DWDM stands for Dense Wavelength Division Multiplexer. DWDM’s allow you to multiplex or demultiplex more than one wavelength over one fiber similar to the CWDM mentioned in questions 3 and 4. The word “Dense” is referring to the very narrow channel spacing measured in Gigahertz (GHz) as opposed to nanometer (nm).
DWDM’s typically use channel spacing of 100GHz with a working channel passband of +/-12.5GHz from the wavelengths center. This allows you to add multiple wavelengths on to one fiber within the 1550nm band which are wavelengths between approximately 1525nm–1565nm (C band) and/or 1565nm–1625nm (L band) adhering to the DWDM ITU-T G.694.1 frequency grid.
DWDM’s will also use 200GHz spacing essentially skipping every other channel in the DWDM grid. They have also gone one step further using an Optical Interleaver to get down to 50GHz spacing doubling the channels capacity from 100GHz spacing.
