An inventive controllable optical multiplexer for control of multiplexing channels in a fiber-optic communication system provided with 2N of wavelength-division multiplexing channels, whose optical frequencies are re-tunable at a constant wavelength shifting between adjacent channels, comprises a multi-stage structure of optical filters provided with elements for controllable re-tuning transmission coefficients. The optical filters are embodied in the form of single stage, and multi-stage asymmetric Mach-Zehnder interferometers. The electro-optical and thermo-optical phase shift devices are used for controllable re-tuning the transmission coefficients of the optical filters. The demultiplexer can be produced according to integrated optic technologies in the form of a monolithic solid-state device.