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

Abstract

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-μm 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 4×10^-13 at 1 s.