With fiber optic cabling becoming an increasingly important aspect of Local Area Network communications infrastructures, LAN installers have to be well schooled in the fundamentals of terminating fiber and installing fiber connectors. In fiber network installations, workmanship is absolutely critical to achieve acceptable results. Even a small imperfection or microscopic dirt on the face of the fiber can create significant problems with optical propagation that lead to failure of the link.
While having the proper tools is a critical factor for success, using the proper techniques is also paramount. As many LAN installers migrate from a focus on copper to handling fiber installations, it is all too easy to pick up bad habits that can lead to inefficiency, result in substandard quality and may also create safety risks.
This article will provide a hands-on tutorial regarding the best practices for quickly, safely and correctly creating fiber optic connections that meet accepted standards of quality workmanship and assure optimal coupling efficiency. The following sections are intended to help field installers avoid problems by providing a solid base of information that can act as a learning tool and a reference source for both new and experienced field technicians handling fiber-cabling termination.
Creating and Maintaining a Safe Work Environment for Handling Fiber
The first and foremost consideration that all installers must keep in mind is the need for adequate safety measures when handling and terminating fiber. Not only should technicians protect themselves during the installation process, they also need to leave the completed installation area in a safe condition for other people who follow behind them.
Fundamental safety tools include a dark work surface, such as a black work mat, and a proper trash receptacle for fiber scraps that is clearly marked as to its contents (not just a piece of black tape to stick them on). Unfortunately, too many technicians have been incorrectly trained in the field to just flick off the cleaved fiber scraps with their fingers; a practice that is grossly unsafe in buildings or schools where the subsequent occupants could be harmed by coming into contact with the nearly unnoticeable sharp fiber scraps.
The technician should always have a pair of Teflon-tipped tweezers close at hand for removal of fiber splinters. Safety glasses are an absolute requirement. Good safety practices also include washing of hands as soon as they are done handling the fiber and avoiding all food and drink during the fiber handling process.
Later on, when checking fibers with a microscope, technicians should always make sure that the other end is not connected to a power source. Installers must remember that laser light is not visible so they may not have any warning of the danger and therefore should make no assumptions without double-checking on the power status of the link. As an extra precaution, it is also a good idea to select field microscopes with built-in IR eye protection to guard against inadvertently looking into a “hot” laser.
Cutting and Stripping Fiber Cabling
The first steps in terminating fiber are to cut, strip and prep the cable. Technicians should use cutting/stripping tools that match the specific size of cable being terminated and which can perform multiple operations without having to switch tools. For example, the MiniLite-Strip is essentially two tools in one, with a small stripping v-notch to remove the buffer and coating material from 125um fiber and a large stripping v-notch to strip a wide range of outer cable jacket insulation. The blade area is heat-treated for durability and laser marked for easy identification of the stripping options. Or the Fiber Optic Stripper can handle 900/250µm fiber with replaceable precision-ground blades that cleanly cut and strip a wide range of insulations and coatings.
Even with the best-adjusted and calibrated stripper, installers still need to learn the proper technique. Continuing to keep the pressure on after the buffer has been cut can place lateral pressure on the fragile glass core. Experienced technicians learn to “feel” for the slight loss of resistance when the tool cuts through the buffer, allowing them to ease up and avoid breaking the glass fiber. Veteran fiber installers also know the importance of keeping the stripper cutting face clean because even a small particle of dirt or debris can lead to broken or scored glass core. Therefore most experienced installers keep an old toothbrush handy in order to give the blades a precautionary cleaning before each round of stripping operations. (Safety note: never use canned air for cleaning tools in a fiber installation environment.)
Another important technique when stripping fiber is to avoid “pushing from the cut” in the manner technicians typically use to strip insulation from an electrical conductor. The tendency to bend the arm and wrist in a sweeping motion can twist the fiber cabling and create excess friction between the buffer and glass fiber, causing the fiber to curl and/or break. A better method is to “draw the glass fiber out of the buffer” in a nice linear pulling motion. Also, for some beginning installers, it may make sense to simply cut the buffer in smaller segments (¼ to ⅜ inch at a time) and pull it off a piece at time—creating less friction and minimizing the tendency to curl.
