Frequency combs and applications

A laser frequency comb allows the conversion of the very rapid oscillations of visible light of some 100´s of THz down to frequencies that can be handled with conventional electronics. This capability has enabled the most precise laser spectroscopy experiments yet and found a variety of scientific and technical applications. Among them are stringent tests of quantum electrodynamics, the determination of fundamental constants and the search for possible slow changes of these constants. Using an optical frequency reference in combination with a laser frequency comb has made it possible to construct all optical atomic clocks, which are now outperforming even the best cesium atomic clocks. In future direct frequency comb spectroscopy might enable high resolution laser spectroscopy in the extreme ultraviolet for the first time. Frequency combs are also used to calibrate astronomical spectrographs and might reach an accuracy that is sufficient to observe the change of the expansion rate of the universe in real time.