**2.2 Wavelength division multiplexing**

Fiber-optic communications require a different kind of multiplexer called a wavelength division multiplexer (WAD) [2, 4]. It is an analog multiplexing technique. It is designed for high data rate capability fiber cable. In this technique the bandwidth of the communication channel should be greater than the combined bandwidth of the individual channels. Here signals are converted to light signals; each light which has different wavelengths is transmitted through the same fiber cable. WDM transmission system divides the optical fiber bandwidth into a number of nonoverlapping optical wavelengths; these are referred to as WDM channels. WDM mixes all incoming signals having different wavelengths and are transmitted over a common channel. A demultiplexer does the reverse operation and separates the wavelengths. This multiplexing mechanism provides a much higher available transmission capacity.

**Figure 3** shows the representation of WDM system that consists of both multiplexer and demultiplexer.

WDM multiplexing and demultiplexing are similar to the refraction of light through a prism as shown in **Figure 4**.

Wavelength division multiplexing is divided into two types, unidirectional WDM and **bi-directional WDM**. In unidirectional WDM, the data is sent only from one side and received on the other side. Multiplexing of the wavelength occurs on the sender side, and demultiplexing of the wavelengths takes place on the receiver side. In bi-directional WDM the data can be sent from both sides which means that both sides can do multiplexing and demultiplexing as shown in **Figure 5**.
