Advanced buffer tube material upgrades fiber routing, access and entry

Advanced buffer tube material upgrades fiber routing, access and entry

Compared to existing buffer materials, a strong but pliable buffer tube material withstands tighter bends and provides less stiffness

jim holder and

richard power

alcatel telecommunications cable

Because increasing fiber-optic cable and network installations have mandated improved cable handling, craft friendliness and mid-span entry solutions to accommodate the rising number of cable entries, a second-generation Alcatel buffer tube material, or ABM2, has been developed using a thermoplastic resin blend.

Serving as a protective tube around the optical fibers, the improved buffer tube material has been field-tested for cable splicing, handling and mid-span entry access. In fact, measurement results indicate improved operational characteristics over common buffer tube materials, such as standard polybutylene terephthalate, or PBT, and polybutylene terephthalate/ polycarbonate, or PBT/PC, relative to kink resistance, tube flexibility and fiber access.

Kink resistance is an important installation parameter when field technicians have to work with small, compact splice enclosures that contain high-fiber-count cables. Presently, routing tubes (spiral wrap) and associated labeling are widely used in directing fiber-optic buffer tubes to a splice tray. This routing practice has been implemented because the kink resistance of available buffer tube materials does not adequately withstand the tight bends required in compact enclosures.

Because it can withstand small bend diameters, ABM2 provides network planners with the option of eliminating the use of closure routing tubes. Its pliability property also lowers the risk of buffer tube kinking when subjected to multiple splice tray entries. By routing ABM2 buffer tubes directly to the splice tray, tube color identification is thereby made available for future fiber splice applications. In addition, eliminating closure routing tubes saves network installation time and material costs.

Kink resistance

Buffer tube installation measurements for kink resistance made at 25C demonstrate that ABM2 offers 76% and 139% improvements over PBT and PBT/PC buffer tube materials, respectively. In other words, ABM2 is capable of smaller bend diameters without kinking compared to the other two buffer tube materials.

Measurements were made using a modified kink performance loop test. In the test setup, a 700-millimeter buffer tube length was marked 225 mm from each end. The tube was then shaped into a loop configuration with a starting diameter of 79.5 mm. Then, the tube sample was placed into a tensile tester, and the ends of the tube were pulled in opposite directions to reduce the diameter of the loop. Kink is defined as the point at which the load decreases rapidly and the tube kinks. Also tested at -10 and +60C, the ABM2 buffer tube material exhibited smaller bend diameters than the other two materials.

Environmental checks

In an environmental test, hydrolytic stability (resistance to moisture degradation while aging) measurements were taken for the PBT, PBT/PC and ABM2 buffer tube materials. This test was run for 45 days at a relative humidity of 85% and a temperature of 85C.

In accordance with Bell Communications Research Generic Requirements for Optical Fiber and Fiber-Optic Cable (GR-20-CORE, Issue 1), a tensile test for elongation at break is proposed to measure the extent of material degradation in an accelerated (high temperature and humidity) aging environment. The current minimum elongation requirement after aging, as established by Bellcore, is 10%. Upon completion of the test period, ABM2 demonstrated a minimum elongation of 300% under the environmental test conditions.

Higher hydrolytic stability in field applications means buffer tube material should remain flexible for a longer time as the material ages under prolonged heat and humidity conditions. Long-term flexibility is a desirable material trait, particularly when splice enclosures must be entered to access older, possibly brittle, buffer tubes. Furthermore, environmental testing at the temperature extremes of -40 and +70C confirmed that no measurable fiber attenuation increase or buffer tube movement was exhibited by ABM2 routed inside a splice enclosure.

Flexibility tests

For fast, easy and reliable installation of fiber-optic cables, buffer tube materials must be easy to handle and coil, especially in confined areas such as manholes and pedestals. High stiffness properties make buffer tube materials difficult to handle and manipulate. The stiffness of buffer tube materials also adds to the complexity of storing buffer tubes in splice enclosures with limited storage area. Indeed, adequate buffer tube storage is an important installation issue because of the increased use of high-fiber-count cables.

