Ramsey Fringes in Saturation Spectroscopy

Provided one has been able to develop/acquire frequency and intensity stabilized sources of coherent radiation, there remain two main problems in the pursuit of narrow lines: transit time broadening and Doppler broadening. The first-order Doppler effect may be essentially eliminated by various techniques: saturated absorption, two-photon spectroscopy, highly collimated beams, ... Transit time (interaction time) broadening is more difficult to eliminate. Still, it has recently proved possible to resolve the radiative recoil-induced doublets in the three main hyperfine components of methane at 3.39 pm. This high resolution (2 parts in 1011), derived from an external absorption cell with a 30 cm aperture, nevertheless remained 2 orders of magnitude removed from the natural lifetime line width. Larger cells and associated optics being cumbersome, one is induced to find alternative schemes to reduce transit time broadening and so approach natural line width resolution.