Abstract: An optical fiber with a graded index crystalline core (2,3) is made by filling a tube (1) with a mixture of two salts for which the one with the higher melting point has the lower refractive index. A graded composition results from partition effects when the fused salts are slowly cooled so that solidification proceeds uniformly inwardly from the tube wall. Fiber is pulled conventionally from the reesulting preform, and later is passed through a short hot zone to convert the core into single crystal form.

Abstract: An optical glass fiber is formed of an inner layer of germania doped glass within the bore of the outer cladding tube of silica. The tube with the higher index of refraction inner layer is rotated while being heated to collapse the bore to form an optical fiber preform. The preform is then drawn into fiber in a separate operation. A first silica layer can be deposited within the tube bore before the germania doped layer. The deposition of germania takes place under hydrogen free conditions. Appropriate heat treatment of the tube avoids excessive evaporation of germania and provides a graded transition of the refractive index in the boundary between core and cladding.