Optical fiber

1. An optical fiber comprising:

a core;

an inner cladding surrounding the core, the inner cladding having a refractive index smaller than a refractive index of the core; and

an outer cladding surrounding the inner cladding, the outer cladding having a refractive index smaller than the refractive index of the core and greater than the refractive index of the inner cladding,

wherein a ratio of a caustic radius to a MAC-value at a bending radius of 10 mm at a wavelength of 1625 nm is 2.70 μm or more.

2. The optical fiber according to claim 1 , having a mode field diameter of 8.2 μm or more and 9.9 μm or less at a wavelength of 1310 nm.

3. The optical fiber according to claim 1 ,

wherein an outer radius b of the inner cladding is 15.5 μm or more and 22.5 μm or less,

a value Δ dep obtained by subtracting a maximum relative refractive index difference of the outer cladding from an average relative refractive index difference of the inner cladding is −0.11% or more −0.03% or less, and

the MAC-value is 6.26 or more and 7.56 or less.

4. The optical fiber according to claim 1 ,

wherein an outer radius b of the inner cladding is 18.0 μm or more and 22.0 μm or less,

a value Δ dep obtained by subtracting a maximum relative refractive index difference of the outer cladding from an average relative refractive index difference of the inner cladding is −0.09% or more and −0.04% or less, and

the MAC-value is 6.30 or more and 7.25 or less.

5. The optical fiber according to claim 1 , having:

a cable cut-off wavelength of 1260 nm or less; and

a zero-dispersion wavelength of 1300 nm or more and 1324 nm or less.

6. The optical fiber according to claim 1 ,

wherein the core has a diameter of 7.3 μm or more and 9.0 μm or less, and

a value Δ core obtained by subtracting a maximum relative refractive index difference of the outer cladding from an average relative refractive index difference of the core is 0.32% or more and 0.40% or less.

7. The optical fiber according to claim 1 ,

including a position separated by a caustic radius at a bending radius of 15 mm at a wavelength of 1625 nm from a center of the core either in the inner cladding or in a transition portion between the inner cladding and the outer cladding.

8. The optical fiber according to claim 1 ,

including a position separated by a caustic radius at a bending radius of 15 mm at a wavelength of 1625 nm from a center of the core in the outer cladding excluding a transition portion between the inner cladding and the outer cladding,

wherein a refractive index profile defined by a relationship between a distance and a refractive index has a negative slope at the position defined by the caustic radius, the distance being defined as a distance along a direction from the center of the core toward an outer peripheral surface of the outer cladding.

9. The optical fiber according to claim 1 ,

including a position separated by a caustic radius at a bending radius of 15 mm at a wavelength of 1625 nm from a center of the core in the outer cladding excluding a transition portion between the inner cladding and the outer cladding,

wherein a residual stress profile defined by a relationship between a distance and a residual stress has a positive slope at a position defined by the caustic radius, the distance being define as a distance along a direction from the center of the core toward an outer peripheral surface of the outer cladding.

10. The optical fiber according to claim 1 ,

wherein in an outermost part of the outer cladding, a residual stress decreases from a midpoint toward an outer peripheral surface of the outer cladding, the midpoint being located between a boundary of the inner cladding and the outer cladding and the outer peripheral surface of the outer cladding, the outermost part being located on an outer side with respect to the midpoint.

11. The optical fiber according to claim 1 , further comprising

a primary resin layer surrounding the outer cladding.

12. The optical fiber according to claim 11 ,

wherein the primary resin layer has a refractive index of 1.44 or more and 1.51 or less at a wavelength of 546 nm.

13. The optical fiber according to claim 11 , further comprising

a secondary resin layer surrounding the primary resin layer.

14. The optical fiber according to claim 13 ,

wherein an absolute value of a refractive index difference between the primary resin layer and the secondary resin layer at a wavelength of 546 nm is 0.07 or less.

15. The optical fiber according to claim 13 ,

wherein the primary resin layer has a Young's modulus of 1 MPa or less, and

the secondary resin layer has a Young's modulus of 800 MPa or more.