Fig. 3: Comb-mode-resolving echellogram.
Main image, a 20.2 megapixel camera frame recorded from a cross-dispersion spectrograph, with annotations indicating the index and center wavelength of each echelle order. The free spectral range of each order is 3–4 nm, equivalent to ~200 comb modes and indicated by the dashed white lines. Insets, magnified regions of the echellogram confirming that the 30 GHz comb modes are resolved across the full range of modes orders 120–143, corresponding to wavelengths from 392–472 nm. To compensate for the 8-bit dynamic range of the camera, orders 139–143, which are associated with the intense pump SHG light, were recorded at lower gains than orders 120–138. Regions bounded by yellow dashed lines share the same camera gain. In the main image, the color map is linear with intensity; a nonlinear color map has been used to enhance the visibility of the weaker comb modes in the insets.
