Randomly layered active medium as a broadband amplifier

A flat-layered active medium in which two types of layers with different refractive indices alternate is considered. The thickness of a layer is assumed to be random and large as compared to the wavelength of propagating radiation. A wave propagating along the normal to the layers in such a medium is exponentially enhanced over lengths of the order of many layer thicknesses. In contrast to the familiar case of a periodic flat-layered active medium, waves with any (not necessarily definite resonant) frequency are amplified identically in a wide frequency range. By way of an example, convective instability of space-charge waves in a flow of charged particles moving through a randomly layered medium is considered. The predicted effect can be regarded as an analogue of Anderson's localization, when increasing solutions rather than exponentially decreasing ones are selected in view of the activity of the medium. (C) 2002 MAIK "Nauka/Interperiodica".