a method of fabricating an optical fiber incorporating a volatile constituent , involving : providing a preform comprising a cladding glass having an axial aperture and a core glass arranged in the axial aperture , wherein the working temperature of the core glass lies below the working temperature of the cladding glass ; and drawing the preform into an optical fiber at a drawing temperature that lies between the working temperatures of the core and cladding glasses and above the softening temperature of the cladding glass , wherein the core glass prior to drawing includes a dioxide or higher oxide compound of the volatile constituent having a gibbs free energy of disassociation into a monoxide compound of the volatile constituent that is negative at the drawing temperature , whereby the dioxide or higher oxide compound tends to disassociate into the monoxide compound during drawing . the volatile constituent may be sn or pb . the method may also be adapted for incorporating p as the volatile constituent . with this method , the core material melts while the cladding glass remains solid but in a deformable state . melting the core material , provides more freedom in the choice of combinations of core and cladding glasses . the starting material for the core may be a powder or a solid rod , i . e . the invention may be embodied as a powder - in - tube or a rod - in - tube method .