Defocusing as a means of light removal from an amplifying medium

Defocusing as a means of light removal from an amplifying medium is studied. The wave propagation in a simple layer and in an Epstein layer is considered. It is shown that defocusing leads to additional nonsaturatable, nonresonante losses. When defocusing is optimum, it is profitable to operate in the saturation regime with no limitations on the length of the active medium. Defocusing naturally leads to a single-mode regime and to a high directivity of the output radiation. Experiment showed that defocusing is present in the superluminescence of a dye excited by a narrow knife-shaped pumping beam. The properties of defocusing layers and fibers are, in some respects, familiar to those of unstable resonators.