Optical Networks - _3_3 Multiplexers and Filters_37

Optical Networks - _3_3 Multiplexers and Filters_37 - 3.3...

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3.3 Multiplexers and Filters 121 SOP Faraday rotator SWP SWP λ /2 plate Fiber in Fiber out Faraday rotator SWP SWP λ /2 plate Fiber in Fiber out (a) (b) Figure 3.5 A polarization-independent isolator. The isolator is constructed along the same lines as a polarization-dependent isolator but uses spatial walk-off polarizers at the inputs and outputs. (a) Propagation from left to right. (b) Propagation from right to left. 3.3 Multiplexers and Filters In this section, we will study the principles underlying the operation of a va- riety of wavelength selection technologies. Optical filters are essential compo- nents in transmission systems for at least two applications: to multiplex and de- multiplex wavelengths in a WDM system—these devices are called multiplexers/ demultiplexers—and to provide equalization of the gain and filtering of noise in optical amplifiers. Furthermore, understanding optical filtering is essential to under- standing the operation of lasers later in this chapter. The different applications of optical filters are shown in Figure 3.6. A simple filter is a two-port device that selects one wavelength and rejects all others. It may have an additional third port on which the rejected wavelengths can be obtained. A multiplexer combines signals at different wavelengths on its input ports onto a com- mon output port, and a demultiplexer performs the opposite function. Multiplexers and demultiplexers are used in WDM terminals as well as in larger wavelength crossconnects and wavelength add/drop multiplexers . Demultiplexers and multiplexers can be cascaded to realize static wavelength crossconnects (WXCs). In a static WXC, the crossconnect pattern is fixed at the time
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122 Components Wavelength multiplexer l 1 l 2 l 3 l 4 l , l , l , l 1 2 3 4 Wavelength filter l 1 l , l , l , l 1 2 3 4 l , l , l 2 3 4 (b) (a) Figure 3.6 Different applications for optical filters in optical networks. (a) A simple filter, which selects one wavelength and either blocks the remaining wavelengths or makes them available on a third port. (b) A multiplexer, which combines multiple wavelengths into a single fiber. In the reverse direction, the same device acts as a demultiplexer to separate the different wavelengths. l , l , l , l 1 2 3 4 2 2 2 2 l , l , l , l 1 2 3 4 1 2 2 1 l , l , l , l 1 2 3 4 1 1 1 1 l 1 l 2 l 3 l 4 l , l , l , l 1 2 3 4 2 1 1 2 Demultiplexer Multiplexer Figure 3.7 A static wavelength crossconnect. The device routes signals from an input port to an output port based on the wavelength. the device is made and cannot be changed dynamically. Figure 3.7 shows an example of a static WXC. The device routes signals from an input port to an output port based on the wavelength. Dynamic WXCs can be constructed by combining optical switches with multiplexers and demultiplexers. Static WXCs are highly limited in terms of their functionality. For this reason, the devices of interest are dynamic rather than static WXCs. We will study different dynamic WXC architectures in Chapter 7.
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