duced with OADM BUs, with bands of wavelengths being designated to specific paths through the network, as shown in the following figure. As before, the different paths have significantly different lengths. The hybrid Raman amplifier can be configured to suit this, with a Noise Figure which varies with wavelength as shown in the next figure. Fibre loss and non-linearity also reduce a little with increasing wavelength. By positioning signals which travel along the longest paths at the longest wavelengths one ensures that these signals can still achieve the required GOSNR.
SUMMARY
Technology currently offers a rich variety of choices to help different users achieve their aims in shared systems. The latest coherent modules offer improved performance in many ways, the range of constellations with lower granularity of capacity being particularly interesting. Fibres are now available with low loss and large effective area to minimise non-linear effects. Amplifier technology now includes separate EDFA C+L and hybrid Raman-EDFA. A portfolio of different technologies is clearly good, in that it offers more options, but at the same time it makes the process of analysis more difficult. Varying the technology between different segments and/or fibre pairs offers an opportunity to minimise compromises and there are clearly more possibilities than those described in this short article. *For some time now there have been interesting publications of a mixture of electronic processing which can reduce non-linear effects, Theses range from shaping the constellation (which is relatively simple) to attempting to compute, and then negate, the non-linear effects (which is very complex). There have also been a number of publications which advocate systems designs which operate at modest power levels, an approach which is also less demanding on overall system power. STF TONY FRISCH is Chief Technical Officer of Xtera. Tony joined Xtera in 2004 initially managing Marketing and Proposals for terminal equipment and upgrades and then responsible for products such as Repeaters and Branching Units, and now serves as CTO. Tony started work at BT ’s Research labs investigating cable problems and then moved to Alcatel Australia, becoming involved in testing and commissioning submarine systems. A move to Bell Labs gave him experience in terminal design and troubleshooting, after which he went back to Alcatel France, where he worked in Alcatel Submarine Networks’ Technical Sales before moving to head Product Marketing.
The Deep-Sea Cables BY RUDYARD KIPLING
The wrecks dissolve above us; their dust drops down from afar -Down to the dark, to the utter dark, where the blind white sea-snakes are. There is no sound, no echo of sound, in the deserts of the deep, Or the great grey level plains of ooze where the shell-burred cables creep. Here in the womb of the world -here on the tie-ribs of earth Words, and the words of men, flicker and flutter and beat -Warning, sorrow and gain, salutation and mirth -For a Power troubles the Still that has neither voice nor feet. They have wakened the timeless Things; they have killed their father Time; Joining hands in the gloom, a league from the last of the sun. Hush! Men talk to-day o’er the waste of the ultimate slime, And a new Word runs between: whispering, “Let us be one!” NOVEMBER 2018 | ISSUE 103
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