FEATURE
SUBMARINE NETWORK CABLE EVOLUTION
J
ust as the industry’s idea of the “ideal” terrestrial fiber has evolved over time (G.652 SMF G.653 DSF G.655 NZDSF G.655 LEAF G.652 SMF G.654 LA), the design of submarine network cables has evolved as well to match leading-edge technology for submarine line terminal equipment (SLTE). Figure 1 shows the dynamics of evolution between the fiber, wet plant, and SLTE. Before the advent of coherent technology, SLTE tended to operate with intensity-modulated direct-detection (IM-DD) transmission, such as nonreturn-to-zero (NRZ), return-to-zero (RZ), and variations of duobinary transmission. Regardless of the specific transmission technique, chromatic dispersion (CD) tends to be the limiting optical impairment at data rates of 2.5 Gb/s to 10 Gb/s. CD is the effect whereby longer wavelengths have a higher velocity through the fiber than shorter wavelengths. Therefore, a given optical pulse (e.g., an NRZ or RZ symbol) would be dispersed as it travels along the fiber. Figure 2A shows a terrestrial cable designed for IM-DD transmission in which the typical geographic path of the cable
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SUBMARINE TELECOMS MAGAZINE
BY GEOFF BENNETT comprises conventional, positive-dispersion fiber. In order to compensate for CD, spools of negative-dispersion dispersion-compensating fiber (DCF) are located in the amplifier sites along the route. But the key point is that in this terrestrial design, the spools of DCF are not part of the route length – you can imagine the light simply spinning around the spools in each amplifier site. This is partly determined by the nature of terrestrial amplifier design, in which a mid-stage link to the DCF is located between the pre-amp and booster amp stages. In Figure 2B we see a dispersion-managed submarine network cable. Rather than locating the DCF in the midstage spool location, lengths of positive- and negative-dispersion fiber alternate along the length of the cable itself. The instantaneous level of CD is managed along the length of the cable so that it is just enough to offset non-linear effects while not accumulating to a magnitude where the receiver is swamped with a dispersed signal. Around 2010, the first coherent SLTE systems became commercially available, and these included the ability to compensate for CD in the receiver – typically up to around 50 s/m. In a coherent system, it’s actually advantageous to