The document compares the characteristics and requirements of light emitting diodes (LEDs) and laser diodes (LDs) as optical sources for fiber optic communication systems. LEDs produce incoherent light through spontaneous emission, while LDs produce coherent light through stimulated emission, requiring population inversion and optical feedback. LDs have higher output power, narrower spectral linewidth, faster modulation speeds, and more directional light emission than LEDs. However, LEDs are simpler, more reliable, cheaper devices compared to LDs. The choice between LEDs and LDs depends on the specific needs and tradeoffs of linearity, coupling efficiency, bandwidth, cost, and other factors in a given fiber optic communication system.

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2. Taiz University, YEMEN
• Optical sources convert electrical energy in the form of a current into optical
energy which allows the light output to be effectively coupled into the optical
fiber.
Two types:
(a) Light emitting diode (LED) - incoherent source.
(b) Laser diode (LD) - coherent source.
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Requirements :
 Sufficient output power
• Overcome component losses
• Laser (mW range) has much higher power than LED (μW
range)
 Narrow spectral linewidth
• Minimizes fiber dispersion and increases transmission
capacity in WDM systems
• Laser has much narrower linewidth (typically 1−3 nm) than
LED (typically 30−50 nm)
• The spectral linewidth depends on device structure
 Must accurately track the electrical input signal to minimize distortion
and noise. Ideally the source should be linear.
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Requirements :
 Directional light output
• Increases coupling efficiency
• Laser (spreading at an angle of 10-20°) couples more light into fiber
than LED (spreading out at larger angles)
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Requirements :
 Useful emission wavelength region
• For low fiber attenuation and small fiber dispersion and
where the detectors are efficient. (typical windows:
780−850 nm, 1300 nm, 1550 nm)
• Emission wavelength depends on semiconductor material
from which the light source is made
 Modulation
• Easily modulated at high bit rates » greater information
capacity
• Speed. Lasers are faster than LEDs
 Stable light output
 Cheap and reliable
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• In this context the requirements for the laser
source are far more stringent than those for the
LED. Unlike the LED, the laser is a device, which
amplifies light. Hence the derivation of the term
LASER as an acronym for Light Amplification by
Stimulated Emission Radiation.
• By contrast the LED provides optical emission
without an inherent gain mechanism which
results in incoherent light output.
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• The frequency of the absorbed or emitted radiation f is related to the
difference in energy E between the higher energy state E2 and the lower
energy state E1 by the expression:
E = E2 - E1 = hf, where h = 6.626 x 10-34 J.s is Planck's constant
• Absorption: Atom excited to higher energy state (i.e. E1→E2) when
bombarded by photon with energy hf
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• When a photon with energy (E2 – E1) is
incident on the atom it may be excited into
the higher energy state E2 through absorption
of the photon.
• Alternatively when the atom is initially in the
higher energy state E2 it can make a transition
to the lower energy state E1 providing the
emission of a photon at a frequency
corresponding to equation E = E2 - E1 = hf,
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• This emission process can occur in two ways:
1) Spontaneous emission in which the atom returns to the lower energy
state in an entirely random manner.
LED
The random nature of the spontaneous emission process where
light is emitted by electronic transitions from a large number of
atoms gives incoherent radiation.
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2) Stimulated emission when a photon having an energy equal to the
energy difference between the two states (E2 – E1) interacts with the
atom in the upper energy state causing it to return to the lower state
with the creation of a second photon.
LASER
It is the stimulated emission process which gives the laser its special
properties as an optical source.
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LASER
1. The photon produced by stimulated emission is generally of an
identical energy to the one which caused it and hence the light
associated with them is of the same frequency - Monochromatic
2. The light associated with the stimulating and stimulated photon is
in phase and has the same polarization – Coherent.
• Furthermore this means that when an atom is stimulated to
emit light energy by an incident wave, the liberated energy can
add to the wave in a constructive manner, providing
amplification.
• Therefore, in contrast to spontaneous emission, coherent
radiation is obtained.
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Wavelength of emission, λ = (1.24/Eg)
where Eg = gap energy in eV.
Different material and alloys have different bandgap
energies
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Material λ(µm) Eg(eV)
GaInP 0.64-0.68 1.82-1.9
GaAs 0.9 1.4
AlGaAs 0.8-0.9 1.4-1.55
InGaAs 1.0-1.3 0.95-1.24
InGaAsP 0.9-1.7 0.73-1.35

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LASER different from LED
It Requires:
Population inversion.
Optical feedback.
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 Population Inversion
• Under the normal conditions the lower energy level E1 of
the two level atomic system contains more atoms than
the upper energy level E2.
• This situation which is normal for structures at room
temperature is illustrated in next Figure.
• However, to achieve optical amplification it is necessary to
create a non-equilibrium distribution of atoms such that
the population of atoms in the upper energy level is
greater than that of the lower energy level (i.e. N2 > N1 ).
• This condition which is known as population inversion.
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Populations in a two-energy-level system: (a) Boltzmann distribution for a system in
thermal equilibrium; (b) a nonequilibrium distribution showing population inversion

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• This process is achieved using an external energy
source and is referred to as 'pumping'.
• A common method used for pumping involves the
application of intense radiation.
• Population inversion may be obtained in systems with
three or four energy levels.
• The energy level diagrams for two such systems which
correspond to two non-semiconductor lasers are
illustrated in next Figure.
• To aid attainment of population inversion both systems
display a central metastable state in which the atoms
spend an unusually long time
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Energy-level diagrams showing population inversion and lasing for two nonsemiconductor lasers:
(a) three-level system – ruby (crystal) laser; (b) four-level system – He–Ne (gas) laser

