The WDM enables the simultaneous transmission of multiple optical signals with different wavelengths over a single optical fiber, while the optical amplifiers amplify these optical signals of varying wavelengths, facilitating efficient and long-distance optical signal transmission. This chapter addresses the operating principles of WDM. Wavelength division multiplexing (WDM) is a technology for increasing the transmission capacity of optical fiber communications by sending multiple data channels simultaneously through a single fiber, each on a different wavelength of light. This collection encompasses a variety of research papers, conference proceedings, and technical articles that explore both foundational.