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The appearance of the frequency comb technology, awarded the Nobel Prize in Physics 2005, has enormously revolutionized the metrology of optical frequencies, eliminating the need for complicated frequency chains. By direct linking to the unit of time, the second, through frequency standards (Cs, Rb), by using femtosecond mode-locked lasers and frequency comb technology, the Spanish Centre of Metrology (CEM) has established a new way of practical realisation of the National Standard of Length, the metre. By stabilising and characterising two free parameters – the repetition frequency fr and the offset frequency f0, the frequency comb generator thereby was successfully put into operation. After such realization, the accuracy of the length unit will be increased in two orders of magnitude, that is 2 × 10−13 instead of 2.1 × 10−11. In this paper we present the results of applying comb generator to the absolute measurement of the three Zeeman stabilized He–Ne lasers operating at 633 nm with a nominal frequency of 473.612 THz. A comparison of these results with those obtained by the current system based on standard iodine stabilized lasers is in good compatibility. A treatment for the evaluation of measurement uncertainty of laser frequency in calibration using a comb in accordance with Guide of Uncertainty Measurement ISO/BIPM is also presented

Realisation of the metre by using a femtosecond laser frequency comb: applications in optical frequency metrology