122 and is conducted to the photodector 2 a significant component of the radiation of wavelength u03bb 1 is blocked from the photodetector 2 and a significant component of the radiation having the other wavelength u03bb 2 is blocked from the laser diode 1 the multiplex demultiplex module shown in plan view in fig6 differs from the module of fig5 in that a wavelength selecting directional coupler 30 is used instead of a wavelength selecting branching grating 3 the laser diode 1 and the photodetector 2 in the module of fig6 are the same as in the module of fig5 and are therefore identified with the same reference symbols the same being true for the optical fiber 8 the strip waveguide section 120 and end face 121 of fig5 correspond to the strip waveguide 1200 and end face 1210 in fig6 the other strip waveguide section 1220 in fig6 corresponds to the branching strip waveguide section 122 in fig5 in the region 1230 of the branching in fig6 the branching strip waveguide section 1220 is coupled to the strip waveguide section 1200 via the wavelength selecting optical directional coupler 30 which forms the deflector element for this purpose for example in the region 1230 of the branching the two strip waveguides sections 1200 and 1220 proceed at such a small spacing next to each other along selected sections thereof so that radiation of the wavelength u03bb 2 can be coupled between those sections whereas radiation of wavelength u03bb 1 from the laser diode 1 is not coupled the strip waveguide sections 1200 and.