Titania-doped quartz glass and making method

A method for manufacturing a titania-doped quartz glass using a horizontal furnace comprising a burner, the burner comprising a central multi-fold tube section including at least three tubes, a reactant gas injecting central tube, a first combustion-supporting gas feed tube outside the central tube, and a first combustible gas feed tube outside the first combustion-supporting gas feed tube, and a multi-nozzle section including a first shell tube enclosing the central multi-fold tube section, a second shell tube enclosing the first shell tube, and a plurality of combustion-supporting gas feed tubes disposed inside the first and second shell tubes, the spaces surrounding the combustion-supporting gas feed tubes serving as combustible gas feeder,

• said method comprising the steps of subjecting a silicon-providing reactant gas and a titanium-providing reactant gas to oxidation or flame hydrolysis with the aid of a combustible gas and a combustion-supporting gas, to form silica and titania fine particles, depositing the silica and titania fine particles on a rotating target, concurrently melting and vitrifying the particles to form a titania-doped quartz glass ingot, wherein
• the silicon-providing reactant gas, the titanium-providing reactant gas, and the combustion-supporting gas are fed through the reactant gas injecting central tube in such amounts that the titania-doped quartz glass may have a titania content of 3 to 10% by weight,
• the combustible gas is fed at a linear velocity of less than or equal to 100 m/sec, and
• the combustible gas and the combustion-supporting gas are fed in such amounts that a H2/O2 ratio may fall in the range:
• 1.7≦H2/O2<2, wherein
• the flow rates of the combustible gas, the combustion-supporting gas, the silicon-providing reactant gas, and the titanium-providing reactant gas each are controlled within a variation of ±1%,
• the temperatures of air fed into the furnace for cooling, exhaust gas taken out of the furnace, and ambient air surrounding the furnace each are controlled within a variation of ±2.5° C.,
• the distance between the burner and the ingot growth face is 250 to 330 mm,
• the angle included between the ingot growth axis and the axis of the reactant gas injecting tube of the burner is 126° to 140°,
• the target is rotated at a speed of 5 to 200 rpm without relative oscillation of the burner and the target and without oscillation of the ingot in a plane perpendicular to the growth axis direction,
• said method further comprising the steps of hot shaping the ingot, slicing the ingot to a predetermined thickness, and annealing at a cooling rate of 1 to 20° C. to or below 300° C.