiber includes a core, paired stress applying parts disposed on both sides of the core, and a clad encompassing the core and the paired stress applying parts. When the polarization maintaining fiber has a fiber length of 2 m and a bend radius of 140 mm, the polarization maintaining fiber has a cut-off wavelength equal to or greater than 1.41 m and less than 1.55 m, and when the polarization maintaining fiber has a bend radius of 5 mm and twists at a rate of one rotation per 31.4 mm of fiber length, the polarization maintaining fiber has a bending loss equal to or less than 7 dB at a wavelength of 1.55 m.

The optical fiber includes a glass fiber and a coating resin layer. The coating resin layer includes a primary resin layer and a secondary resin layer. The glass fiber has an outer diameter of from 124 m to 126 m. The secondary resin layer has an outer diameter of from 145 m to 170 m. The primary resin layer has an in situ elastic modulus of from 0.1 MPa to 0.5 MPa. The secondary resin layer has an in situ elastic modulus of from 1200 MPa to 2800 MPa. A maximum value of amplitude of an amount of eccentricity is 6 m or less in a spectrum obtained by measuring the amount of eccentricity of the glass fiber and by applying Fourier transform to a waveform representing the amount of eccentricity.

The optical fiber includes a glass fiber and a coating resin layer. The coating resin layer includes a primary resin layer and a secondary resin layer. The glass fiber has an outer diameter of from 79 m to 81 m. The secondary resin layer has an outer diameter of from 120 m to 170 m. The primary resin layer has an in situ elastic modulus of from 0.1 MPa to 0.4 MPa. The secondary resin layer has an in situ elastic modulus of from 1200 MPa to 2800 MPa. A maximum value of amplitude of an amount of eccentricity is 10 m or less in a spectrum obtained by measuring the amount of eccentricity of the glass fiber and by applying Fourier transform to a waveform representing the amount of eccentricity.