Dual-comb precision calibration of an astronomical Fabry–Pérot cavity

Astronomical precision spectroscopy supports searches for exoplanets and may enable the direct observation of the Universe’s expansion. A key challenge in these efforts is the precise and long-term calibration of astronomical spectrographs. White light-illuminated Fabry–Pérot cavities, cross-calibrated with hollow-cathode lamps, are reliable calibrators. However, they exhibit temporal variability and nontrivial dispersion of the resonance frequencies, and the calibration lamps provide only sparse spectra with complex line shapes, which are also prone to aging. In contrast, laser frequency combs (“astrocombs”) can offer SI-referenced accuracy and precision, but the requirement of resolvable comb lines implies pulse repetition rates >10 GHz, which is challenging to implement. Here, we explore an alternative technique where lower-repetition rate dual-frequency comb lasers (comb lines unresolvable by the spectrograph) measure the modes of a 12 GHz Fabry–Pérot cavity across a spectral span of 30 THz (175–205 THz). This effectively transfers the comb’s accuracy to the cavity and enables precise near real-time tracking of the cavity modes. These measurements are of immediate relevance to currently used calibration techniques, and with further advances, this approach could potentially support spectrograph calibration at the level needed to search for Earth-like exoplanets.