The use of two different wavelengths in one direction or bi-directional is called wavelength multiplexing. Two independent signals are travelling on the same fiber at different wavelengths without any disturbance to each other.

With TDM the individual channels are interleaved, one channel after the other by transporting all information in a serial matter. Time Domain Multiplexing causes a higher necessary bandwidth because the bandwidth of the individual channels is added.

TDM - Time Domain Multiplexing - information of four channels interleaved

With CWDM the individual channels are transported parallel at different wavelengths with a narrow spectral characteristic. Each channel is converted to a unique wavelength, usually in a separating distance of 20 nm. Using these narrow bands the individual signals have no interference to each other, the information of all channels is transmitted simultaneously.

Coarse wavelength multiplexing is specified by the standard ITU-T G694.2.

CWDM - Coarse Wavelength Multiplexing - with four different signals on different wavelengths traveling at the same time

There are 18 wavelengths defined for CWDM by ITU-T G694.2: The wavelengths are 1270, 1290, 1310, 1330, 1350, 1370, 1390, 1410, 1430, 1450, 1470, 1490, 1510, 1530, 1550, 1570, 1590, 1610 nanometers.