18 GHz Ultraviolet Astrocomb via Chip-Integrated Harmonic Generation

Calibration of astronomical spectrographs with extreme precision enables the search for Earth-like exoplanets and quantifying the expansion of the universe via radial velocity measurements. Laser frequency combs (astro-combs) [1], [2] provide adequate calibration markers for this task. However, the typical resolution of astronomical spectrographs on the order of 10 GHz requires these combs to operate at equivalently high repetition rates. Gen-erating broadband combs in this regime becomes extremely challenging due to the low pulse energies that are available for nonlinear frequency conversion [3]. Here, we demonstrate frequency combs at a repetition rate of 18 GHz that are transferred via $\chi^{2}$ -based harmonic generation in periodically-poled, integrated lithium niobate waveguides [4], [5] from the telecommunication wavelength band to visible and UV wavelengths.