The review systematizes the experimental material and reports the fundamental theoretical results on the study of nonlinear resonance effects in spectra of atomic systems situated in the radiation field of lasers. The theoretical part explains the topics of greatest importance and significance for understanding of the experimental studies: the Stark effect in an optical field; the nonequilibrium velocity distribution of the atoms; the shape of the spectral line for a transition adjacent to a transition combining with a strong field; the simultaneous manifestation of the Stark effect and of Doppler broadening in the form of a very sharp structure on the line contour; a brief mention of the influence of collisions, recoil, and other factors on the sharp structure in spectra. The experimental part reports the results of experiments on the observation of the Stark effect in the optical region of the spectrum, the fine structure of the profile of a Doppler-broadened line and its properties, namely the dependence on the observation direction, the anisotropy in the field of a standing wave, the polarization properties, etc. in spectra of spontaneous or stimulated emission, absorption, and generation.

@article{Beterov:1973,
	author = {I. M. Beterov and R. I. Sokolovskii},
	title = {Nonlinear effects in the emission and absorption spectra of gases in resonant optical fields},
	publisher = {Physics-Uspekhi},
	year = {1973},
	journal = {Phys. Usp.},
	volume = {16},
	number = {3},
	pages = {339-349},
	url = {https://ufn.ru/en/articles/1973/3/c/},
	doi = {10.1070/PU1973v016n03ABEH005185}
}