Technology in long-span submarine systems
By Tony Frisch
40
Submarine systems have already evolved a great deal. Only a few years ago the first WDM links offered 4-8 channels at 2.5 Gbit/s. Today contracts are signed for capacities of 80 x 10 Gb, and in the lab a variety of interesting technical solutions gives capacities which are 4-5 x greater still. What are realistic targets and how do we achieve them? Perhaps a better question initially is why we are searching for more capacity when it seems that we already have more than we need? At present it seems clear that new cable orders are stalled as the potential of existing cables is exploited. Unless capacity growth stops, however, we will eventually need new cables. Assuming that in the long-term demands continue to increase at an ever-increasing rate, as they have in the past, we may need a lot more capacity. No supplier can risk not developing higher capacities even when it’s unclear when the technology will be used. Consider current repeatered systems; these are based on Erbium doped fibre amplifiers (EDFAs) which offer a bandwidth of around 30 nm. With considerable effort this can be stretched to 40 nm, the difficulty