Ultrastable vacuum-gap Fabry–Perot cavities operated in air

We demonstrate a vacuum-gap ultrastable optical reference cavity that does not require a vacuum enclosure. Our simple method of optical contact bonding in a vacuum environment allows for cavity operation in air while maintaining vacuum between the cavity mirrors. Vacuum is maintained long term, with no observed degradation in cavity stability for over 1 year after bonding. For a 1550 nm laser stabilized to a 9.7 mL in-vacuum bonded cavity, the measured Allan deviation is 2.4×10 −14 at 1 s, and its phase noise is thermal-noise-limited from 0.1 Hz to 10 kHz, reaching about −105dBc/Hz at 10 kHz offset frequency. This represents the highest stability of any oscillator operated without a vacuum enclosure, to our knowledge. Furthermore, we demonstrate a 0.5 mL in-vacuum bonded cavity created using microfabricated mirrors and cavity dicing, with phase noise reaching −95 dBc/Hz at 10 kHz offset frequency. By relieving the need for high-vacuum enclosures, we greatly enhance the portability and utility of low-noise, compact, cavity-stabilized lasers, with applications ranging from environmental sensing to mobile optical clocks to ultralow-noise microwave generation.