Space: The Next Frontier in Optical Networks | Corning

Space: The Next Frontier in Optical Networks

Space: The Next Frontier in Optical Networks

by Roshene McCool MEng CEng, Optical Fiber Market and Technology Development Manager, Corning Incorporated and Matthew Guinan, Outside Plant Cable Market and Technology Development Manager, Corning Incorporated 

Introduction - The Connectivity Distribution and Densification Challenge

Massive connection point distribution and optical fiber cable densification is occurring in access and data center networks. Each connection point needs an optical fiber, so the number of fibre strands needed to deliver network connectivity is spiraling upwards, while space and physical pathways to route these fibers is fixed or rapidly being consumed. Consequently, fitting as many fibers into as small a cable as possible is key, but doing this requires highly complex engineering. In this article, we will explore the factors driving ever-increasing optical fiber counts, and consider two highly spatially-efficient cable designs that help operators use their physical space more efficiently to meet the connectivity distribution and densification challenge.

Access Networks

Access networks serve bandwidth to subscribers directly and the 2017 Cisco Complete Visual Networking Index (VNI) Forecast demonstrates that consumers are currently driving bandwidth growth at predicted per annum rates of:

  • 47% in mobile networks
  • 24% in IP networks
  • 34% in machine to machine connections

In response, network operators are implementing optical fiber networks that terminate closer to their subscribers – at the home, business, antenna or street cabinet. In many cases the fibre infrastructure has terminations serving both fixed line and mobile customers and, increasingly, more machine to machine communications associated with the internet of things (IoT). This is known as the converged network.

These new access networks are characterised by many connections distributed across regional districts up to 20 km in span. The proliferation of connection end points and the need to distribute those end points across a regional district is driving high fiber counts within the cable feeder pathways that connect subscribers back to local data centers, exchanges and head ends. At the same time, the physical space available in these feeder pathways remains fixed or is being consumed by network overlays. This creates pressure on the existing infrastructure and a need to maximize the density of optical fiber within cables along these pathways.


  Figure 1 - access networks are characterised by many connections distributed across areas up to 20 km in span  


Data Center Networks

Inside data centers, spine-and-leaf architectures are driving millions of connections. There is a preference to retain point to point connectivity between data centers for switch to switch connectivity. Therefore, the large number of connections within the data center is driving the need for high-fiber-count interconnect cables between data centers. These Data Center Interconnect (DCI) cables can contain 3,000 optical fibers or more.

Concurrently, a trend towards decentralisation is distributing data centers to locations closer to the user interface. The result is a distributed data center architecture connected by high-fiber-count DCIs. As in access networks, DCI applications are seeing a push towards increasing fiber counts and the distribution of end point connections; but again, physical space within the existing data center infrastructure is limited. Indeed, the very-high-fiber-count cables (3000+ fibers) must fit into ducts measuring Ø (diameter) 50 mm that are commonplace between data centers.

Spatial Efficiency Requires Complex Optical Cable Engineering

The key challenge currently facing optical cable engineers is to fit as many optical fibers 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.


Figure 2 - competing optical and mechanical performance parameters determine the ideal optical cable solution for a given application


High-Density Micro Cables

Micro cables are up to 60 percent smaller and 70 percent lighter than traditional stranded loose tube cables. As optical fiber counts have risen over time, cable diameters have been driven down through concerted industry effort. Today, Corning’s smallest standard loose tube cable (72 fibers, ø 10.3 mm) is in fact bigger than its largest micro cable (288 fibres, ø 9.7 mm). This represents 3.5x greater fiber density (measured in fibers per square millimeter).


  Figure 3 – 72-fiber loose tube cable and 288-fiber high-density micro cable  

Optical Fiber Miniaturisation

This example of optical cable miniaturisation is actually enabled by optical fiber miniaturisation. Traditional recommendation ITU-T G.652 single-mode fibers feature a light-carrying core of 9.2 µm, surrounded by a glass cladding to keep light from escaping, that brings the glass strand outer diameter to 125 µm and a final acrylate coating layer to protect the glass, that brings the final fiber diameter to approximately 250 µm. In recent years, manufacturers have produced G.652 fibres with a thinner coating that reduces the overall fiber diameter to 200 µm. The 125 µm cladding diameter is maintained for splicing purposes, but the 33 percent reduction in cross-sectional area means more fibers can be packed into each buffer tube for more fiber capacity in a cable, or duct.

