Bidirectional spike-free stimulated emission from a solid-state ring laser with a nonlinear absorber

An experimental investigation was made of the amplitude-frequency characteristics of a rotating YAG:Nd³⁺ ring laser with a nonlinear absorber made of LiF:F₂^–. The spiking instability of lasing was suppressed by an active negative feedback loop which created losses of the counterpropagating optical waves in proportion to the sum of their intensities. It was established that in the case of free lasing and under active mode-locking conditions the feedback loop made it possible to increase considerably the permissible value of the initial absorption coefficient and of the intensities of the counterpropagating waves at which the spike fluctuations were still absent. Strong self-diffraction of the counterpropagating waves in the nonlinear absorber eliminated completely the competitive suppresion of one of the waves when the difference between their frequencies was increased. Consequently, this rotating solid-state laser with the nonlinear absorber characterized by a short relaxation time was capable of stable generation of beats in a wide range of frequency differences between the counterpropagating waves.