Core/clad glass optical fiber preforms free of bubbles and soot at the coclad interface are fabricated by inserting a glass core rod into a cladding glass tube sized so that space remains between them, sealing the top and bottom of the tube onto the core rod to form a sealed space between...http://www.google.ca/patents/US5735927?utm_source=gb-gplus-sharePatent US5735927 - Method for producing core/clad glass optical fiber preforms using hot isostatic pressing 
