Absolute frequency measurement of an SF6 two-photon line using a   femtosecond optical comb and sum-frequency generation
  We demonstrate a new simple technique to measure IR frequencies near 30 THz using a femtosecond (fs) laser optical comb and sum-frequency generation. The optical frequency is directly compared to the distance between two modes of the fs laser, and the resulting beat note is used to control this distance which depends only on the repetition rate fr of the fs laser. The absolute frequency of a CO2 laser stabilized onto an SF6 two-photon line has been measured for the first time. This line is an attractive alternative to the usual saturated absorption OsO4 resonances used for the stabilization of CO2 lasers. First results demonstrate a fractional Allan deviation of 3.10-14 at 1 s. 
Anne Amy-Klein (LPL), Andrei Goncharov (LPL), Mickael Guinet (LPL),   Christophe Daussy (LPL), Olivier Lopez (LPL), Alexande...