A good case can be made that the fast progress made in optical telecommunication systems over the past decade has been mainly due to the introduction of optical amplification, and more specifically due to the erbium-doped fiber amplifier (EDFA). This development allowed for transmission distances to multiply, and for line capacities to increase by orders of magnitude, thanks to dense wavelength division multiplexing (DWDM). These days, it is possible to transmit hundreds of DWDM channels down a fiber link over many thousands of kilometers. Line amplifiers placed at every 80 km or so regenerate the signal level for all channels simultaneously. Transmission distance is no longer limited by signal power, but

by amplifier noise and by the accumulation of transmission impairments such as chromatic dispersion and fiber nonlinearities. In this way, optical amplifiers have radically changed the economics of optical fiber transmission. Before, a regenerator (basically a receiver-transmitter pair) was needed for each separate wavelength channel at each regeneration site. Now, a single amplifier can be used by many channels at once, dramatically reducing the cost per channel. Most, if not all, modern DWDM systems use optical amplification or are at least prepared for its use. Not so with CWDM.