Optical Fiber Communication

Lecture-12

Prepared by Bijoy Kumer Karmaker 1

Light-Diodes (LEDs)
• They are used in fiber-optic communications, mostly because of their small size and long life. However,
their low intensity, poor beam focus, low-modulation bandwidth, and incoherent radiation—in
comparison with laser diodes, that is—restrict their usage to a specific sector of communications
technology: relatively short-distance and low-bandwidth networks.

Prepared by Bijoy Kumer Karmaker 2

Energy Bands of an Intrinsic Semiconductor:
When the absolute temperature is zero and no
external electric field is applied, all electrons are
concentrated at the valence band and there are no
electrons at the conduction band. This is because
none of the electrons possess enough extra energy
to jump over the energy gap. But when some
external energy —either through temperature or by
an external electric field—is provided to the
electrons at the valence band, some of them acquire
enough energy to leap over the energy gap and
occupy energy levels at the conduction band. We
say these electrons are ’’excited” These excited
electrons leave holes (positive charge earners) at the
valence band, as Figure 9.1(b) shows.

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Energy Bands of an Intrinsic Semiconductor:
When an excited electron falls from an upper energy level to a lower
one. it releases a quantum of energy called a photon. The
𝒉𝒄
relationship among ∆𝑬𝒍 , ∆𝑬𝒑 , 𝒂𝒏𝒅 𝝀 is given by: ∆𝑬 = ∆𝒑 = 𝒉𝒇 = ,
𝝀
where ∆𝑬 is the difference between the two energy levels, ∆𝑬𝒑 is the
photon's energy, and 𝝀 is the wavelength.
The same idea holds for semiconductors. If an excited electron falls
from a conduction band to a valence band, it releases a photon
whose energy,𝑬𝒑 , is equal to or greater than the energy gap. 𝑬𝒈 .
Since not just one but many energy levels at the conduction and
valence bands can participate in the radiation process, many close
wavelengths, 𝝀𝒊 , can be radiated. The result of this multiwavelength
radiation is a wide spectral width, ∆𝝀, of light emitted by the
semiconductor. This explanation is depicted in Figure 9.2.
Thus, to make a semiconductor radiate, it is necessary to excite a
significant number of electrons at the conduction band. This can be
done by providing external energy to the material. The most suitable
form of this external energy is electric current flowing through a
semiconductor. Prepared by Bijoy Kumer Karmaker 4
Light Radiation-The p-n Junction:
We can insert atoms of another material into a semiconductor so that either a majority of electrons
(negative charge carriers) or a majority of holes (positive charge carriers) will be created. The former
semiconductor is called the n type, where n stands for negative, and the latter is called the p type, where p
stands for positive. We call these n type and p type doped, or extrinsic, semiconductors in contrast to a
pure, or intrinsic, semiconductor, which consists of atoms of one material. The inserted foreign materials
are called dopants.

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Light Radiation-The p-n Junction cont..
When an n-type semiconductor is brought into physical contact with a p type, a p-n junction is created. At the
boundary of the junction, electrons from the n side diffuse to the p side and recombine with holes and. at the
same time, holes from the p side diffuse to the n side and recombine with electrons. Thus, a finite width zone,
called the depletion region, forms. Here, there arc no mobile electrons or holes. Since positive ions at the n side
and negative ions at the p side within the depletion region are left without electrons or holes, these ions create
an internal electric field called a contact potential. We characterize this field by depletion voltage. 𝑽𝑫 . Figure
9.3(a) illustrates this explanation.

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Light Radiation-The p-n Junction cont..
The most important point to keep in mind is: An electron-hole recombination releases a quantum of energy—a
photon. In other words, to make a semiconductor radiate, it is necessary to sustain electron-hole recombination.
But the depletion voltage prevents electrons and holes from penetrating into a depletion region; therefore,
external energy must be supplied to overcome this voltage barrier. This external voltage, called forward biasing
voltage, V. is shown in Figure 9.3(b). Obviously. V must be greater than 𝑽𝑫 .
To achieve permanent light radiation, the following dynamic process must occur: Mobile electrons from the n
side, attracted by the positive terminal of V, enter the depletion region. Simultaneously, mobile holes from the p
side, attracted by the negative terminal of V, enter the same depletion region. Electron-hole recombinations
within a depletion region produce light.

