Optical fiber

In an optical fiber having a core consisting essentially of vitreous silica, said core having jacket of plastic which is optically effective at least on a length of 10 m reckoned from the light input end of the fiber and which has a refractive index n.sub.M which is smaller than that of the core, the improvement wherein said core consists essentially of an inner portion and an outer portion, which are integral with one another and differ with respect of their refractive indices, said outer portion consisting essentially of vitreous silica having a refractive index n.sub.ext which is smaller than the refractive index n.sub.K of the remaining inner portion but greater than the refractive index n.sub.M of said jacket, said vitreous silica of said core consisting essentially of synthetic vitreous silica made from silicon halides, the OH-content of said synthetic vitreous silica amounting to less than 10 ppm, and wherein n.sub.ext lies in the range from n.sub.K -(n.sub.K -n.sub.M).multidot.(0.25 to 0.8), the thickness of said outer portion amounts between 0.8 .mu.m to 8 .mu.m and the thickness of said inner portion is greater than the thickness of said outer portion.