On the mechanism of transverse-mode beatings in a Fabry — Perot laser

The mechanism of emergence of fundamental-mode and first-mode beatings in the case of a step-wise increase in the pump rate is studied under the stationary single-mode lasing conditions. Investigation is based on the numerical solution of nonstationary wave equations in a resonator in the quasi-optic approximation and on the equation for a relaxation-type medium as well as on the use of the first two Hermite — Gaussian polynomials ψ_0,1(x) to obtain the distribution projections I_0,1(t), g_0,1(t) of the radiation intensity and gain, respectively. It is shown that the transverse-mode beatings emerge at early stages of two-mode lasing, the appearance of radiation intensity oscillations in the active medium preceding the development of the gain oscillations. The time of the passage of two-mode lasing to the stationary regime is determined. The phase shift π/2 between the oscillations I₁(t) and g₁(t) is found for the established beating regime and the modulation depth ΔI averaged over the output aperture of the radiation intensity in the established two-mode regime is shown to be proportional to the pump rate excess k over the single-mode lasing threshold. A scheme for controlling the mode composition of laser radiation is proposed, which is based on the rules for determining I_0,1(t) by the sensor signals. The efficiency of the scheme is studied. The scheme employs two field intensity sensors mounted inside the resonator behind the output aperture.