A fiber preform structure, preferably of an optical glass for use in making fibers and lenses, constructed with a central core of a first material, a surrounding tube of a second material, and a deeply placed bonded layer integrally formed between the core and tube preferably by a heat driven interdiffusion of the first and second materials. The deeply placed interface layer of the resulting preform structure exhibits material characteristics related to the interdiffused material characteristics of the rod and tube materials. The interdiffusion is preferably performed while supporting the combined rod and tube structure. The preform is rotated during heating to maintain the geometric symmetry of the preform and of the interface layer. An encapsulating carrier is used to support the preform in all dimensions during heating.