2.1 Wavelength Division Multiplexing: Basic Principles

Telecommunications makes wide use of optical techniques in which the carrier wave belongs to the classical optical domain. The wave modulation allows transmission of analog or digital signals up to a few gigahertz or gigabits per second on a carrier of very high frequency, typically 186 to 196 THz. In fact, the bit rate can be increased further, using several carrier waves that are propagating without significant interaction on the same fiber. It is obvious that each frequency corresponds to a different wavelength. This technique is called frequency division multiplexing (FDM) or wavelength division multiplexing (WDM). The latter term is currently preferred in most cases. DWDM is reserved for very close frequency spacing (typically less than 100 GHz corresponding to 0.8 nm at wavelengths near 1.5 ?m). The term "frequency division multiplexing" is used in a few cases, such as multiplexing with optical frequency shift keying and coherent detection. But the terminology is not completely stabilized.

With WDM, it is possible to couple sources emitting at different wavelengths ? ₁, ? ₂, ? _j, ? _n into the same optical fiber. After transmission on the fiber, the ? ₁, ? ₂, ? _n signals can be separated towards different detectors at the fiber extremity (Figure 2.1). The component at the entrance must inject the signals coming from the different sources into the fiber with minimum losses: This is the multiplexer. The component separating the wavelengths...