Amplifying waveguides and the concentration of radiation on supersmall scales

A uniform cylindrical waveguide containing an amplifying medium is shown to have no frequency cutoff. There, waves of an arbitrary frequency increase in the direction of propagation, and the increment obtained for small waveguide cross sections is inversely proportional to the waveguide diameter. A nonuniform waveguide consisting of amplifying and absorption sections is also analysed. The transmittance of this waveguide for below-cutoff waves is considerably higher than the transmittance of a uniform passive waveguide.