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LED principle of action:
A light-emitting diode, LED. is a semiconductor diode made by creation of a junction of n-type and p-type
materials. Thus, the principle of an LED's action works precisely the same way the creation of permanent light
radiation: The forward biasing voltage, V, causes electrons and holes to enter the depletion region and recombine.
Alternatively, it can say that the external energy provided by V excites electrons at the conduction band. From
there, they fall to the valence band and recombine with holes 9.2(a)). Whatever point of view you prefer, the net
result is light radiation by a semiconductor diode. This concept is displayed by the circuit of an LED (Figure 9.4(a)).
What's the difference between an LED and a regular diode?” The difference is that in a regular diode these
recombinations release energy in the thermal—rather than the visible—portion of the spectrum. This is why these
electronic devices are always warm when you turn them on. In an LED, however, these recombinations result in
the release of radiation in the visible, or light, part of the spectrum. We call the first type of recombination
nonradiative, while the second type is called radiative recombination.

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Homstructure LED:
In homostructure LED the n-type and p-type
semiconductors are made from the same substrate. By
adding various dopants, we can make either an n type of
semiconductor, with excessive electrons (that is, negative
charge carriers) or a p type of semiconductor. with
excessive holes (that is, positive charge carriers). Both
semiconductor types have the same energy gap. The p-n
junction of such semiconductors becomes what’s known as
a homojunction. The possible structures of an LED made
from such a semiconductor—homostructures—are shown
in Figure 9.5(a) and 9.5 (b).

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Drawbacks of Homestructure LED
A homostructured LED has two major drawbacks:
First, its active region is too diffuse, which makes the device’s efficiency very low. This is
because electron-hole recombinations take place in various locations, that is. over a large
area, a situation that requires high current density to support the desired level of radiated
power.
Second, this type of LED radiates a broad light beam. This makes the coupling of this light into
an optical fiber extremely inefficient and is the reason why you cannot find an LED with a
homojunction in practical applications.

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Heterostructured LED
Commercially manufactured LEDs that radiate well-
directed light with acceptable efficiency use
heterojunctions. Heterostructured LEDs are made from
different types of semiconductor materials, each type
having a different energy gap. Figure 9.5(c) shows a
heterostructure made from two different semiconductors.
Two basic concepts are introduced with this
heterostructure: the confinement of electron- hole
recombinations within a highly restricted active region
and the conduction of radiated light in one direction.

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Heterostructured LED cont..
The first is achieved by placing a semiconductor with a small
energy gap between the two layers of the substrate
semiconductor with the larger energy gap. Figure 9.5(c) shows
that gallium arsenide (GaAs). whose 𝑬𝒈 = 1.42 eV, is placed
between the aluminum gallium arsenide (AlGaAs) layers,
whose 𝑬𝒈 = 1.92 eV. As one can see from Figure 9.5(c)
electrons injected from n-type AlGaAs confront an energy
barrier at the junction where GaAs and p-type AlGaAs meet
and are reflected back into the active region. The same
mechanism works for holes.
The conduction of light in one direction is achieved because
the GaAs semiconductor has a higher refractive index (here.
3.66) than the substrate semiconductor (here. 3.2). Thus, the
active region works as a waveguide similar to the way a fiber
traps light within the core using the corecladding interface.
The same concept is implemented for another popular
heterostructure. indium phosphide-indium gallium arsenide
phosphide (InP-InGaAsP) Prepared by Bijoy Kumer Karmaker 12
Surface and Edge Emitting LED:
There are two basic arrangements of an LED: surface
emitting (SLED) and edge emitting (ELED). The depletion
region and surrounding area, where electron-hole
recombinations take place, are known as an active
region. Light produced by these recombinations radiates
in all directions, but only a transparent window of the
upper electrode (Figure 9.5la]) or an open edge (Figure
9.5(b)) allows light to escape from the semiconductor
structure. All other possible directions (in the case of
SLED) and the opposite edge (in the case of ELED) are
blocked from light by the LED’s packages.

