Optical Ramsey Resonances Obtained Through an Equidistant Train of Laser Pulses

5 Optical Ramsey Resonances Obtained Through an Equidistant Train of Laser Pulses 

The Ramsey principle for the interaction of an atom moving in time through different electro-magnetic fields can be also realized in a completely different way. Atoms in a collimated beam are irradiated with an equidistant pulse train from a laser beam travelling collinearly with the atomic beam (Fig. 9.64). The distance Af between successive pulses corresponds to the flight time of molecules between the separated fields in the classical Ramsey method. 

This modification of the Ramsey technique was used by Hansch and coworkers for precision measurements on the hydrogen atom. The fundamental radiation of a cw dye laser at A. = 486 nm was frequency-doubled and through an acousto-optical modulator formed into an equidistant sequence of pulses. These pulses were sent into a high finesse resonator where they formed a standing wave with a corresponding pattern of spatially separated amplitude peaks [1270]. The achieved line width of the two-photon transition at X = 121 nm was below 5 kHz (Fig. 9.65). 

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