FIBER OPTIC CABLE TERMINATION

Once the fiber 0ptic cable runs are in, they are ready to be terminated. Fiber optic connectors offer a mechanical means to terminate optical fibers to other fibers and to active devices, there by connecting O/E Transducers, receivers and cables into working links. Fiber Optic Connectors are terminated onto the fiber optic cable via splicing or connectorizing. To connectorize, a connector is attached to the end of a raw fiber. Connectors can be plugged in and out of patch panels, or station outlets. In splicing, two bare fibers are joined together with a mechanical or fusion splice. A splice is a permanent joining method, used either to connect two cable runs together to make the run longer, or to add a pigtail connector onto the cable. A pigtail is a short piece of cable with a fiber optic connector factory-attached to one end. Because of the great potential for misalignment, most singlemode terminations are factory-made onto pigtails. The TIA-568-A standard does allow for fusion or mechanical splicing methods using pigtails, allowing for a maximum loss of .3 dB.A general rule of thumb is that singlemode fibers are spliced, and multimode fibers are connectorized. In most multimode applications, cable will be directly terminated with a fiber optic connector.

PREPARING FIBERS FOR SPLICING OR CONNECTORIZATION

To minimize the optical power loss across the intended fiber mating, certain conditions must be met. First, the fiber ends must be optically flat and smooth. Second, the end-to-end presentation of both fibers must align and the gap (air space) be minimized. Reflectivity must especially be minimized in high speed digital systems and analog video systems incorporating laser sources, where the connectorized fiber end face can become a highly reflective surface. To prevent these large reflections from interfering with system performance, newer types of optical finishing techniques have been developed to reduce reflections. For these applications, it is typical to use pigtails with pre-terminated factory polishes designed to reduce reflection. These should be fusion spliced to the cable, and housed in a splice tray incorporated into a patch panel.

1. Remove the fiber optic cable's protective jackets and buffers to allow access to the optical fiber. The diagram above shows the outer jacket and inner jackets are removed, exposing the Kevlar strength member,the buffer tube and the fiber. The fiber still has protective coatings which will also have to be removed. Standard cable strippers can be used to remove the outer jackets. Make sure the blades or cutting members are not damaging the buffer tubes.The Kevlar® can be trimmed using scissors or Kevlar cutters. The amount removed can vary depending upon the design of the strength member of the fiber optic cable. If the fiber optic cable does not incorporate a strength member, the Kevlar® can be used as one.
2. The buffer tubes, like the outer jackets, can be removed by mechanical stripping tools. Use care not to kink or damage the internal coated fibers.
3. Once the coated fiber is exposed, remove the protective coatings to start the actual fiber splicing. Most coated fibers can be stripped using mechanical or chemical methods. Take care to use tools and procedures that will not damage the fibers.
4. After the coating is removed, clean the fiber with isopropyl alcohol to assure the fiber is clean, as contaminants on the fiber wall can cause the fiber to mis-align itself in the alignment fixture. The fiber is now ready to be spliced or connectorized.

SPLICING

There are two types of splicing: fusion and mechanical. Fusion splicing is the joining and fusing of two fibers by placing them between two electrodes, and discharging an electric arc over the fibers. This splice technique is non-reflective. In mechanical splicing, the optical glass fibers are glued or mechanically gripped in place but not fused(melted) together.Singlemode plugs are usually spliced to the fiber cable. When terminating, a great deal of care must be taken to align and center the core of the fiber. To get the optimum alignment, usually the fiber is sized to the outside diameter (O.D.) of the cladding (usually 125 +/- 1 micron), its core(9 +/- 1 micron), and the inside diameter (I.D.) of the connector ferrule (the component of a fiber optic connection that holds a fiber in place and aids in its alignment). To avoid the great potential for misalignment, most singlemode terminations are factory-made onto pigtails and then spliced via splice trays to the cable. In addition, low reflection polishes (such as PC, SPC and APC) must be performed in controlled manufacturing environments to maintain low loss and reflection levels.

BASIC CONNECTORIZING

The connectorizing process may vary with connector style and manufacturer, but generally all follow the same basic procedure - see dagram below. Through the years, many techniques have been developed to improve on both the performance and the installation time to install fiber optic connectors. Each technique has its own advantages and disadvantages for users. Also, the installed cost is different for each one, depending on the component cost and level of skill required. The most common techniques are listed below.
• Epoxy is the most common of connectorization techniques. This method uses either heat-cure epoxy or an anaerobic epoxy to cure the fiber into the ferrule. After curing, the fiber is scribed and polished to a fine flat end surface. Several new connector types have the epoxy pre-loaded into the connector. Regarding installed cost, the parts for this method are the least expensive, but labor is more intensive than other techniques.
• Cleave and Crimp uses a pre-loaded fiber stub into the ferrule, allowing the user to prepare the fiber and jacket and then cleave the fiber to a pre-established length. The fiber is then inserted into the plug and crimped into place. This method requires the least skill on the part of the installer, but uses more expensive components.
• Ultra Violet (UV) Adhesive is similar to the epoxy technique, with the exception that the fiber is bonded via a UV adhesive and the use of a UV source such as a UV lamp or sunlight to cure the epoxy.
• Epoxyless are new no-epoxy connectors which use a unique body technique where the fiber and cable are crimped to the plug body. The plug is then mounted into a tool which forces a plunger forward, forcing a resilient sphere to provide a compression fit over the fiber. The plug is then scribed and polished.

FIBER OPTIC CONNECTORS
The primary task of a fiber connector is to minimize the optical loss across the interface of the coupled fibers. This loss is expressed in decibels (dB). High performance connectors are classified (and required by TIA-568-A) as those with less than .75 dB of loss. Losses occur from in exact end-to-end mating of the fibers, and the surface condition of fiber ends. Secondly, the fiber connector is to provide mechanical and environmental protection and stability to the mated junction. And thirdly, the connector design should permit rapid and uncomplicated termination of a cable in a field setting.
An ideal connector would encompass the following features:
• Utilize a fiber alignment scheme yielding low loss
• Be physically small
• Be of rugged construction
• Be easily field terminated
• Be field repairable;*
• Have good thermal characteristics;
• Offer excellent fiber/cable strain relief;
• Be of moderate cost;
• Be compatible with standard tooling and fiber cables.
*(Factory-terminated cable assemblies provide users with
the choice to field connectorize or to splice pigtail assemblies
using fusion or mechanical splices).