Two-photon frequency comb spectroscopy of the 6s-8s transition in cesium

We report a new absolute frequency measurement of the Cs 6s-8s two-photon transition measured using frequency comb spectroscopy. The fractional frequency uncertainty is 5 x 10(-11), a factor of 6 better than previous results. The comb is derived from a stabilized picosecond laser and referenced to an octave-spanning femtosecond frequency comb. The relative merits of picosecond-based frequency combs are discussed, and it is shown that the AC Stark shift of the transition is determined by the average rather than the much larger peak intensity. (c) 2007 Optical Society of America.