Higher-order distributed feedback and generation of light

An investigation is made of a higher-order distributed feedback which appears in a medium characterized by nonharmonic modulation of the parameters due to different radiation diffraction processes. The averaging method is used to obtain reduced equations for the amplitudes of opposite waves and expressions for the gain (absorption coefficient) as well as for the coupling between the waves, subject to all the gratings induced in a medium. Competition between the contributions of various diffraction processes to distributed feedback of arbitrary order is studied. An analysis is made of the coupling coefficient of waves in a dye solution laser with a second-order distributed feedback due to gratings formed by the populations of levels participating in a resonance transition and also thermal gratings induced in the active medium because of saturation of spatially inhomogeneous pumping. A calculation is made of the parameters of the medium for which the competition between the various diffraction processes makes the greatest contribution to a higher-order distributed feedback