Published In July 2001
To most of us, when we think of fiber optics, we think first of the telephone companies who use fiber for their long-distance lines and connections between local offices. Some of you may be familiar with fiber optics as the backbone cabling for cable access television (CATV) that makes cable modems for Internet connections possible. And many of you are already installing fiber optics for premises cabling systems for local area network (LAN) connections. This month, we’d like to explore some more applications for fiber optics outside these traditional ones. Video and audio One of the first “non-telco” applications was video. Because it requires such high bandwidth, video has typically used coaxial cable. To make coax lower loss for longer links, it must be larger diameter, which means bulkier and heavier. Using simple optical-to-electrical and electrical-to-optical converters, video can be transmitted on fiber optics. The first application of this type was done for portable cameras used by television (TV) companies for their remote vans. Coax limited the camera to about 50 feet from the van, but fiber optic converters gave the cameraman and reporter a radius of over 5,000 feet! The small size and ruggedness of the fiber optic cable (similar to that used in military field applications) revolutionized remote TV news. Today, closed circuit television (CCTV) routinely uses this technology to allow longer distances or multiple cameras to share one fiber optic cable. Bi-directional links allow controlling the camera and receiving the video over one fiber. Most airports and many large office buildings use this technique because they need longer distances from cameras to monitoring equipment than is possible with coax. A new application for fiber is audio. Peak Audio has developed a digital audio system called Cobranet that sends uncompressed high fidelity audio signals over Fast Ethernet cables, just like structured cabling. Unlike analog audio connections, which suffer from attenuation and distortion when transmitted long distances, these digital signals can be transmitted without degradation, making them ideal for auditoriums, stadiums, and other large buildings. Using multimode optical fiber, the sound not only improves, the cost is also significantly less! If you have looked at the back panel of a new generation digital video (DV) camera, DVD, CD, or MiniDisc system, you might have noticed an optical port. This is IEEE-1394, also known as Firewire. It’s digital transmission over plastic optical fiber (POF). POF is inexpensive, comparable to coax, and easy to install. Its limitations are distance (about 50 meters or 150 feet) and bandwidth, less than a few hundred megabits per second. But it is great for a stereo system, DV editing, or even hooking up an entertainment system in a home. Of course, being optical fiber, it is impervious to noise pickup and ground loops. Fiber optic sensors Fiber optics can be used for sensors. Sensors are devices that measure physical parameters like temperature or pressure. Most sensors are electrical by nature, like thermocouples, which measure temperature by the reaction of dissimilar metals to heat. Others, like thermistors or load cells, sense physical changes by changes in resistance of the sensor. Fiber optics can also be used to measure many physical parameters. For temperature, optical measurement senses the infrared radiation emitted from an object. You may have used an infrared thermometer to troubleshoot electrical problems, like overheating transformers. With fiber optics, the measurement can be made remotely by piping the infrared radiation from the point of measurement over an optical fiber. The glass fiber will withstand very high temperatures, so the measurement electronics can be remote from the place where the measurement is actually taking place, protecting both the instrument and the user. When installing fiber optic cable, we are always careful not to bend it too tightly, as that can induce unwanted loss. But if we place the cable in a location where it will be stressed, we can calibrate this bending loss to measure that stress. Fiber optics have been buried in roads to measure vehicle loading and imbedded in bridges to sense not only stress but failures in the structure. Other fiber optic sensors can measure liquid level, chemical composition, or even electrical current or high voltage. Utilities have been using these electrical sensors for years on high-voltage distribution systems to manage the power grid. The combination of sensor, fiber, and electronics makes fiber optic sensors expensive, but for some applications, they are the only possible solution. Lighting The ability of the optical fiber to transmit light has been used for half a century to “pipe light.” Early applications were for medical or nondestructive inspection. Now, it is widely used for signs and lighting in hazardous areas like swimming pools. Since fiber can be made to emit light from either the end or the side and in many colors, it offers unique decorative possibilities also. While the bulk of fiber optic applications are the “traditional” ones, new and unique applications are also taking advantage of fiber’s special properties. Knowing more about fiber and its applications will allow you to find new opportunities. HAYES is the founder of Fotec, the fiber optic test equipment company and the Cable U training programs. He can be contacted at Jh@jimhayes.com.