The installer also needs to be sure all of the coating has also been removed from the glass fiber. With some tools this can require multiple passes, however newer tools such as the MiniLite-Stripper are designed to effectively remove both the buffer and the coating in a single pass.
Cleaning and Preparing the Fiber
The next step is cleaning and preparing the fiber cabling for mating with the connector ferrule. Fiber Optic Cleaning is critical because there is only a 1 to 2 micron clearance in between the fiber and the connector ferrule. Even a very minute amount of debris on the fiber will interfere with fitting the fiber into the ferrule. The trimmed jacket at the base of the exposed fiber should be cleaned as well, in order to make sure that the epoxy will also adhere to the jacket for added strain relief.
Cleaning should only be done with an approved solution designed specifically for fiber such as “tech-grade” Isopropyl alcohol (99 percent pure). Never use standard isopropyl alcohol (70 percent pure) because the other 30 percent may consist of water, lanolin or other substances that can contaminate the fiber and keep the epoxy from adhering properly to the glass. To support fast and efficient cleaning, many installers have turned to using specially designed split-tip swabs that are pre-loaded with 99 percent isopropyl alcohol. The split-tip fits snugly around the fiber to thoroughly clean it and a single swab can be used for many cleanings or until the alcohol is gone.
Mating the Fiber into the Connector Ferrule & Curing the Epoxy
The next step is to apply the epoxy into the connector and insert the fiber. It is also a good idea to apply a small amount of epoxy to the base at the trimmed jacket for strain relief. There are a variety of approaches that can be used for curing the epoxy. Some experienced installers have developed special methods that can significantly accelerate the curing time and therefore increase overall fiber installation productivity. Traditionally, field-curing ovens use a relatively low temperature in the range of 150-200°F, which can require from 5 to 15 minutes of curing time. Some veteran installers carry a 3M Hot-Melt oven, with a temperature of up to 400°F, which can cut the cure time down to as little as 90 seconds.
Ultimately, the choice of which method to use for attaching fiber connectors comes down to a number of tradeoffs, which involve balancing the curing time against the pot life and factoring in the ability to leverage standard tools and methods. For example, crimp on connectors can eliminate the need for using epoxy but drive up the cost of the connector itself as well as requiring proprietary tools. Room cure epoxy can eliminate the need to have a field-heating oven but can take as much as 60-90 minutes to cure, with the added complication of only a short (5-8 minute) pot life, meaning that the mixture sometimes can set up faster than it can be applied. Heat cure epoxies are no different from room cure except that they are formulated to have a long pot life but cure quickly in high temperatures. This typically gives the installer the best balance when terminating many fiber connections by providing flexibility for applying the epoxy many times before it starts to set up, along with the ability to cure each connection in a matter of a few minutes with a high-heat field oven. Experienced installers also can take advantage of this flexibility by interleaving a number of different operations (epoxy, curing, cooling, etc.) in parallel to further boost overall throughput and efficiency.
Scribing the Connection and Removing Excess Fiber
Once the epoxy has cured and the connection has cooled, the next step is to scribe the glass and remove the excess fiber. The objective of scribing is to assure a clean break across the entire face, without shattering. This involves scoring the glass close to the connector end and then running the fingers up the connector and pulling away the excess glass along the linear axis to create a clean break near the connector face.
Depending upon the specific requirements and personal preferences, different installers may choose between ruby, sapphire or carbide scribing tools. Ruby and the sapphire are industrial gemstones polished to a super-sharp edge. There is essentially no difference between the two as far as cutting properties are concerned. From a personal preference standpoint, some people like to use a lighter scribe such as the sapphire and others prefer the ruby scribe’s darker line that provides contrast against the glass. Advanced carbide scribes, such as the IDEAL 45-359, deliver a smooth superfine cutting edge that is equal to either the ruby or the sapphire gemstones.
