Method of preparing a foreproduct for the production of an optical lightconductor

In a process for preparing a foreproduct useful in the production of an optical, self-focusing lightconductor in which the index of refraction of the lightconductor material diminishes with increasing distance from the conductor axis such that, when plotted against the conductor diameter, the refractive index curve is parabolic and the apex of the parabola lies over the center of the conductor diameter wherein a plurality of layers of doped silica are deposited on a cylindrical support, said layers being formed of a mixture of a vaporous silicon compound and a vaporous doping agent at elevated temperature, the improvement which comprises depositing at least one layer of undoped silica between layers of doped silica and following each deposition of each doped or undoped silica layer heating the deposited silica to a temperature of at least 900.degree. C. to vitrify the same, each layer being deposited in a thickness ranging from 0.1 to 20 .mu.m, such that the number of layers of undoped silica which are to be deposited on a layer of doped silica before another layer of doped silica is deposited increases with increasing distance from the lightconductor axis if a doping agent is used which produces an increase in the refractive index of vitreous silica and decreases with increasing distance from the lightconductor axis if a doping agent is used which produces a decrease in the refractive index of vitreous silica.