Space: The next frontier in optical networks
Massive connection point distribution and optical fibre cable densification is occurring in access and data centre networks. Each connection point needs an optical fibre, so the number of fibre strands needed to deliver network connectivity is spiralling upwards, while space and physical pathways to route these fibres is fixed or rapidly being consumed.
Data centre networks
Inside data centres, spine-and-leaf architectures are greatly increasing the number of optical connections into the millions. There is a preference to retain point-to-point connectivity between data centres for switch-to-switch connectivity. Therefore, the large number of connections in the data centre is driving the need for high-fibre-count interconnect cables between data centres. These Data Centre Interconnect (DCI) cables can contain 3,000 optical fibres or more.
Concurrently, a trend towards decentralisation is distributing data centres to locations closer to the user interface. The result is a distributed data centre architecture connected by high-fibre-count DCI cables. As in access networks, DCI applications are moving towards increasing fibre counts and the distribution of endpoint connections; but again, physical space within the existing data centre infrastructure is limited. Indeed, the very-high-fibre-count cables required (3,000+ fibres) must fit into ducts measuring ø50mm (diameter) that are commonplace between data centres.
Spatial efficiency requires complex optical cable engineering
The key challenge currently facing optical cable engineers is to fit as many optical fibres into as small a cable as possible. Yet, competing optical and mechanical performance parameters and industry design and installation standards leave a very small window for the ideal cable solution. However, two spatially-efficient cable constructions which strike this balance are high-density micro cables and extreme-density ribbon cables.
Micro cables are up to 60 per cent smaller and 70 per cent lighter than traditional stranded loose tube cables. As optical fibre counts have risen over time, cable diameters have been driven down through concerted industry effort. Today, the largest micro cable contains 432 fibres with an outer diameter of 10.8mm, which is just 0.5mm bigger than the smallest standard loose tube cable containing 72 fibres with an outer diameter of 10.3mm. This represents 5.5 times greater fibre density (measured in fibres per square millimetre). This example of optical cable miniaturisation is actually enabled by optical fibre miniaturisation. Traditional Recommendation ITU-T G.652 single-mode fibres feature a light-carrying core of 9.2µm, surrounded by a glass cladding to keep light from escaping, that brings the glass strand outer diameter to 125µm and a final acrylate coating layer to protect the glass, that brings the final fibre diameter to approximately 250µm. In recent years, manufacturers have produced G.652 fibres with a thinner coating that reduces the overall fibre diameter to 200µm. The 125µm cladding diameter is maintained for splicing compatibility purposes, but the 33 per cent reduction in cross-sectional area means more fibres can be packed into each buffer tube for more fibre capacity in a cable, or duct.
Micro cables are installed in microducts, which deliver spatial-efficiency in two key scenarios:
Overbuild – when installing additional cables into an already occupied duct, a microduct override provides more optical fibres than an equivalent-sized loose tube cable. A 96-fibre loose tube cable measures approximately 12mm, whereas a 12/10mm (outer diameter of 12mm, inner diameter of 10mm) microduct accommodates a 288-fibre high-density micro cable (with an outer diameter of 8mm) for three times more fibres.
New Build – when new duct infrastructure is necessary, multi-path microducts provide greater capacity in the same footprint as traditional conduits. A common 40/33mm duct is approximately the same size as a 7 x 10/8mm multipath microduct bundle (ø ~33mm) and whilst both cost the same to deploy initially, the microduct bundle offers six extra dig-free pathways over the traditional conduit for fast, inexpensive future capacity upgrades.
Extreme-density ribbon cables
In a ribbon cable, the standard 12 coloured optical fibres are encapsulated in an array, or ribbon. Multiple ribbons are stacked to achieve fibre counts up to 3,456 fibres in a single cable. Until recently, ribbon cables offered a maximum of 1,728 fibres with an outer diameter of 32mm. But today, a new generation of extremedensity ribbon cable offers twice as many fibres in the same cable size, for twice the density in a ø50mm (2-inch) duct.
As optical fibre counts hit multiple thousands, splice time becomes a key concern, as to splice 3,456 fibres individually would take over 100 hours. Fortunately, ribbon cables enable mass fusion splicing, where 12 fibres are fused in a single step, reducing total splice time by as much as 80 per cent. This means a 3,456-fibre cable can be spliced in less than 20 hours, but this efficiency can be enjoyed on any scale, as mass fusion splicing is faster than single fusion splicing at any fibre count of 12 or higher.
Roshene McCool is optical fibre market and technology development manager; and Matthew Guinan is outside plant cable market and technology development manager at Corning Incorporated