• The Basics of Fiber Optic Cables
• Fiber for Physicists and Engineers
• Installing Optical Fiber
• Fiber Cable: Attributes, Geometry, and Testing.
• Optical Fiber to the Desktop
• Fiber Optics in Harsh Environments

1) Guide to Fiber Attributes, Geometry, and Testing.

With standard field practices and termination equipment, the installation of fiber optic cable is simpler, faster, and less costly than ever; and testing after installation is a breeze. The fact is that, today, optical fiber technology surpasses that of copper. Fiber is different, but actually easier to work with.

Freedom From Worry
Optical fiber transmission involves changing electrical signals into pulses of light, using an optoelectronic transmitter, and sending the pulses down the core of an optical fiber. Because the core and its surrounding cladding glass have different compositions, the light is trapped within the core. It has nowhere to go but down the length of the fiber. At the opposite end, a receiver changes the pulses back to electrical signals.

When it comes to network installation, optical fiber cable offers many benefits. First, its small size and light weight actually make it easy to install. Despite some lingering misconceptions, optical fiber cable is quite strong. Its pull strength is 200 lbs. for a two-fiber cable. And the bend radius for two-fiber cable and four-pair UTP copper cable is the same.

Besides, current cable designs provide ample protection for fibers, so there's little need to worry about damage. Recently, for example, a crane was driven into an installation site and stopped atop a Siecor optical fiber cable and the crane executed a 900 turn while on top of the cable, before cable installers could stop the crane. The fiber cable was slightly deformed but all the fibers remained operational.

Perhaps most important, optical fiber is reliable enough to be virtually worry-free. Because it is dielectric, fiber eliminates most of the concern over factors affecting link performance. It is immune to crosstalk, electromagnetic interference (EMI), radio frequency interference (RFI), impedance mismatches, transmission frequency variability, and ground loops (all pitfalls of copper-based systems).

Fiber's immunity to EMI means installers need have few concerns about where they run optical fiber cable. No need to worry about coming too close to electric motors or fluorescent lights, for example. This helps make for smoother, simpler jobs, with easier installation, fewer testing problems, and no call-backs.

Only four performance factors should concern installers. The four are bandwidth; environmental effects, such as temperature dependence; continuity (unbroken transmission of a signal from one point to another); and attenuation (acceptable signal loss over distance).

Installers need concern themselves only with continuity and attenuation. Using quality optical fiber, cable, and connectors helps to minimize these concerns, or even prevent them altogether. Besides, testing for continuity and attenuation is very simple.

One process for manufacturing fiber involves using ultrapure, vapor-deposited chemicals. Some companies use computer systems to continuously measure each fiber's dimensions along its entire length, while others measure at either end of a spool.

Whatever method is used, the important result is predictable consistency in fiber profile and geometry. Fiber geometry, the physical characteristics of a fiber, is vital when connectorizing or splicing, joining the core of one fiber with the core of another while losing as little light as possible. Consistent fiber geometry helps get splices right the first time during installation.

2) A Fiber Geometry Primer

The key dimensions of fiber geometry are cladding outer diameter, core/clad concentricity, and cladding non-circularity.

Outer diameter.
Tight tolerances in cladding outer diameter determine the precision with which each fiber fits into ferrule-type connectors. If the fiber is too thick, it won't fit, and connectorizing times are increased. If it is too thin, the cores won't align properly, and power losses are increased. In effect, tight tolerances on both the fiber and the connector optimize link loss performance (see Figure 2).

Core/clad concentricity.
Core/clad concentricity is a measure of how well the fiber core is centered in the cladding glass. Because the outer cladding is referenced when connectorizing and splicing to align the two cores, tight centering tolerances translate to closer alignments and less power loss.

3) Some Testing Is Necessary

Optical fiber technology is formidable and impressive. But fiber cables still have to be tested for continuity and attenuation. The good news is that field-testing fiber is simple, quick, accurate, and inexpensive.

Continuity.
Testing for continuity confirms that each fiber is connected at the correct place. The test can be performed on multimode fibers with a flashlight and a walkie-talkie. You simply select the color-coded fiber to be tested, darken the closet, shine the flashlight at the fiber's end, and wait for confirmation that your partner sees the light.

Attenuation.
Installers should confirm actual power loss against acceptable levels for each optical fiber link segment. In the horizontal, a typical link segment runs from the telecommunications outlet to the horizontal cross-connect. It should include the cable, connectors, and adapters.

In practice, attenuation and continuity testing can be combined, because if you can measure attenuation, you must have continuity. Some even argue that attenuation testing itself can be eliminated, because fiber is so consistent and predictable.

However, attenuation testing is usually worthwhile. Customers have a right to know that installations were performed to specifications and will support high data rate transmissions.

The only equipment required to perform attenuation testing is a light source, power meter, test cords, and an adapter. Remember that it is not necessary to measure attenuation in both directions. You need measure in one direction only, and for horizontal links, only at one wavelength.

Testing begins with cleaning connectors and adapters and zeroing out the equipment to establish a reference measurement. The power meter and test cord are then moved to the far path panel, where the equipment is assembled and a power reading is recorded.

This power level is compared with the reference measurement to obtain the end-to-end attenuation. The process is simple, quick, and uncomplicated.