Abstract: The inventions relate to a method for operating a controllable selective optical add/drop channel for fiber-optic communication system provided with 2N of wavelength-division multiplexing channels, whose optical frequencies are retunable at a constant frequency separations ?v between adjacent channels, with the aid of the inventive controllable optical add/drop multiplexers (70, 80, 90) that comprise multi-stage structures of optical filters ({75-i}, {85-i}, {95-i}), that are connected in different manner and provided with devices, for example electro-optical and thermo-optical phase shift devices, for controllable tuning the transmissions characteristics thereof. The optical filters are embodied in the form of single-stage (20), two-stage (40) and/or multi-stage (60) asymmetric Mach-Zehnder interferometers. The controllable optical add/drop multiplexer can be produced according to integrated optic technique in the form of monolithic solid-state device.