A light guide is produced by the out-diffusion of boron oxide through a free surface of a uniform borosilicate mass. The resulting structure consists of increased index silica rich regions near the surface in contact with lower index regions of a composition approaching that of the unmodified borosilicate. The out-diffusion process, generally carried out at a temperature approaching the softening point of the glass, may be applied to a borosilicate tube to result in a fiber preform. Fibers drawn from such a preform may be characterized by a silica core region, a borosilicate clad region, and a silica outer shell. Use of the procedure results in a graded index structure.