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.