Selection of optical modes in a ribbon fibre with a modulated gain

The gain spectrum of optical modes is theoretically analysed in an antiwaveguide ribbon fibre in which the amplifying regions are made coincident with periodically arranged regions of a reduced refractive index. A class of resonance periodic structures is selected in which the gain of one mode with the field uniformly distributed over channels exceeds the gain of all other modes by the value independent of the number of channels up to their critical value, which was also found analytically.