Ribbon-cable and mass-fusion-splicing technologies accrue extended benefits
When united with ribbon-cable techniques, mass-fusion splicing proves four times more productive than single-fiber fusion and two times more productive than "field-ribbonized" loose fiber mass fusion
Ronald G. Lindsay, Jr. and scott D. Robbins
Sumitomo electric lightwave corp
stephen c. mettler, charles f. cottingham and kenneth w. jackson
at&t bell laboratories
Ribbon-cable and mass-fusion-splicing technology advancements offer improved splicing productivity, cable placement and preparation, fiber and splice organization, mid-span fiber access, splicing and storage for rearrangements, and fiber identification. Because of design and process improvements, central-tube ribbon cable is now preferred over other types of high-fiber-count cables. Tighter parameter values for fiber core-to-cladding concentricity error (offset), cladding diameter and curl enable ribbon cable to achieve--with mass splicing--singlemode splice losses that would have been unattainable in the 1980s.
The latest mass-fusion techniques make it possible to splice entire ribbons in a little more time than it takes for single-fiber fusion. These techniques provide uniform splice quality, better than 0.1-decibel average splice loss, negligible material cost, numerous software options and installer friendliness. When used with ribbon cable, mass-fusion splicing has proved, via industry studies, to be approximately four times more productive than single-fiber fusion, and almost twice as productive as mass-fusion splicing of ribbonized cable fashioned in the field from loose fibers.
These gains in productivity are expected to benefit the installation of fiber-in-the-loop systems, which require large fiber counts, frequent mid-span access, high splice quality and unskilled splicing methods. To that end, progress continues to be made in miniaturizing mass-fusion machines, providing robotic features and maintaining costs at almost the same level as for single-fiber fusion machines. Likewise, fiber-optic ribbon-cable technology breakthroughs are occurring with novel ribbon sizes, cable designs, manufacturing methods and installer procedures.
As fiber-optic cable extends deeper into the distribution plant, installation demands have focused on high-fiber-count cables and more efficient splicing techniques. For FITL applications, ribbon-cable designs are preferred to loose-fiber-cable designs because of high-packing density, easier fiber identification, splicing productivity, fiber/splice organization and robustness. The FITL marketplace also demands ribbon-at-a-time splices that are reliable, easy to learn, fast to complete in the field and low in loss.
During the past decade, breakthroughs in ribbon construction and ribbon-at-a-time splicing have paved the way for ribbon cable`s growing popularity and wider applications. To date, more than 2 million fiber-kilometers of ribbon cable have been placed and spliced, with an annual growth in sales of 50%. The latest fiber ribbons are compact, robust and installer friendly. They use a removable matrix material to bond the fibers in lieu of the "tape-sandwich" technique implemented in the first-generation ribbons of the 1970s and early 1980s. New ribbon sizes have proliferated, with 2-, 4-, 8- and 16-fiber ribbons augmenting the standard 12-fiber ribbon design.
Field splicing of ribbon cables has advanced because of mass-fusion splicing--a process that simultaneously fuses the fibers of one ribbon to those of another. It helps field installers meet rigorous time demands for fiber project completion.
Mass-fusion machine technology advancements are providing improved field productivity, low and uniform splice losses, premium splice quality and strength, and such options as the ability to program splices of different fiber and ribbon types.
A ribbon consists of an even number of coated fibers arranged adjacent to one another in a row, and held in place with a transparent acrylate bonding matrix material. Immediately preceding mass splicing, the bonding material and fiber-coating material can be stripped in one piece from the end of the ribbon. The flexible matrix material permits handling of the ribbons in a manner similar to that used for individual fibers. Yet, the ribbon is mechanically stronger than the individual fibers. The matrix material is selected for compatibility with cable-filling compounds and to ensure reliability in adverse environments. Ribbons are manufactured using a separate coating process after their component fibers have been individually spooled and tested.
