Fibre innovation: the foundation of next generation networks
We are all becoming used to living in a super-connected world. Our societies are changing thanks to social networks, cloud computing, video streaming, and television ? la carte. However, the ever increasing number of bandwidth-hungry applications places unprecedented levels of pressure on networks from the backbone to the access. As such, it is necessary to look at this world not from the perspective of the latest smart phone or ?app?, but from the fundamental technology at the foundation of this super connectivity: namely optical fibre.
Research conducted by Infonetics forecasts a significant increase in the number of links operating at 40G and 100G by 2013. Delivering such high data rates means the attenuation of optical fibre becomes a key concern. This data predicts that almost half of the network will still be operating at 10G, where maintaining low dispersion is key in ensuring a profitable link for the carrier.
Delivering high data rates: low attenuation fibres and the OSNR challenge
A basic rule of thumb is that, in the absence of system advances, each tenfold increase in data rate, such as a move from 10G to 100G, requires a 10 dB increase in optical signal to noise ratio (OSNR). A further challenge is that a similar transition from 10G to 400G will require 16 dB of OSNR gain. Recent advances in modulation formats, digital signal processing (DSP) and coherent systems have helped achieve OSNR gains of up to 6 dB, but for some network links this may not be enough to deliver data rates beyond 100G nor does it answer the demand for low cost non-coherent 100G.
The OSNR of a system is proportional to the signal power in the fibre, which in turn is proportional to the effective area of the fibre. OSNR is also inversely proportional to the total signal attenuation of the system, which increases with span length. Thus, to improve OSNR through fibre innovation, we must increase fibre effective area and or reduce fibre attenuation.
Reducing the attenuation will have no impact on the standards compliance of a fibre, but increasing the fibre?s effective area can make it non-compliant with the basic terrestrial ITU fibre standards like G.652 and G.655. New lower attenuation fibres, some also with larger effective areas, are already available and deliver significant OSNR gains relative to standard G.652 or G.655 fibres .
Submarine networks are usually custom greenfield builds, so standards compliance is less of a concern. This enables the use of very large effective area fibres such as Corning? Vascade? EX2000 fibre (110 um2), which coupled with a pure silica core to enable ultra-low loss (0.162 dB/km), can deliver up to 5.4 dB OSNR gain over a 100 km span. A more recent submarine fibre development, Vascade? EX3000 fibre, features an even larger effective area ( ?145 um2) and lower loss (?0.16 dB/km) enabling up to 6.9 dB of gain over 100 km: illustrating just how much optical fibre innovation can offer to the OSNR challenge.
In terrestrial systems, compliance with existing fibre standards becomes essential due to the need for backwards compatibility with legacy networks. Thus significant increases in effective area are not possible and efforts must focus on reducing fibre loss to achieve OSNR gains. New ultra-low-loss G.652 fibres like Corning? SMF-28? ULL fibre (0.17 dB/km typical at 1550 nm) deliver 3.4 dB of gain over 100 km and are well-suited to terrestrial systems. Other G.652.D and G.655 compliant low attenuation fibres are now available like SMF-28e+? LL fibre (0.18 dB/km at 1550 nm) and LEAF? fibre (0.19 dB/km) that deliver OSNR gains of 2 dB and 3.8 dB respectively relative to standard G.655 or G.652.D fibres.
Low attenuation fibres: how your network can benefit
By simply lowering the attenuation of optical fibre you can deliver OSNR gains that have real benefits in terms of network capacity, performance, cost, and technology robustness.
Lower attenuation delivers more OSNR margin for upgrading to 40G, 100G and beyond, such that lower cost transmission design can sometimes be used. Upgrading a network link of 800 km or 1000 km in length to a higher data rate can compromise its reach, forcing the addition of an optical-to-electric regeneration stage mid-way in the link at considerable operator cost. However, the use of low-loss fibres is delivering sufficient spare margin to allow operators to data-rate upgrade such networks with minimal or no compromise on the original link length.
Corning tested its Vascade EX3000 fibre in a 112 Gb/s link, with sixteen channels, 100 km spans and EDFA amplification only. The reach at this high data rate was an impressive 7200 km, the longest distance recorded for this type of configuration, clearly demonstrating that low attenuation and large effective area offer significant benefits in terms of reach at high data rates
In another 100G demonstration by Ciena using ultra-low attenuation SMF-28 ULL fibre, they demonstrated a 1500 km reach even when using long 125 km spans and EDFAs only (no Raman amplification) (Figure 2). This demonstration showed that ultra-low attenuation fibre can extend distances by 30 per cent to 35 per cent.
In a network deployment in EMEA a carrier deployed a ring network between three cities using SMF-28 ULL fibre, with distances between each city ranging from 135 km to 145 km. Though the carrier paid a premium for the innovative 48-fibre count cable, this ultra-low loss fibre gave a 4.8 dB advantage in span loss and avoided constructing three amplifier sites in remote areas which would be very costly to build, power, and cool.
As each fibre was lit, the carrier made additional savings from not having to install three additional amplifiers on each fibre. This yielded an initial saving (with one fibre pair lit) of almost $1.5 million and potential savings of over $8 million (excluding NPV) once the cable was fully lit. Thus the carrier got a system with ultra-low loss fibre enabled advanced OSNR performance that also enabled significant reductions in capex and opex.
The additional system margin enabled by the use of low attenuation fibres can also be used to provide extra resilience to repairs. With each cable cut costing 0.2 dB in loss, a 100 km cable with only 1 dB of spare system margin will go ?dark? after five cuts, but the use of a low attenuation fibre can enable at least an additional 3 dB of margin, enabling a highly valuable additional 15 cuts, thus greatly delaying the time before cable replacement is required.
In access networks, despite the much shorter link distances, a simple reduction in attenuation on a G.652.D fibre can deliver additional reach resulting in up to 20 per cent more subscriber, cabinet or antenna coverage area. A lower attenuation also provides additional margin to facilitate network upgrades.
Lower dispersion in next generation applications
Whilst some networks are already operating at 40G and 100G speeds, a significant number of networks remain at 10G with some vision towards 40G. For such routes with modest traffic demand, the use of low-cost non-coherent systems with in-line dispersion compensation, rather than advanced coherent 100G systems, is most cost-effective. Here non-zero dispersion-shifted G.655 fibres are most effective.
Lower dispersion optical fibres enable the removal of dispersion compensation modules (DCM). In a basic 560 km, eight-span 10G optical transmission link with G.652 fibre, switching to G.655 fibre would enable removal of five DCMs required in the original architecture. This would also enable the use of lower cost single-stage amplifiers at each of these points.
When the first fibre pair is lit, the equipment savings immediately cover the G.655 cable premium to yield net positive savings which increase to $2 million (excluding NPV) once the 12 fibre pairs are lit. Hence G.655 fibres continue to be popular in certain applications.
Low dispersion G.655 fibres are also bringing key benefits to 100G where non-coherent 100G systems are an attractive very low cost option. In a recent demonstration, ADVA have shown that the use of G.655 fibres can extend non-coherent 100G system reach from 40 km on G.652 fibres to 200 km (with some amplification) enabling application of low cost non-coherent 100G systems in regional networks.
The role of optical fibre in delivering the networks of the future
Innovation and advances in optical fibre attributes make a significant difference to a network. Lower attenuation can deliver higher OSNR and optimised selection of fibre type can enable lower costs and advanced network performance.
We are driving relentlessly towards a super-connected world, which is driving capacity demand throughout access, metro, and backbone networks. For a super-connected future, fibre innovation is critical. Next generation fibres will facilitate the cost and performance gains required for future networks and ultimately enable the super-connected world.
