Fiber-Optic Techniques - LAN Wiring - page 246

certain incident angle, called the critical angle , the ray of light is reflected from the
interface, rather than refracted into the medium. That is exactly the reason that the
coin in the water seems to disappear when you try to view it from a low angle to
the water's surface. By the way, the angle at which the light is bent depends on the
wavelength (color) of the light, which is how prisms (and rainbows) work. This phe-
nomenon actually causes lots of problems for multimode fiber, as we will see later.
For years, we concentrated on the incident angles that were necessary for the
effective transmission and refraction of light from one medium to another, through
the interface. This was useful for the design of eyeglasses, binoculars, telescopes, and
microscopes. However, if the transmission medium is formed into a solid strand of
glass or plastic, we can use the reflective and refractive principles to guide the rays
of light within the strand , along its length.
Carefully constructed fibers of glass or plastic can efficiently transfer light for
long distances along their length, with very little loss of light outside the fiber. You
are probably familiar with the decorative lamps made with hundreds of short plas-
tic fibers. The fiber strands glow very slightly along their length when a light source
illuminates the strand ends placed at the base of the lamp, but the fiber ends shine
brightly. Similar fibers are used to transmit light along the network pathway of
LANs. We simply modulate (vary) the light beam in order to carry data.
Optical fibers can be either glass or plastic, but for most of this discussion, we
will refer to the fiber-optic material as glass, since silica compounds are used in most
current LAN implementations.
These silica fibers are sometimes called light pipes , because light seems to be
conducted through the flexible fiber, just like water through a tube. Optical fiber is
not actually a pipe for light, as stated previously. Light passes through the glass, not
through a pipe formed from the glass. The refractive properties of the glass are used
to direct the light through the center portion of the fiber, as shown in Fig. 11.2.
Depending on the type of fiber core, the incident light beam reflects or refracts its
way down the length of the fiber, staying roughly in the center portion of the core,
much as water in a pipe (thus the analogy). An appropriate light source is used to
send a communications signal through the optical fiber.
Here is how data transmission over a fiber works. A data-modulated electrical
signal, such as a LAN packet, is first applied to the light source, which converts the
signal into light. The beam of light from the source is then directed at one end of
the fiber, which transmits it through the fiber all the way to the other end. At the
far end of the fiber, a light-sensitive device is used to detect the light from the fiber
and convert it back to the electrical signal.
The device that actually creates the light is called the source and the device that
receives the light is called the detector . Light-emitting diodes (LEDs) and laser