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 Optical Feedback and Laser Oscillation
• Light amplification in laser occurs when a photon colliding with an
atom in the excited energy state causes the stimulated emission of
a second photon and then both these photons release two more.
• Continuation of this process effectively creates avalanche
multiplication, and when the electromagnetic waves associated
with these photons are in phase, amplified coherent emission is
obtained.
• To achieve this laser action it is necessary to contain photons
within the laser medium and maintain the conditions for
coherence.
• This is accomplished by placing or forming mirrors at either end of
the amplifying medium.
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The Semiconductor Injection Laser
• Stimulated emission by the recombination of the injected
carriers is encouraged in the semiconductor injection laser
(ILD) by the provision of an optical cavity in the crystal
structure in order to provide the feedback of photons.
• This gives the injection laser several major advantages
over other semiconductor sources that may be used for
optical communications.
These are:
1. High radiance due to the amplifying effect of stimulated
emission. Injection lasers will generally supply milliwatts of
optical output power.
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2. Narrow linewidth of the order of 1 nm or less which is useful in
minimizing the effects of material dispersion.
3. Modulation capabilities which at present extend up into the
gigahertz range and will undoubtedly be improved upon.
4. Relative temporal coherence which is considered essential to
allow heterodyne (coherent) detection in high capacity systems,
but at present is primarily of use in single mode systems.
5. Good spatial coherence which allows the output to be focused by
a lens into a spot which has a greater intensity than the dispersed
unfocused emission. This permits efficient coupling of the optical
output power into the fiber even for fibers with low numerical
aperture.
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The ideal light output against current characteristic for an injection laser

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• The LED is a device with a p-n junction that emits light
when it is forward biased.
• In the forward biased p-n junction, electrons and holes are
injected into the active region, where they recombine to
produce light.
• At present LEDs have several further drawbacks in
comparison with injection lasers.
• These include:
(a) generally lower optical power coupled into a fiber
(microwatts)
(b) relatively small modulation bandwidth (often less than
50MHz)
(c) harmonic distortion
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However, although these problems may initially appear to make the
LED a far less attractive optical source than the injection laser, the
device has a number of distinct advantages which have given it a
prominent place in optical fiber communications:
(a) Simpler fabrication. There are no mirror facets.
(b) Cost. The simpler construction of the LED leads to much
reduced cost which is always likely to be maintained.
(c)Reliability. The LED does not exhibit catastrophic degradation
and has proved far less sensitive to gradual degradation than
the injection laser. It is also immune to self pulsation and
modal noise problems.
(d) Simpler drive circuitry. This is due to the generally lower
drive currents and reduced temperature dependance which
makes temperature compensation circuits unnecessary.
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(e)Less temperature dependence. The light output
against current characteristic is less affected by
temperature than the corresponding characteristic for the
injection laser. Furthermore the LED is not a threshold device and
therefore raising the temperature does not increase the threshold
current above the operating point and hence halt operation.
(f) Linearity. Ideally the LED has a linear light output against
current characteristic unlike the injection laser. This can prove
advantageous where analog modulation is concerned.
• These advantages coupled with the development of high radiance
medium bandwidth devices has made the LED a widely used optical
source for communications applications.
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LED Structures
• There are five major types of LED structure although
only two have found extensive use in optical fiber
communications.
• These are the etched well surface emitter, often simply
called the surface emitter, or Burros (after the
originator) LED, and the edge emitter.
• The other two structures, the planar and dome LEDs,
find more application as cheap plastic encapsulated
visible devices for use in such areas as intruder alarms,
TV channel changes and industrial counting.
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Spectral width of LED types
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Typical spectral output characteristics for InGaAsP surface- and edge-emitting
LEDs operating in the 1.3 μm wavelength region

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• When deciding whether to choose an LED or an LD as the light
source in a particular optical communication system, the main
features to be considered are the following:
(a)
• The optical power versus current characteristics of the two
devices differ considerably.
• Near the origin the LED characteristic is linear, although it
becomes non-linear for larger power values.
• However, the laser characteristic is linear above the
threshold.
• Single-mode lasers show an excellent linear characteristic
above the threshold.
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• Linearity of the source is important for analog systems,
but is less important for digital systems.
• The power-to-current characteristic of an LD depends
greatly on temperature, but this dependence is not so
great for an LED.
• The power supplied by both devices is similar (about 10-
20 mW).
• However, the maximum coupling efficiency of a fiber is
much smaller for a LED than for a LD; for a LED it is 5-10
percent, but for a LD it can be up to 90 percent.
• This difference in coupling efficiency has to do with the
difference in radiation geometry of the two devices
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(b)
• As an LED emits spontaneous radiation, the speed of
modulation is limited by the spontaneous recombination
time of the carriers.
• LEDs have a large capacitance and modulation bandwidths
are not very large (a few hundred megahertz).
• The capacitance can be reduced by biasing the diode with
a forward current, which increases the modulation speed.
• For a laser above the threshold the electrons remain in the
conduction band for a very short time, due to the
stimulated recombination; therefore, very fast modulation
is possible (up to 10 GHz).
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(c)
LDs have narrower spectra than LEDs, and the single-
mode lasers, in particular have a very narrow
spectrum. This explains why the pulse broadening at
transmission through an optical fiber is very small.
Therefore, with an LD as a light source, wideband
transmission systems can be designed. The spectrum of
an LD remains more stable with temperature than that
of an LED.
(e)
At current prices, LEDs are less expensive than LDs.
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