Abstract
The advent of frequency combs, in the early 2000s, was a revolution for the metrology of optical frequencies (a few hundreds of terahertz), and stimulated the development of a new generation of atomic clocks, this time referenced to an optical frequency. These combs, based on lasers delivering femtosecond long pulses, are today the most efficient and reliable way to compare an optical clock to microwave primary frequency standards, or directly two optical clocks operated at different frequencies. In this paper we report on the work carried out at LNE-SYRTE to characterize these devices. We describe the steps necessary to have the combs performing such that they do not limit the frequency comparison between the various clocks of our laboratory.