A single mode optical transmission line consisting of a core and a clad, both formed from essentially the same type of multicomponent glass, e.g. borosilicate, for use with electromagnetic energy having a wavelength between 0.5.mu.m and 2.0.mu.m. The core has a higher refractive index than the clad because the relative concentrations of the constituents of the glass composition are varied between core and clad, e.g., the ratio of SiO.sub.2 /B.sub.2 O.sub.3 concentrations is higher in the core than in the clad.