  Narrow-linewidth yet tunable laser oscillators are one of the most important
tools for precision metrology, optical atomic clocks, sensing and quantum
computing. Commonly used tunable coherent oscillators are based on stimulated
emission or stimulated Brillouin scattering; as a result, the operating
wavelength band is limited by the gain media. Based on nonlinear optical gain,
optical parametric oscillators (OPOs) enable coherent signal generation within
the whole transparency window of the medium used. However, the demonstration of
OPO-based Hertz-level linewidth and tunable oscillators has remained elusive.
Here, we present a tunable coherent oscillator based on a multimode coherent
OPO in a high-Q microresonator, i.e., a microcomb. Single-mode coherent
oscillation is realized through self-injection locking (SIL) of one selected
comb line. We achieve coarse tuning up to 20 nm, and an intrinsic linewidth
down to sub-Hertz level, which is three orders of magnitude lower than the
pump. Furthermore, we demonstrate that this scheme results into repetition rate
stabilization of the microcomb. These results open exciting possibilities for
generating tunable coherent radiation where stimulated emission materials are
difficult to obtain, and the stabilization of microcomb sources beyond the
limits imposed by the thermorefractive noise in the cavity.
