 Figure 1. Stripped fiber optic cables, ready for termination. |
By Pete Doyon
Common fiber optic cable constructions and preparing each as part of the cable assembly process
Fiber preparation is the process of preparing a fiber optic cable for termination. The steps include, but are not limited to, stripping the outer jacket, cutting the Kevlar, and stripping the buffer and/or coating. This article looks at the most common fiber optic cable constructions and how each must be prepared as part of the cable assembly process. The steps that are critical to success are discussed, and the problems associated with manual fiber preparation and the benefits of automation are considered.
Many process steps precede the fiber preparation step, including measuring and cutting the fiber optic cable to length, coiling and tying it, and labeling the cable assembly. Because many of these steps have already been automated for the copper wire and cable industry, only minor modifications were necessary to make the same equipment suitable for fiber optic cable handling. With this part of the process puzzle considered solved, it is not discussed further.
Glass Optical Fiber
Glass optical fiber, the type typically used in the telecommunications industry, has a nominal outside diameter of 125 µm. As part of the manufacturing process, a protective plastic coating is applied to the bare fiber after the drawing process, but prior to spooling. The most common coating is a UV-cured acrylate, which is applied in two layers, resulting in a nominal outside diameter of 250 µm for the coated fiber (see Figure 1).
 Figure 2. Automatic machine for stripping primary and secondary coatings. |
The coating is highly engineered, providing protection against physical damage caused by environmental elements, such as temperature and humidity extremes, exposure to chemicals, point stress, and so on, while also minimizing optical loss. Without it, the manufacturer would not even be able to spool the fiber without breaking it.
The coating must protect the fiber over its installed life, yet still meet standards for stripability, which require that the peak strip force to mechanically remove 30 mm of coating not exceed 2.0 lb force.
The primary coating is stripped using a hand tool or stripping machine with blades having a precision bore sized slightly larger than the 125 µm fiber diameter. During the stripping process, the 250 µm coating gets scraped off of the glass and breaks up into many fine particles. The coating cannot be removed in one cohesive tube, but it can be stripped in one pass without the need for preheating the fiber beforehand. This method works equally well for end stripping or window stripping and can be done by manual or automatic methods. Regardless of the method, because glass is so brittle the utmost care must be taken in stripping the fiber. The slightest scratch will provide a point for a crack to propagate, ultimately resulting in a broken fiber.
Cleaning the fiber with a clean lint-free rag soaked with isopropyl alcohol is recommended to remove any loose particles of coating from the glass before terminating it. Because the cleaning process reduces the tensile strength of the fiber, only one wipe is recommended to minimize the damage.
The 250 µm-coated fiber is the building block for many common fiber optic cable constructions. It is often used as is, especially when additional mechanical or environmental protection is not required, such as inside of optical devices.
Buffered Fiber
For additional physical protection and ease of handling, a secondary coating of polyvinyl chloride (PVC) or Hytrel (a thermoplastic elastomer that has desirable characteristics for use as a secondary buffer) is extruded over the 250 µm-coated fiber, increasing the outside diameter up to 900 µm. This type of construction is known as "tight-buffered fiber." Typical fiber preparation requirements call for stripping 20 mm or more of the primary and secondary coatings down to the bare fiber. In order to remove both coatings at the same time, a controlled heat is applied to the fiber to soften the coatings. When done properly, both coatings "tube off" as one package, leaving a clean bare fiber.
Some applications call for window stripping a section of the 900 µm-buffered fiber at some point other than the fiber end. In this case, it is not possible to displace the coating layers to create a stripped window. Instead, an automatic machine with a dual slitting blade set is required to axially slit both sides of the 900 µm buffer down to the 250 µm-coated fiber. Two radial cuts are then made to define the boundaries of the window strip. When the fiber emerges from the machine, the preslit secondary coating must be manually removed to expose the primary-coated fiber. Finally, a hand tool is used to carefully strip the primary coating down to the bare fiber.
Jacketed Cable
Jacketed cable is typically constructed with two additional layers that are applied over a 900 µm-buffered fiber. The first layer is made up of Kevlar fibers, which act as the tensile strength member of the cable. The second layer is an outer jacket, typically made of PVC material that is extruded over the inner layers to complete the cable package. The jacket can be easily removed with a manual stripping tool or inline using an automatic machine.
The Kevlar fibers usually must be trimmed so the remaining fiber strands protrude about 8 mm from the jacket strip position. Although scissors are often used to trim the Kevlar, automatic machines are available (see Figure 2) that use air from a vacuum to pull the Kevlar to one side and trim it. This method is preferable over manual trimming to ensure that Kevlar fibrils (subfibers) are disposed of safely (so they cannot enter the respiratory tract).
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Often, a duplex cable is constructed by extruding two simplex cables (a cable construction containing only a single fiber) side by side into a zipcord-type package. During cable assembly, the zipcord is usually peeled apart manually so that two simplex cables can be terminated. The slitting operation can also be performed automatically on an inline system that measures, cuts, strips and coils the cable.
Flat Fiber Ribbon Cable
Glass optical fibers are often grouped together in a flat array of four, eight, 12, 18 or more fibers to increase density for packaging and termination. The flat ribbon is manufactured by color-coding the individual 250 µm-coated fibers and bonding them together in a matrix material. The typical fiber preparation for this cable type is to strip all coatings down to the bare fibers in a single pass. This requires a semiautomatic stripping device that has a heating system to soften the coatings so they can be removed cleanly. Precise temperature control of the heating system and ultraprecision blades are necessary for successful stripping results.
Manufacturers Automate
Most of the fiber preparation procedures described in this article can be accomplished using hand tools or semiautomatic or fully automatic machines (see Table 1). A skilled fiber optic technician can achieve satisfactory results using simple hand tools or automatic machines. With the low unemployment situation, fiber optic cable assembly shops are having difficulty finding, training and retaining skilled technicians. In order to gain control over the entire cable assembly process, manufacturers are looking to automate each fiber preparation step to eliminate operator variability.
Conclusion
The fiber optic cable assembly industry has experienced explosive growth in volume. There is a lot of competition and the price for finished fiber optic cable assemblies has dropped steadily. The cost for the components has dropped significantly as well, yet the labor portion of the assembly has remained almost constant.
Fiber optic cable termination preparation includes stripping the outer jacket, cutting the Kevlar, and stripping the buffer and/or coating. A process that relies heavily on manual processes is difficult to control, making it even more difficult to achieve a high yield. By automating the steps in the process, fiber optic cable assembly shops will be able to achieve higher quality at a lower cost.
PETE DOYON is Vice President of Product Management, Schleuniger Inc., 87 Colin Dr., Manchester, NH 03103; (603) 668-8117; Fax: (603) 668-8119; Web site: www.schleuniger.com.
SPEC SHEET
End Applications:
Fiber optic cable assemblies
Related Products:
Manual and automatic stripping tools
Main Point:
The price for finished fiber optic cable assemblies has dropped steadily, yet the labor portion has remained almost constant. Common fiber optic cable constructions and how each must be prepared as part of the cable assembly process are covered. Steps for fiber preparation include stripping the outer jacket, cutting the Kevlar, and stripping the buffer and/or coating. Performing these steps properly, whether manually or automatically, is critical to success.
