Novel demultiplexer configurations based on concatenated Mach-Zehnders and a ring resonator are presented. These new devices exhibit high channel isolation, flexibility, no loss, and megahertz-adjustable full width at half maximum (FWHM) suitable for narrow channel spacing. General design equations are described. Examples of a 1×4-2.5-GHz demultiplexer and a 2.5-GHz/5-GHz interleaver are reported. These designs have channel isolation >40 dB, 128 MHz FWHM, 10 dB gain. These devices can be conveniently implemented using the silicon or InP integrated optic technologies. A fabrication tolerance analysis is carried out. The worst figures for the previous designs are 220 MHz FWHM, 23-dB channel isolation, and 5.7-dB gain and can be readjusted with a gain control.