Frequency measurement of the 5S1/2‐5D3/2 two‐photon transition in rubidium and prospects for measuring (He‐Ne)/I2 lasers at λ=633 nm

We have studied the hyperfine structure of the 5S1/2‐5D3/2,5/2 two‐photon transition in rubidium. Stabilizing a titanium‐sapphire laser on some of these transitions we have carefully investigated their metrological potential. The development of a new frequency standard at λ=778 nm may be expected soon. This will have performance similar to that of the (He‐Ne)/CH4 laser at λ=3.39 μm. With these two references at one’s disposal it will be possible to measure, in a simple way, the absolute frequency of the (He‐Ne)/I2 laser at λ=633 nm with an accuracy of some parts in 1012: this is of great interest for length metrology.