| 摘要 |
A radius of a first core 21 in a large-diameter end surface EF1 of a tapered portion 31 is denoted by r1S, a radius of a second core 22 is denoted by r2S, a relative refractive index difference of the first core 21 with respect to a clad 23 is denoted by &Dgr;1, a relative refractive index difference of the second core 22 with respect to the clad 23 is denoted by &Dgr;2, a refractive index volume of the first core 21 is denoted by V1S, and a refractive index volume of the second core 22 is denoted by V2S, r2S/r1S is set to be 3 or more and 5 or less, V2S/V1S is set to be 1.07r22−13.5 or more and 1.07r22−11.5 or less, and r2S/r1S is set to be −3×&Dgr;1/&Dgr;2+10 or more. |
