Advanced optoelectronics and optical communication

Submitted By
Sanjida Jahan Tanha
BKH2217Msc101F
Dept. of Electrical and Electronic Engineering
Noakhali Science and Technology University
Course Name: Advanced Optoelectronics and Optical
Communication
Course Code: EEE5121
Submitted To
Kamaruzzaman
Assistant Professor
Dept. of Electrical and Electronic Engineering
Noakhali Science and Technology University

Contents
• Laser Operation
• Population Inversion In Degenerate Semiconductor
• Laser Cavity
• Operating Wavelength
• Threshold Current Density
• Power Output
• Elimentary Laser Diode Characteristics
• Hetero Junction Lasers
• Optical And Electrical Confinement
• Single Frequency Solid State Lasers- Distributed Bragg
Reflector (DBR)
• Distributed Feedback (DFB)
• Optical Laser Amplifier

LASER
(Light Amplification by the Stimulated Emission of Radiation)
• Laser is an optical oscillator. It comprises a resonant optical
amplifier whose output is fed back into its input with matching
phase. Any oscillator contains:
1- An amplifier with a gain-saturated mechanism
2- A feedback system
3- A frequency selection mechanism
4- An output coupling scheme
• In laser the amplifier is the pumped active medium, such as biased
semiconductor region, feedback can be obtained by placing active
medium in an optical resonator, such as Fabry-Perot structure, two
mirrors separated by a prescribed distance. Frequency selection is
achieved by resonant amplifier and by the resonators, which
admits certain modes. Output coupling is accomplished by making
one of the resonator mirrors partially transmitting.

Laser Operation
• Three main process for laser action:
1- Photon absorption
2- Spontaneous emission
3- Stimulated emission
Optical Fiber communications, 3rd
ed.,G.Keiser,McGrawHill, 2000

Photon absorption
• Absorption of radiation is the process by which electrons in the ground
state absorbs energy from photons to jump into the higher energy level.
• The electrons orbiting very close to the nucleus are at the lower energy
level or lower energy state whereas the electrons orbiting farther away
from the nucleus are at the higher energy level. The electrons in the lower
energy level need some extra energy to jump into the higher energy level.
This extra energy is provided from various energy sources such as heat,
electric field, or light.

Photon absorption
• Let us consider two energy levels (E1 and E2) of electrons. E1 is
the ground state or lower energy state of electrons and E2 is the
excited state or higher energy state of electrons. The electrons
in the ground state are called lower energy electrons or ground
state electrons whereas the electrons in the excited state are
called higher energy electrons or excited electrons.
•

Photon absorption
• In general, the electrons in the lower energy state can’t jump into the
higher energy state. They need sufficient energy in order jump into
the higher energy state.
• When photons or light energy equal to the energy difference of the
two energy levels (E2 – E1) is incident on the atom, the ground state
electrons gains sufficient energy and jumps from ground state (E1) to
the excited state (E2).
• The absorption of radiation or light occurs only if the energy of
incident photon exactly matches the energy difference of the two
energy levels (E2 – E1).
•

Spontaneous Emission
• Spontaneous emission is the process by which electrons in the excited state return to the ground
state by emitting photons.
• The electrons in the excited state can stay only for a short period. The time up to which an excited
electron can stay at higher energy state (E2) is known as the lifetime of excited electrons. The
lifetime of electrons in excited state is 10-8
second.
• Thus, after the short lifetime of the excited electrons, they return to the lower energy state or
ground state by releasing energy in the form of photons.
• In spontaneous emission, the electrons move naturally or spontaneously from one state (higher
energy state) to another state (lower energy state) so the emission of photons also occurs
naturally. Therefore, we have no control over when an excited electron is going to lose energy in
the form of light.
• The photons emitted in spontaneous emission process constitute ordinary incoherent light.
Incoherent light is a beam of photons with frequent and random changes of phase between them.
In other words, the photons emitted in the spontaneous emission process do not flow exactly in the
same direction of incident photons.
•

Stimulated Emission
• Stimulated emission is the process by which incident photon interacts with the excited electron and
forces it to return to the ground state.
• In stimulated emission, the light energy is supplied directly to the excited electron instead of
supplying light energy to the ground state electrons.
• Unlike the spontaneous emission, the stimulated emission is not a natural process it is an artificial
process.
• In spontaneous emission, the electrons in the excited state will remain there until its lifetime is
over. After completing their lifetime, they return to the ground state by releasing energy in the form
of light.
• However, in stimulated emission, the electrons in the excited state need not wait for completion of
their lifetime. An alternative technique is used to forcefully return the excited electron to ground
state before completion of their lifetime. This technique is known as the stimulated emission.
•

