Femtosecond fiber laser based methane optical clock

An optical clock based on an Er3+ fiber femtosecond laser and a two-mode He-Ne/CH4 optical frequency standard (lambda=3.39 mu m) is realized. Difference-frequency generation is used to down convert the 1.5-mu m frequency comb of the Er3+ femtosecond laser to the 3.4-mu m range. The generated infrared comb overlaps with the He-Ne/CH4 laser wavelength and does not depend on the carrier-envelope offset frequency of the 1.5-mu m comb. In this way a direct phase-coherent connection between the optical frequency of the He-Ne/CH4 standard and the radio frequency pulse repetition rate of the fiber laser is established. The stability of the optical clock is measured against a commercial hydrogen maser. The measured relative instability is 1x10(-12) at 1 s and for averaging times less than 50 s it is determined by the microwave standard, while for longer times a drift of the He-Ne/CH4 optical standard is dominant.