Every year around Christmas, stores roll out artificial trees made with optical fibers. You know the ones—they glow and shine colored light out the ends of the branches. Who would think that these trees are made of a type of optical fiber that offers easy-to-install cheap communications links?
Plastic optical fiber, generally referred to as POF, is a large-core fiber made of very pure plastic. Most POF is a simple step-index fiber made from ultra-pure plastics by a simple extrusion process. While the optical principles it uses are the same as for glass fibers, the differences in materials make its performance quite different.
POF transmits light at a different wavelength than glass fiber. While glass fiber works at 850, 1,300 and 1,550nm in the infrared regions, POF transmits best at 650nm, using visible red light. POFs attenuation is a lot higher than glass fiber—150 dB/km instead of 1-3 dB/km, and its bandwidth limits it to shorter links. However low-cost links that transmit 125 Mb/s over 100km are commercially available.
The really big advantage of POF is its simple and inexpensive termination. Because POF is much larger than glass fiber (mostly 1mm in diameter, about 15 times larger than the usual multimode fiber and 100 times larger than single-mode fiber) aligning fibers to transmitters or receivers is easy, and it couples plenty of power from inexpensive LED sources. Connections can be done by simply cutting the fiber and crimping on an inexpensive plastic connector that snaps into place, without the polishing necessary for glass fiber.
As a downside, unpolished POF connections from fiber to fiber are not so good, typically 1 to 2 dB compared to less than 0.5 dB for glass fibers. But since most POF links are simple point-to-point connections, that is not a problem.
If POF is cheap and easy to install, why don’t we see more of it? Well, it has a lot of uses, but not in obvious places. It has been used for years inside such electronic instruments as oscilloscopes and industrial machinery as welders and semiconductor processing equipment, where its immunity to electrical interference is important. It has been used in other high-reliability systems such as medical instruments as well.
If you look at the back of home stereo components, you will find optical ports for POF links. TOSLINK and some FireWire links use POF to make digital connections among consumer electronics. These links may find more consumer acceptance in home entertainment networks, especially as digital TV becomes widely available.
One big application has appeared. Several POF networks are used in vehicles, including Flexray and MOST. Flexray connects all the driveline control devices, such as the engine, transmission, ABS brakes, traction control and other driving aspects of the car. MOST is a 25 Mb/sec network (soon to be upgraded to 400 Mb/sec) that links such communications devices as radios, CD and DVD players, cellphones and human interface devices (control panels, displays, speakers, etc.).
Flexray and MOST use fiber to reduce the noise problems inherent in vehicles and save considerable amounts of weight, up to 20 pounds per vehicle. About 5 million MOST devices alone will be shipped in such 2003 luxury vehicles as Mercedes-Benz, BMW, Porsche and Saab. Volumes will grow fast as MOST device prices drop so they will be adopted for mainstream cars and appear in the automotive aftermarket.
For more common applications like desktop network connections, UTP copper has not been seriously challenged by POF, simply because the makers of networking gear have not embraced it themselves and offered compatible products. They see little, if any, advantage over UTP and understand the difficulty of creating distribution and installation networks for a new communications medium.
POF, however, may become very familiar to many electrical contractors. It has been used in signs and architectural lighting for some time, but new technical developments and more aggressive marketing are expanding its use.
Its size and flexibility allows much more creative design of lighting fixtures or incorporation of lighting into architectural features. Its ability to emit light directly from the end or along the side and carry light of any color gives it lots of flexibility in application. And, of course, it is nonconductive, so it can safely be used around water, like in pools or hot tubs.
POF lighting applications need light sources, of course, since the fiber is only a “light pipe.” Various types of electrical lighting can be used in remote locations to provide the actual light, requiring the services of an electrician for installation. EC
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.