It covers various types of optical multiplexers and demultiplexers, including wavelength division multiplexing (WDM) devices, arrayed waveguide gratings (AWGs), and photonic integrated circuits for multiplexing and demultiplexing optical signals. This technique enables bidirectional communications over a. Wavelength Division Multiplexing (WDM) is a technique in fiber-optic communication systems that enables multiple optical signals with different wavelengths to be combined, transmitted, and separated over a single optical fiber. This makes it possible to scale capacity cost-effectively by using existing infrastructure more efficiently.