Design of an optical reference cavity with low thermal noise limit and flexible thermal expansion properties

An ultra-stable optical reference cavity with re-entrant fused silica mirrors and a ULE (Ultra Low Expansion) spacer structure is designed for the ultranarrow clock laser for an Al + optical standard. The designed cavity has a low thermal noise limit of 1 × 10 -16 and a flexible zero crossing temperature of the effective coefficient of thermal expansion (CTE). The CTE zero crossing temperature difference between a composite cavity and a pure ULE cavity can be tuned from −10°C to 23°C, which enables operation of the designed reference cavity near room temperature without worrying about the CTE zero crossing temperature of the ULE spacer. This design can also be applied to cavities with different lengths. Vibration immunity of the cavity is also achieved through structure optimization.