Absolute frequency measurement in the 28-THz spectral region with a femtosecond laser comb and a long-distance optical link to a primary standard

A new frequency chain was demonstrated to measure an optical frequency standard based on a rovibrational molecular transition in the 28-THz spectral region accessible to a CO2 laser. It uses a femtosecond-laser frequency comb generator and two laser diodes at 852 nm and 788 nm as intermediate oscillators, with their frequency difference phase locked to the CO2 laser. The rf repetition rate of the femtosecond laser was compared with a 100-MHz signal from a hydrogen maser, located at BNM-SYRTE. The 100-MHz signal is transmitted by amplitude modulation of a 1.55-mum laser diode through a 43-km telecommunication optical fibre. As a first example, the absolute measurement of a saturation line of OsO4 in the vicinity of the P(16) laser line of CO2 is reported with a relative uncertainty of 10(-12), limited by the CO2/OsO4 frequency day-to-day reproducibility. The current limit on the stability of the frequency measurement is 4x10(-13) at 1 s.