High bandwidth optical waveguide having B.sub.2 O.sub.3 free core and method of fabrication

The method of forming a high bandwidth optical filament comprising the steps of

• providing a cylindrical tubular starting member formed of a base glass and at least one dopant, the index of refraction of said tubular starting member being in excess of the index of refraction of said base glass,
• forming a barrier layer on the inside wall surface of said tubular starting member consisting essentially of a base glass, and at least one additional dopant,
• maintaining the quantity of said base glass and dopant within said barrier layer substantially uniform throughout the thickness of said barrier layer, the index of refraction of said barrier layer being equal to or less than the index of refraction of said tubular starting member,
• forming a B.sub.2 O.sub.3 free core layer having a gradient index of refraction over said barrier layer thereby forming an interface therebetween, said core layer having at said interface an index of refraction approximately equal to or less than that of said barrier layer, there being no step increase in the index of refraction of the core at said interface, said B.sub.2 O.sub.3 free core layer consisting essentially of a high purity base glass and at least one dopant,
• simultaneously varying the quantity of said one or more dopants gradually from said interface toward the central axis of the article so formed in a predetermined manner so as to result in a desired gradient index of refraction across the cross section of said core layer,
• heating the article so formed to a temperature sufficient to collapse the central hole thereby forming a solid preform,
• heating the structure so formed to the drawing temperature of the materials thereof, and
• drawing the heated structure to reduce the cross section of area thereof to form an optical filament having predetermined desired characteristics.