System and method for operating a single unit of light-amplifying medium, structured to produce light with a complex spatial spectrum including multitude of high-order spatial modes, in external cavity configured, in conjunction with an optical etalon installed intra-cavity, to reduce spatial spectrum of such light to provide an output containing smaller number of high-order spatial modes and, optionally, only the lowest spatial mode at power levels on the order of 1 W or higher (for example, tens or hundreds of Watts).