With all its advantages, plastic optical fiber still can’t do all the things glass fiber can do.
Plastic optical fiber, or POF, is finally making its way into the optical fiber market. It requires no special tools, no expensive training, and no long procedures. You can cut it with side-cutters, a razor blade, or a pair of scissors. A maker of POF connectors even claims you can terminate a plastic fiber in 30 sec!
If you have any experience with glass optical fiber, you’re probably thinking, “This sounds too good to be true.” Well, in a way, you’re right. While having all these advantages, POF cannot do all that glass fiber can do.
For some applications, POF is ideal; for others, it’s useless. How do you know which is best for your application? Let’s look at a general overview of the plastic fiber technology and how you can apply it.
So, what are POF limitations? The biggest limitation of POF is its high attenuation—not a distortion of the signal, but a loss of light. Plastic optical fiber doesn’t transmit light as well as glass fiber. While glass fiber can transmit light for tens of kilometers without difficulties associated with loss, POF is seldom useful in lengths over 100 m. Attenuation for glass fiber is generally in the range of 3 dB/km or less. Attenuation for POF is generally in the range of 50 dB/km to 100 dB/km or more.
Where can you use POF? Since plastic fiber is virtually useless for long runs, you can’t use it for most telephone transmissions, cable TV, or building-to-building links. For short links, however, POF is best. Some common applications include local area networks (LANs) and point-to-point links, as shown in Fig. 1 (original article).
LAN cabling is probably the more important application. In the past ten years, optical fiber has taken over almost every facet of the data communications industry with one big exception—the last link between the communications closet (telephone closet) and the desktop. Even vertical runs in multistory buildings are now almost always fiber. Only the horizontal runs (those contained on one level of a building) are still copper.
Obviously, most network administrators would love to have the bandwidth advantages of optical fiber, but fiber costs more than copper. Regardless of the reasons for a premium on fiber (among other advantages, you don’t have to replace it in a few years), new fiber installations still cost more than copper installations. Since initial cost rules the data cabling market, fiber won’t be widely used until it’s priced equally with copper wiring.
Ideally suited to compete with copper Category cabling for the horizontal run market, POF is cheaper than Category cabling and easier to install. The only thing still standing in the way of fiber-to-the-desk is the cost of the network interface card (NIC). This is the electronic board in any computer connected to a network. Basically, it connects the computer to the system.
An NIC card for copper wiring costs about $100, while a fiber card (which translates optical signals into electronic signals) is about $150.00. The question now is whether the cost saving per run of plastic fiber makes up for the extra $50.00 for the NIC.
Point-to-point links, if short and simple, are also ideal applications for POF. The only links exceeding POF’s current signal capacity of 3 Gb/sec are those used for long-distance telephone and Internet traffic. For almost everything else, POF will work.
Industrial data transmission is perhaps the most common use of point-to-point links. Fiber is especially important in factories because it’s immune to electromagnetic interference (EMI). Since POF installs easily, factory electricians can install it with a minimal training.
Consumer equipment digital links are ideal for POF because of its unlimited signal transmission, superb signal quality, and easy installation.
Optical interconnects are another point-to-point link suitable for POF. For example, patching signals between pieces of equipment is easier with plastic than glass.
Vehicle links are also suitable for POF. Modern cars, trucks, and airplanes use signal transmission and light transfer. Plastic makes it cheap and easy. Your car may already have POF in it.
Medical equipment manufacturers are changing over to POF because the necessary links are short and transmission quality is very important.
Developing plastic fiber. Because of glass fiber’s difficulties with terminating, splicing, etc., people began searching for ways to make it easier to install. One of the first ideas was using clear plastic rather than glass. Motorola developed plastic fiber in the 1980’s, but never put much effort into marketing it. The first generation of POF transferred light correctly, but the attenuation (loss of light) was so high, only very short runs proved viable. Consequently, its production stopped quickly.
In the past several years, some fiber manufacturers (primarily Japanese) worked on new types of plastic fiber with much lower attenuation. Two big developments resulted: Graded-index fiber and low-NA POF.
Graded-index POF contains many layers of plastic, each with a lower index of refraction (the most dense plastic in the middle, and less-dense layers as you move toward the surface of the fiber). Since light travels faster in the less-dense layers of plastic, the light rays refracted to the outside of the fiber race to match those traveling in the center. The result: a capability for high-speed data transmission over a long distance.
Fig. 2 (original article) shows three types of glass and plastic optical fibers. At the bottom is graded-index POF. At the top is step-index POF, the original (discontinued) type of POF. Step-index fiber has only two densities of plastic—one for the core and one for the cladding. Because of this arrangement, any light sent through them bounces through the fiber. Since some rays travel down the middle of the fiber while others bounce from side to side, some of the rays reach the far end before the others. We call this signal distortion modal dispersion. Dispersion refers to the spreading of the data pulse (some parts arriving before the others), and mode refers to the light’s path.
Although designed to eliminate the problem of modal dispersion, graded-index plastic fiber is not easy to make and was not available until recently.
Low-NA fiber is based on the development of fibers with low numerical apertures (NAs) . NA is a measurement of the angle at which light can enter the core of an optical fiber. If the light enters at too great an angle, it passes through the core/clad boundary and is lost in the cladding.
Fig. 3 (original article) shows the difference between high NA fiber and low NA fiber. Notice the high NA fiber allows light to enter (and remain) in the core from much larger angles. This leads to an increase in pulse-spreading. You can see why having a fiber with a low NA is advantageous—there is less modal dispersion. So, plastic optical fiber with a low NA allows for higher bandwidth (the amount of signal sent through the fiber). Low NA fiber allows less light to enter the fiber. Most transmitters are more than powerful enough, so this isn’t a problem.
The current state of POF. Engineers now know the right types of plastics and manufacturing techniques to develop graded-index and low-NA plastic fiber with transmission rates of 3 Gb/sec in runs exceeding 100 m.
Referring to Fig. 2 again, look at single-mode fiber. This type of fiber has incredibly high transmission capabilities. Until recently single-mode POF didn’t exist. But today, single-mode POF may make longer transmission distances possible, while making signal boosting easier and cheaper.