Control of zero-crossing temperature of coefficient of thermal expansion and reduction of mechanical residual stress for TiO2–SiO2 glass optical cavity

We developed a procedure for fabricating a Fabry–Perot optical cavity with less long-term length variation. We fabricated a homogenized TiO2–SiO2 ultralow-expansion (ULE) glass ingot from a commercial TiO2–SiO2 ULE glass ingot with striae. We also controlled the zero-crossing temperature of the coefficient of thermal expansion of the homogenized ingot to approximately 30 °C by heat treatment at 970 °C for 72 h to change the fictive temperature of the ingot. A 100-mm-long cavity spacer was carefully fabricated while reducing the mechanical residual stress introduced by machining. A cavity composed of the spacer and two mirrors of 39-layered Ta2O5/SiO2 films exhibited a high finesse of 420,000 at 729 nm and a high length stability of 13 fm/day after 500 days from the beginning of the measurement at a constant temperature of 29.78 °C.