Measuring the frequency of light with mode-locked lasers

We have stabilized the modes of a comb of optical frequencies emitted by a made-locked femtosecond-laser and used it as a ruler to measure differences of up to 45.2 THz between laser frequencies in a new type of frequency cha in. Directly converting optical to radio frequencies, we have used it for a n absolute frequency measurement of the 1S-2S transition in the hydrogen at om. Here, an intuitive model of the comb's properties is given and essentia l techniques for its stabilization and efficient detection of brat signals are presented. (C) 1999 Elsevier Science B.V. All rights reserved.