Stimulated Emission
• When incident photon interacts with the excited electron, it forces the excited electron to return to
the ground state. This excited electron release energy in the form of light while falling to the ground
state. In stimulated emission, two photons are emitted (one additional photon is emitted), one is
due to the incident photon and another one is due to the energy release of excited electron. Thus,
two photons are emitted.
• The stimulated emission process is very fast compared to the spontaneous emission process.
• All the emitted photons in stimulated emission have the same energy, same frequency and are in
phase. Therefore, all photons in the stimulated emission travel in the same direction.
• The number of photons emitted in the stimulated emission depends on the number of electrons in
the higher energy level or excited state and the incident light intensity.
• It can be written as:
• Number of emitted photons α Number of electrons in the excited state + incident light intensity.
•

Spontaneous and Stimulated emission
•

Lasing in a pumped active medium
• In thermal equilibrium the stimulated emission is essentially
negligible, since the density of electrons in the excited state is
very small, and optical emission is mainly because of the
spontaneous emission. Stimulated emission will exceed
absorption only if the population of the excited states is greater
than that of the ground state. This condition is known as
Population Inversion. Population inversion is achieved by
various pumping techniques.
• In a semiconductor laser, population inversion is accomplished
by injecting electrons into the material to fill the lower energy
states of the conduction band.

Population inversion
• Population inversion is the process of achieving greater
population of higher energy state as compared to the lower
energy state. Population inversion technique is mainly used for
light amplification. The population inversion is required for laser
operation.
• Consider a group of electrons with two energy levels E1 and E2.
• E1 is the lower energy state and E2 is the higher energy state.
• N1 is the number of electrons in the energy state E1.
• N2 is the number of electrons in the energy state E2.
• The number of electrons per unit volume in an energy state is
the population of that energy state.

• Population inversion cannot be achieved in a two energy level
system. Under normal conditions, the number of electrons (N1) in
the lower energy state (E1) is always greater as compared to the
number of electrons (N2) in the higher energy state (E2). N1 > N2
• When temperature increases, the population of higher energy state
(N2) also increases. However, the population of higher energy state
(N2) will never exceeds the population of lower energy state (N1).
• At best an equal population of the two states can be achieved which
results in no optical gain.
• N1 = N2
• Therefore, we need 3 or more energy states to achieve population
inversion. The greater is the number of energy states the greater is
the optical gain.

• Therefore, we need 3 or more energy states to achieve population
inversion. The greater is the number of energy states the greater is
the optical gain.
• There are certain substances in which the electrons once excited;
they remain in the higher energy level or excited state for longer
period. Such systems are called active systems or active media
which are generally mixture of different elements.
• When such mixtures are formed, their electronic energy levels are
modified and some of them acquire special properties. Such types
of materials are used to form 3-level laser or 4-level laser.
•

Laser Diode
• Laser diode is an improved LED, in the sense that uses stimulated
emission in semiconductor from optical transitions between distribution
energy states of the valence and conduction bands with optical
resonator structure such as Fabry-Perot resonator with both optical
and carrier confinements.

Semiconductor Laser Diode
• Definition: It is specifically fabricated p-n junction diode. This diode
emits laser light when it is forward biased.
• Principle:
• When a p-n junction diode is forward biased, the electrons from n –
region and the holes from the p- region cross the junction and
recombine with each other.
• During the recombination process, the light radiation (photons) is
released from a certain specified direct band gap semiconductors like
Ga-As. This light radiation is known as recombination radiation.
• The photon emitted during recombination stimulates other electrons
and holes to recombine. As a result, stimulated emission takes place

Semiconductor Laser Diode: Construction
• Figure shows the basic construction of semiconductor laser. The active
medium is a p-n junction diode made from the single crystal of gallium
arsenide.
• This crystal is cut in the form of a platter having thickness of 0.5μmm
• The platelet consists of two parts having an electron conductivity (n-
type) and hole conductivity (p-type).
• The photon emission is stimulated in a very thin layer of PN junction (in
order of few microns). The electrical voltage is applied to the crystal
through the electrode fixed on the upper surface.
• The end faces of the junction diode are well polished and parallel to
each other. They act as an optical resonator through which the emitted
light comes out.
•

