## Wavelength Division Multiplexing (WDM)

In optical fibre communications, it has been very difficult until recently to generate and detect individual carrier frequencies of light with sufficient spectral stability to be sandwiched within the finite bandwidth available on long lengths of fibre and still resolved at the receiver. Recently, however, the laser source, repeater amplifier and detector technology have all improved to permit several independently modulated light carriers to be sent over a single fibre. In optical circles, this is referred to as Wavelength Division Multiplexing (WDM) rather than frequency division multiplexing. Each individual light carrier would typically be supporting data rates of up to 10 Gbps with users time multiplexed onto the channel. WDM thus offers the possibility of several hundreds of gigabits transmission over a single fibre and also bi-direction transmission over the same fibre.