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Optical Link Design



          270  Chapter Sixteen




















                      Figure 16.4. A 2.5-Gbps link running over a 30-km path length
                      with a short optical jumper cable at each end.

                      TABLE 16.3. Spreadsheet for Calculating the 1310-nm SONET Link Power Budget

                      Component/loss parameter   Output/sensitivity/loss  Power margin, dB
                      Coupled laser diode output       2dBm
                      APD sensitivity at 2.5Gbps       32dBm
                      Allowed loss [ 2   ( 32)]                            30.0
                      Jumper cable loss (2   1.5dB)    3dB                 27.0
                      Splice loss (5   0.1dB)          0.5dB               26.5
                      Connector loss (4   0.6dB)       2.4dB               24.1
                      Cable attenuation (30km)         18dB                 6.1 (final margin)


                      is whether to operate at 1310nm or to use more costly 1550-nm components.
                      Therefore the first step is to calculate the 1310-nm power budget. Suppose the
                      installed fiber for the link meets the G.655 specification and that at 1310nm
                      the fiber attenuation is 0.6dB/km versus 0.3dB/km at 1550nm. For the 30-km
                      cable span, there is a splice with a loss of 0.1dB every 5km (a total of 5 splices).
                      The engineer selects a laser diode that can launch  2dBm of optical power into
                      the fiber and an InGaAs avalanche photodiode (APD) with a  32-dBm sensitivity
                      at 2.5Gbps. Assume that here, because of the way the equipment is arranged, a
                      short optical jumper cable is needed at each end between the transmission cable
                      and the SONET equipment rack. Assume that each jumper cable introduces a
                      loss of 1.5dB. In addition, there is a 0.6-dB connector loss at each fiber joint
                      (two at each end because of the jumper cables for a total of four connectors).
                        Table 16.3 gives the spreadsheet for calculating the 1310-nm link power
                      budget. The final link margin is 6.1dB. Therefore operation at 1310nm is ad-
                      equate in this case. Note that there still is the question of whether the link
                      meets the bandwidth requirements, which is addressed in Sec. 16.3.



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