Boosting Astrocomb Reference Performance with a Wideband, Frequency-Stabilized WGM Etalon

Astrocombs are highly precise optical reference systems used for spectrograph calibration, enabling accurate and consistent measurement of stellar absorption line frequencies over 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. This work demonstrates a specially designed crystalline whispering-gallerymode resonator (WGMR) that serves as an astrocomb etalon, featuring a coupling wavelength span of two octaves and a quasi-single-mode family. The system’s compact design allows for dualmode thermometric frequency stabilization and vacuum packaging, effectively minimizing thermal and pressure-induced frequency drifts. Longterm frequency stability, measured via dual mode RF beat, is measured to be better than 2×10−12, with a projected long-term modal frequency stability of ≲ 10−10 (equivalently 3 cm s−1 in Doppler velocity). This architecture provides a novel capability as compact, ultra-wideband spectral reference for detecting small planets in long-period orbits around nearby stars.