Patent ID: 7532797

Claim:
An optical fiber comprising at least: a central core portion having a maximum refractive index N 1 and an outer diameter 2 a ; a depressed portion, provided on an outer periphery of said central core portion, having a minimum refractive index N 2 and an outer diameter 2 b ; and a cladding portion, provided on an outer periphery of said depressed portion, having a maximum refractive index N 3 , wherein said respective maximum refractive indices of said central core portion, said depressed portion and said cladding portion satisfy the following relationship: N 1 >N 3 >N 2 , wherein a relative refractive index difference Δ 1 of said central core portion with respect to said cladding portion is larger than 1.0%, and a relative refractive index difference Δ 2 of said depressed portion with respect to said cladding portion is smaller than −0.3%, and wherein, at a wavelength of 1.05 μm, the second derivative β 2 of the propagation constant β with respect to frequency ω is positive, the third derivative β 3 of the propagation constant β with respect to frequency ω is negative, and the ratio (β 3 /β 2 ) of the third derivative β 3 to the second derivative β 2 is −0.002 ps or less.