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Constructing the WDM Network Puzzle



          226  Chapter Thirteen


                                 Spectral band  C-band  Multi-λ  Optical power
                                   splitter   EDFA     VOAs     monitors
                      Transmission
                         fiber                    DCF
                                                   DCF




                                                  DCF
                                                   DCF
                         Distributed
                         Distributed
                          Raman            L-band                   Spectral band
                           Raman
                         pump lasers        EDFA                     combiner
                         pump lasers
                                                    Control electronics
                                                     Control electronics
                      Figure 13.9. Adding a Raman amplification mechanism extends the gain
                      response of an EDFA into the L-band.

                      can be extended into both the S-band and the L-band. Figure 13.9 illustrates
                      the concept for operation in the C- and L-bands. Here a multiple-wavelength
                      distributed Raman amplifier pump unit is added ahead of a band-splitting
                      device. The Raman amplification boosts the power level in the L-band. After
                      passing through the band splitter, the gains of the L-band wavelengths are fur-
                      ther enhanced with an L-band EDFA. Following the amplification process, the
                      wavelengths are recombined with another wideband multiplexing unit.
                        Other components used in this amplification system include dispersion-
                      compensating fibers or DCFs (see Sec. 13.2.8) and gain equalization units. Gain
                      equalization is accomplished by monitoring the optical power levels of each
                      wavelength entering and leaving the C- and L-band EDFAs and then using a
                      series of VOAs to adjust the wavelength power levels individually.



          13.2.6. Metro optical amplifiers
                      A high-performance EDFA for long-haul wideband links requires peripheral
                      components such as gain-flattening filters, cooled high-power (greater than
                      300-mW or  25-dBm) pump lasers, and sophisticated frequency and temperature
                      control electronics. On the other hand, an EDFA for metro applications can be
                      a narrowband device that typically amplifies from one to eight wavelengths.
                      Since this covers a range of only a few nanometers, gain-flattening filters are
                      not needed and uncooled pump lasers with a lower output power (around
                      80mW or  19dBm) can be used in metro EDFAs.
                        Figure 13.10 shows a schematic of a narrowband EDFA. A C-band/980-nm
                      coupler combines the output from a 980-nm pump laser with metro-based wave-
                      lengths in the C-band. A pump reflection filter following the erbium-doped fiber
                      prevents the 980-nm light from being coupled into the transmission fiber. The
                      optical isolator prevents reflected C-band light from coupling back into the


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