LOW-FREQUENCY BEATS PRODUCED BY INTERFERENCE OF LASER MODES - OBSERVATIONS AT THE SINGLE-PHOTON LEVEL

A helium-neon laser operating in three or more axial modes can generat e, at a photo detector with a nonlinear response to intensity, beat si gnals with frequencies corresponding to the second differences between the frequencies of these modes. This paper presents a quantum-field t heoretical treatment of the formation of such low-frequency beats, as well as experimental data demonstrating that interference effects corr esponding to the production of such beats can be observed even at ligh t levels at which the probability of at least one photon from each of the three modes being present simultaneously in the optical path is ne gligible.