An increase in the sensitivity of the saturated absorption method in the multimode regime

A possibility for an increase in the signal-to-noise ratio in the laser spectroscopy that is free of Doppler broadening and is based on the saturated absorption is considered. The application of the counter-propagating laser beams in the multimode regime is proposed. The number of atoms that effectively interact with the field, and, hence, the intensity of a narrow resonance in the line shape can be increased due to the interaction of the counterpropagating modes with different frequencies. It is demonstrated that, for the intrinsic photon noise, the signal-to-noise ratio can be increased by a factor of √N, where N is the number of modes. For the remaining noises (fluctuations of the radiation power, noise of photodetector, etc.), the signal-to-noise ratio can be increased by a factor of N.