| 摘要 |
A core 10 comprises: an inner core portion 11 containing the central axis C of the core 10; and an outer core portion 12 which surrounds the inner core portion 11 and has a lower refractive index than the inner core portion 11. A clad 20 comprises: an inner clad portion 21 which has a constant refractive index in a radial direction and is in contact with the core 10; and an outer clad portion 22 which surrounds the inner clad portion 21. The refractive index of the inner core portion 11 in a region that is in contact with the boundary with the outer core portion 12 is higher than the refractive index of the outer core portion 12; the refractive index of the outer core portion 12 gradually decreases from the inner periphery side toward the outer periphery side; and the refractive index of the inner clad portion 21 is set to be equal to the refractive index of the outer core portion 12 at the outermost periphery and not higher than the refractive index of the outer clad portion 22. |
