The component separating the wavelengths is the demultiplexer. The multiplexer may be replaced by a simple optical coupler, but losses will increase. Obviously, when the light propagation is reversed, the multiplexer becomes the demultiplexer, and conversely. It is important to note, however, that the coupling efficiency is not necessarily preserved in reverse operation. For example, if the multiplexer uses single-mode entrance fibers and a multimode output fiber, the coupling losses would be excessive in the reversed use.
Multiplexers designed with identical input and output fibers are usually reversible.
However, wavelength division multiplexers using interference filters cannot be used when the number of channels is too high or when the wavelengths are too close. The dispersive element was a grating embedded in a monoblock of silica, and the optical fibers were directly fixed to the block. Hence an incident beam with several wavelengths is angularly separated in different directions. The diffraction angle depends on the groove spacing and on the incidence angle. Principle of multiplexing by diffraction on an optical grating.