Prepared by Bijoy Kumer Karmaker 13

Surface and Edge Emitting LED cont..
Lambertian sources are a type of light source that emits light uniformly in all directions. This means that the intensity of light emitted from a
Lambertian source is the same regardless of the viewing angle.
SLED radiates light as a Lambertian source Its power
distribution is described by the following formula:
𝑷 = 𝑷𝟎 𝐜𝐨𝐬 𝜽
where 𝜽 is the angle between the direction of
observation and the line orthogonal to the radiating
surface; thus, 𝑷 = 𝑷𝟎 when 𝜽 = 𝟎𝟎 . Half of the power of
the Lambertian source is concentrated in a 𝟏𝟐𝟎𝟎 cone.
ELED radiates as a Lambertian source in the plane parallel
to the edge and produces a much narrower beam in the
plane perpendicular to the edge, as Figure 9.6(b) shows.
A Lambertian source is simply a reference model that
describes in a general way a homostructurcd SLED. In
reality, a heterostructured LED radiates a much better
directed beam. Figure 9.6(c) depicts a sample of a real
spatial pattern of radiation. Because of the form of its
radiant pattern, a SLED is more suitable to use with a
multimode fiber, while an ELED can be used with a
singlemode fiber.
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Coupling light into a fiber:
It is quite evident that we are interested in having as powerful an input light signal as possible because,
given fiber attenuation, a more powerful signal travels a greater distance. It would seem that to accomplish
this, we would need a more powerful light source, but this is not the whole truth. The key to the distance a
signal travels is not just the power radiated by the source, but the power coupled into an optical fiber
because this is the real input signal being transmitted. With inefficient coupling, you may lose most of the
light power radiated by your LED, thus making the quality of the LED absolutely unimportant from the
transmission standpoint.
If you approximate the radiation pattern of a SLED by a Lambertian model, then light power (𝑷𝒊𝒏 ) coupled
into a step-index fiber with a numerical aperture (NA) can be calculated by the following formula:
𝑷𝒊𝒏 = 𝑷𝟎 𝑵𝑨 𝟐
Where 𝑷𝟎 𝒊𝒔 when 𝜽 = 𝟎𝟎 .

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Coupling Light from an LED into an Optical Fiber.

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End of Class 12

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Class 13

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Laser diodes (LDs):
• The acronym laser means light amplification by the stimulated emission of radiation. The first working ruby
laser was developed in 1960 by the American scientist Theodore Meiman. The theoretical and practical
foundations for this development were made by the American Charles Townes and the Russians Alexander
Prokhorov and Nikolay Basov, who shared the Nobel Prize for Physics in 1964 for their work.
• Interestingly, the laser is not a light amplifier, as the term suggests, but, rather, a light generator.
• The laser is a device that amplifies (or. as we now know, “generates”) light by means of the stimulated
emission of radiation.

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Spontaneous Radiation:
Spontaneous "means that radiation occurs without external cause. That’s exactly what happens in an LED:
Excited electrons from the conduction band fall, without any external inducement, to the valence band, which
results in spontaneous radiation.
The properties of spontaneous radiation follow naturally from the way it occurs:
First, the transition of electrons from many energy levels of conduction and valence bands contributes to the
radiation produced, thus making the spectral width of such a source very wide. This is why a typical LED’s ∆𝝀 is
about 60 nm at an operating wavelength of 850 nm and about 170 nm at an operating wavelength of 1300 nm.
Second, since photons are radiated in arbitrary directions, very few of them create light in the desired direction,
a factor that reduces the output power of an LED. This means that current-to-light conversion occurs with low
efficiency and an LED has relatively low out¬ put power (intensity).
Third, even those photons that contribute to output power do not move strictly in one direction; thus, they
propagate within a wide cone, yielding widespread radiated light. For this reason, LEDs are modeled as
Lambertian source.
Fourth, this transition, and therefore photon radiation, occurs at any time, in other words. photons are created
independently of one another. Hence, no phase correlation between different photons exists and the total light
radiated is called incoherent.

