Fiber optic cable has typically been categorized as fragile,like glass, which the actual fiber is, of course. But unlike drinking glasses that break when dropped or windows that lose every battle with a kid's baseball, glass optical fiber is incredibly strong and flexible. Remember, the material that provides the strength in fiberglass boats, ladders, etc., is glass fiber.

Fiber's strength can be traced back to its manufacturing process. Most common glasses are made by melting silica (SiO2, which is basically sand) in a big pot, pouring it into a mold and cooling it fast enough to prevent crystallization, leaving an amorphous solid. While the process is cheap and quick, the glass produced is not pure, which limit its strength and ability to transmit light. The impurities absorb as much as 1,000 times more light than optical fiber and concentrate stresses that reduce its strength and can cause cracks.

Optical fiber is made by an entirely different process. It starts with a pure silica rod or tube and the rest of the material is deposited at high temperature using ultra-pure gasses to create a preform. The preform is solidified, then placed in a drawing tower where the fiber is pulled into long strands. Careful control of the process makes the glass incredibly pure, transparent and strong.

How strong? The glass fiber's theoretical maximum (tensile) strength is about 2 million pounds per square inch, but the practical limit is 10 to 20 percent of that. The cross-sectional area is so small,about 20 millionths of an inch,that the actual maximum fiber strength is about five to 10 pounds of tension.

But no matter how strong the hair-thin fiber may be, its small size makes it necessary to protect it from moisture and scratches. Moisture can increase attenuation and make the fiber brittle. Surface scratches cause faults that make the fiber fail. The first level of protection is a hermetic plastic coating applied to the fiber just as it comes off the drawing tower.

Optical fiber is then made into a cable. The cable provides protection from the environment, not just moisture or dirt, but other liquids, crushing or bending forces and, of course, pulling tension encountered during installation.

All fiber optic cables include strength members, typically aramid fibers such as DuPont's Kevlar. Kevlar is strong, but its most important characteristic is that it does not stretch under tension. When pulled, it will not stretch, then relax when the tension is removed, which would cause kinks in the cable and put stress on the fiber. The amount of aramid fiber inside a typical indoor cable allows up to 200 pounds of pulling tension, while outdoor cables are rated at 600 to 800 pounds of tension. An additional strength member is used as a stiffener to maintain the allowable cable-bend radius and prevent kinking when pulling around corners.

You must always remember to never pull fiber optic cable by the jacket, unless it is specifically rated for that. Some cables that have armor or strength members imbedded in the jacket can be pulled by using a Kellums grip on the jacket, but it's necessary to also tie the pulling eye to the cable with the strength members to spread the pulling tension around.

Fiber is also flexible because of its tiny diameter and high strength. You can take simplex or zipcord cable and tie it in knots. The tight bends will cause loss in the fiber, but the soft aramid strength members surrounding the fiber and the soft jacket will protect the fiber and prevent it from breaking. Remove the knot and the cable will go back to its original condition. You can try this yourself using a fiber optic power meter and source to see how much loss is induced by the knot and how it goes away when the cable is untied. With many cables you can use a visual fault locator (a visible laser source) and actually see the light lost through the fiber jacket.

Fiber optic cables are designed to withstand crushing forces. The soft jacket and padding provided by the strength members protect the fibers. Fiber optic cables are used under carpets in casinos where money carts are regularly pushed over them. Special tactical cables are used on the battlefield where they can withstand the weight of tanks.

Overall, fiber optic cables have been designed to be stronger than perhaps necessary. But that's insurance against heavy-handed installation. Successful fiber installation is assured by following two guidelines:

¥ Do not exceed the maximum pulling tension.

¥ Allow for a minimum bending radius of 20 times the cable diameter while under pulling tension. EC

HAYES is a VDV writer and trainer as well as president of the Fiber Optics Association. Find him at www.JimHayes.com.