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 in the 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.

中文翻译：

---

基于微梳的自注入锁定光参量振荡器

窄线宽可调谐激光振荡器是精密计量、光学原子钟、传感和量子计算最重要的工具之一。常用的可调谐相干振荡器基于受激发射或受激布里渊散射；因此，工作波长带受到增益介质的限制。基于非线性光学增益，光学参量振荡器 (OPO) 能够在所用介质的整个透明窗口内生成相干信号。然而，基于 OPO 的赫兹级线宽和可调谐振荡器的演示仍然难以实现。在这里，我们提出了一种基于高 Q 微谐振器（即微梳）中的多模相干 OPO 的可调谐相干振荡器。单模相干振荡是通过一根选定的梳状线的自注入锁定（SIL）来实现的。我们实现了高达 20 nm 的粗调和低至亚赫兹水平的固有线宽，这比泵浦低三个数量级。此外，我们证明该方案可以使微梳的重复率稳定。这些结果为在难以获得受激发射材料的情况下产生可调谐相干辐射以及微梳源的稳定性超出腔内热折射噪声所施加的限制开辟了令人兴奋的可能性。