Light is combination of different colors. If we use fiber optics , the carrier signals are being multiplexed into the optical fiber by different kind of wavelengths.

This is an analog multiplexing technique and is done conceptually in the same manner as FDM but uses light as signals.

In WDM we work on dividing the bandwidth into multiple number of waves and then sending and receiving data based on that.

A WDM system splits the bandwidth or your carrier into the waves of media .You can send the data using the wave that is meant for you to send data on a particular wavelength.

WDM system are mainly of two kinds Coarse and Dense. Coarse one provides 16 channel multiple transmission over the silica fibers.

Dense wavelength use the C-Band transmission and the channel spacing is more denser.