Dynamic spectral filter

Results of the discussions with N.B. Delone on the perturbation of the atomic transition by a strong nonresonant noise field of electromagnetic radiation are presented. Four examples that are conceptually and mathematically close to each other are considered: spectrum of radiation with the diffusion of phase, spectrum of radiation with the sine phase modulation, atomic absorption lineshape in the presence of a strong nonresonant noise field, and the spectral filter whose maximum-transmission frequency is varied with time (dynamic spectral filter). For all of the examples, the counterintuitive scenario of the formation of the resulting spectrum is demonstrated: the instantaneous radiation frequencies (or the instantaneous positions of the perturbed level) are not summarized in the output spectrum. The analysis is based on the limitation that is typical of the Fourier transform and lies in the fact that significant time is needed for the observation of small frequency shifts.