Fibre-optic micro cables - the past, present and future
By Justin Quinn*, Product Development Engineer
Wednesday, 10 June, 2015
Fibre-optic cables have evolved over the years as the demand for bandwidth has increased exponentially. This article details the evolution of micro cables and looks ahead to future designs.
The fibre density - the ratio of the number of optical fibres to the cable outer area - has continued to increase. Today, ultrahigh-density micro cables are commonly used in applications where space is limited. This can include micro cables that are blown into ducts or routed through data centres. In each case, cables are designed to be compact, yet provide the robustness required to protect the optical fibres.
The primary purpose of a fibre-optic cable is to protect the optical fibre contained within. During installation and throughout the life of a cable, it may be exposed to a variety of mechanical forces and environmental conditions.
As such, stakeholders have worked together to develop several documents that identify typical conditions to which cables may be exposed, depending on the application, and define performance expectations under such conditions.
While traditional aerial and direct buried applications require a robust cable that is capable of withstanding severe forces as described in industry documents, these cables are often overdesigned for many applications.
Particularly in FTTx environments, where higher fibre density and reduced cable diameter are preferred, many applications allow for alternative designs that may not be as rugged as traditional cables. Such cables are often referred to as micro cables.
This article focuses on micro cables that are designed for three different applications: outside plant (OSP) cables for use in microducts, indoor cables for use outside of ducts and indoor cables for use within microducts.
OSP cable
Traditional OSP cable
OSP cables typically consist of loose groups of fibres within buffer tubes, where multiple tubes are SZ stranded together over a central strength member (CSM). The stranded core may be covered with additional strength members and/or corrugated armour along with the outer jacket(s). Figure 1 shows the construction of a typical OSP cable, suitable for direct burial.
Traditional OSP cables are often designed and tested to meet the requirements of GR-20-Core. In particular, this standard defines test methods and criteria to determine impact and compression resistance, tensile strength and operating temperature ranges for cables. The buffer tube material and dimensions can have a significant impact on each of these characteristics.
Optical fibres use light waves to transmit signals. As the light wave travels down the fibre, some of the power is naturally lost as light escapes from the core. In addition, macro bends and micro bends of the fibre can cause additional attenuation losses. For example, as the buffer tube contracts at the cold operating temperature, the fibre may buckle if there is not enough free space within the buffer tube, creating significant macro-bending loss. Alternatively, if fibres are pulled taut against a rough buffer tube surface under the tensile loads and bends applied during and after installation, fibres may experience significant micro-bending loss.
In order to overcome these issues, buffer tubes have traditionally utilised a large inner diameter (ID) to provide considerable free space for the fibres, resulting in a large outer diameter (OD). The large OD allows for a large central strength member (CSM), which contributes significantly to the tensile strength of the cable and limits contraction at cold temperatures. However, this design results in a large cable OD and, therefore, low fibre density. Fibre density is defined as the ratio of the number of fibres to the cross-sectional area of the cable. For the 72 fibre, traditional OSP cable shown in Figure 1, the fibre density is 46 cm-2.
OSP microduct cable
Fibre-optic ducts, typically plastic tubes, have been used since 1981 as a conduit to route cables and limit their exposure to many external forces.1
Initially, the ducts used traditional OSP cables, but they were often overdesigned for these applications. Naturally, cable manufacturers quickly began developing smaller, less robust cables to take advantage of this technology. As cable sizes continued to decrease, duct manufacturers were able to develop smaller ducts.
In the 1990s, this led to the development of multitube ducts. Where a traditional 1¼″ duct was suitable for one 288 fibre cable, the 1¼″ multiduct, containing five microducts, was suitable for five 72 fibre micro cables. This increased the fibre density of the duct system by 25% and started a trend that has continued into the 2010s.
Microduct cables were included in the third issue of GR-20-Core, which was released in 2008, although no requirements were defined. The fourth issue, released in 2013, refers to the requirements of IEC 60794-5-10 and allows for testing at lower levels than traditional OSP cable (since cables are installed within protective ducts). This is important to understand since, for a given fibre count, the size of the individual components - buffer tube, CSM and outer jacket - must decrease for fibre density to increase.
A reduction in the buffer tube size may result in a less compression-resistant tube and less free space for the fibres within the tube, making the cable less resistant to compressive loads. Figure 2 shows the difference in compression resistance for a traditional buffer tube with fibre density of 210 cm-2 (low density) versus a smaller tube, made with the same materials, with fibre density of 580 cm-2 (high density).
Additionally, the reduced tube size and free space within the tube reduces the fibre strain free window, or the amount of strain applied to the cable before the fibres begin to experience strain. In loose tube cables where the buffer tube is stranded over a CSM, the fibres will actually pull to the inside wall of the tube, since this is the shortest path, prior to experiencing strain. Assuming the buffer tube has no excess fibre length (EFL), the strain free window, ∆H, can be estimated by calculating the relative change due to the difference in the helix along the centreline of the tube and the helix when the fibres pull to the inside wall of the tube.
Further, a reduction in size of the CSM, a significant contributor to the overall strength of the cable, results in higher cable strain than traditional OSP cables under the same tensile load. When combined with the reduced strain free window, OSP microduct cables are rated at lower tensile loads to limit the strain on the fibres. However, since these cables are jetted or blown into the microducts, reduced tensile ratings are acceptable and significant increases in fibre density are achievable.
Future development of microduct cables
Recent development of 200 µm optical fibres will allow additional increases in fibre density, both with traditional OSP cables and microduct cables.
