| Abstract: |
Frequency combs, like many ground-breaking technologies, are simple in concept; they results from the spectrum of any regular train of optical pulses. What is remarkable is that this simple picture can be actually realized in a number of different experimental systems. The original application of frequency combs was to compare an optical clock with another rf or optical clock. Such comparisons can reach 10-19 or lower fractional uncertainties, limited only by the Doppler shifts associated with the thermal contraction or expansion of the experimental apparatus [1-3]. However, the utility of frequency combs is not limited to optical clocks; they provide a broadband optical source with well-defined phase coherence across the spectrum and are being explored for a growing number of applications. The variety of applications and approaches are many and this commentary will touch on only a few of the more metrological ones, omitting such significant areas as attosecond laser sources [4]; the reader is directed to some of the many review articles for more details including the 2005 Nobel lectures of Hänsch and Hall [5-8]. |
