DE2907650

In an optical fiber having a core whose material consists essentially of vitreous silica, said core having a 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 nM which is smaller than that of the core, the improvement wherein the core has an outer layer whose refractive index next is smaller than the refractive index nK of the remaining inner core portion which is constant and greater than the refractive index nM of the jacket, and next lies in the range from nK-(nK -nM). (0.25 to 0.8) and whose thickness amounts to between 0.8 mu m and 8 mu m.