Abstract: A multicore optical fiber that includes: a core that is formed of silica glass doped with at least fluorine, the core having a relative refractive index difference with respect to a refractive index of silica as a reference being ?0.30% to ?0.10%; and a clad that is formed of silica glass doped with at least fluorine. The clad includes a first clad surrounding an outer periphery of the core and a second clad provided outside the first clad. A relative refractive index difference between the first clad and the core is 0.8% or more. A refractive index of the second clad is higher than a refractive index of the first clad and lower than a refractive index of the core.

Abstract: A polarization-maintaining optical fiber of the present invention includes a core, a pair of stress-applying parts provided on both sides of the core, and a cladding surrounding the core and the stress-applying parts, and is used in a wavelength range of 400 to 680 nm. The diameter of the cladding is 125 ?m, the diameter of the stress-applying part is 33 to 37 ?m, a distance between the pair of stress-applying parts is 8.6 to 15.4 ?m, a relative refractive index difference between the core and the cladding is 0.35 to 0.45%, and a cut-off wavelength is less than or equal to 400 nm.

Abstract: The polarization-maintaining fiber of the invention includes a core (1) made of germanium doped silica glass; a stress-applying part (3) made of boron doped silica glass; a cladding (2) made of pure silica glass; and a polyimide coating layer (4) with a thickness of 10 ?m or less that surrounds the outer periphery of the cladding (2).

Abstract: The polarization-maintaining fiber of the invention includes a core (1) made of germanium doped silica glass; a stress-applying part (3) made of boron doped silica glass; a cladding (2) made of pure silica glass; and a polyimide coating layer (4) with a thickness of 10 ?m or less that surrounds the outer periphery of the cladding (2).