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.