The basic tradeoff is that gemstone scribes are slightly less expensive than carbide however they are more fragile. Dropping ruby or sapphire scribes on a hard surface can chip the cutting surface but a carbide blade is much more rugged and durable. All of IDEAL’s ruby, sapphire and carbide scribes have a unique dual-edge feature that allows each blade to be reversed to obtain a second cutting edge whenever the first one has become worn or damaged.
Polishing Processes
After the fiber has been scribed and removed, the face is polished through a series of steps to achieve a smooth surface. The first step involves an “air polish” using a 12 micron grit lapping film, in which the film is held up by one corner and the face of the connector is gently rubbed back and forth against the suspended lapping film. The objective of the air polishing step is simply to bring the level of the glass down to the level of the glue bead at the connector opening. This step only takes about 20-30 seconds and can be checked by gently rubbing a finger over the surface to assure that any jagged edge left from the scribe break has been smoothed down flush with the bead.
The next step is to use a polishing puck on a rubber or neoprene pad, along with progressively finer lapping film to smooth the fiber face down to the required level. The lapping film is placed face up on the pad and the puck is placed on the film. Then the connector is inserted into the polishing puck and is moved in “figure 8 motions” across the surface of the film to polish down both the glue bead and the glass. It is important to check the surface regularly to avoid over-polishing. Most epoxy is infused with a blue dye so it is readily apparent when the glue bead has been eliminated. The inherent give in the polishing pad allows the polishing process to produce a slightly dome-shaped result that eliminates the glue around edges and creates a smooth face for propagation of the light through the fiber.
For multimode fiber, the polishing process should progress at least down to 3 micron lapping film, with 0.5 micron being optional (always check the recommendations of the connector and fiber manufacturers). For singlemode fiber, a final polishing step with 0.5 micron film should be mandatory to minimize coupling loss and assure adequate light propagation. Some installers also finish the process with a final “wet polish” by applying a small amount of 99 percent pure isopropyl alcohol to the 0.5 micron lapping film.
The lapping films should always be cleaned between before each use. The underlying neoprene pad also should always be clean in order to avoid any grit or debris that can cause bumps in the polishing action. After polishing, the entire connection should then be cleaned with 99 percent pure isopropyl alcohol or similar solution—including both the fiber face and connector ferrule. Here again, do not use canned air and never blow on the connector face in an attempt to clean it!
Inspecting with Field Microscopes
Each connection should then be inspected using a good field fiber microscope (with built-in eye protection). For multimode fiber, the minimum magnification should be 100x. For singlemode, the magnification should be at least 200x. The installer should look for a well-defined “bulls-eye” where the center of the bulls-eye is the core of the fiber and the next ring is the cladding and the final outer ring is the connector itself. The inspection also should assure that there are no scratches, pits, chips, nicks or glue residue. The examples below show a comparison of plucked, dirty and clean fiber faces.
When selecting a field microscope, installers should look for those that include multiple adapters to handle a variety of different standard connector types. In addition, some of the newer field inspection microscopes use LEDs instead of incandescent sources for backlighting because white LEDs provide a more pure light source and make it easier on the technicians’ eyes—especially after repeated usage in the field.
As soon as the connector has been inspected, it should be immediately covered with a clean dust cap in order to protect it from dirt or damage.
Testing for Basic Continuity
Field installers also should be responsible for conducting a continuity check of the fiber link, using a basic Fiber Optic Continuity Tester—typically with a powerful Krypton light source to provide long range testing. The continuity tester should include a soft ferrule gripping membrane to protect the finished ferrules while firmly securing the connector in the tester body. In addition, visible laser-light source testers can be valuable for checking the integrity of the jacket and detecting breaks in the cladding along the whole length of the link.
Ultimately, field installers are fundamentally responsible for delivering high-quality consistent workmanship and a functioning network infrastructure, while maintaining a safe and productive work environment. By obtaining the right tools, learning the right techniques and seeking out in-depth training on proper procedures, field technicians responsible for installing fiber networks can meet all of these objectives while simultaneously achieving a well-deserved high degree of pride in their work.
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