To test for flexibility, two 145-mm-diameter coils were formed using a continuous length of buffer tube. Next, the coils were taped together in four locations, 90 degrees apart, around the circumference of the combined loops. The width of the tape was kept minimal. Then, the coils were twisted 180 degrees, forming a "figure eight" configuration.

The top and bottom loops of the "figure eight" were then compressed toward each other and placed into a standard compression test unit with the compression plates spaced 50.8 mm apart. The ends were compressed and the force was measured until the top and bottom loops of the "figure eight" were 6 mm apart. A lower applied compression force translates into increased flexibility.

Simulating resistance

This flexibility test was selected to simulate the resistance that buffer tubes exert when placed into a splice enclosure. Temperatures of -10 , +25 and +60C were selected to cover a range of operating environments. The flexibility of a buffer tube material minimizes the difficulty associated with storing buffer tubes in splice enclosures with limited storage capacity. When storing buffer tubes in splice enclosures, a flexible material offers less resistance and does not exhibit the tendency to "spring back." Moreover, the "spring-back" effect can lead to kinking when buffer tubes are forced into a tight storage area inside the splice enclosure.

At all three test temperatures, the ABM2 buffer tube material required less force to compress its "figure eight" configuration. Therefore, the ABM2 is more flexible and easier to handle and coil, especially in confined spaces. For example, at 25C, the flexibility tests confirmed that PBT buffer tubes are 105% stiffer than ABM2 buffer tubes; PBT/PC buffer tubes are 194% stiffer, making them more difficult to handle.

Another important aspect of fiber-optic cable entry is the ease of buffer tube access. Stiff and rigid buffer tube materials have a tendency to break while a field technician is attempting to cut through the buffer tube material to gain access to the stored fibers. A highly elastic buffer tube therefore enables field technicians to more easily shave the material.

Ease of buffer tube entry--the pulling force needed to cut and open the buffer tube with a fiber access tool--proves 40% lower for ABM2 compared to PBT, and 18% lower compared to PBT/PC, at 25C.

Fiber access tests were also performed at -10 and +60 C. At the three temperatures, the pulling force was measured by attaching a force gauge to a fiber access tool. Then, the tool was used to shave the different buffer tube materials to gain access to the enclosed fibers. A low pulling force is less likely to break the buffer tubes or fibers.

In cold temperatures, a common practice for accessing fibers within buffer tubes involves warming the tubes with a hot air gun. Obviously, heating increases buffer tube material elasticity and thereby reduces the likelihood of breaking the tube with a fiber access tool. This practice is not necessary with ABM2 buffer tube material, however, because of its favorable elastic characteristics at low temperatures.

Switchback points

Another feature that makes fiber-optic cable entry easier, faster and accessible is the ability to identify the reverse oscillating lay, or ROL, of both armor and non-armor cables. To aid in this identification, the buffer tube switchback points are marked with the word "switchback" on the cable`s outer jacket.

For field technicians, the visual location of reverse oscillating lay becomes important for mid-span access and imperative in taut sheath accessing. Taut sheath application occurs when no cable slack is planned into the cable span during installation. Under these conditions, little or no excess buffer tube/fiber length is available for a field technician to use because of the helical construction of the fiber-optic cable core.

The reverse oscillating lay markings pinpoint the section of buffer tube where fiber length can be maximized by locating the point at which the buffer tubes can be unwound from the core. Because field technicians want to optimize buffer tube slack length for fiber splicing, the markings eliminate guesswork and permit faster fiber access. Without these markings to locate the buffer tube reversal points, the field technician usually has to make several cable entries to obtain optimal buffer tube length. This practice results in lost time and increased costs.

Fiber access tool

When performing mid-span fiber access, entering a buffer tube is a delicate task for a field technician because a broken tube or optical fiber causes costly time, labor and task delays. When accessing a buffer tube, a fiber access tool should incorporate the following features:

Quick blade change, with no blade adjustment required

Full view of cutting operation

Buffer tube sizing range of 2.5 mm (0.098 inches) to 3.2 mm (0.125 inches) to accommodate both 6- and 12- fiber tube sizes

Buffer tube sizing slots included in the body

Lightweight, contoured body

Work effectively in low temperatures. u

Jim Holder is cable development engineer and Richard Power is customer support engineer at Alcatel Telecommunications Cable, in Claremont, NC.

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