A single optical fibre is capable of transporting multiple signals, each at its own wavelength. At the fibre entrance, the signals fed by a number of transmitters, each operating at a specific wavelength, are combined by a wavelength multiplexer and launched into the transmission fibre. At the fibre exit, a wavelength demultiplexer routes these signals again according to their wavelengths to one or more receivers. If the demultiplexer selects only one wavelength channel, it is usually called a wavelength filter. When the fibres at the input and at the output of the device are identical, a wave-length multiplexer can typically be used as a wavelength demultiplexer in the opposite direction also, and vice versa.