Boosting astrocomb reference performance with a wideband, frequency-stabilized WGM etalon

Astrocombs are highly precise optical reference systems used for astronomical spectrograph calibration, enabling accurate and consistent measurement of stellar absorption line frequencies across a broad spectral range. By comparing a star’s spectrum to the astrocomb spectrum, minute radial velocity shifts indicative of an exoplanet’s presence can be detected. The highest precision and accuracy astrocombs are generated by laser frequency combs. However, in the visible band, frequency comb calibrators have stringent operational lifetime limitations due to degradation of the supercontinuum-generating medium, and are thus typically used to calibrate Fabry–Pérot etalons, which are then used for simultaneous observations with target stars. This work demonstrates a specially designed crystalline whispering-gallery-mode resonator (WGMR) that serves as an astrocomb etalon, featuring a coupling wavelength span of two octaves and a quasi-single-mode family. Several innovations have been incorporated into the system’s compact design to achieve frequency stability, in addition to wide-range wavelength coupling. These include dual-mode thermometric frequency stabilization, in addition to vacuum packaging, effectively minimizing thermal and pressure-induced frequency drift. Long-term frequency stability, measured via a heterodyne RF beat note, exceeds 2×10 −12 , with a projected long-term modal frequency stability of ≲10 −10 (equivalently 3 cm/s in Doppler velocity). This level of stability obviates the need for frequent cross-calibration with a laser reference, as was required with other astrocomb etalons. Overall, the architecture of the WGMR etalon provides a new and powerful capability, beyond what has previously been achieved by microresonator-based etalons, as a compact, ultra-wideband spectral reference for detecting small planets in long-period orbits around nearby stars. It also offers a path forward for a low size, weight, and power (SWaP) spectral reference appropriate for spaceflight.