The characterization of Earth-like exoplanets and precision tests of
cosmological models using next-generation telescopes such as the ELT will
demand precise calibration of astrophysical spectrographs in the visible
region, where stellar absorption lines are most abundant. Astrocombs--lasers
providing a broadband sequence of ultra-narrow, drift-free, regularly spaced
optical frequencies on a multi-GHz grid--promise an atomically-traceable,
versatile calibration scale, but their realization is challenging because of
the need for ultra-broadband frequency conversion of mode-locked infrared
lasers into the blue-green region. Here, we introduce a new concept achieving a
broad, continuous spectrum by combining second-harmonic generation and
sum-frequency-mixing in an aperiodically-poled MgO:PPLN waveguide to generate
gap-free 390-520 nm light from a 1 GHz Ti:sapphire laser frequency comb. We
lock a low-dispersion Fabry-Perot etalon to extract a sub-comb of bandwidth
from 392-472 nm with a spacing of 30 GHz, visualizing the thousands of
resulting comb modes on a high resolution cross-dispersion spectrograph.
Complementary experimental data and simulations demonstrate the effectiveness
of the approach for eliminating the spectral gaps present in
second-harmonic-only conversion, in which weaker fundamental frequencies are
suppressed by the quadratic \{chi}^((2)) nonlinearity. Requiring only ~100 pJ
pulse energies, our concept establishes a practical new route to broadband
UV-visible generation at GHz repetition rates.Comment: 14 pages; 4 figure