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Spontaneous Radiation cont..
These four main properties of spontaneous radiation—wide spectral width, low intensity, poor
directiveness, and incoherence—make it impossible as light sources for long-distance communication
links.

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Stimulated Radiation:
First, an external photon forces a photon with similar energy (𝑬𝒈 ) to be emitted. In other words, the external
photon stimulates radiation with the same frequency (wavelength) it has. This property ensures that the
spectral width of the light radiated will be narrow. In fact, it is quite common for a laser diode’s ∆𝝀 to be about
1 nm meter at about 1300 nm and 1550 nm.
Second, since all photons propagate in the same direction, all of them contribute to output light. Thus, current-
to-light conversion occurs with high efficiency and a laser diode has high output power. (In comparison, to make
an LED radiate 1 mW of output power requires up to 150 mA of forward current; a laser diode, on the other
hand, can radiate 1 mW at 10 mA).
Third, the stimulated photon propagates in the same direction as the photon that stimulated it; hence, the
stimulated light will be well directed.
Fourth, since as stimulated photo is radiated only when and external photon triggers this action, both photos
are said to be synchronized, that is, time-aligned. This means that both photos are in face and so the stimulated
radiation is coherent.

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Thus, in contrast to spontaneous radiation, stimulated radiation has narrow spectral width, high intensity
(power), a high degree of directivity, and coherence. This is why laser diodes, which radiate stimulated light,
find use in long-distance communication links.

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Lasing Effect:
A semiconductor diode functions like a laser if the following conditions are met:
• Population Inversion
• Stimulated Emission
• Positive Feedback

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Lasing Effect cont..
Positive feedback: Positive feedback To radiate stimulated light
with essential power, we need not one photon but millions and
millions. Here is, how such radiation is achieved: a minor at one
end of an active layer is plaed, as Figure 9.10(b). Two photons—
one external and one stimulated— are then reflected back and
directed to the active layer again. These two photons now work as
external radiation and stimulate the emission of two other
photons. The four photons are reflected by a second minor, which
is positioned at the other end of the active layer. When these
photons pass the active layer, they stimulate emission of another
four photons. These eight photons are reflected back into the
active layer by the first mirror and this process continues ad
infinitum. (Figure 9.10(b) illustrates these explanations.)
Thus, the two mirrors provide positive optical feedback—positive
because the feedback adds the output (stimulated photons) to the
input (external photons).

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Lasing effect cont..
In Population Inversion fig. 9.10(b) the number of stimulated photons rises very fast. To sustain this
dynamic process, an incalculable number of excited electrons at the conduction band is needed.
In fact, lasing action needs more electrons at the high-energy conduction band than at the lower-energy
balance band. This situation is called population inversion because, normally, the valence band is much
more heavily populated than the conduction band. To create this population inversion, high-density
forward current is passed through the small active area.
Population inversion is a necessary condition to create a lasing effect because the greater the number of
excited electrons, the greater the number of stimulated photon that can be radiated.

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Characteristics of Light radiated by a Laser Diode:
Monochromatic: The spectral width of the radiated light is very narrow. Indeed, the line width for a laser
diode can be in tenths or even hundredths of a nanometer.
Well directed: A laser diode radiates a narrow, well-directed beam that can be easily launched into an
optical fiber.
Highly intense and power-efficient: A laser diode can radiate hundreds of milliwatts of output power. A
new type of laser diode, the VCSEL, radiates 1 mW at 10 mA of forward current, making current-to-light
conversion 10 times more efficient than it is in the best LEDs.
Coherent: Light radiated by a laser diode is coherent; that is, all oscillations are in phase.