Semiconductor Laser Diode:Working
• Figure shows the energy level diagram of semiconductor laser
• When the PN junction is forward biased with large applied voltage, the
electrons and holes are injected into junction region in considerable
concentration
• The region around the junction contains a large amount of electrons in
the conduction band and a large amount of holes in the valence band.
• If the population density is high, a condition of population inversion is
achieved. The electrons and holes recombine with each other and this
recombination’s produce radiation in the form of light.
• When the forward – biased voltage is increased, more and more light
photons are emitted and the light production instantly becomes
stronger. These photons will trigger a chain of stimulated recombination
resulting in the release of photons in phase.
•

Semiconductor Laser Diode:Working
• The photons moving at the plane of the junction travels back and forth
by reflection between two sides placed parallel and opposite to each
other and grow in strength.
After gaining enough strength, it gives out the laser beam of
wavelength 8400o A (GaAS)
The wavelength of laser light is given by
Where Eg is the band gap energy in joule.

Elemantary Laser Diode Characteristics
• Elementary laser diode characteristics refer to the fundamental
properties and behaviors that are inherent to laser diodes, which
are a type of semiconductor laser. These characteristics are
crucial for understanding the basic operation and potential
applications of laser diodes. Here are the elementary
characteristics of a laser diode:
• Lasing Threshold: The minimum current or optical power
required to achieve lasing in a laser diode. Below this threshold,
only spontaneous emission occurs.
• Threshold Current: The minimum current needed to achieve
lasing. Beyond this current, stimulated emission dominates, and
coherent laser light is produced.

•Output Power vs. Current: Laser diodes exhibit a nonlinear
relationship between output power and current. Above the
threshold current, the output power increases significantly with
small increases in current.
•Spectral Output: Laser diodes emit light at a specific wavelength
determined by the bandgap of the semiconductor material used in
the active region.
•Beam Divergence: The angle at which the laser beam diverges
from the axis. Laser diodes often have higher beam divergence
compared to other types of lasers.

• Beam Divergence: The angle at which the laser beam diverges
from the axis. Laser diodes often have higher beam divergence
compared to other types of lasers.
• Beam Profile: Laser diode beams typically have a Gaussian or
elliptical intensity profile.
• Modulation Bandwidth: The frequency at which a laser diode can
be modulated or switched on and off. Modulation bandwidth is
crucial for applications such as optical communication.
• Temperature Sensitivity: Laser diodes are sensitive to changes in
temperature, which can affect their output characteristics and
wavelength.

•Lifetime and Reliability: The operational lifetime of a laser diode is
influenced by factors like current, temperature, and usage
conditions. Longevity and reliability are critical considerations.
•Coherence Length: The distance over which the laser light
maintains its coherence, or phase relationship. Coherence length is
inversely proportional to the spectral linewidth.
•Efficiency: Laser diodes are generally efficient in converting
electrical power into optical power.
•Direct Modulation: Laser diodes can be directly modulated by
varying the input current, allowing for rapid switching and
modulation of the output light.
•Wavelength Tunability: Some laser diodes allow limited tuning of
the emission wavelength by adjusting temperature or current.

• Spectral Linewidth: The range of wavelengths present in the
laser output. Narrow linewidth is desirable for applications
requiring precise spectral characteristics.
• Single-Mode vs. Multimode: Laser diodes can operate in single-
mode (one dominant spatial mode) or multimode (multiple
spatial modes) operation.
• Spatial Mode: Describes the spatial distribution of the laser
beam's intensity profile. Single-mode lasers have a well-defined
spatial mode.

Laser Operation & Lasing Condition
• To determine the lasing condition and resonant frequencies, we
should focus on the optical wave propagation along the
longitudinal direction, z-axis. The optical field intensity, I, can be
written as:
• Lasing is the condition at which light amplification becomes
possible by virtue of population inversion. Then, stimulated
emission rate into a given EM mode is proportional to the
intensity of the optical radiation in that mode. In this case, the
loss and gain of the optical field in the optical path determine the
lasing condition. The radiation intensity of a photon at energy
varies exponentially with a distance z amplified by factor g,
and attenuated by factor according to the following
relationship:
)
(
)
(
)
,
( z
t
j
e
z
I
t
z
I 
 
 [4-19]

h


 
 
z
h
h
g
I
z
I )
(
)
(
exp
)
0
(
)
( 

 

 [4-20]
1
R 2
R
Z=0 Z=L
 
 
)
2
(
)
(
)
(
exp
)
0
(
)
2
( 2
1 L
h
h
g
R
R
I
L
I 

 

 [4-21]
2
2
1
2
1
t,
coefficien
absorption
effective
:
t
coefficien
gain
:
g
factor,
t
confinemen
Optical
:












n
n
n
n
R
α
1
n
2
n
Lasing Conditions:
1
)
2
exp(
)
0
(
)
2
(



L
j
I
L
I

[4-22]

