Mux/Demux DWDM Dense Wavelength Division Multiplexing

DWDM MUX or Multiplexer combines multiple data signals into one signal

DWDM DEMUX or Demultiplexer divides a single signal into multiple data signals

Due to the physical properties of light at different wavelengths, multiplexers can be used in dual and single strand network configurations because the data signals cannot collide even if travelling bi-directionally. 

DWDM supports up to 96 channels at a frequency range called the C-band

In DWDM configuration wavelengths of between 1530nm and 1565nm are most commonly used.