to the axis 45 of the waveguide 4 and to the axis 55 of the waveguide 5 and axis 65 to the waveguide 6 as illustrated the waveguides 5 and 8 are basically thick light waveguides and the other waveguide 6 and 7 of each pair of waveguides are thinner than the thicker waveguides and also the main waveguide 4 the parting line 80 as well as the filter layer 100 extends at 45 u00b0 to each of the axes 45 55 65 75 and 85 after coating of the parting surface with the layers forming the filter layer 100 and after joining the two parts 20 and 30 back together with the waveguide axis 65 intersecting the waveguide axis 75 the multiplexer demultiplexer is obtained the multiplexer demultiplexer 300 is such that the filter layer 100 will be transmissive to one defined wavelength u03bb 1 and reflective to another defined wavelength u03bb 2 thus the waveguides 5 and 6 which have axes extending parallel to the axis 45 of the waveguide 4 will handle light of the wavelength u03bb 1 while the waveguides 7 and 8 which are at an angle 90 u00b0 to the axis 45 will handle light of the wavelength u03bb 2 as illustrated the waveguides 6 and 7 which are the thin waveguides are arranged to be adjacent to each other also the diameter of the waveguides 5 and 6 of the first pair and the waveguides 7 and 8 of the second pair are expediently selected so that sum of their diameters is equal to the diameter of the main waveguide 4 care must also be exercised that all of the curved light waveguides that the.