Fig. 3: Ultraviolet astrocombs.

a Setup for ultraviolet (UV) astrocomb generation. Rb rubidium-based atomic references (main text, Methods); CW continuous-wave laser, PM phase modulator, IM intensity modulator, WS waveshaper, EDFA erbium-doped fiber amplifier, Col. fiber-to-free-space collimator, M mirror, CU calibration unit splitting the comb equally into A and B fibers, CCD SOPHIE charged coupled device detector. All fibers following the lithium niobate chip and feeding the spectrograph are multimode fibers. b Composite CCD detector image, obtained with two different settings of the flux-balancing prism, showing fourth- and third-harmonic in the UV and visible (VIS) wavelength ranges. The inset indicates how wavelengths increase along the individual cross-dispersed echelle orders. The magnified CCD sub-images show the distinguishable but slightly overlapping frequency comb lines on the detector, originating from the two spectrograph fibers, which are both fed by the same comb. c Extracted 1-dimensional comb spectra of both harmonics; the insets show a magnified (linear scale) portion of the spectrum and the 18 GHz-spaced comb lines. The UV detector cutoff is marked.