JOHN WILLIAMSON EXTENDING WAVELENGTHS
the baud rate (bit rate per symbol on the modulation constellation) and realise a trade-off in link capacity versus span length. “By adjusting the baud rate in fine increments, roughly around 0.1 Gbaud, system flexibility increases to operate at a desirable span length for a given capacity, and within a desired optical channel plan,” he offers. “In subsea cable systems, this allows the operator to increase the number of deployed channels (and the resulting total lit capacity), increase system link margins, or reduce the number of operating optical transponders.” Infinera’s Shore is a supporter of turning up additional fibres in parallel. “Rather than a single pair of fibres between two locations where you expend a lot of energy to maximise the capacity of that single fibre pair, you use multiple fibre pairs between locations,” he asserts. ”This enables network operators to focus more on using cost and power efficient solutions.” Shore reasons that with this approach, spectrum becomes a much lower premium. “The value of spectrum is lower enabling people to better leverage much more cost-effective optical engines that maybe aren’t as spectrally efficient.” This, he believes, paves the way for increased use of pluggable coherent optical engines. “This class of optical engines are extremely cost effective, and extremely power efficient, but are generally quite a bit less spectrally efficient.” He likens the difference between pluggables and the embedded optical engines historically designed to maximise spectral efficiency as the difference between thumb drives and computer hard drives. WHAT IS MORE? There’s a spectrum of opinion about what’s next in terms of how to head off potential optical cable capacity short-falls. Janson believes the trends
to consider are the development of advanced fibre cables, including issues surrounding inherent fibre impairments and practical, physical limitations of SDM. “It is also worth watching future developments in SLTE coherent detection and signal processing as they will impact the capacity capabilities of future systems.” Inniss predicts that it’s likely that operators serving different market segments will, at different times, adopt different capacity boosting solutions for these segments. “Thinner fibre and MCF, for example, will go into subsea first,” he calculates. According to Kidorf, one trend is to reduce the power and space required for undersea amplifiers and to increase the electrical power-carrying capability of the cable. “I also look forward to an increased eco-system for multi-core components (a multi-core amplifier?) so that N-core fibre does not require N independent amplifiers in the repeater,”
mechanisms will be viable and which won’t. Here it’s worth repeating the remarks of Kidorf that, despite 30% (or more) growth rate of required capacity on some subsea routes, this is accompanied by a decline in the price charged for capacity. “There is not a fantastic growth rate in the total money exchanged for undersea traffic, but, for now, we are getting more Gbit/s for the same money.” At the same time, C- and L-band may not always be friendly neighbours. “There are some interplays that happen when you try to put C- and L-bands on the same fibre where the C-band signals actually lose power to the L-band signals. You get this kind of Raman effect where the L-band signals steal power from the C-band” notes Shore. It’s also the case that C- and L-bands are not the only available wavelengths in town. Inniss says that the S-, E-, O- and U-bands are being assessed to understand the challenges. “WDM can scale beyond the C and L-bands,” he acknowledges.” But, again, there are downsides here. “Cost is a big challenge,” allows Inniss. “Transponders, amplifiers, and ROADMs are needed for other bands. They are not readily available today.” OTHER ROUTES OUT OF OPTICAL GRID-LOCK A number of other avenues aimed at side-stepping would-be optical cable capacity log jams are being implemented or explored. As well as the SDM and multicore fibre instanced by Kidorf, one of Nokia’s takes on maximising spectral utilisation in subsea transmission is what is termed ‘water filling’. As explained by Chris Janson, Senior Product Marketing Manager, Subsea Systems, Nokia Optical Networks, water filling makes very fine adjustments in link baud rate to optimise spectral efficiency, line capacity, and optical span reach. He observes that for a fixed bit rate, shaped modulation format, and FEC scheme, you can adjust
SUPERSIZING THE C-BAND Over time, the optical networking
industry has extended the capacity of C-band operations. According to Infinera literature, the deliverables of the original C-band (4 THz amplification giving 80 channels at 50 GHz spacing) were upped in Extended C-band to a flat gain of 4.8THz, giving 96 channels at 50GHz. More recently advances have been made in C-band amplifiers, extending the effective range to about 6.1 THz and offering up to 120 channels 50 GHz. These solutions typically referred to as Super C-band. “The benefit of this approach is that network operators can get 25 to 30% more capacity while still using a single set of amplifiers, filters, and ROADMs,” comments Shore. “Additionally, tuneable lasers that can now be tuned across the entire 6.1 THz of spectrum.”
Daryl Inniss Member of the ECOC Market Focus Committee
Dave Brown Senior Product Marketing Manager, Line Systems, Nokia Optical Networks
Howard Kidorf Managing Partner, Pioneer Consulting
Rob Shore Senior Vice President, Marketing, Infinera.
Chris Janson Senior Product Marketing Manager, Subsea Systems, Nokia. Optical Networks
www.opticalconnectionsnews.com
17
ISSUE 36 | Q1 2024
Made with FlippingBook flipbook maker