Patent ID: 6842566

Claim:
An optical fiber with a built-in rating, comprising: a core having a refractive index n 0 ; an inner cladding that surrounds the core and has a refractive index n 1 ; an intermediate cladding that surrounds the inner cladding and has a refractive index n 2 ; and an outer cladding that surrounds the intermediate cladding and has a refractive index n 3 , wherein n 0 >n 3 ≧n 1 >n 2 , grating is formed at least in a part of the inner cladding, and the relative refractive index difference between the inner cladding and the intermediate cladding, (n 2 −n 1 )/n 1 , is more than −1.0% and less than or equal to 0.3%.