Patent ID: 8192808

Claim:
An apparatus for manufacturing an optical preform via an internal vapor deposition process, comprising: a hollow quartz substrate tube having (i) a supply side through which glass-forming precursors are supplied to the interior of said substrate tube and (ii) a discharge side from which glass-forming precursors that have not been deposited on the interior of said substrate tube are discharged; an energy source that is movable relative to the length of said substrate tube between a reversal point at said substrate tube's supply side and a reversal point at said substrate tube's discharge side; wherein the improvement comprises an insert tube having a central portion and a truncated cone portion, the maximum external diameter of said central portion being less than the minimum internal diameter of said substrate tube and greater than the minimum external diameter of said truncated cone portion; wherein said insert tube is disposed within the interior of said substrate tube, said insert tube being positioned at the discharge side of said substrate tube such that said truncated cone portion faces said substrate tube's supply side and said central portion faces said substrate tube's discharge side.