Method for making halogen doped optical element

A method of forming an optical element, the method comprising:

• producing silica-based soot particles using chemical vapor deposition, the silica-based soot particles having an average particle size between about 0.05 microns and about 0.25 microns and a particulate surface area greater than about 20 m2/g, and the silica-based soot particles having a titania content from about 1.0 wt. % to about 14 wt. %;
• pressing the silica-based soot particles with a pressing mechanism to form a soot compact from the silica-based soot particles, the pressing mechanism asserting a pressure on the silica-based soot particles of less than about 1000 psi, the soot compact having a density from about 0.50 g/cc to about 1.20 g/cc;
• doping the soot compact with a halogen in a closed system to form a doped soot compact, the doped soot compact comprising the halogen,
• wherein, during the doping of the soot compact with the halogen, maintaining a total pressure in the closed system of about 0.25 atm or greater;
• consolidating the doped soot compact by simultaneously increasing the temperature of the doped soot compact and decreasing a concentration of a dopant containing gas in the closed system, while still actively flowing the dopant containing gas in the closed system, to form a glass article;
• and
• annealing the glass article to form the optical element,
• wherein the optical element has a slope of coefficient of thermal expansion versus temperature at 20° C. of less than about 1.0 ppb/K2.