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Two-photon excitation of gas atoms by two counter-propagating frequency modulated laser beams is investigated with the help of second order perturbation theory. Simple spectra nearly free of Doppler broadening can be obtained from a region of the sample where the sum frequency of the two beams is constant. The comb of equally spaced laser modes can thus be locked to a resonant two-photon transition, providing calibration markers for the measurement of large optical frequency intervals.
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Hänsch et al. (1980) studied this question.
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