Compact sub-hertz linewidth laser enabled by self-injection lock to a sub-milliliter FP cavity

A narrow linewidth laser (NLL) of high frequency stabil-ity and small form factor is essential to enable applications in long-range sensing, quantum information, and atomic clocks. Various high performance NLLs have been demon-strated by Pound-Drever-Hall (PDH) lock or self-injection lock (SIL) of a seed laser to a vacuum-stabilized Fabry-Perot (FP) cavity of ultrahigh quality (Q) factor. However, they are often complicated lab setups due to the sophisticated sta-bilizing system and locking electronics. Here we report a compact NLL of 67-mL volume, realized by SIL of a diode laser to a miniature FP cavity of 7.7 x 108 Q and 0.5-mL volume, bypassing table-size vacuum as well as thermal and vibration isolation. We characterized the NLL with a self -delayed heterodyne system, where the Lorentzian linewidth reaches 60 mHz and the integrated linewidth is similar to 80 Hz. The frequency noise performance exceeds that of commercial NLLs and recently reported hybrid-integrated NLL real-ized by SIL to high-Q on-chip ring resonators. Our work marks a major step toward a field-deployable NLL of supe-rior performance using an ultrahigh-Q FP cavity.(c) 2023 Optica Publishing Group