The invention of frequency comb lasers in the year 2000 has produced a revolution in many fields of research, ranging from precision optical frequency metrology to attosecond science. Frequency comb lasers are based on phase and repetition rate stabilized mode locked lasers. With these devices it is e.g. possible to measure optical frequencies of hundreds of THz directly in comparison to an atomic clock.
Several frequency combs are available based on Ti:sapphire and fiber lasers, which are used in various ways. One important application is as a facility to calibrate lasers for precision experiments. These experiments include a search for variation of the fundamental constants a (the fine structure constant) and m (the proton-to-electron mass ratio) by precision spectroscopy in molecular hydrogen and atomic ions. In these cases laser light is transported by optical fibers to the frequency combs, which are then used to count the optical frequency relative to an atomic clock.
Another line of research aims at extension of the frequency comb principle to much shorter wavelengths for which no narrow bandwidth lasers exist, such as extreme ultraviolet (wavelength < 100 nm). This makes more precise tests of quantum-electrodynamic and nuclear size effects possible in e.g. the ground state of helium and hydrogen-like ions. The idea here is to use amplified and upconverted pulses from the frequency comb laser directly for excitation. Through high-harmonic generation we have shown that it is possible to generate a comb spectrum at short wavelengths. This principle was first demonstrated with an experiment on krypton at 212 nm [S. Witte et al., Science 307, 400 (2005)], and recently at 51 nm with direct comb excitation in helium [D. Z. Kandula et al., Phys. Rev. Lett. 105, 063001 (2010)].
Fig: Left: frequency comb laser near 800 nm based on Ti:sapphire. Right: extreme ultraviolet frequency comb excitation in helium at 51 nm based on harmonic upconversion of a comb at 773 nm. The timing between the upconverted pulses from the comb laser is scanned on an attosecond timescale, which leads to a Ramsey-like modulation of the excitation probability.