Fabrication of lightguide soot preforms

A method of forming a lightguide preform by vapor-phase axial deposition comprising the steps of:

• feeding glass-forming reactants as a gas through a double-flame torch;
• directing gases through the torch toward a support structure to form an outer flame front between an outermost gas and an intermediate gas, said outer flame front having a first temperature;
• directing an additional gas through the torch toward the support structure to form an inner flame front between an intermediate gas and said additional gas, said inner flame front having a second temperature and being substantially surrounded by said outer flame front;
• forming a stream of glassy soot by directing said reactants through the torch and substantially surrounded by the outer flame front toward the support structure, said reactants being of appropriate composition and having appropriate properties to react at the temperatures at which they are subjected so as to form a glassy soot stream of glassy soot particles, part of which is deposited on the support structure to form a sootform;
• consolidating the soot-form to form a lightguide preform, wherein:
• said second temperature of the inner flame front is sufficiently higher than said first temperature of the outer flame front so as to establish a significant temperature gradient at the location on the support structure at which glassy soot is deposited, which gradient decreases in temperature in a direction from the center of the double flame toward the outside of the double flame, and is sufficiently pronounced that excess non-deposited glassy soot particles of the glassy soot stream are directed away from the support structure by thermophoretic forces.