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Optical Networks - _3_7 Switches_41

Optical Networks - _3_7 Switches_41 - 3.7 Switches 205...

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3.7 Switches 205 front-end amplifier can handle. This may not be an important consideration for many optical communication links since the power level seen by the receivers is usually more or less fixed. However, dynamic range of the receivers is a very im- portant consideration in the case of networks where the received signal level can vary by a few orders of magnitude, depending on the location of the source in the network. The transimpedance amplifier has a significantly higher dynamic range than the high-impedance one, and this is another factor in favor of choosing the transimpedance amplifier. The higher dynamic range arises because large variations in the photocurrent I p translate into much smaller variations at the amplifier input, particularly if the amplifier gain is large. This can be understood with reference to Figure 3.65(b). A change I p in the photocurrent causes a change in voltage I p R L across the resistance R L (ignoring the current through the capacitance C ). This results in a voltage change across the inputs of the amplifier of only I p R L /(A + 1 ) . Thus if the gain, A , is large, this voltage change is small. In the case of the high-impedance amplifier, however, the voltage change across the amplifier inputs would be I p R L (again ignoring the current through the capacitance C ). A field-effect transistor (FET) has a very high input impedance and for this reason is often used as the amplifier in the front end. A pin photodiode and an FET are often integrated on the same semiconductor substrate, and the combined device is called a pinFET. 3.7 Switches Optical switches are used in optical networks for a variety of applications. The different applications require different switching times and number of switch ports, as summarized in Table 3.3. One application of optical switches is in the provisioning of lightpaths. In this application, the switches are used inside wavelength crossconnects to reconfigure them to support new lightpaths. In this application, the switches are replacements for manual fiber patch panels, but with significant added software for end-to-end network management, a subject that we will cover in detail in Chapters 8 and 9. Thus, for this application, switches with millisecond switching times are acceptable. The challenge here is to realize large switch sizes. Another important application is that of protection switching , the subject of Chapter 9. Here the switches are used to switch the traffic stream from a primary fiber onto another fiber in case the primary fiber fails. The entire operation must typically be completed in several tens of milliseconds, which includes the time to detect the failure, communicate the failure to the appropriate network elements handling the switching, and the actual switch time. Thus the switching time required is on the order of a few milliseconds. Different types of protection switching are
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206 Components Table 3.3 Applications for optical switches and their switching time and port count requirements.
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