Shed Some Fiberoptic Light

Fiberoptic Cable Has Copper Wire Beat for Speed and Immunity. Just Know Which Type Your Application Needs

By Loren Shaum

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Since its commercial introduction 30 years ago, fiberoptic technology has become a valuable method for transmitting information fast and without much loss of data integrity.

Light pulses travel over glass fibers using a principle known as total internal reflection. Because light travels in a straight line and cabling often is bent, the light bounces or is reflected off the fiber's mirrored walls or cladding. This means the light signal is not absorbed outside the glass fibers and can travel with little degradation.

The signal begins at the network's transmitter, which can accept electronic pulses from copper wire and translate it to equivalently coded light pulses.

Regardless of the transmitter used, a fiberoptic cable is subject to signal losses, primarily through light scattering or dispersion. The faster the light fluctuates and the longer the distance traveled, the greater the risk of dispersion. Light strengtheners, called repeaters or optical regenerators, often are necessary to refresh the signal in certain applications.

Durability Counts

Under harsh conditions, the ruggedness and durability of previous-generation fiberoptic cables had come into question. Typical loose-tube or "indoor" tight-buffered cables work for most straightforward, non-critical applications. However, factories might need the performance advantages of higher-quality, abuse-resistant, tight-buffered, fiberoptic cables. With improved bend, crush, impact and chemical resistance — across a broad thermal operating range — ruggedized, tight-buffered cables can speed installation, reduce attenuation loss, and maximize up-time.

"I was involved in one of the first wastewater treatment plants that ever used fiberoptic cable," recalls Charlie Motz, senior engineer with system integrator Control Instruments (C2i), Smyrna, Ga. "But we don't bother with loose-tube stuff anymore. Now we use ruggedized, tight-buffered cable because it's quicker to install, is more reliable and provides greater value in the long run."

Low Cost, High Risk

Almost as old as glass fiber itself, loose-tube fiberoptic cable still is used in commercial and industrial enterprise applications and for many long-haul backbones — often because of its relatively low cost. "But in a demanding plant environment, the temporary cost differentials vanish in the specter of interrupted output caused by data loss or cable failure," says Bob Booze, vice president of sales operations and marketing at Optical Cable. "The cost of cabling represents a small fraction of a communication system when factoring in network equipment, cable connectivity hardware, and installation and testing. A 10-20% price premium for ruggedized, tight-buffered cable amounts to only a few percentage points increase."

Installation Hazards

Unlike some long haul-applications where the cable basically is dropped into the ground and covered with dirt with minimum connectorization, placement of fiber in a factory requires many bends, pulls and connections. Here, loose-tube designs can fail quickly. For the most part, if major cable stress or damage occurs during the installation process, the contractor presumably knows about it right away, and will quickly fix or replace the damaged link.

More insidious, though, is microbend or residual stress that can occur during or after installation. Often too small to notice initially, the cumulative stresses on cable during rough handling can return to haunt a plant with higher loss transmissions, missing data, and broken fibers. At worst, a complete shutdown in the communications link can occur.

Also, instrumentation and controls engineers increasingly need 10-Gbps transmission for some links in the factory, which can require the latest 50-µm, multi-mode, OM-3 fiber and, in some cases, single-mode fiber. However, these fibers are much more bend-sensitive than previous generation 62.5-µm fibers, requiring cable quality that goes well beyond minimum standards.

In-Service Abuse

"Once installed in a factory, the integrity of cable runs remains anything but static as harsh environments and even gravity can play havoc," says Booze. "If anything, today's 10-Gigabit communication links increasingly spotlight the fragility of all but the most sturdy fiberoptic cables. Even after installation, any kind of stress, whether minor mechanical loads or temperature extremes, can result in microbends or other fiber stress that in turn might lead to increased cable loss and transmission errors, or even eventual fiber failure and breakage."

For instance, explains Booze, heavy cables lying on top of the high-speed link in a cable-tray can cause cumulative trauma to the glass fibers. In addtion, even within vertical runs, cable can be subjected to added stress because gravity causes axial migration that can slowly weakens the fiber. This can go on until a good installation degrades to the point where the link no longer functions properly.

"Consider the fact that 1-Gigabit lengths that are limited to about 300 m, and OM3 10-Gigabit lengths that often are used to extend 1 Gigabit links beyond 1,000 m, have less than 43 dB of total allowable channel insertion loss — per the IEEE 802.3 Ethernet specification — and it becomes obvious that even the slightest increase in attenuation can sabotage the communication link," says Booze.

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