Graded index MM fiber
In the case of multimode (MM) fibers, it is common to have a graded refractive index.
In the fiber manufacturing process, this graded refractive index profile is usually achieved by doping the silica core with a graded level of germanium. The higher the level of germanium doping, the higher the refractive index in that particular core region.
In the case of germanium-free designs for harsh environments, this graded index profile can also be achieved by down-doping with fluorine.