Page 62 - Optical Switching And Networking Handbook

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Basic Fiberoptics Technologies 47

Because of cost, the LED is a favored device in LANs. Table 2-5 sum-
marizes LED characteristics. These devices fit many communica-
tions applications.

Table 2-5
Characteristic Description
Summary of LED
Low cost LEDs are much less expensive than communications
Characteristics
lasers. This is highly controversial. Communications LEDs
and lasers are not much different in structure and are
comparable in manufacturing cost. However, most LEDs
cannot be used with single-mode fiber, whereas lasers can
be used with either single-mode or multimode fiber.
Relatively wide LEDs output a band of wavelengths instead of a single
range of wavelength. This was discussed earlier. The wavelengths
frequencies can be reduced by using filters. By filtering the light, a
narrow band of wavelengths can be used. Everyone proba-
bly has been exposed to a form of filtering with sun-
glasses, sun-blocking agents, or even in photography. The
filters give the desired effect by blocking specific frequen-
cies in the light spectrum.
Low power output The maximum light output of a LED typically is much
lower than a laser. However, a new class of LEDs with
higher output has become available. The regulation of
devices in production is less severe with LEDs, although
things always change.

Digital LEDs cannot produce pulses short enough to be used at
high speeds. LEDs can operate at speeds of up to 622
Mbps. Digital modulation is straightforward. The device
“turns on” when the voltage is strong enough to power it
and turns off when the voltage drops off. This light on,
light off can represent the two digital states used in com-
munications (0 and 1).
Lenses required A lens is needed to focus the light onto the end of a fiber.
LEDs are not suitable for use with single-mode fiber
because it is too hard to get the light into the narrow
opening of the core and focus the light down to the other
end.

Analog too LEDs also can be analog, which is an advantage over
lasers. By modulating an analog signal continuously on
the fiber, the information can be carried on the analog car-
rier. Most people think that fibers are purely for digital
transmissions.