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Optical Link Design



                                                                         Optical Link Design  271


                       Laser  WDM                  EDFA        WDM    Optical  Optical
                      transmitters  mux         (gain = 20 dB)  demux  filters  receivers


                         •                                                 •
                         •             OADM                                •
                                        OADM
                                                        GFF
                                  24-km       24-km     GFF  60-km
                                  fiber        fiber         fiber
                                     Added/dropped
                                      wavelengths  1-dB
                            P S                                             P R
                                                  loss
                             3 dB  6 dB  4 dB  6 dB    3 dB  15 dB  3 dB  3 dB
                      Figure 16.5. Example of optical power losses of various components in a WDM link.



          16.2.4. DWDM link example
                      As a more complex example, consider the WDM link shown in Fig. 16.5. Assume
                      this is a four-channel link with each channel running at 10Gbps. The system
                      operates in the C-band and contains an optical add/drop multiplexer (OADM),
                      an EDFA with a gain of 20dB, a gain-flattening filter (GFF) following the
                      EDFA, and optical filters at the receivers. The fiber has an attenuation of
                      0.25dB/km. The individual laser diode transmitters have fiber-coupled outputs
                      of P S   2dBm, and the individual InGaAs APD receivers need a power level of
                      at least P R   24dBm to maintain a 10  11  BER.
                        The task is to make sure there is sufficient power margin in the link. The vari-
                      ous power losses are shown at the bottom in Fig. 16.5 and are listed in Table
                      16.4. The final margin is only 2.0dB. This is not sufficient, particularly since no
                      allowance was made yet for any possible power penalties. Thus an optical ampli-
                      fier is needed, for example, just ahead of the wavelength demultiplexer.



          16.3. Rise-Time Budget
                      A rise-time budget analysis is a convenient method for determining the disper-
                      sion limitation of an optical link. This is particularly useful for a digital link. In
                      this approach the total rise time t sys of the link is the root-sum-square calcula-
                      tion of the rise times from each contributor t i to the pulse rise-time degradation,
                      that is, if there are N components in a link that affect the rise time then

                                                           N  2   1/2
                                                   t sys      i                         (16.5)
                                                            t
                                                         i 1
                        The five basic elements that may limit the system speed significantly are the
                      transmitter rise time t TX , the modal dispersion rise time t mod of multimode fiber,
                      the chromatic dispersion (CD) rise time t CD of the fiber, the polarization mode
                      dispersion (PMD) rise time  t PMD of the fiber, and the receiver rise time  t RX .



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