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We have shown that the modes of a femtosecond mode-locked laser are distributed uniformly in frequency space and can be used like a ruler to measure large optical frequency differences. To measure absolute optical frequencies we use nonlinear optics for the conversion into a large frequency interval. Unlike the complex harmonic frequency chains used in the past this new approach uses only a few laser sources (ideally just one) and is nevertheless capable of measuring almost any optical frequency with the same set up. We applied the new technique to determine the absolute frequencies of the cesium D/sub 1/ line at 335 THz, of several components in I/sub 2/ around 563 THz, the sharp /sup 115/In/sup +/5s/sup 21/S/sub 0/-5s5p/sup 3/P/sub 0/ transition at 1267 THz and the hydrogen 1S-2S transition at 2466 THz.

We have determined the frequency of the a/sub 1/ component of the P(54)32-0 transition of molecular iodine with an estimated relative standard uncertainty below 1/spl middot/10/sup -11/ using a phase-coherent optical frequency chain from the CH/sub 4/ frequency standard to the visible. This allows the realisation of the meter by visible frequencies with an uncertainty reduced by more than one order of magnitude compared to previous measurements (1999).