A Basic Fiber Optic Communication System
Cable TV shows, phone calls, or Internet files normally travel through copper wire cables in the form of electrical signals. In a fiber optic system, a transmitter converts these electrical signals into pulses of light. It shoots them down glass fibers, until they reach the far end of the line. Here, receivers re-convert the light pulses to electrical signals, which then are turned back into voice, video, and data files.
How Does a Hair-thin Thread of Glass Carry So Much Information?
If you flash light in front of a window, it travels through instantaneously. However, most glass is not pure enough to allow light to travel very far. Remember how fuzzy things look through those thick glass blocks in bathroom walls? But fiber optic glass is so pure that almost nothing distorts the light signals traveling down its path. This means fiber optic technology can carry lots of information for very long distances—60 miles or more—before the signals have to be boosted.
To keep the light pulses from being lost out of the fiber, engineers add a layer of a different material around the inner core of pure glass. This “cladding” reflects the light back toward the center and prevents it from escaping. Further layers of coatings protect the fiber from bends, cracks, etc., that would weaken the signals.
Single-mode and Multimode Fibers
Optical fibers come in two main types. Single-mode fiber has a small core that forces the light waves to stay in the same path, or mode. This keeps the light signals going farther before they need to be beefed up, or amplified. Most long-distance, or long-haul, fiber optic telephone lines use single-mode fiber.
The second type, called multimode fiber, has a much larger core than single-mode fiber. This gives light waves more room to bounce around inside as they travel down the path. The extra movement eventually causes the pulses to smear, losing part of the information. That means multimode fiber signals can’t travel as far before they need to be cleaned up and re-amplified. Multimode fibers can carry only a third or less the information-carrying capacity—or bandwidth—than single-mode fiber, and they won't work for long distances.
Network engineers prefer multimode fiber for shorter-distance communication, such as in an office building or a local area network (LAN), because the technology is less expensive. However, with the growing demand for more bandwidth between computers and over the Internet, single-mode fiber is becoming more popular for smaller networks, too.
Fiber Optic Light Sources
In order to send messages through optical fibers, the light pulses have to be strong enough and keep their shape long enough so that they don’t lose too much information as they travel. Two main types of light sources can do the trick—laser diodes and light-emitting diodes, or LEDs.
LEDs, like the tiny green and red lights on computer equipment, are less powerful than lasers, but cheaper to make. LED signals work well enough for short-distance fiber optic networks, and they are cheaper than laser diodes.
The laser diodes used for fiber optic systems are similar to the ones inside CD and DVD players. Unlike LEDs, lasers send out coherent light, which means all the light particles, or photons, have exactly the same wavelength. Coherent light pulses keep their shape longer, so they lose less information and can operate at faster data speeds. Laser diodes can also deliver more power than LEDs, which keeps the signal traveling farther before it loses strength.
Fiber Optic communication systems promise clearer phone calls, faster Internet connections, and many more standard and HDTV channels.
Holly Martin - Holly B. Martin's blog, www.scienceshift.com explores the world of science beyond the fringe.
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