Patent ID: 7076139

Claim:
An optical fiber provided with a refractive index profile having a central core; a middle part provided around the outer periphery of said central core and having a lower refractive index than that of said central core; and a cladding provided around the periphery of said middle part and having a higher refractive index than said middle part and a lower refractive index than said central core; wherein said optical fiber has an effective core area of at least 120 Î¼m 2 in an employed wavelength band selected from the range of 1.53 to 1.63 Î¼m, and has a cut-off wavelength that is capable of substantially single mode propagation in said employed wavelength band, and when the radius of the central core is designated as r 1 and the radius of the middle part is designated as r 2 , then 3.0â‰¦r 2 /r 1 â‰¦5.0, and, when specific refractive index differences for the central core and the middle part are designated as Î” 1 and Î” 2 respectively where the refractive index of the cladding is taken as the standard, then Î” 1 is at most 0.30% and Î” 2 is âˆ’0.05 to âˆ’0.15%.