  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 $2.4\times 10^{-14}$ at 1 s and its phase noise is
thermal-noise-limited from 0.1 Hz to 10 kHz, reaching about -105 dBc/Hz at 10
kHz offset frequency. This represents the highest stability of any oscillator
operated without a vacuum enclosure. 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.
