Monolithic optical resonator for high spectral purity laser and low-noise photonic millimeter-wave synthesis

Optical resonators as indispensable tools in optical metrology usually benefit from an evacuated and highly isolated environment to achieve peak performance. Here, we demonstrate a monolithic Fabry-Perot (FP) resonator in ambient environment, characterized with $4.5$~cm$^3$ volume, $2\times10^5$ finesse. \hl{We achieve 18 Hz integrated laser linewidth and $7\times10^{-14}$ frequency stability measured at 0.2~s averaging time, the highest spectral purity and stability have ever been demonstrated in the content of monolithic reference resonators.} By locking two separate lasers to distinct modes of the same cavity, generation of 96~GHz microwave signals is achieved with phase noise -100~dBc$/$Hz at 10~kHz frequency offset, more than an order of magnitude lower than previously achieved with injection-locked lasers. The compact monolithic FP cavity is promising for applications in spectrally pure, high-frequency microwave photonic references as well as optical clocks and other metrological devices, and can replace more technically demanding evacuated FP cavities that have filled those roles to date.