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Basic Structures and Types of Laser Diodes
The basic construction of a laser diode is shown in
Figure 9.12(a). If it looks similar to the edge-emitting
LED shown in Figure 9.5(b), it in fact is, except for two
major differences: First, the thickness of an active
region in a laser diode is very small, typically on the
order of 0.1 µm. Second, a laser diode’s two end
surfaces are cleaved to make them work as mirrors.
Since the refractive index of GaAs — the material
making up the active region—is about 3.6, more than
30% of incident light will be reflected back into the
active region at the GaAs-air interface. Thus, no special
mirrors are required, these surfaces, called laser facets,
provide positive feedback. This basic type of laser diode
is called a broad-area LD.

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Basic Structures and Types of Laser Diodes cont..
Gain-guided Laser: A strip contact is used to confine charge
carriers—electrons and holes— even more securely within the
laser diode’s small active region. (fig. 9.129[b].) This
construction restricts the current flow within this narrow
region. Since current flow produces gain in an active region,
this type of laser diode is called gain-guided.
Index-guided Laser: To even further circumscribe the active
region, surround it with a material with a lower refractive
index. Such an LD is called index-guided. Its structure is very
similar to the core-cladding arrangement in an optical fiber.
These surrounding layers are called cladding layers and the
term sandwich is usually used to describe this structure. The
most popular construction of an index-guided LD, where the
cladding layer’s thickness varies, is known as a ridge
waveguide, RWG. In an index-guided laser diode, the small
active region is buried between several lower refractive index
layers. Such a structure is called a buried heterostructure
(BH).
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Quantum-well laser Diodes:
To make lasing action more efficient, a special fabrication
technique is used to form an especially thin active region, one
on the other of 4 to 20 nm of thickness. Such devices are called
quantum-well (QW) laser diodes. The quantum-well techniques
modifies the density of energy level available for electrons and
holes. The result is a much larger optical gain. From the P-N
junction standpoint, a quantum-well diode is characterized by
the lower potential energy of its electrons and holes, thus
making their recombination easier. In other words, less forward
current is required to reach and sustain lasing action in this
type of laser diode. The main advantage of quantum-well laser
diode are more efficient current-to-light conversion, better
confinement of the output beam, and the potential to adiate a
variety of wavelengths.

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End of Class 13

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Class 14

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 Laser Modulation
There are two types of laser diode modulation: digital and
analog. In the simplest digital-modulation scheme, logic 0 is
represented by a dark period and logic 1 by a flash of light. To
attain this state, an information signal changes the forward
current of a laser diode from values below threshold to values
above threshold. This is shown in figure. In analog
modulation, designers want to use the linear portion of a P-I
curve to avoid nonlinear distortion of an output signal. This is
attained by applying dc biasing current, 𝑰𝒃 , along with the
information signal, as in figure.

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 Laser Modulation Cont..

After the drive current exceeds the threshold value, population inversion is created, the transition of excited
electrons from conduction to valence band occurs, and photons are radiated. These radiated photons travel
within the laser cavity and escape from there, creating an output information signal in the form of flashes of
light.

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Transmitter Modules:
A transmitter module includes a light source, coupling optics, a
signaling circuit, and a power-control circuit. All these
components are packed into one module as in figure.
Data conversion unit: The transmitter’s data conversion unit
performs three major functions: encoding, parallel-to-serial
conversion, and reshaping the electric format of the data.
• Encoding means representing data in a physical format
(pulses). This is necessary because data are transmitted in
different line codes.

• The second function of the data-conversion unit is parallel-to-serial conversion. Data enter in parallel format,
but a laser diode can be driven only by serial pulses of modulation current. Thus, a parallel-in serial-out
converter (PISO), which is often called a multiplexer, is used to convert data into the serial format.

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• The third function of the data-conversion unit is reshaping the electric format of data. Either a
comparator or a buffer can be used for this purpose.
Laser Driver: Data prepared for light transmission passes into a laser-driver. We need this circuit because
a lased diode is a current-driven rather than a voltage-driven device, while the power supply is always a
voltage source. Thus, the first function of a laser driver is to convert outside voltage into the current
needed to drive the laser.
Modulation circuit: Modulation is controlled by simply changing the driving current from the bias level to
maximum.
Controlling and monitoring circuits: Transmitter circuits allow users to control and monitor transmitter
performance. The control signal transmitter disable allows the user to shut down the transmitter while
keeping the module in standby mode.

