Two types of high-purity synthetic silica glasses were annealed with different processes in precision annealing furnace.The thermal stress and structure of samples were analyzed by two types of stress instruments and Fourier transform infrared spectrometer.After being annealed at 1 070 ℃ for 6 days and then cooled slowly,optical path difference(OPD) caused by stress in the center and edge of Type Ⅲ silica glass with a diameter of 150 mm and thickness of 50 mm decreased from 6 to 2 nm/cm.Meanwhile,with the same annealing process,fictive temperature of small-size Type Ⅲ glass decreased to 939 ℃,and structural stability of silica glass was improved.In addition,after being annealed at 1 100 ℃ for 5 hours and then cooled slowly,internal stress in Type Ⅳ silica glass with a thickness of 1 mm was basically eliminated,and its fictive temperature decreased from 1 421 to 966 ℃.
