Abstract: A method for manufacturing an optical fiber preform and fiber. According to the method, a first glass rod is formed, preferably by an OVD method, with a refractive index delta preferably between 0.2% and 3%. A glass sleeve tube is formed, preferably by an MCVD or PVCD method. The first glass rod is inserted into the sleeve and an alkali metal vapor is flowed between the sleeve tube and the first glass rod. Additional glass may optionally be formed on the inside surface of the sleeve tube prior to inserting the first glass rod and flowing the alkali metal vapor. The additional glass may be up-doped, down-doped, or both. The sleeve may then be collapsed onto the first glass rod to form a second glass rod doped with an alkali metal oxide. The second glass rod is drawn to form a third glass rod. Additional glass may then be formed on the third glass rod to form an optical fiber preform from which optical fiber may be drawn.

Abstract: Disclosed is an optical fiber having a core with an alkali metal oxide dopant in an peak amount greater than about 0.002 wt. % and less than about 0.1 wt. %. The alkali metal oxide concentration varies with a radius of the optical fiber. By appropriately selecting the concentration of alkali metal oxide dopant in the core and the cladding, a low loss optical fiber may be obtained. Also disclosed are several methods of making the optical fiber including the steps of forming an alkali metal oxide-doped rod, and adding additional glass to form a draw perform. Preferably, the draw preform has a final outer dimension (d2), wherein an outer dimension (d1) of the rod is less than or equal to 0.06 times the final outer dimension (d2). In a preferred embodiment, the alkali metal oxide-doped rod is inserted into the centerline hole of a preform to form an assembly.