Threshold gain & current density











2
1
1
ln
2
1
R
R
L
gth 
th
g
g 
:
iff
lase"
"
to
starts
Laser
[4-23]
For laser structure with strong carrier confinement, the threshold current
Density for stimulated emission can be well approximated by:
th
th J
g 
 [4-24]
on
constructi
device
specific
on
depends
constant
:


Optical output vs. drive current

Semiconductor laser rate equations
• Rate equations relate the optical output power, or # of photons per unit
volume, , to the diode drive current or # of injected electrons per
unit volume, n. For active (carrier confinement) region of depth d, the
rate equations are:

emission
stimulated
ion
recombinat
s
spontaneou
injection
rate
electron
loss
photon
emission
s
spontaneou
emission
stimulated
rate
Photon
















Cn
n
qd
J
dt
dn
R
Cn
dt
d
sp
ph
sp


[4-25]
density
current
Injection
time
life
photon
mode
lasing
the
into
emission
s
spontaneou
of
rate
process
absorption
&
emission
optical
the
of
intensity
the
expressing
t
Coefficien
:
:
:
:
J
R
C
ph
sp


Threshold current Density & excess electron density
• At the threshold of lasing:
• The threshold current needed to maintain a steady state threshold
concentration of the excess electron, is found from electron rate
equation under steady state condition dn/dt=0 when the laser is just
about to lase:
0
,
0
/
,
0 



 sp
R
dt
d
th
ph
ph n
C
n
Cn 









1
0
/
25]
-
[4
eq.
from [4-26]
sp
th
th
sp
th
th n
qd
J
n
qd
J






0 [4-27]

External quantum efficiency
• Number of photons emitted per radiative electron-hole pair
recombination above threshold, gives us the external quantum
efficiency.
• Note that:
)
mA
(
)
mW
(
]
m
[
8065
.
0
)
(
dI
dP
dI
dP
E
q
g
g
g
th
th
i
ext









[4-29]
%
40
%
15
%;
70
%
60 


 ext
i 


Laser Resonant Frequencies
• Lasing condition, namely eq. [4-22]:
• Assuming the resonant frequency of the mth
mode is:
,...
3
,
2
,
1
,
2
L
2
1
)
2
exp( 



 m
m
L
j 





n
2

1,2,3,...
2

 m
Ln
mc
m

Ln
Ln
c
m
m
2
2
2
1




 





 
[4-30]
[4-31]

Spectrum from a laser Diode
width
spectral
:
2
)
(
exp
)
0
(
)
( 2
0




 




 

g
g [4-32]

Laser Diode Structure & Radiation Pattern
• Efficient operation of a laser diode requires reducing the # of
lateral modes, stabilizing the gain for lateral modes as well as
lowering the threshold current. These are met by structures that
confine the optical wave, carrier concentration and current flow
in the lateral direction.
• The important types of laser diodes are: gain-induced, positive
index guided, and negative index guided.

(a) gain-induced guide (b)positive-index waveguide (c)negative-index waveguide

Hetero-Junction Lasers
• Multiple layers have to be built-up in the laser
structure to develop more efficient lasers
operating at room temperature. These devices
are called heterojunction lasers, and can be
operated continuously at room temperature to
meet with the optical communication needs.
• two interfaces of different indexes of refraction,
one on top and one below the active region, so
two junctions are formed in what is called a
heterostructure laser diode, or a double
heterostructure, since there are two confining
interfaces. The double-heterostructure
arrangement confines intracavity light in only one
direction (top and bottom) of the GaAs layer,
further improvement in performance can be made
by manufacturing the device so that a confining
dielectric interface exists on all four sides of the
active region in a buried heterostructure laser

• Double Heterostructure Fabrication: For a double heterostructure, suitable
materials with different bandgaps and compatible lattice constants must be
chosen.
• AlxGa1-xAs Example: AlxGa1-xAs is a basic example, with bandgap controlled
by Al fraction (x) and constant lattice constant to prevent defects during growth.
• Active and Confinement Layers: A double heterostructure laser consists of an
active layer (low Al content) and confinement layers (high Al content) to ensure
effective carrier confinement.
• Wavelength Tuning: Photon energy is higher than substrate bandgap, enabling
wavelength tuning by adjusting Al fraction in active layer for lasers in 800-900
nm range.
• Cladding Layer Importance: To avoid absorption losses, the lower cladding
layer should be sufficiently thick since photon energy exceeds substrate
bandgap (GaAs).

