Nonlinear mixing of radiation in a medium having gain without population inversion

V. S. Grigor'ian, Andrei I. Maimistov, Iu. M. Skliarov

Kazuo Kuroda

Nonlinear optics normally involves the change in the susceptibility of a material due to the presence of optical radiation. In many cases, the light-matter interaction produces a change in the refractive index of the medium, as occurs in photorefractive media, as well as in Kerr-like media, Brillouin-active media and absorbing media with thermallyinduced refractive index changes. Such nonlinear refractive index variation corresponds to a change in the real part of the optical susceptibility %. In resonant materials, the optical radiation induces stimulated emission (or absorption) causing transfer of population between different energy states of the medium. This can lead to saturation of the amplification (or absorption) corresponding to changes in the imaginary part of the susceptibility x= i2odk where the amplitude gain coefficient a(I) is a saturable function of the intensity /, and k is the magnitude of the wavevector of the optical radiation. The interference of two (or more) coherent beams I(x) in the laser amplifier can cause a modulation of the population inversion N(x). This modulation of the inversion is commonly known as spatial hole burning in lasers and leads to formation of a gain-grating a(x) = 1/2GN(X) where G is the stimulated emission cross-section. The gain grating can act as a very efficient diffractive optical element exhibiting high diffraction efficiency that may be even greater than unity [16] and also very high phase conjugate reflectivity using four-wave mixing (FWM) geometries [5-7].

Gennadii K. Vlasov, G. M. Chernyavskii, E. I. Chizhikova +2 more

Michail G. Galushkin, А. А. Ионин, А. А. Котков +1 more

Numerical simulation is used as the basis of an analysis of nonlinear optical properties of the active medium in intracavity four-wave mixing of the radiation of a pulsed electron-beam-controlled discharge CO 2 laser on saturated-gain and refractive-index diffraction gratings. The reflection coefficient of the phase-conjugated signal is determined for various cavity Q -factors, specific input energies, and pressures of the laser-active mixture. A comparison is made of the theoretical and experimental results. It is found that the rate of formation of amplitude gratings is governed primarily by the initial population inversion and by the intensities of the interacting waves. It is shown that transient phase gratings make the dominant contribution to the phase-conjugate reflection coefficient at high pressures of the mixture.

Pochi Yeh

Andrei L. Mikaelyan, M.L. Ter-Mikaelyan