Wavelength multiplexing is a good and affordable method of transmitting multiple signals across the same fiber. Each wavelength (color) transports a signal. In this way, you can significantly increase the capacity of existing fiber lines. Combined with Optical Add / Drop Multiplexer (OADM), one can pick and / or add single wavelengths on the way between endpoints. A major advantage of using passive wavelength multiplexing is also that the method is protocol and speed independent.
WDM Wavelength Division Multiplexing:
Uses 2 wavelengths - 1310nm and 1550nm.
CWDM Coarse Wavelength Division Multiplexing:
Uses 20 wavelengths from 1271nm to 1611nm with 20nm spaces.
DWDM Dense Wavelength Division Multiplexing
Uses up to 80 wavelengths from approx. 1528nm to 1561nm with approx. 0.4nm space. Channels are given in frequency from approx. 196thz to 192THz with 50 or 100GHz spaces.