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Optical Networks: A Practical Perspective - Part 17

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Optical Networks: A Practical Perspective - Part 17. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 130 Components Fabry-Perot cavity Input signal Reflections Transmitted waves add in phase Figure 3.16 Principle of operation of a Fabry-Perot filter. 3.3.5 constants for the cladding modes are discussed in Ven96b . The amount of wavelength-dependent loss can be controlled during fabrication by controlling the UV exposure time. Complicated transmission spectra can be obtained by cascading multiple gratings with different center wavelengths and different exposures. The example shown in Figure 3.15 was obtained by cascading two such gratings Ven96a . These gratings are typically a few centimeters long. Fabry-Perot Filters A Fabry-Perot filter consists of the cavity formed by two highly reflective mirrors placed parallel to each other as shown in Figure 3.16. This filter is also called a Fabry-Perot interferometer or etalon. The input light beam to the filter enters the first mirror at right angles to its surface. The output of the filter is the light beam leaving the second mirror. This is a classical device that has been used widely in interferometric applications. Fabry-Perot filters have been used for WDM applications in several optical network testbeds. There are better filters today such as the thin-film resonant multicavity filter that we will study in Section 3.3.6. These latter filters can be viewed as Fabry-Perot filters with wavelength-dependent mirror reflectivities. Thus the fundamental principle of operation of these filters is the same as that of the Fabry-Perot filter. The Fabry-Perot cavity is also used in lasers see Section 3.5.1 . Compact Fabry-Perot filters are commercially available components. Their main advantage over some of the other devices is that they can be tuned to select different channels in a WDM system as discussed later. Principle of Operation The principle of operation of the device is illustrated in Figure 3.16. The input signal is incident on the left surface of the cavity. After one pass through the cavity as 3.3 Multiplexers and