Hello Giovanni
Dark fiber is a term used to refer to fiber optic cables that are installed for future use. It is called “dark” because it is initially not in use, thus it is not “lit up” by diodes or lasers. Dark fiber is often installed within “right of way” paths (roads, railroads, sewer systems, electrical cabling runs etc) whenever those infrastructures are being upgraded. It costs less to put in cabling when trenches are being dug up anyway for other works, rather than digging up trenches solely for the installation of the cables. The cables are installed, and then “lit up” as needed depending on future requirements. Dark fiber is simply a strategy used for infrastructure deployment in order to reduce installation costs.
Dense Wavelength Division Multiplexing (DWDM) is a completely different concept. It is a technology that is used to allow multiple data streams to be sent over the same fiber strand simply by using different wavelengths for each signal. Because light doesn’t interfere with itself, these multiple wavelengths can be simultaneously sent, thus drastically increasing the bandwidth of any particular fiber optic strand. DWDM depends upon the capability of light emitters and receivers to generate as well as distinguish between, wavelength pulses of differing wavelengths.
I hope this has been helpful!
Laz