Optical fiber preforms which can be drawn into optical fibers of desired dimensions are fabricated by applying a vacuum to a cladding tube and drawing molten glass from a crucible into a bore of the cladding tube while a portion of the cladding tube is within a furnace preferably through a small hole in the top of the furnace. The method and apparatus are particularly applicable to highly non-linear fiber (HNLF) glasses and highly doped or rare earth glasses since materials therein are generally expensive and only a small quantity of molten glass is required but can be applied to virtually any optical fiber construction where the core glass has a lower melting or softening point than that of the cladding tube. Sources of contamination, breakage and other preform defects are substantially avoided and toxic substances, if present are readily confined.