What Does 'Figure 8' Mean?

Every technology has its own jargon, of course. Some of it we joke about because it’s mostly TLAs—the abbreviation for “three letter abbreviation,” the favorite way tech people have of showing how cool they are. I remember a paragraph in a telephone industry magazine once that had not a single noun in it—all TLAs, that required a side column to define them all. But not all jargon is TLAs, as there are some technical terms like core and cladding in fiber optics that are descriptive if you understand the underlying technology.

One term in fiber optic jargon is particularly confusing, because it has a dual meaning—“figure 8.” If you have learned to terminate optical fiber with an adhesive/polish termination method, you are familiar with polishing in a figure 8 pattern. If you are installing long outside plant (OSP) cables, you may have to “figure 8” the cable at an intermediate pulling point. Let’s examine both of them more carefully.

Polishing connectors in a figure 8 pattern

When polishing a fiber optic connector, the general rule is to polish in a figure 8 on the polishing film. Why a figure 8? That motion causes the direction of polishing to move around in all directions on the end of the ferrule, preventing polishing marks from polishing in only one or two directions. The random, uneven nature of the figure 8's as you polish is even better, not only for the random direction of motion but because it moves the connector over more of the polishing film, avoiding polishing on top of areas with residue from earlier polishing strokes.

Not everybody agrees on polishing in figure 8's, however. Some say it is unnecessary and polishing in circles is best. Some say do it for multimode but polish in circles for single-mode, which is how I learned. Single-mode polishing is quite different in fact, done on diamond film with a wet polishing slurry, which worked for me. Polishing machines generally allow you to choose one or the other and maybe other random patterns.

Figure 8-ing OSP cables

Today you can purchase OSP fiber optic cable in long continuous runs, 10 kilometers long or more, if you have the heavy equipment to handle the size of the cable reels on the job site. This works great for avoiding splicing on long runs, especially for direct burial cable. But if you are pulling underground cable in conduit or innerduct, you probably cannot pull lengths anywhere near that long because of friction between the cable and the duct. A few kilometers is not an unusual limit.

The usual method of dealing with this situation is to use a midspan pull. You pull cable in a length of duct, coil it up outside the end, then continue another pull. At each midspan location, however, you must coil up the cable during the pull before pulling from that coil to on the next span.

Figure 8-ing is the method used to coil the cable without putting a twist in the cable or tangling it. The process is simple. After pulling the cable out of a conduit, you lay it on the ground in a figure 8. The loop on one end of the 8 puts a half-twist into the cable. The loop on the other end takes the twist out, meaning no overall twist in the cable length.

After pulling all the cable out of the conduit, you have a big pile of cable in a figure 8 pattern with a cable on the top leading into the conduit you just pulled the cable from and the other end of the cable underneath all the pile of cable. Here is what it looks like while being taught at one of the Fiber Optic Association's schools, Triple Play Fiber Optics.

Next you have to get to the end that you want to pull into the next length of conduit. You do that by picking up the pile of cable and flipping it over, leaving the open end read to feed off the pile of cable. Attach that end of the cable to the pulling eye and pull into the next length of conduit.

Here is the secondary pull being done on a cable by Telecom Malaysia. The tech is guiding the cable off the loops as it is pulled into the next length of conduit. You can see the other end of the cable from the first pull disappearing into the manhole.

You should note the cable is on a tarp. It’s common to lay the loops on a tarp to keep the cable clean and the tarp can help when picking up the loops of cable to turn over.

This image shows the process done manually as it is often done. Equipment exists to help in pulling of the cable at the intermediate point and at least one company makes a trailer with an arm that can make the figure 8 loops for you.

About the Author

Jim Hayes

Fiber Optics Columnist and Contributing Editor
Jim Hayes is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com .

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