Duct Miniaturisation

Micro cables are installed in microducts, which deliver spatial-efficiency in two key scenarios:

1.   Overbuild

When installing additional cables into an already occupied duct, a microduct override provides more optical fibers than an equivalent-sized loose tube cable. A 96-fiber loose tube cable measures approximately ø 12 mm, whereas a 12/10 mm (outer diameter of 12 mm, inner diameter of 10 mm) microduct accommodates a 216-fiber high-density micro cable for 125 percent more fibers.

2.   New Build

When new duct infrastructure is necessary, multi-path microducts provide greater capacity in the same footprint as traditional conduits. A common 40/33 mm duct is approximately the same size as a 7 x 10/8 mm multipath microduct bundle (ø ~33 mm) 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.


  Figure 4 – microduct and micro cable overbuild and new build deployment scenarios  

Extreme-Density Ribbon Cables

In a ribbon cable, the standard twelve colored optical fibers are encapsulated in an array, or ribbon. Multiple ribbons are stacked to achieve fiber counts up to 3,456 fibers in a single cable. Until recently, ribbon cables offered a maximum of 1,728 fibers with an outer diameter of 32 mm but today, a new generation of extreme-density ribbon cable offers twice as many fiberrs in the same cable size, for twice the density in a 50 mm (2-inch) duct.


  Figure 5 – 1,728-fibre stranded tube ribbon cable and 3,456-fibre extreme-density ribbon cable  

Splicing En Masse

As optical fiber counts hit multiple thousands, splice time becomes a key concern as to splice 3,456 fibers individually would take over one hundred hours. Fortunately, ribbon cables enable mass fusion splicing, whereby 12 fibers are fused in a single step reducing splice time by as much as 80 percent. This means a 3,456-fiber 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 fiber count of 12 or higher.

Not All Extreme-Density Ribbon Cables are Created Equal

Many extreme-density ribbon cable designs feature a ‘net design’ ribbon, whereby the optical fibers are only intermittently connected so that they collapse upon one another to achieve the required packing density. This structure is less robust than proven solid ribbons and individual fibers can separate during handling. Some manufacturers recommend that net design ribbons are turned into solid ribbons using glue before splicing. This process, known as ‘ribbonisation’, can add significant time and cost to an installation.

Moreover, due to the packing density in such designs microbend attenuation is a real concern. Consequently, many extreme-density ribbon cables feature Recommendation ITU-T G.657 optical fiber for extra bend resistance that feature low (often 8.6 micron) Mode-Field Diameter (MFD). This can cause test and measurement compatibility issues when attempting to splice to legacy G.652.D fibres with an MFD of 9.2 micron; bidirectional testing is required leading to increased testing and troubleshooting time, a significant issue for high-fiber-count 3,456+ fiber cables.

However, choosing an extreme-density ribbon cable with a proven, solid ribbon from Corning would eliminate the need to preribbonise, yielding up to 30 percent faster installation than net design constructions. These cables also utilise fiber which provides G.657.A1 bend performance with a backwards compatible G.652.D-rated 9.2 micron MFD. This gives an operator 3,456 fibers in a 50 mm (2-inch) duct without sacrificing infrastructure robustness, installation efficiency or backwards-compatibility.


As connection point distribution and densification occurs in access and data center networks, more optical fibers are deployed along more pathways. But the physical duct space in which optical cables are deployed is fixed or rapidly being consumed. In response, the optical fiber cable industry has delivered two highly spatially-efficient cable solutions that help meet this distribution and densification challenge. High-density micro cables provide scalable, high-fiber-count capacity where space is most scarce; while extreme-density ribbon cables with thousands of individual fiber strands provide fast splice installation and the highest fiber counts available in a single cable.