Lasers with hertz linewidths at time scales of seconds are critical for metrology, timekeeping, and manipulation of
quantum systems. Such frequency stability relies on bulk-optic lasers and reference cavities, where increased size
is leveraged to reduce noise but with the trade-off of cost, hand assembly, and limited applications. Alternatively,
planar waveguide–based lasers enjoy complementary metal-oxide semiconductor scalability yet are fundamentally
limited from achieving hertz linewidths by stochastic noise and thermal sensitivity. In this work, we demonstrate
a laser system with a 1-s linewidth of 1.1 Hz and fractional frequency instability below 10^−14 to 1 s. This low-noise
performance leverages integrated lasers together with an 8-ml vacuum-gap cavity using microfabricated mirrors.
All critical components are lithographically defined on planar substrates, holding potential for high-volume
manufacturing. Consequently, this work provides an important advance toward compact lasers with hertz
linewidths for portable optical clocks, radio frequency photonic oscillators, and related communication and
navigation systems.