Hertz-Integral-Linewidth Lasers based on Portable Solid-state Microresonators

Optical reference resonators serve as a cornerstone in various scientific fields. In recent years, there has been an increasing demand for compact ultrastable reference resonators capable of operating in ambient environments, enabling applications beyond the laboratory, such as navigation, portable optical clocks, and remote sensing. Here, we present a compact ultrastable whispering-gallery-mode \ce{MgF2} reference resonator with a high loaded quality factor of $2.24\times 10^9$. The device is packaged in a compact form of 50$\times$77$\times$90 mm and supports stable optical coupling with polarization-maintaining fiber, which enables robust operation under ambient conditions. Laser stabilization using this resonator yields a phase noise of -105 dBc/Hz at a 10 kHz offset frequency, an integral linewidth of 4 Hz, and a fractional frequency stability of $2.5\times 10^{-14}$ at a 10 ms averaging time. With the high performance and rapid manufacturability, our work offers a promising solution for ultrastable optical frequency references beyond laboratory settings.