Methods, apparatus and precursors for producing substantially water-free silica
soot, preforms and glass. The methods and apparatus make substantially water-free fused silica preforms or glass by removing water as a
reaction product, removing water from the
atmosphere, removing water from the transport process, or combinations thereof. In a first embodiment, substantially water-free
soot, preforms or glass are achieved by using a
hydrogen-free fuel, such as
carbon monoxide, in the
deposition process. In another embodiment, a
soot producing burner has parameters that enable operation on a substantially
hydrogen-free fuel. End burners, which minimize
water production, are also described. Such water-free methods are useful in depositing
fluorine-doped soot because of the low water present and the efficiency in which
fluorine is incorporated. In another embodiment, glassy
barrier layer methods and apparatus are described for minimizing
dopant migration, especially
fluorine.
Laser and induction methods and apparatus for forming the
barrier layer are depicted. A
chlorine, fluorine and silica precursor, such as chlorofluorosilane, may be utilized to form fluorinated soot. Other methods and apparatus are directed to combinations of conventional and substantially water-free processes. One embodiment is directed to
combustion enhancing additives for addition to the substantially
hydrogen-free fuels. The methods and apparatus in accordance with the invention are particularly useful for producing
photomask substrates and
optical fiber preforms.