Summary229SOAFilterfsfsfpfp2ffps-2-2sp-2-Figure 3.79Wavelength conversion by four-wave mixing in a semiconductor opticalampliﬁer.efﬁciency goes down signiﬁcantly as the wavelength separation between the signaland probe is increased. We will study the conversion efﬁciency of four-wave mixingin Section 5.8.4.SummaryWe have studied many different optical components in this chapter. Couplers, iso-lators, and circulators are all commodity components. Many of the optical ﬁltersthat we studied are commercially available, with ﬁber gratings, thin-ﬁlm multicavityﬁlters, and arrayed waveguide gratings used in commercial WDM systems.Erbium-doped ﬁber ampliﬁers (EDFAs) are widely deployed and indeed servedas a key enabler for WDM. EDFA designs can incorporate multiple stages andgain-ﬂattening ﬁlters and provide midstage access between the multiple stages toinsert other elements such as dispersion compensating modules and wavelengthadd/drop multiplexers. Distributed Raman ampliﬁers are being used in conjunction
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