A method of forming an optical fiber, including: exposing a soot core preform to a dopant gas at a pressure of from 1.5 atm to 40 atm, the soot core preform comprising silica, the dopant gas comprising a first halogen doping precursor and a second halogen doping precursor, the first halogen doping precursor doping the soot core preform with a first halogen dopant and the second halogen precursor doping the soot core preform with a second halogen dopant; and sintering the soot core preform to form a halogen-doped closed-pore body, the halogen-doped closed-pore body having a combined concentration of the first halogen dopant and the second halogen dopant of at least 2.0 wt %.

A method of forming an optical fiber, including: exposing a soot core preform to a dopant gas at a pressure of from 1.5 atm to 40 atm, the soot core preform comprising silica, the dopant gas comprising a first halogen doping precursor and a second halogen doping precursor, the first halogen doping precursor doping the soot core preform with a first halogen dopant and the second halogen precursor doping the soot core preform with a second halogen dopant; and sintering the soot core preform to form a halogen-doped closed-pore body, the halogen-doped closed-pore body having a combined concentration of the first halogen dopant and the second halogen dopant of at least 2.0 wt %.

On an EUV light-reflecting surface of titania-doped quartz glass, an angle () included between a straight line connecting an origin (O) at the center of the reflecting surface to a birefringence measurement point (A) and a fast axis of birefringence at the measurement point (A) has an average value of more than 45 degrees. Since fast axes of birefringence are distributed in a concentric fashion, a titania-doped quartz glass substrate having a high flatness is obtainable which is suited for use in the EUV lithography.

A glass body that includes titania and silica, wherein an average hydroxyl concentration amongst a plurality of segments of the glass body is from about 0 ppm to about 100 ppm and a peak-to-valley of hydroxyl concentration amongst the plurality of segments is about 60 ppm or less, the hydroxyl concentration being measured using a Fourier transform infrared spectroscopy in transmission, and the plurality of segments including every adjacent segment across a length and a width of the glass body, the length being about 25 mm or more and the width being about 25 mm or more, and the glass body comprising a chlorine concentration of about 100 ppm or less.

The present invention relates to a silica glass substrate including: a first main surface and a second main surface that are facing each other, in which the silica glass substrate has a density of 2.0 g/cm3 or less, the silica glass substrate includes a plurality of bubbles, the silica glass substrate has an average diameter of first recessed portions of 30 m or less, the first recessed portions being formed by the bubbles exposed on the first main surface, and the silica glass substrate has the number of the first recessed portions of 200/mm2 or less on the first main surface.

Titania-doped quartz glass is manufactured by mixing a silicon-providing reactant gas and a titanium-providing reactant gas, preheating the reactant gas mixture at 200-400 C., and subjecting the mixture to oxidation or flame hydrolysis. A substrate of the glass is free of concave defects having a volume of at least 30,000 nm.sup.3 in an effective region of the EUV light-reflecting surface and is suited for use in the EUV lithography.

On an EUV light-reflecting surface of titania-doped quartz glass, an angle () included between a straight line connecting an origin (O) at the center of the reflecting surface to a birefringence measurement point (A) and a fast axis of birefringence at the measurement point (A) has an average value of more than 45 degrees. Since fast axes of birefringence are distributed in a concentric fashion, a titania-doped quartz glass substrate having a high flatness is obtainable which is suited for use in the EUV lithography.