Transient energy exchange between opposite waves and narrowing of the stimulated emission spectrum of a laser to the absorption line of a resonant medium in the resonator

A theoretical analysis is made of the transient influence of a strong light wave on the transmission of a weak wave (having the same frequency and traveling in the opposite direction) by a resonant absorping medium with a homogeneously broadened line. It is shown that when the intensity of the strong wave exceeds by a factor of 3.6 the saturation intensity of the absorption transition, the weak wave may be amplified because of the transfer of energy from the strong wave as a result of the backdiffraction by light-induced transient gratings of the absorption coefficient and of the refractive index of the resonant medium. This energy exchange between the opposite waves may be one of the reasons for the locking of the emission frequency of a wide-band laser in the absorption region of a resonant medium introduced into its resonator.