Optical fiber: blown or conventional? Costs, and ease of installation and repair are determining factors

Communications News, July, 2004 by Kevin Barks

Blown optical fiber (BOF) is an alternative to the standard installation of a fiber-optic cable plant. British Telecom first developed BOF in the early 1980s, and it was used first commercially in Europe and then Japan. BOF is used in both military and commercial environments, each with different challenges.

BOF does not produce tensile stresses on the fiber because instead of applying all the stress and pressure at the end of the cable, the fiber floats through re-installed microducts under no tension. By installing fiber in this manner, the potential of creating microbends and cracks that could lead to increased attenuation is eliminated. Other benefits of BOF are added flexibility in installation options, capacity for future growth, lower cable attenuation and a reduction in splices.

British Telecom has leased this technology to two companies. One is Sumitomo Electric Corp. of Triangle Park, N.C., which refers to it as FUTUREFLEX Air Blown Fiber (ABF).

The other company licensed by British Telecom is General Cable/ NEXTGEN Fiber Optics. General Cable/NEXTGEN, with a U.S. office in Highland Heights, Ky., has licensed its BLOLITE product to shipyards like Northrop-Grumman Newport News and Portsmouth Naval, both in Virginia. BLOLITE has been adopted by the Navy as one of the cabling methods for all new LAN backbones for both large and small platforms, as well as in more than 100 blown optical fiber installations in the United States.

The concept of blowing fiber cables has been around for some time. BOF was thought of as a subcomponent of loose tube fiber-optic cable technology. This is one of the most widely used methods in the optical fiber cable industry to protect optical fibers from water, temperature and external influences that cause mechanical damage and deterioration of the optical performance of fibers.

LOOSE TUBE TECHNOLOGY

Loose tube can be composed of one or more plastic layers, and is filled with a water-blocking jell. BOF, however, is not recommended in an aerial application due to potential condensation developing inside the tubes. Loose-tube cable is designed to protect the fibers from the rigors of installation, as well as the installed environment.

The microducts in the BOF cabling are made of a sheath material that can be polyethylene for external use or low-smoke zero halogen for interbuilding or intrabuilding, and military installations. Tests on the tubes include temperature, humidity and water immersion. The fibers are broken out from the tube at interconnection boxes, and a furcation or breakout unit may be used to build up the fiber, providing additional protection and strength so a connector can be attached for routing or connection to the end device.

In the Sumitomo configuration, tube cables act as the highway for the fiber--a key element of the fiber link. Cables are available in various sizes, with the fiber blown using low-pressure compressed air or nitrogen.

Once the tubes are installed, fiber bundles or individual fiber strands containing either single-mode, 50-micron or 62.5-micron multimode fibers are blown in. These fibers can be blown in at rates of up to 150 feet per minute. Currently, the maximum distance a group of eight fibers can be blown is up to 3,280 feet horizontally and 1,000 feet vertically. Modifications to the blowing unit can add four additional fibers that can be blown simultaneously. This will decrease the horizontal distance to 1,640 feet.

General Cable/NEXTGEN BOF technology is based on simultaneously blowing individual optical fibers. To facilitate the blowing process, an additional, textured-acrylate coating is extruded onto standard optical fibers. This additional coating provides color and a dimpled surface, which creates a viscous drag that picks up the fiber by friction. This allows the fibers to float in a stress-free manner through the center of the tubes during the installation process.

TWO DIFFERENT APPROACHES

A tractor feed is used to feed the fibers into the tubes until the friction is sufficient enough to pick up the fiber and move it through the tubes. In the Sumitomo blowing process, where fibers are blown in as a bundle, a tip is placed on the end of the fiber bundle, keeping it from getting caught on the joints of the push-fit connectors where the tube cables connect together.

The fibers are manufactured so they will feed through the blow head without jamming it and slide along the inner tubes. Sumitomo places a low-friction surface material on the fiber bundles, while NEXTGEN puts it on the walls of the tubes.

Another advantage of BOF is that a large amount of dark fiber for future growth does not need to be installed. In addition, fewer installation technicians may be required. With a conventional system, a minimum of a four-person team might take three days to complete a 3,000-foot installation. With BOF, a two-person team can perform the installation in less than one day after the microducts are installed.

From a design perspective, BOF creates continuous cable runs, so there is no need for costly splicing. Any path can be created once the tube plant is in place. As the network changes, fiber and drops can be added or systems can be reconfigured. The need to plan a network in advance or guess at the future requirements five years down the road is not necessary.