Best Practices for Terminating Fiber Optic Cabling

April 1, 2004
As fiber optic cabling becomes an increasingly important aspect of local area network (LAN) 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 critical to achieve acceptable results. Even a small imperfection or microscopic dirt on the face of the fiber can create

As fiber optic cabling becomes an increasingly important aspect of local area network (LAN) 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 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, which can cause the link to fail.

While the proper tools are important for success, the proper techniques are even more critical. As many LAN installers shift their focus from copper to fiber installations, it will be all too easy to pick up bad habits that can lead to inefficiency, result in substandard quality, and create safety risks.

This article will provide a hands-on tutorial for quickly, safely, and correctly creating fiber optic connections that meet accepted standards of quality workmanship and ensure optimal coupling efficiency. The following sections will 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 who terminate fiber optic cabling.

Creating and maintaining a safe work environment. Adequate safety measures are paramount when handling and terminating fiber. Not only should you protect yourself during the installation process, you also need to leave the installation area in a safe condition for other people who follow behind you.

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. Using a piece of black tape to stick your scraps on isn't an acceptable work practice. And 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 occupants could be harmed by the nearly unnoticeable yet sharp fiber scraps.

Later on, when checking a fiber with a microscope, you should always make sure the other end isn't connected to a power source. Laser light is harmful to the eye, but it isn't visible, so you may not even realize you're looking into it until it's too late. As an extra precaution, consider using a field microscope 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. You should use cutting/stripping tools that match the specific size of the cable you're terminating. It's also a good idea to consider using a tool that can perform multiple operations, which eliminates the need to constantly switch tools.

Even with the best-adjusted and calibrated stripper, you'll 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's cutting face clean because even a small particle of dirt or debris can lead to a broken or scored glass core. It's a good idea to 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 to be aware of when stripping fiber is to avoid “pushing from the cut” in the manner you typically use to strip insulation from an electrical conductor. Bending your 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. If you're a beginner, it may make more sense for you to simply cut the buffer in smaller segments (0.25 in. to 0.375 in. at a time) and pull it off one piece at time, which creates less friction and minimizes the tendency to curl the cable.

You must also be sure to remove all of the coating from the glass fiber when stripping the outer layers away. With some tools this can require multiple passes. Newer tools 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 optical fiber for mating with the connector ferrule. Cleaning is critical because there's only a 1-micron to 2-micron clearance between the fiber and the connector ferrule. Even a very small amount of debris on the fiber will interfere with fitting the fiber into the ferrule. It's important to also clean the trimmed jacket at the base of the exposed fiber to make sure the epoxy will also adhere to the jacket. This provides for added strain relief.

Cleaning should only be done with a swab and an approved solution designed specifically for fiber, such as “tech-grade” isopropyl alcohol (99% pure). Standard isopropyl alcohol (70% pure) may contain water, lanolin, or other substances that can contaminate the fiber and keep the epoxy from adhering properly to the glass.

Mating the fiber into the connector ferrule and curing the epoxy. The next step is to apply the epoxy into the connector and insert the fiber. It's also a good idea to apply a small amount of epoxy to the base at the trimmed jacket for strain relief. You can use a variety of approaches to cure the epoxy.

Field-curing ovens traditionally use a relatively low temperature in the range of 150°F to 200°F, which can require 5 minutes to 15 minutes of curing time. Some installers are now using hot-melt ovens with temperature as high as 400°F, which can cut the cure time down to as little as 90 seconds. This will improve overall fiber installation productivity.

Ultimately, your 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 of the epoxy 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. They also require proprietary tools. Room cure epoxy can eliminate the need to have a field-heating oven, but can take as much as 60 minutes to 90 minutes to cure, with the added complication of only a short (5-minute to 8-minute) pot life, meaning that the mixture can sometimes set up faster than it can be applied. Heat cure epoxies are no different from room cure except that they're formulated to have a long pot life but cure quickly in high temperatures. This typically gives you 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 minutes with a high-heat field oven. Experienced installers can take advantage of this flexibility by interweaving a number of different operations, such as epoxy, curing, or cooling, 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 is to ensure a clean break across the entire face, without shattering. This involves scoring the glass close to the connector end and then running your 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 your personal preferences, you may choose between ruby, sapphire, or carbide scribing tools. Ruby and sapphire are industrial gemstones polished to a super-sharp edge. There's 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 like the sapphire and others prefer the ruby scribe's darker line that provides contrast against the glass. Carbide scribes are equal to either the ruby or the sapphire gemstones in cutting capability.

The tradeoff is that gemstone scribes are slightly less expensive than carbide. However, they're more fragile. Ruby or sapphire scribes can chip if dropped on a hard surface. A carbide blade is much more rugged and durable.

Polishing processes. After you've scribed the glass, it's time to polish the face to a smooth surface. The first step involves an “air polish” with a 12-micron grit lapping film. Hold the film up by one corner and gently rub the face of the fiber back and forth against the suspended lapping field. This step brings the level of the glass down to the level of the glue bead at the connector opening and only takes about 20 seconds to 30 seconds. Check the polish by gently rubbing a finger over the surface to ensure 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. You should place the lapping film face-up on the pad and then place the puck on the film. You then insert the connector into the polishing puck and move it in figure 8 motions across the surface of the film to polish down both the glue bead and the glass. It's important that you check the surface regularly to avoid over-polishing. Most epoxy is infused with a blue dye so it's readily apparent when the glue bead has been eliminated. The inherent give in the polishing pad allows you 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.

When working with multimode fiber, your polishing process should progress at least down to 3-micron lapping film; 0.5-micron is optional. Always check the manufacturer's recommendations for each connector you're working on. When working with singlemode fiber, always perform a final polishing step with 0.5-micron film to minimize coupling loss and ensure adequate light propagation. Some installers also finish the process with a final “wet polish” by applying a small amount of 99% pure isopropyl alcohol to the 0.5-micron lapping film.

You should always clean the lapping films before each use. Make sure the underlying neoprene pad is also clean to avoid accumulation of grit or debris that can cause bumps in the polishing action. After you finish with the polishing process, clean the entire connection — including both the fiber face and connector ferrule — with 99% pure isopropyl alcohol or similar solution. Again, don't use canned air, and never blow on the connector face in an attempt to clean it.

Inspecting with field microscopes. You should now inspect each connection with a good field microscope. For multimode fiber, the minimum magnification should be 100x. For singlemode fiber, the minimum magnification should be at least 200x. When viewing the connection through the microscope, look for a well-defined “bulls-eye” where the center of the bulls-eye is the core of the fiber, the next ring is the cladding, and the final outer ring is the connector itself. Your inspection should also ensure that there are no scratches, pits, chips, nicks, or glue residue on or in the connection.

When selecting a field microscope, consider one that offers multiple adapters to handle several connector types. 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 your eyes — especially after repeated usage in the field.

After inspecting the connector, immediately cover it with a clean dust cap to protect it from dirt or damage.

Testing for basic continuity. You're also responsible for conducting a continuity check of the fiber link with a basic fiber optic continuity tester. The continuity tester should include a soft ferrule gripping membrane to protect the finished ferrules while firmly securing the connector in the tester body. You may also want to consider using a visible laser-light source tester to check the integrity of the jacket and detect breaks in the cladding along the entire length of the link.

Ultimately, you're 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, you can meet all of these objectives and take pride in your work.

Munch is technical advisor, and Reinert is product manager of fiber optic tools with Ideal Industries in Sycamore, Ill.

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