Self-pulsations of radiation in a solid-state laser with a nonlinear thin-film Q switch

Abstract

The conditions for realization of the regime of self-oscillations of the intensity of a continuously pumped solid-state laser have been qualitatively analyzed using a system of kinetic equations. It has been established that, in the case where a thin semiconductor layer absorbing light at frequencies close to the lasing frequency is placed into the cavity of such a laser, self-oscillations of its radiation intensity arise as a result of the amplitude-phase self-modulation of the laser light field.Keywords: relaxation dynamics of lasers, intracavity self-modulation of radiation, self-modulation broadening of the amplification line.Introduction. Lasers emitting a continuous sequence of short pulses are used in metrology and modern apparatus for transmission of information. Generation of stable trains of contrasting subpicosecond-and picosecond light pulses calls for top laser technologies. Currently, planar semiconductor structures have come into use as passive Q switches in solid-state lasers generating ultrashort pulses in the regime of mode locking [1][2][3]. It is also known [4] that certain solid-state lasers can generate a regular sequence of fairly short pulses on excitation of relaxation oscillations in them at a fairly low external modulation of the pumping level or the cavity Q factor [5]. The physical processes occurring in the cavities of cw solid-state lasers emitting ultrashort pulses were investigated in [6, 7] on the basis of simulation and comprehensive analysis of the operation of saturable Q switches based on semiconductor structures.Formulation of the Problem. The aim of the present work is to determine the range of parameters of a solid-state laser with a thin-film passive Q switch at which in it there arise self-oscillations of its light intensity. Such a laser begins to operate in the regime of intracavity self-Q-switching on excitation of relaxation oscillations in it as a result of the change in the effective transmission of its Q switch caused by the change in the radiation intensity. However, the transmission of a semiconductor Q switch can change not only as a result of the saturated absorption of light in its material, but also due to the high criticality of the very thin layer of this material to the variations in the frequency of oscillations in the cavity. Such variations in the frequency of oscillations can arise in active media characterized by a complex structure of amplification. In the case of a complex profile of the amplification band, even the detuning of the maximum lasing frequency from the fundamental frequency of the active transition can be nonzero. It has been established in [8,9] that the refractive index of semiconductors and activated crystals changes resonantly depending on the magnitude and sign of such a detuning. Under these conditions, the dynamics of laser radiation depends on the self-modulation broadening of the amplification line and on the relation between the amplitude and phase of the radiation pulses. In [10], it was established that self-modulation chan...