During the last twenty years, two techniques have been developed to measure the frequency of visible radiation with respect to a Cs primary clock: harmonic frequency chains , and frequency chains operating in the difference frequency domain . The latter technique has turned out to be much more flexible, since its spectral coverage can be restricted to the near-infrared and visible range, where tunable light sources are readily available. In particular, with the advent of optical frequency-comb generators based on Kerr-lens mode-locked lasers  this technique permits the measurement of any unknown frequency in this spectral range.
KeywordsMicrowave Rval Amplit
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- 10.see, for example, G.P. Agrawal, in Nonlinear Fiber Optics (Academic Press, Boston 1989) p. 181Google Scholar