The measurement of large optical frequency differences and the design of a new type of frequency chain

We have used optical frequency interval dividers, optical frequency comb generators and mode-locked lasers to measure large optical frequency differences of up to 45.2 THz between laser frequencies. We have shown that the modes of a mode-locked laser are distributed uniformly in frequency space within the experimental limit of 3.0 parts in 10¹⁷ and that the mode separation equals the pulse repetition rate within the experimental limit of 6.0 parts in 10 ¹⁶. We applied these techniques to determine the absolute frequencies of the cesium D₁ line at 335 THz (895 nm), the hydrogen 1S-2S transition at 2466 THz (121 nm) and the frequency of a methane stabilized He-Ne laser at 88.4 THz (3.39 μm) as well as the isotope shift of the 1S-2S frequency