Method of making optical devices

A method of forming a clad optical waveguide comprising the steps of

• providing a substantially cylindrical mandrel,
• applying a first coating of glass soot to the outside peripheral surface of said mandrel,
• applying a second coating of glass soot over the outside peripheral surface of said first coating, the glass soot of the second coating having a refractive index less than that of the glass soot of the first coating, the viscosities V.sub.1 and V.sub.2 of said first and second coatings of glass soot, respectively, being such that the ratio V.sub.1 /V.sub.2 is between 1/2 and 1/2000 at that temperature at which the glass soot of the second coating reaches a viscosity of 10.sup.10 poise,
• removing said mandrel to form a soot preform having an aperture therein,
• suspending said preform vertically from one end thereof,
• gradually inserting said preform into a furnace, the temperature and rate of insertion being such that gradient consolidation of said preform occurs while said aperture simultaneously closes, and
• heating the structure so formed to the drawing temperature of the materials thereof, and drawing the heated structure to reduce the cross-sectional area thereof, thereby forming an optical waveguide.