Optical frequency combs for atomic clocks and continental frequency dissemination

Nearly 25 years ago, Optical Frequency Combs (OFCs) have revolutionized practically from one day to the next the metrology of optical frequencies. Before that, only a few labs in the world could connect the optical (~1015 Hz) and the microwave (~1010 Hz) domains, at the price of the complex operation of a chain of multiple non-linear frequency conversions. In contrast, OFCs provided a tabletop instrument, compact, reliable and operable by a single scientist. Unmistakeable sign: only a few years had passed before pioneering inventors of OFCs Theodor Hänsch and John Hall were awarded the 2005 Nobel prize in Physics, together with Roy Glauber.