The advantages of fiber-optic wiring and networks are well known. Optical fiber can move more data faster over longer distances than copper wire. Fiber is non-conductive, so it isn't affected by electrical noise, lightning, electromagnetic interference (EMI), or radio frequency interference (RFI). It takes up less space, is unaffected by vibration, and doesn't corrode in the presence of galvanic chemicals or in other harsh environments. Some users note that fiber is more secure because it can't be tapped without registering an interrupt condition. That's the good news.

The bad news is that fiber-optic cable was historically more expensive than copper, harder to install, more delicate, and more subject to debilitating data loss from line breaks. As a result, fiber-optic cable was usually installed as long-distance, high-data-rate backbones in larger industrial and communication networks, often reaching out to and converting data from copper-based systems in the field. However, these disadvantages have lessened in recent years, and now many fiber-optic components are almost as inexpensive as copper, easier to install, and flexible enough to be twisted like a pretzel without breaking or hindering performance. These improvements are moving fiber-optics into many new applications and settings, both on the plant floor and closer to the device level.

Optical fiber includes a high-refractive index core and low-refractive index cladding, which allows light to travel through the core by bouncing repeatedly off the cladding. After passing through the fiber with no loss in light quality, the typical beam is dispersed at a 60° angle toward its traget.

"Any installed advantages of copper have really dwindled. We're seeing fiber going right up to the desktop PCs with fiber-optic network cards between the PCs and the controllers and switches out to the I/O drop," says Alex Johnson, advanced application development director, Invensys Foxboro. "Fiber hasn't reached the sensor or I/O level because the valve to the I/O connection still has to be copper, so it can get power to the device. So, pretty much anywhere you used to have a copper network, you can now use fiber."

While light travels though the high-refractive cores of all fiber-optic cables by refracting off their low-refractive cladding, there are two basic types of optical fiber—plastic and glass, according to Keyence's Fiberoptic Sensors Technical Guide. Plastic optical fiber consists of one or more acrylic resin fibers, 0.25 to 1 mm in diameter, that are encased in a polyethelene sheath. Glass fiber consists of 10- to 100-µ diameter fibers encased in stainless steel tubing, which allows it to be used at up to 350 °C operating temperatures.

Fiber-optic wiring also comes in thinner-core single-mode and thicker-core multi-mode types. By using wavelength division multiplexing (WDM), both single- and multi-mode fibers can simultaneously transmit at numerous wavelengths without interference.

Meanwhile, copper wiring usually consists of a 4-20 mA analog signal moving over traditional 18-gauge, Category-3- to -6 or other types of twisted-pair wires in a variety of configurations. Cat-5's capacity is 100 Mbps for up to 100 meters.

Despite the emergence of fiber-optic wiring, copper is far from being eclipsed. Not only does it continue to be predominant worldwide, but recent technical advances are giving copper new life. For example, Ethernet's ongoing advances on the plant floor are allowing twisted-pair cables to carry more and more sophisticated data. Likewise, the recent approval of the Institute for Electrical and Electronics Engineers' (IEEE) 802.3af power-over-Ethernet (PoE) standard will likely further lessen requirements for and costs of copper wiring because many users will be able to use one set of wires where they used to need two. "Just when you think copper is dead, it improves itself," adds Bob Weiland, Graybar's national marketing manager.

jmontague@reedbusiness.com