Wavelength division multiplexing (WDM) is the second major fiber–optic revolution in the field of telecommunications. WDM is a technology which combines many different segments of wavelength range, called different independent optical channels, into the same optical fiber. The best feature of an optical fiber is that it has a wide spectral region which ranges from 1260 nm to 1675 nm. The light source used in high-capacity optical fiber communication systems emits a narrow wavelength band of less than 1 nm, thus enabling simultaneous transmission of many optical channels using the same optical fiber. WDM allows a huge increase in capacity of an optical fiber as compared to point-to-point link that carries only a single optical channel. Another big advantage of WDM is that different transmission formats can be supported by various optical channels. It means that without the need of common signal format, any data rate can be transmitted simultaneously and independently using the common optical fiber.
This chapter focuses on WDM concepts and components used in high-capacity optic–fiber communication networks. The discussion begins with the principle of wavelength division multiplexing which contains an orthogonal set of optical carriers with a suitable guard band, which a single-mode fiber can propagate. This is followed by a brief discussion on WDM system configuration involving a number of optical devices. An account of applications of WDM systems is presented next. The discussion is carried forward by describing various types of WDM components, including transmitters and receivers. Finally, an analysis of system performance issues and WDM soliton systems are covered.
Principle of Wavelength Division Multiplexing
Wavelength division multiplexing (WDM) is based on the fundamental physical principle which states that many optical rays having different wavelengths can be propagated together over a common optical channel with no interference. The concept of WDM is analogous to the basic concept of frequency division multiplexing (FDM) in which the available bandwidth of a communications channel in its frequency domain is divided into multiple sub-bands (called user channels). It implies that each user channel occupies only a part of the wide frequency spectrum. In WDM, each user channel is recognized by an optical wavelength. Remember the relationship between the wavelength and frequency as, which implies that shorter the wavelength of the signal, higher will be its frequency, and vice-versa.