Low-loss, ribbon-at-a-time splicing depends critically on the geometrical properties of fibers within each ribbon, especially on the accurate alignment of the singlemode 8-micron fiber cores within the surrounding cladding glass. Recent process improvements have permitted fiber manufacturers to tighten their cladding diameter specification to 125 ۫ micron, and the singlemode core-to-cladding concentricity error (also known as offset or eccentricity) to 0.8 micron maximum.
Another important parameter in mass-fusion splicing is fiber curl. Excessive curl (an abnormally small radius of curvature of a fiber when unstressed) can result in poor alignment with the mating fiber and high splice loss during the mass-fusion process. Fiber manufacturers have developed ways to minimize curl. Most vendors specify a minimum fiber radius of curvature of 2 meters--a curl value that ensures uniform fiber cladding contact with the positioning V-grooves of mass-fusion machines.
Manufacturers have also improved the processes that govern ribbon geometry by carefully monitoring such parameters as bonding matrix material thickness and fiber alignment within the bonding matrix. Because mass-fusion splicing relies on cladding alignment, fiber and ribbon geometry improvements have contributed to consistent low-loss splices and more accurate splice-loss estimation.
Fiber identification schemes
The fixed location of the fibers within a ribbon cable contributes to the rapid identification of individual fibers. Also, color-to-color fibers can be spliced faster and more accurately than large-fiber-count loose-fiber cables. Individual fibers are easily recognizable under the transparent matrix material in the numerical order of the standard U.S. color-code sequence. The sequence is the same as the decades-old copper polyethylene insulated conductor code--blue, orange, green, brown, slate, white, red, black, yellow and violet for the first 10 colors, followed by rose and aqua for fiber numbers 11 and 12.
Individual ribbons within a cable are identified via an alphanumeric label that is printed directly on each ribbon and repeated at regular short intervals. This label is also used to provide valuable information, including fiber type and ribbon manufacturer. Multiple ribbons within the same cable are organized in a numeric order that facilitates rapid and positive identification of each ribbon because of its printed label.
A central-tube ribbon-cable construction provides high fiber density that minimizes cable size and weight, optimizes duct space and facilitates the buried or aerial installation of long reel lengths. These factors are important in subscriber-loop systems, where duct space is limited and small cable size can reduce the labor cost of tunneling. Positioning the ribbons in a central tube close to the center line of the cable minimizes the stresses applied to the fibers when the ribbon cable is bent.
Manufacturers offer central-tube ribbon cables that contain from one to 18 ribbons; for example, 216 fibers are provided by 18 12-fiber ribbons. The ribbons are normally stacked in a rectangular array, and a small level of twist is imparted to the ribbon stack to minimize stress on individual fibers whenever the cable is bent.
During cable manufacture, a central-core tube (or buffer tube) is extruded around the ribbon stack. In outside-plant installations, a filling compound is used to displace the air space between the ribbons and the tube to prevent water entry in case the sheath is accidentally damaged. To promote cable flexibility and mid-span ribbon access, a polyolefin material is used for the central-core tube.
Ribbon cables are available in a variety of metallic and all-dielectric sheath constructions. Armored cables, containing one or more corrugated metal layers under the outer layer of polyethylene, are necessary for most direct-buried applications, particularly if gnawing rodents inhabit the area. These cables can withstand mechanical abuse much better than their all-dielectric counterparts of comparable size.
All-dielectric cables offer immunity from lightning or high-voltage power line damage, and require no electrical bonding or grounding at splice closures. The express-entry type of armored and dielectric sheath has become popular worldwide; it permits installers to easily enter the cable and quickly access the ribbons for splicing. Ease of mid-span access--important in subscriber-loop applications--is aided by the express-entry sheath design. The express-entry cable family finds usage in pole-line, direct-burial and duct-pull applications.
