A method and apparatus is disclosed for the manufacture of an
optical fiber preform having incorporated therein a comparatively
high concentration of
rare earth metal dopant material, and which thus can be drawn and processed into an
optical fiber having low
numerical aperture, low core attenuation, and high pumping
power absorption. The high concentrations of
rare earth metal dopant material are accomplished through a ''
hybrid vapor
processing'' (HVP) method or a ''
hybrid liquid
processing'' (HLP) method, either being practiced in combination or independently of one another. The HVP method involves the
vaporization of a
rare earth metal halide by the
exposure thereof to a sufficiently elevated temperature, independently, or contemporaneously with the transport of the
resultant rare earth metal
halide laden vapor, into a glass-forming oxidation
reaction zone on a flowing
stream of essentially an unreactive
inert gas, such as
helium. According to the HLP method, a first amount of rare earth metal
dopant is provided according to the HVP method and / or other vapor source of rare earth metal dopant which is mixed with glass-forming vapors to form a deposited
soot layer on the internal surface of a
glass tube. The
soot-deposited tube is then impregnated with a dopant solution comprising, a second amount of rare earth metal dopant. The tube is then thermally collapsed resulting in an optical preform with an enhanced amount of rare earth metal dopant incorporated at a comparatively
high concentration. The apparatus comprises means, such as tubes, for introducing the rare earth metal dopant as a vapor, formed from a
solid state form of the dopant, into the main glass deposition tube separately from glass-forming material vapors and
oxygen for the reaction within the main tube.