Transmission Basics  |  Components  |  Applying Fiber Optic Technology  |  System Performance  

Determining Fiber Size

Quick identification of the exact size and type of a given piece of optical fiber is a routine but necessary task. If one has access to the fiber itself, the first step in identification is to remove any outer jacket material that may exist and carefully remove the plastic buffer from the fiber. (Note: If you do not have access to the fiber itself and can only view the fiber end, then proceed to the section on Core Size.) To do this, use fiber strippers designed for the task, or use a razor blade. (It takes practice to remove the plastic with a razor blade, but it can be mastered after a few repetitions.) Always cut along the fiber axis towards the cut end of the fiber. Fiber has tremendous strength in tension but is very weak in all other directions. Always stroke the razor blade away from your body. Use the razor blade to remove a sliver of plastic, then rotate the fiber 90° and repeat the process until the fiber cladding is fully exposed. Once the bulk of the plastic coating is removed, carefully clean the bare fiber with a tissue soaked in alcohol. (Note: Use only industrial grade 99% pure isopropyl alcohol. Commercially available isopropyl alcohol, for medicinal use, is diluted with water and a light mineral oil. Industrial grade isopropyl alcohol should be used exclusively.) Always wipe along the fiber axis with continuous strokes to the end of the fiber.

Cladding Size

Once the fiber is clean, take a clean machinist micrometer, such as the one in Figure 1, and carefully measure the outer diameter of the fiber. This outer diameter is the cladding diameter of the fiber. Be certain that the metal faces of the micrometer are clean. Do not over tighten the micrometer as the fiber will fracture. Table 1 shows the possible results for the most common fiber sizes and the interpretation of the results.

Figure 1 - Micrometer


Table 1 - Common Fiber Cladding Size & Corresponding Micrometer Reading

Fiber Type Nominal Cladding Diameter Tolerance Low Micrometer Reading High Micrometer Reading
Single-mode 125 µm ±1 µm 4.88 mils 4.96 mils
Multimode 125 µm ±4 µm 4.76 mils 5.08 mils
Multimode 140 µm ±4 µm 5.35 mils 5.67 mils
Multimode 230 µm ±5 µm 8.86 mils 9.25 mils

Thus, if the fiber cladding diameter measures 5.38 mils, then the fiber is almost certainly multimode with a 140 µm cladding diameter. Cladding diameter is the most important parameter when selecting the fiber optic connector size. The cladding diameter determines the size of the hole in the fiber optic connector.

Core Size

Once the cladding diameter is determined, one must find the core size, unless the cladding diameter uniquely identified the fiber size. This step requires a microscope capable of about 50X magnification. A high intensity light or penlight is useful to light the fiber end. The idea is to get a good look at the end of the fiber, and judge the fiber size from what is seen. This technique works best if the fiber is in a connector so that the fiber end is polished and flat. If that is not the case, it may be necessary to cleave the fiber so that the end can be examined. Clamp the fiber end or fiber optic connector at the focal plane of the microscope and shine the light onto the fiber end. It is sometimes useful to light the far end of the fiber as well if it is accessible. Once focus is established, compare the view with the five drawings shown in Figure 2.

Figure 2 - Relative Core/Cladding Size

Relative Core/Cladding Size

This figure represents scale drawings of the relative size of the core and cladding on the six most popular fiber types for fiber optic communication. Of these six, the last two, PM single-mode 9/125 µm and single-mode 9/125 µm, are the most common and 110/125 µm is the least common of the six types. With a little practice, it is easy to quickly and accurately identify the fiber types. When looking through the microscope, the fiber core will be very dark if the fiber is illuminated only at the microscope side. If the distant end of the fiber is illuminated, then the fiber core will appear brightly lit. Simply compare the image in the microscope to the scale drawing to the left, and match the relative size of the core to cladding. With basically only five sizes of fiber to worry about, it becomes a relatively simple matter to judge the correct size. There is also a range of much larger fibers, but these have limited use in communications applications. Other large fiber sizes include 200/230 µm, 400/430 µm and 1000/1050 µm. The latter fiber is very nearly a glass rod. There are other imaginative ways of determining fiber size. For instance, if one has a multimode LED light source available (e.g. a fiber optic video transmitter), a simple light injection test can be performed to quickly determine the fiber size. A surface-emitting LED is best for this purpose since its light injection level varies most dramatically with fiber size. The LED should be powered at a fairly constant current and then attached to a few known size fiber optic cables. For more information on LEDs see Light-emitting Diodes. In each case, note the relationship between fiber size and launched power. Once a small database of results is available, then proceed to attach the unknown fiber optic cables. It is generally very easy to distinguish between fiber sizes. One possible drawback of this method is the possibility of high loss due to a bad fiber optic connector or a stressed or broken optical fiber. Launching the LED into both ends of the fiber optic cable will usually improve the chances of a correct result.