Patent ID: 7174078

Claim:
An optical fiber for transmitting light, said optical fiber having an axial direction and a cross section perpendicular to said axial direction, said optical fiber comprising: (1) a first core region comprising a first core material having a refractive index N co,1 , (2) a microstructured first cladding region surrounding the first core region, said first cladding region comprising a first cladding material and a plurality of spaced apart first cladding features or elements that are elongated in the fiber axial direction and disposed in the first cladding material, said first cladding material having a refractive index N cl,1 , and each said first cladding feature or element having a refractive index being lower than N cl,1 , whereby a resultant geometrical index N ge,cl,1 of the first cladding region is lowered compared to N cl,1 , (3) a second core region surrounding said first cladding region, said second core region comprising a second core material having a refractive index N co,2 , and (4) a second cladding region surrounding the second core region, said second cladding region comprising a second cladding material having a refractive index N cl,2 , wherein the first core material, the first cladding material and the first cladding features, the second core material, and the second cladding material are selected and arranged so that N co,1 >N ge,cl,1 , N co,2 >N ge,cl,1 , and N co,2 >N cl,2 .