Indoor cables
Traditional tight buffered cables
In transitioning from OSP cables to indoor cables, the requirements change significantly, so the construction of these cables changes as well. Notably, indoor cables must be flame retardant, so that in the event of a fire, the cables will not propagate the flame and smoke throughout the structure. Further, cables must be more flexible to facilitate installation and fibres should allow for connectorisation.
To support these alternative requirements, traditional indoor cables utilise 900 µm tight buffered fibres, semi-rigid or non-rigid strength members, and flame-retardant buffer and jacket materials. Figure 3 shows the construction of a typical indoor backbone cable with a fibre density of 25 cm-2.
Traditional indoor cables are often designed and tested to meet the requirements of Telcordia’s GR-409-Core. Again, this standard defines test methods and criteria to determine impact and compression resistance, tensile strength and operating temperature ranges for cables.
Similar to OSP cables, materials and dimensions of indoor cables can have a significant impact on each of these characteristics. While this standard does distinguish between light-duty interconnect cables and heavy-duty backbone cables, it does not include requirements for indoor duct cables.
Indoor micro cables
Since optical fibres are capable of carrying so much bandwidth, low fibre count cables are suitable for most single family residences. However, some applications, such as data centres, require high fibre count cables and benefit greatly from high fibre density, as they require less space and are less restrictive to airflow.
Because these cables may be installed directly, that is outside of ducts, they are designed to meet the requirements for backbone cables. In this sense, cables meet the same mechanical, environmental and safety requirements as traditional, tight buffered indoor cables. However, because of the application and connector options, early micro cables were developed where bare fibres replaced tight buffered fibres. As seen in Figure 4, the change from tight buffered fibre to bare fibre provides an avenue to reduce the cable diameter and increase fibre density. However, additional changes are required. For one, the multimode fibres used in many indoor cables are more sensitive to macro bending and micro bending than singlemode fibres used in most OSP cables. In traditional indoor designs, the tight buffer over the fibre minimises the fibre’s exposure to these conditions. To limit these effects without the tight buffer, the choice of flame-retardant jacket material becomes critical.
Traditional flame-retardant jacket materials used for indoor cables include fluoropolymers and polyvinyl chloride (PVC). There are advantages to each, but engineered PVC compounds are preferred because of their cost and customisable physical properties.
In addition to mechanical protection, the flame-retardant jacket materials limit flame spread and smoke generation when cables are exposed to fire. Commonly in data centres, cables are installed in plenum locations, and cables must be rated for use in these locations.
Throughout the late 2000s and into the 2010s, cable manufacturers and PVC compounders have collaborated to develop higher performing flame-retardant materials, allowing for continued increase in fibre density and fibre count per cable.
With proper materials and design, indoor micro cables can meet the same mechanical, environmental and safety performance levels as traditional indoor cables with significantly reduced cable OD and increased fibre density. Further, micro cables allow for higher fibre counts, including the GR-409-Core horizontal backbone compliant 288 fibre cable developed earlier this year.
Indoor microduct cables
For many of the same reasons that microduct cables are beneficial in OSP environments, they are also beneficial in indoor environments. Similar to OSP microduct cables, indoor microduct cables are jetted or blown into ducts with compressed air. However, in this case, the microducts are tubes made of flame-retardant plastics and can be independently safety listed, enabling future system expansions.
Once again the microduct prevents these cables from exposure to many mechanical forces. However, the most recent revision of GR-409-Core, released in 2008, does not address indoor microduct cables. While the previously discussed indoor micro cables can be used within ducts, development of ultrahigh fibre density indoor microduct cables, also called air-blown fibre, began in 2009. These cables are designed and tested to meet the requirements of interconnect cables.
Material selection is again critical as these cables must meet the same safety requirements as other indoor cables, while providing the proper stiffness for proper installation. Replacing buffered subunits with bundled fibres allows for ultrahigh fibre density. Figure 5 compares a 72 fibre indoor microduct cable with a fibre density of 453 cm-2 to the previously discussed indoor micro cable.
Future development of indoor micro cables
In addition to development with 200 µm fibres, work is ongoing with Spider Web Ribbon (SWR). Unlike traditional ribbon, SWR consists of 250 µm fibres fixed intermittently with UV curable resin. As such, the ribbon can be easily formed into a bundle within a cable and unrolled for mass fusion splicing. Alternatively, the fibres can be easily separated.2 Development work is also ongoing with halogen-free flame-retardant materials.
Conclusion
Since 1974, when the first optical cable system was deployed in Long Beach, California, optical fibres, cables and systems have changed to meet the needs of customers.3
In particular, micro cables, smaller cables with higher fibre density than traditional cables, are evolving, with current designs suitable for a variety of applications, including backbone networks and FTTx. These cable constructions support the need for higher fibre density and smaller cable diameter. Additionally, since cables can be blown in when needed, installers can realise a reduction in initial investment costs. Utilising improved, bend-insensitive fibres, higher performing flame-retardant materials, and innovative cable and duct designs, today’s micro cables comply with industry-developed performance standards and appropriately protect the optical fibres.
References
1. http://www.duraline.com/content/history
2. M. Yamanaka, K. Osato, K. Tomikawa, D. Takeda, M. Isaji, N. Okada, “Ultra-high density optical fiber cable with ‘Spider Web Ribbon,’” Proceedings of the 61st IWCS Conference, pp. 37-41.
3. Vivek Alwayn, Optical Network Design and Implementation. (2004).
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