  Ultralow noise, yet tunable lasers are a revolutionary tool in precision
spectroscopy, displacement measurements at the standard quantum limit, and the
development of advanced optical atomic clocks. Further applications include
LIDAR, coherent communications, frequency synthesis, and precision sensors of
strain, motion, and temperature. While all applications benefit from lower
frequency noise, many also require a laser that is robust and compact. Here, we
introduce a dual-microcavity laser that leverages one chip-integrable silica
microresonator to generate tunable 1550 nm laser light via stimulated Brillouin
scattering (SBS) and a second microresonator for frequency stabilization of the
SBS light. This configuration reduces the fractional frequency noise to
$7.8\times10^{-14} 1/\sqrt{Hz}$ at 10 Hz offset, which is a new regime of noise
performance for a microresonator-based laser. Our system also features
terahertz tunability and the potential for chip-level integration. We
demonstrate the utility of our dual-microcavity laser by performing optical
spectroscopy with hertz-level resolution.
