2. Transmitters 39
Edge emitting LEDs have greater coupling efficiencies because their
source area is smaller. Their geometry is similar to a conventional diode
laser, because light travels back and forth in the plane of the active region
and is emitted from one anti-reflection coated cleaved end. These LEDs
differ from lasers in that they do not have a feedback cavity. Because of
their smaller source size, they can be coupled into single mode fiber with
modest efficiency 004%. ELEDs are typically used as low-coherence
sources for fiber sensor applications, rather than in communications,
because of their broad emission spectrum.
Device characteristic curves for LEDs include the forward current vs.
applied voltage, the light emission or luminous intensity vs. forward cur-
rent, the spectral distribution and the luminous intensity vs. temperature
and angle. An example of these is shown in Figure 2.6. Since LEDs are
chosen for low cost applications, they are modulated by varying the drive
current, rather than using an external modulator.
2.3 Lasers
There are many different types of lasers, only some of which are useful
for communication systems. The term laser is an acronym for Light
Amplification by Stimulated Emission of Radiation. In order to form a
basic laser system, we require a media which can produce light (photons)
through stimulated emission. This means that electrons in the media are
excited to a higher than normal energy state, then decay into a lower
energy state, releasing photons in the process. These photons will have
the same wavelength, or frequency, as well as the same phase; thus,
they will add up to create a coherent optical beam which emerges from
the laser. Although we speak of laser light as being monochromatic,or
having a single wavelength, in reality a laser produces a narrow spread
of different wavelengths, or an emission spectra. The material which
creates photons in this way, called the gain media, is most commonly
a gas (in the case of medical lasers) or a semiconductor material (in the
case of some medical devices and almost all communication systems).
Electrons are excited by supplying energy from some external source,
a process called pumping the laser. Most lasers are pumped with an
electrical voltage, though another optical beam can sometimes be used to
provide the necessary energy. Since the electrons are normally found in
their lower energy state, pumping them into a higher energy state creates
what is known as a population inversion. The final element in creating
40 Fiber Optic Essentials
Spectral
distribution
Relative intensity
Wavelength λ (nm)
10
0.8
0.6
0.4
0.2
0
600 620 640 660 680 700
Luminous intensity vs.
Ambient temperature
% Relative luminance
Ambient temperature (°C)
180
160
140
120
100
80
60
–50 –25 0 25 50 75 100
Relative luminous
intensity vs. angle
100
80
60
40
20
020406080
Relative intensity (%)
Number of degrees off angle (%)
I Current
200 mA
150
mA
100
mA
50
mA
–50
pA
–100
pA
20 10
12
V Volts
Typical I-V characteristics
(a) (b)
(d)(c)
Figure 2.6 Characteristics of an LED: spectral distribution, luminous intensity vs.
temperature and angle, and I vs. V characteristics.
a laser is an optical feedback mechanism, such as a pair of mirrors, to
reflect light many times through the gain media and produce a stronger
output beam. Laser light is coherent, meaning that all the photons are at
the same wavelength (frequency) and all are in phase with each other.
Lasers have very low divergence (a laser spot will spread out very little
over long distances).
In most fiber optic communication systems, we are interested in the
type of lasers based on solid state devices, similar to LEDs in that they
use PN heterojunctions to emit photons. In LEDs, PN junctions are used

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