Posted: 11/15/1998
Optical Cable:
Fiber's Knight in Shining Armor
By Dean Yamasaki
Optical cables are fiber's knights in shining armor. They protect optical fibers from damage during installation, and for years after, against the ravages of the environment.
But no single optical cable design can do it all. Outdoor and indoor applications each have unique needs that can be met best by particular optical cable options. For example, flame-retardant jacket materials required for indoor applications are not as rugged as cable jacket materials typically employed for outdoor applications. So designers have created various cable options for different needs.
Stranded loose-tube and tight-buffered cable are the two prevalent optical cable designs. Loose-tube cables are designed for outdoor applications, and this robust design protects the fiber from a host of environmental and mechanical hazards. Tight-buffered cables are best used indoors. This design's main strengths are direct termination simplicity and flame retardance--key elements of any indoor application.
Travails of the Outside Plant
In general, optical fiber cables installed in an outdoor environment are exposed to more severe mechanical and environmental conditions than those in the protected, climate-controlled indoor environment. Outdoor installations are subjected to demanding combinations of environmental conditions such as ultraviolet (UV) radiation, standing water, cable-gnawing rodents and extreme temperatures. Outdoor installations usually expose cables to more severe mechanical forces during both installation and operation.
Image: Stranded Loose-Tube Cable Design
Source: Siecor Corp.
Changes in the surrounding temperature will initiate expansion or contraction forces within the cable structure. But the loose-tube cable design mitigates the influences of external effects on the optical fiber. It compensates for movement in the cable structure without inducing mechanical forces on the fiber.
In comparison, a tight-buffered cable design does not decouple the optical fibers from the effects of expansion and contraction of the other cable components. The thermoplastic "tight" buffer is directly coupled to each fiber. Tight-buffered cables do not isolate the fibers from external forces, so temperature-related expansion and contraction effects directly applied to any component of a tight-buffered cable are translated to the optical fiber. Tight-buffered cables typically are more sensitive to temperature extremes and mechanical disturbances than loose-tube cables.
Ice-crush hazards affect optical cables in locations where standing water and freezing temperatures coexist. In a confined space, water expanding as it turns to ice can create significant tensile and compressive forces. An optical cable is particularly vulnerable if water can reach the cable core and subsequently freeze. Ice formation in the cable-core area imparts stress close to the optical fibers and may result in unacceptable attenuation or fiber breakage.
A loose-tube cable is designed to provide maximum protection against water penetration and migration. Water is blocked by surrounding the cable core with a water-swellable material to stop the entry and migration of water should the cable's outer jacket be breached. Standard tight-buffered cables do not have waterblocking protection, making them susceptible to damage caused by water penetration and migration.
Of course, no plastic material is completely impervious to water and a tight-buffer material alone cannot permanently isolate an optical fiber from the influence of moisture. If water penetrates the cable jacket and subsequently freezes, the individual tight-buffered fibers are subjected to microbending, increased attenuation or damage.
Optical fiber cables placed outdoors, especially in aerial applications, must withstand direct exposure to sunlight and its damaging UV radiation. UV light, in combination with heat and moisture, can adversely affect optical cable performance. Without protection, UV light degrades the mechanical performance of cables. Carbon black, compounded directly into the thermoplastic jacket material, is the best defense against UV degradation of the jacket material. Standard tight-buffered cables, typically yellow or orange, do not incorporate carbon black into the jacket material and are susceptible to UV damage.
Loose-tube cable design also performs well during installation and maintenance situations in which cable is subject to tensile, flexure, twisting, crushing, impact and bending forces. The magnitude of these forces usually is more severe in the outside plant. By isolating the fiber from external forces, the loose-tube design ensures maximum cable life in an outdoor environment. Again, tight-buffered cables do not isolate the fibers from external forces and typically are more sensitive to mechanical disturbances than loose-tube cables.
The cable's outer jacket is the first line of defense to protect the optical fibers. The outer jacket of the cable must be rugged enough to withstand initial installation forces as well as years of outdoor punishment. Polyethylene (PE) typically is used for the jacket of dedicated outside-plant loose-tube cable. PE is rugged and provides both a low coefficient of friction and a higher abrasion resistance, which is good during installation.
Tight-buffered cables usually employ polyvinylchloride (PVC) jackets formulated to provide flame retardance for indoor applications. These PVC materials are not as rugged as PE materials used for dedicated outdoor cables due to their flame retardancy additives. The more durable medium-density PE (MDPE) material is better suited to protect an optical cable outside.
Installing the New Guard
Loose-tube optical cables have been in the field for more than two decades, but they continue to improve with technical advances.
The growing use of optical fiber to support broadband applications has resulted in more diverse splicing and termination environments. Locations for these procedures frequently include tight-fitting pedestals and smaller enclosures dictated by the application. Today's installations are more likely to expose the cable to high-heat/high-humidity conditions that potentially can degrade the buffer-tube material performance through hydrolysis.
Hydrolysis reduces the flexibility of the buffer-tube material and could result in brittleness. Recently developed loose-tube cables incorporate an advanced engineering thermoplastic buffer tube material designed to resist hydrolysis and provide improved long-term performance in the expected environment presented by today's broadband applications. This material also improves the kink resistance of the buffer tube and aids technicians in routing buffer tubes in smaller closures and hardware without affecting field technician efficiency.
A second technical innovation, which results in improved craft efficiency, is Dry, a water-swellable material that helps prevent water migration along the outside of the buffer tubes within the cable core. A gel material still is used inside each buffer tube and works with the Dry material to protect the optical fiber from moisture. Testing has demonstrated that cables with this Dry technology will meet the same industry-accepted water penetration testing as their gel-filled counterparts.
Replacing the gel on the outside of the buffer tubes with a dry, water-swellable material reduces cable preparation time by eliminating the time-consuming process of wiping the cable core with a degreasing agent. In contrast to the gel-removal process, the Dry material can be removed quickly and easily with a pair of scissors. Several studies have indicated that up to a 50 percent reduction in preparation time can be realized from this one innovation.
Dean Yamasaki is an applications engineer at Siecor Corp. in Hickory, N.C. He can be reached at (704) 327-5000.
