A new concept of an integrated-optical multichannel wavelength multiplexer for single-mode-fibre systems is presented. The operation principle of the cascaded multiplexer is based on the periodic wavelength dependence of the two-mode-interference (TMI) in a two-mode waveguide, which is adiabatically coupled to single-mode waveguides. The multiplexer is analysed analytically and numerically (BPM), and is realised experimentally by using Ti: LiNbO3 channel waveguides.