Wavelength Division Multiplexing (WDM)
Wavelength division multiplexing (WDM) is the combination of many optical signals into one transmission medium. Rather than sending a signal at 1550nm or 630nm, or sending both using two individual terminals, WDM enables the signals to be combined and sent using one transmitter. WDM is commonly used in fiber-optics and is an emerging technology in free-space optics (FSO).
To visualize how it works, think of a white flashlight. White’s spectrum is a composition of red, green, and blue color curves. Say that the flashlight spectrum can be isolated at 450nm, 530nm, and 650nm. These spectrums can be individually modulated to send a signal much like a laser at that wavelength. Then, by turning on the flashlight, all three signals are transmitted in the same direction at the same time under the umbrella of the white spectrum. At the receiver end, by filtering out all light except for 450nm, 530nm, or 650nm, the transmitted signal at that wavelength can be received. In this way, wavelength division multiplexing can be described as “putting several signal-carrying colors of light into one transmission medium and separating them at the receiver.”
WDM is extremely secure, particularly in fibers. However, security issues still exist. Primary issues include an intruder transmitting at the same wavelength and power as a legitimate user or an intruder transmitting at a different wavelength to induce interactive effects which damage the legitimate user’s signal strength. 
Coarse WDM (CWDM)
CWDM is a compressed form of WDM where only 8 channels (wavelengths) are used, typically spaced 20nm apart. This makes transmission cheaper and lower power, but it cannot transmit as far and cannot send as much information as DWDM.  CWDM is typically used in cable television and for intrametropolitan applications.
Dense WDM (DWDM)
DWDM refers to optical signals multiplexed in the 1525-1610nm waveband to take full advantage of erbium-doped fiber amplifiers (EDFA).  The center of DWDM communication is 193THz or approximately 1550nm. True to its name, DWDM signal separation is very fine. In the late 1990s, DWDM could only fit 40 channels with a separation of 0.8nm (100GHz) between signals. Today, DWDM networks have up to 160 channels with a separation as small as 0.2nm (25GHz) between signals.
Due to the immense density, ideal center wavelength, and ease of amplification with EDFAs, DWDM is used in data centers, core networks of telecommunication, and cable and cloud services.
Given the relatively new and comparably unexplored nature of FSO, the work done on combining FSO and WDM is recent and has yet to make major impacts on the market. A team in China created an FSO-WDM system that could communicate 200 Gb/s using power from directly modulated TOSAs (transmitter optical sub-assemblies).  Studies have also been done on the performance of FSO-WDM networks in simulated conditions , on solving the “last mile” problem in fiber networks , and on mitigating scintillation in FSO-WDM systems.