Tunable laser frequency combs for astronomical spectrograph characterization and calibration

Tunable laser frequency combs that can map the point spread function across a spectrograph’s entire bandwidth are necessary for improving the instrumental systematics to reach near-infrared radial velocity precision levels < 1 m/s. We demonstrate a scannable 30 GHz electro-optic (EO) comb by tuning a combination of laser center frequency and mode spacing. This tandem tuning is necessary to avoid cavity-comb mode walk-off in a Fabry-Pérot cavity, which is used to suppress the broadband noise between the comb lines. This tuning is well-described by modeling the transmission of the EO comb through the cavity, and the entire supercontinuum spanning 700 – 1350 nm remains unchanged across the 30 GHz tuning range. We intend to implement this tunability for the 30 GHz EO comb deployed at the Habitable Zone Planet Finder (HPF) spectrograph.