A method of manufacturing an optical waveguide perform includes exposing a soot preform to an atmosphere including a chlorine-containing compound and thereby doping the soot preform with chlorine, wherein the absolute pressure of the atmosphere is greater than about 1.013null102 kPa. An apparatus for manufacturing an optical waveguide preform using a soot preform includes a furnace defining a chamber adapted to contain the soot preform and including a heating device operable to heat the chamber. A fluid control system is operable to provide an atmosphere including a chlorine-containing compound in the chamber at an absolute pressure of greater than about 1.013null102 kPa.