Page 99 - Optical Communications Essentials

Light Sources and Transmitters

Light Sources and Transmitters 89

TABLE 6.1. Some LED and Laser Diode Material Mixtures
and Their Characteristics
Material Wavelength range, nm Bandgap energies, eV
GaAs 900 1.4
GaAlAs 800–900 1.4–1.55
InGaAs 1000–1300 0.95–1.24
InGaAsP 900–1700 0.73–1.35

mixtures together with their operating wavelength range and approximate
bandgap energies. Alloys consisting of three elements are called ternary com-
pounds, and four-element alloys are known as quaternary compounds. A specific
operating wavelength can be selected for AlGaAs, InGaAs, and InGaAsP devices
by varying the proportions of the constituent atoms. Thus devices can be tai-
lored to emit at a selected wavelength in the 780- to 850-nm band or in any of
the other transmission bands ranging from 1280 to 1675nm for glass fibers.

6.1.2. Spectral output width
A major difference between light sources is the spectral width of their light out-
put. This is an important factor when one is choosing an optical source since, as
Chap. 4 notes, signal spreading in an optical fiber due to chromatic dispersion is
directly proportional to the wavelength band over which a source emits light.
Recall that chromatic dispersion occurs since each wavelength in a light signal
travels at a slightly different velocity. This effect progressively smears out an opti-
cal signal as it travels along a fiber. Consequently in order to send a high-speed
signal (consisting of very narrow light pulses) over long distances, the source needs
to emit light within as narrow a spectral width as possible. This can be done only
with laser diodes. An LED has spectral widths ranging from 30nm at a central
wavelength of 850nm to around 120nm at a 1550-nm central wavelength. Laser
diodes, on the other hand, can have spectral widths of a few picometers (10 3 nm)
at 1550nm. For a laser diode the spectral width is referred to as the linewidth.

Example Consider a fiber that has a chromatic dispersion D 2ps/(km·nm). Sup-
pose we want to send a 10-Gbps signal over this fiber. At 10 Gbps the pulse widths are
1/(10Gbps) 100ps. If 20 percent pulse spreading is tolerable, then this data rate has
an allowable spread of 20ps before adjacent pulses overlap too much. Then if we use
an inexpensive laser source with a 5-nm linewidth, this signal can travel only 2km
before the pulse overlap limit is reached. For a high-quality laser with a 0.05-nm
linewidth, the chromatic dispersion distance limit is 2000km.

6.1.3. Modulation speed
Direct modulation is the process of using a varying electric signal to change the
optical output level of a device. The term modulation speed refers to how fast a

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.