Process for producing silica glass plate having controlled refractive index distribution

Claim 1

A process for producing a silica glass plate having a controlled refractive index from a porous silica gel plate produced by the sol-gel method, which comprises the steps of: (a) placing said porous silica gel plate in a reactor and reducing the pressure in said reactor to a substantially vacuum state to thereby remove water within said porous silica gel plate, the step being carried out at a temperature of 15° C. to 250° C., (b) introducing germanium tetrachloride gas into said reactor so as to directly contact said porous silica gel plate in a gaseous state at a reactor partial pressure appropriate to establish an absorption equilibrium between a desired concentration of germanium tetrachloride in said porous silica gel plate and the germanium tetrachloride gas introduced into said reactor, to thereby dope said porous silica gel plate and maintaining said partial pressure of germanium tetrachloride for a period of time sufficient to achieve the absorption equilibrium, the step being carried out at a temperature of 15° C. to 50° C., (c) reducing the partial pressure of germanium tetrachloride in step (b) to thereby desorb germanium tetrachloride from the surface of the porous silica gel plate and form a concentration distribution gradient of germanium tetrachloride between a higher level in the interior of said porous silica gel plate and a lower level in the outward portions thereof, the step being carried out at a temperature of 15° C. to 50° C. (d) contacting said porous silica gel plate having said concentration distribution with water within or outside the reactor to thereby hydrolyze the germanium tetrachloride and fix it within said porous silica gel plate as germanium oxide, the step being carried out at a temperature of 10° C. to 100° C., drying the porous silica gel plate doped with germanium oxide, and (e) calcining said porous silica gel plate from step (d) at a temperature of from 900° C. to 1200° C. to render it nonporous to thereby form a silica glass plate having a controlled refractive index.