Applications of Optical Frequency Combs

The optical frequency comb (for which Th.W. Hansch and J. Hall received the Nobel Prize) has meanwhile found numerous applieations, as for instance the very aecu-rate measurement of optical frequencies, whieh has been discussed in the previous section. It is much simpler than former techniques but reaches an accuraey that is two to three orders of magnitude higher. 

A second example is the use of frequency combs in satellites for the global positional system GPS for navigation purposes. The accuracy of navigation depends on the frequency stability of transmitters and receivers. 

An interesting application of frequency combs in astronomy allows the accurate determination of the Doppler-shift of spectral lines from stars due to their velocity relative to the earth. While former measurements had an uncertainty of An = 10 m/s the comb measurements could resolve velocity changes down to 10 cm/s [1338]. 

In order to achieve more accurate values for the ratio of electron mass to proton mass from which the fine-structure constant can be derived, a frequency comb in the infrared region was developed which can be used to measure vibrational-rotational transitions in HD+-ions. The frequency distance q between the modes of the comb, which equals the repetition rate of the femto-second pulses, is stabilized on the resonance frequency of a cryogenic ultra-stable sapphire resonator. For the measurement of the transitions in the HD+-ion diode lasers with external cavity and a grating for achieving single mode operation are used. Their frequency is stabilized onto the centre of the molecular transition and is then compared with the adjacent mode of the frequency comb. 

The frequency comb is now not only directly combined with the Cs-clock but also with an atomic clock which is stabilized on the very narrow transition 5 of the Yb-atom which is more stable than the Cs transition. 

---