The frequency difference (fr) between two neighboring lines in a frequency comb is always exactly the same. It is kept stable by comparing it with an atomic clock. The comb light is guided to the spectrograph in an optical fiber. The light is separated into its its frequency components by the spectrograph and imaged on the CCD detector. The comblike spectrum appears as a row of dots, of which each dot corresponds exactly to one line of the frequency comb. This "laser ruler" can now be used to calibrate the spectrograph. (Image: T. Wilken)