To quicken the splice preparation of express-entry ribbon cables, two ripcords are placed 180 degrees apart along the externally embedded strength members. The installer removes the outer jacket in "banana-peel" fashion after ringing (circularly cutting) the sheath at appropriate points and exposing and pulling the ripcords. In the armored (metallic) express-entry cables, a secondary ripcord helps remove the corrugated metal layer to expose the core tube. The core tube is removed by using ringing and slitting tools, and filling compound is removed from the ribbons by using alcohol wipes.
Ribbons can easily be identified for routing and organizing in the splice closure and for mass splicing. Several centimeters of ribbon are stripped clean of bonding matrix material and fiber coating material, prior to splicing. This operation is done with a stripping tool that applies heat to the ribbon for a few seconds. Heating makes the material soft and easy to strip without damage to the bare fibers.
The matrix material and the dual-layer fiber coating simultaneously "tube" off the fibers in one piece. The fibers are then wiped with an alcohol-soaked soft wipe to remove any residual materials that could interfere with the alignment during splicing. The ribbon is cleaved as a unit (all 12 fibers at once) and placed in the splicing machine.
Using an automated microscopic video system, the machine checks the fibers for end angle and alignment. It then performs the mass-fusion splice if all parameters are acceptable. Next, the machine estimates the splice loss using the fiber alignments measured before and after splicing. Twelve-fiber-at-a-time mass-fusion splicing of factory-made ribbons normally produces actual mean splice losses of 0.05 to 0.10 dB, with a standard deviation of approximately 0.05 dB. The splice is proof-tested for tensile strength and protected with a heat-shrink fiber protection sleeve.
Stripping the matrix material and fiber coatings, cleaving the ribbon ends, fusion splicing the ribbons, shrinking the protection sleeve and storing the completed ribbon splice in the splice tray are completed on a pair of ribbons at a time. To optimize splicing productivity, splicing of a ribbon pair is usually performed while the previous splice is in the protection sleeve process.
Mass-fusion splicing two factory-made ribbons takes slightly more time than splicing two individual fibers. Thus, the productivity gains with mass fusion are potentially large. In large fiber-count splices of individual fiber cable (for example, the loose fiber bundle or loose-tube type), productivity can be improved by "ribbonizing" the fiber ends in the field, allowing the use of mass fusion. Industry studies indicate ribbonizing achieves productivity gains of approximately a factor of two over individual fiber fusion.
However, ribbonizing does not consistently result in splice-loss characteristics equivalent to those achieved by factory-made ribbons. The differences occur because factory-made ribbons involve precisely controlled conditions, which result in stringent dimensional and quality specifications and consistent core alignment. Studies show that ribbonizing loose fibers in the field increases the likelihood of color-code errors relative to factory-made ribbons.
Fiber-optic feeder/distribution networks frequently require mid-span cable re-entry and rearrangements to serve new customers or to provide more services to existing customers. These needs often call for re-entering a splice closure or cabinet that contains working fiber-optic cables and splicing a selected group of dark fibers into a branching cable. In such mid-span access tasks, the use of ribbon cable can improve productivity in several ways. First, the well-organized ribbon color code facilitates rapid and positive fiber identification, reducing the chance of disrupting service on working fibers. Second, the compact size and flexibility of ribbons compared to the buffer tubes of loose-tube cables result in improved fiber organization and slack storage within the closure. When a new branching cable must be stored in the same closure with the original cable, the volume and complexity of slack-loose fibers can be a problem for the splicer.
Another bonus provided by ribbon cable in mid-span entry situations is that mass-fusion splicing of the branching fibers is often an option, and can thus achieve significant productivity gains relative to individual fiber splicing. u
Ronald G. Lindsay, Jr. is a cable applications engineer and Scott D. Robbins is product manager of fusion splicing at Sumitomo Electric Lightwave Corp., Research Triangle Park, NC. Stephen C. Mettler, Charles F. Cottingham and Kenneth W. Jackson are members of the technical staff at AT&T Bell Laboratories in Norcross, GA.