Wave Division Multiplexing (WDM) enables the transportation of multiple optical carrier signals along a single optical link by utilizing different wavelengths, or colors, of laser light. Each wavelength carries its own unique signal. WDM can significantly increase the capacity of Free Space Optics (FSO) signals by providing multiple data channels along a single FSO connection and enhancing the transmission bandwidth.
Additional considerations for integrating WDM into FSO systems include:
Transceivers must be designed to simultaneously receive and transmit multiple wavelengths.
Combination and Separation of Signals
Multiplexers are used at the transmission end to combine multiple wavelengths into a single beam, while demultiplexers are utilized at the receiving end to separate these combined signals back into their original wavelengths. Wavelength filters may also be employed to selectively pass certain wavelengths.
Various modulation techniques can be used to encode data onto different wavelengths of the FSO signal. These include On-off keying (OOK), Phase-shift keying (PSK), Pulse Position Modulation (PPM), Binary Phase Shift Keying (BPSK), Return-to-Zero Differential Phase Shift Keying (RZ-DSPK), Modified Duobinary Return to Zero (MDRZ), and Quadrature Amplitude Modulation (QAM). 
Forward error correction codes are useful as well to mitigate atmospheric turbulence such as Reed–Solomon (RS) codes, concatenated RS codes, turbo codes and low-density parity check codes. 
Spectrum Slicing (SS-WDM)
Spectrum Slicing can be a more cost-effective method than using tunable multi-wavelength sources and can offer enhanced performance. The spectrum is sliced using narrow-band optical filters to provide a unique wavelength for each communication channel. Various methods have been proposed, including the use of superluminescent LEDs, Erbium Doped Fiber Amplifiers (EDFA), and Highly Nonlinear Fiber (HNLF) for supercontinuum generation. The HNLF works by altering its properties depending on the intensity of the light passing through it, broadening the spectrum of the emitted light to create more available wavelengths.
One challenge with this technique is that the power balance among the different wavelength slices can be uneven due to the disparate wavelengths of the created slices.
Arrayed Waveguide Grating (AWG)
AWG functions like a router for different wavelengths of light, operating much like a prism. It separates or demultiplexes the broad spectrum of light that has passed through the HNLF into individual wavelengths, transforming a broad spectrum of light into several narrower ones, each serving as a unique communication channel. However, the number of channels created in experiments so far has been limited.
Channel Equalization Techniques
loss and other impairments vary depending on the wavelength of light, such as in the case of Rayleigh scattering. Equalization is needed to compensate for these wavelength dependent losses and impairments.
Adaptive transmission in general refers changing system settings based on the channel’s conditions reffered to as Channel State Information (CSI). Settings might include the power of the ransmitted signal, size and type of modulation used, and the error correction rate. In the context of WDM, one variable can be the selection of wavelengths used depending on current conditions. This adaptive transmission technique is similar to other methods such as topology control and load balancing.
H. Ishio, J. Minowa and K. Nosu, “Review and status of wavelength-division-multiplexing technology and its application,” in Journal of Lightwave Technology, vol. 2, no. 4, pp. 448-463, August 1984, doi: 10.1109/JLT.1984.1073653.
 K. Elayoubi, A. Rissons, J. Lacan, L. St. Antonin, M. Sotom and A. Le Kernec, “RZ-DPSK optical modulation for free space optical communication by satellites,” 2017 Opto-Electronics and Communications Conference (OECC) and Photonics Global Conference (PGC), Singapore, 2017, pp. 1-2, doi: 10.1109/OECC.2017.8115015.