Comb-mode sweeping in a 650 nm–1030 nm astrocomb

Astrocombs are promising calibration sources for high-resolution astronomical spectrographs, offering stable, uniformly spaced calibration lines whose positions are traceable to an atomic frequency reference. Most spectrograph pixels fall in the gaps between astrocomb lines, remaining uninterrogated by the calibration light; however, it is becoming clear that detector inhomogeneities and intra-order variations in the spectrograph performance make it important to characterize the entire detector area to obtain the best radial-velocity precision. Here, this requirement is fulfilled by the demonstration of an astrocomb architecture in which lines can be swept in any chosen increment in frequency. Based on Fabry-Pérot filtering of a primary comb with a 1 GHz spacing, the astrocomb employs feed-forward locking to overlap a single-frequency laser onto a chosen primary comb line, a process that precisely selects which subset of primary comb modes will be transmitted by the Fabry-Pérot cavity. Operating from 650-1030 nm, and exhibiting continuous performance for >12 hours, the system demonstrates the long-term stability required by a practical astrocomb, while providing the fine sampling of the spectrograph instrument function likely to be necessary for achieving radial-velocity precisions supporting Earth-like exoplanet detection.