  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.
