Non-uniform transmission line spacing in a zero-dispersion Fabry-Pérot cavity: implications for astro-comb...

Astro-combs are critical for achieving cm⋅s^-1 radial velocity (RV) precision in exoplanet detection, as they offer dense reference lines well matched to stellar absorption features. We demonstrate a blue-band astro-comb by filtering a 1 GHz ytterbium fiber frequency comb with a passively stabilized zero-dispersion Fabry-Pérot cavity (FPC) and fully characterize the filtered spectrum using a homemade Fourier transform spectrometer (FTS). Our key finding is that the FPC transmission lines exhibit non-uniform spacing, with deviations mostly within 4 MHz (up to 10 MHz) relative to the average free spectral range of ~44.789 GHz-challenging the conventional assumption of uniformly spaced calibration markers. Despite this non-uniformity, averaging over 1340 resolvable comb teeth suppresses single-line RV errors (2 m·s^-1) to an overall RV precision of 5.5 cm·s^-1. Long-term stability tests over 45 days yield RV precision between 7 and 8 cm⋅s^-1, confirming suitability for exoplanet detection campaigns. This work underscores the need for detailed spectral characterization of astro-combs, as FPC-induced spacing non-uniformity fundamentally influences calibration accuracy and provides critical insights for advancing cm·s^-1-level RV precision.