Two-photon Ramsey fringes at 30 THz referenced to an H maser/Cs fountain via an optical-frequency comb at the 1-Hz level

Aspects of two-photon Ramsey fringes in SF/sub 6/ at 30 THz obtained with a 1-m separation between the two absorption zones are presented. The experiment is referenced to a primary standard, the Hydrogen maser/Cs fountain located at BNM-SYRTE, via a femtosecond laser frequency comb generator. This results in a stability below 1 Hz for 1000 s of averaging, and absolute frequency measurements to /spl plusmn/2 Hz which corresponds to 6/spl times/10/sup -14/ in relative frequency. Resolution of the hyperfine structure now approaches 10 Hz, while the fringe period of 200 Hz provides a frequency discriminator 100 times narrower than any other currently in use in the spectral region.