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 Drawbacks of Internal Modulators:
Although direct modulation offers some benefits, there are at least two serious drawbacks:
1. Bandwidth is restricted by the laser diode’s relaxation frequency.
2. Chirp—the fast variation of the laser’s peak radiating frequency in response to a change in driving
(modulation) current—results to produce a broadening of the light pulse. Chirp is a problem for DFB
lasers, and it is a serious limiting factor in high-speed communication, where DFB lasers are primarily
applied.
3. The third drawback is that: long-distance fiber-optic networks require extra light power to be launched
into the fiber to increase the span between each optical amplifier. Attaining such power requires that a
laser diode operate at high driving current–on the order of 100 mA and higher. But this, in turn, means
that switching to such a current at high speed becomes a major problem

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External Modulator
The radical means to overcome all problems associated with internal modulation is to resort to external-
modulation. This approach leaves a laser diode to radiate a continuous light wave (CW) while a change in light
power occurs outside the laser diode. A block diagram of external modulation is shown in figure:

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Mach-Zehnder(MDM) External Modulators:
Two basic types of external modulators are mainly in use today.
The first one is a stand-alone external modulator called a Mach-
Zehnder (MDN), or lithium niobate 𝑳𝒊𝑵𝒃𝑶𝟑 , modulator. It is
based on the so-called Mach-Zehnder configuration. Its principle
of operation is as follows: The refractive index of lithium niobate
changes when an electrical voltage is applied. Light for this
modulator emanates from the laser diode and splits equally
when it enters the waveguide. When no voltage is applied, both
halves of the incident wave have no phase shift and so they
interfere constructively, forming the original wave. When voltage
is applied, one–half of the incident wave experiences a phase
shift of +𝟗𝟎𝟎 because the refractive index of this portion of the
wave guide decreases, increasing the velocity of the light
propagation and lessening the delay. The other half of the
waveguide receives a −𝟗𝟎𝟎 shift because its refractive index
increases, reducing the velocity of the light propagation and
lengthening the propagation delay. When the halves recombine,
they cancel one another. Such “destructive” interference
demonstrates that we can control output intensity by applying
external voltage. Prepared by Bijoy Kumer Karmaker 39
Disadvantages of MDM modulator:

The major drawbacks of an MDM modulator are:

1. high insertion loss (upto 5 dB)
2. and relatively high modulation voltage ( up to 10 V).
3. It is a stand-alone unit.

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Electroabsorption (EA) external Modulators:
Working principle of EA modulator:
A DFB laser radiates a continuous wave of light, which runs
through a waveguide made from a semiconductor material.
Without applied voltage, this waveguide is transparent to the
light emitted by the DFB laser because its cutoff wavelength,
𝝀𝑪 , is shorter than the wavelength of incident light.
When modulation voltage is applied, a bandgap, 𝑬𝒈 of the
waveguide material decreases. Since the bandgap decreases, the
𝟏𝟎𝟐𝟒
cutoff wavelength increases 𝝀𝒄 = and the waveguide
𝑬𝒈
material starts to absorb the incident light. Hence, by applying
modulation voltage to a semiconductor waveguide, the
absorption property of the waveguide can be change. Fig: Block diagram of a transmitter module with electroabsorption
modulator
The beauty of this type of modulator is that a semiconductor
waveguide can be fabricated onto one substrate with a DFB laser.
The industry refers to this device as a monolithically integrated,
chip.
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Electroabsorption (EA) external Modulators cont..:
The optical output power after the EA modulator is at 0 dBm (1 mW), much less than its direct-
modulated counterpart. The drive voltage of EA is 2 V, much less than that of MDM (10 V).
The Electroabsorption modulator is the most promising modulator for WDM applications.

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End of Class 14

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