Design of an optical reference cavity with flexible thermal expansion tuning properties

An ultra-stable optical cavity is designed as the frequency reference of a clock laser for an Al+ optical clock by finite element analysis (FEA). The cavity has a structure with fused silica (FS) mirrors contacted to FS rings and re-entrant inside the ULE spacer. This design yields a low thermal noise limit and a flexible tuning range of the total coefficient of thermal expansion (CTE) which is suitable for applications with ULE materials that have various zero-crossing temperatures of CTE from 5 °C to 35 °C, and for applications for cavities with various dimensions and configurations.