Patent ID: 6859598

Claim:
An optical fiber including a core region and cladding regions of not less than three layers which surround said core region in order, wherein the optical fiber is a single mode optical fiber; and each of said cladding regions has a mean refractive index different from those of the adjacent regions, at least one of said cladding regions has a lower mean refractive index than both adjacent cladding regions, and at least one cladding region is provided with a plurality of sub medium regions each having a refractive index lower than a main medium constituting this cladding region, wherein said core region is constituted by a substantially homogeneous medium and said cladding regions are constituted by an inner cladding region of not less than two layers made of a substantially homogeneous medium and an outer cladding region surrounding said inner cladding region is provided with said sub medium regions, and wherein said inner cladding region has a two layer structure consisting of a first inner cladding region and a second inner cladding region and a following relationship holds among respective refractive indexes n 0 , n 1 , n 2 of said core region, and said first inner cladding region, said second inner cladding region and a mean refractive index n 3 of said outer cladding region: n 0 >n 2 >n 1 >n 3 .