Method of making glass optical fiber

A method of forming a high purity glass article comprising the steps of

• providing a substantially cylindrical mandrel,
• applying to said mandrel a stratum of particulate glass having a relatively low coefficient of expansion,
• applying a first coating of particulate glass to the outside peripheral surface of said stratum, the coefficient of expansion of said first coating being greater than that of said stratum,
• applying a second coating of particulate glass over the outside peripheral surface of said first coating, said second coating having a refractive index less than that of said first coating,
• removing said mandrel to form a porous preform having an aperture therein,
• heating said porous preform to form an elongated, consolidated glass preform having a longitudinal aperture therethrough,
• etching a surface portion of said stratum glass from the aperture-forming wall of said consolidated preform,
• rinsing the aperture-forming wall of said preform,
• flowing a drying gas through said aperture,
• closing one end of said aperture,
• evacuating said aperture,
• closing the remaining end of said aperture,
• heating at least one end of said consolidated preform and drawing to close said aperture and reduce the diameter of said consolidated preform, thereby forming a large diameter, intermediate fiber,
• adding cladding material to said intermediate fiber, and
• drawing the composite article so formed into an optical waveguide fiber.