Abstract: In an optical frequency comb generator the emission frequency of each one of the set of output lasers is repetitively aligned with an associated one of a set of specifically determined frequencies by comparison of that emission frequency with a frequency indicated by an optical frequency meter. The optical frequency meter is repetitively re-calibrated by reference to the emission frequency of each of at least two frequency tuneable calibration lasers emitting at respectively different frequencies. Each of the calibration lasers has its emission frequency continuously or repetitively aligned with the frequency of an associated spectral absorption line frequency standard.