The energy gain and number of channels of a planar demultiplexer operating in a modern wave-length-division-multiplexing fiber-optic communication line are analyzed. The study involves the Fourier analysis and overlap integrals for complex fields. Reversible and irreversible demultiplexers are considered. The optimal parameters of the devices are determined. These parameters ensure the maximum gain and number of channels. Advantages and disadvantages of the devices are discussed, and desirable values of their parameters are recommended.