• Enhanced Design Flexibility: In1-xGaxAsyP1-y offers greater design
flexibility, achieved by adjusting x and y to lattice-match to InP substrate
and modify bandgap, enabling tunable wavelengths.
• Wavelength Range: In1-xGaxAsyP1-y can fabricate lasers covering
wavelengths from around 1100 nm to nearly 1700 nm, including key
telecommunications wavelengths at 1300 and 1550 nm.

Hetero-Junction Lasers: MOVPE
• The most commonly used technique for growth of heterostructures is
metal-organic chemical vapour phase epitaxy (MOVPE). Hydrides such as
arsine (AsH3), phosphine (PH3) and organometallics such as tri-methyl-
gallium Ga(CH3)3 and tri-ethylindium In(CH3)3 are carried by hydrogen
and react on the surface of the wafer. The material composition is
controlled by adjusting the flow rate of the various sources. Large wafers
can be grown, and some reactors allow multi-wafer handling, making this
technique suitable for large volume manufacturing. MOVPE requires very
stringent safety measures due to the toxicity of the hydrides.
•

Laser Cavity
• The semiconductor laser cavity refers to the optical resonator within the
laser structure that facilitates the generation and amplification of
coherent light.
• The cavity typically consists of two mirrors: a high reflector (HR) mirror
and an output coupler (OC) mirror.
• The active medium (semiconductor material) is placed between these
mirrors, creating a feedback loop for light amplification through
stimulated emission.
• The cavity's length and mirror properties determine the laser's resonant
modes, output characteristics, and efficiency.

Operating Wavelength
• The operating wavelength of a semiconductor laser is the specific
wavelength of coherent light emitted by the laser diode.
• The wavelength is determined by the energy bandgap of the
semiconductor material used in the active region.
• Different semiconductor materials or compositions can be chosen to
achieve specific operating wavelengths, covering a wide spectral range
from visible to near-infrared and beyond.

Threshold Current Density
• The threshold current density is the minimum current density required
to initiate lasing action in a semiconductor laser.
• Below the threshold, only spontaneous emission occurs, and the
emitted light is not significantly amplified.
• Achieving lasing requires reaching a critical population inversion within
the active region, which is influenced by factors like material properties
and cavity design.

Power Output
• Power output refers to the amount of optical power produced by the
semiconductor laser.
• It is determined by factors such as the gain of the active medium, the
efficiency of the laser cavity, and the injection current.
• Power output can range from milliwatts to watts or more, depending on
the design and application of the laser.

Optical And Electrical Confinement
• Optical and electrical confinement are essential techniques used in
semiconductor laser diodes to enhance the laser's performance and
characteristics. These methods ensure efficient light amplification and
confinement of carriers (electrons and holes) within the active region,
leading to improved laser efficiency and reduced threshold current

Optical Confinement
• Purpose: Optical confinement involves creating a higher refractive
index region around the active layer to confine the optical mode and
guide light within the gain region.
• Mechanism: This is achieved by using materials with different refractive
indices. Often, layers with higher refractive indices (e.g., AlGaAs) are
grown on either side of the active layer (typically GaAs) to form
waveguides that trap and guide light.
• Waveguide Structure: The layers with higher refractive indices act as
waveguides, ensuring that light generated in the active region remains
trapped and undergoes multiple reflections for efficient amplification.
• Improved Efficiency: Optical confinement reduces light leakage and
loss, increasing the interaction length between the photons and gain
medium, resulting in higher optical gain and laser efficiency.

Electrical Confinement
• Purpose: Electrical confinement involves controlling the injection and
flow of charge carriers (electrons and holes) within the active region,
enhancing population inversion and stimulated emission.
• Heterojunctions: Semiconductor materials with different bandgaps
and lattice constants are used to create heterojunctions (e.g.,
AlGaAs/GaAs), which trap carriers within the active region due to
potential energy barriers.
• Current Flow Control: The heterojunctions prevent carriers from
escaping the active region, resulting in more carriers available for
recombination and stimulated emission.
• Threshold Reduction: Efficient electrical confinement reduces the
threshold current required to achieve lasing, leading to lower power
consumption and improved laser efficiency.

Single Frequency Solid State Lasers-
Distributed Bragg Reflector (Dbr)
• Single frequency solid-state lasers with Distributed Bragg Reflectors
(DBRs) are advanced laser systems that use a specialized optical
structure called a DBR to achieve single-frequency operation. DBRs
are crucial for controlling the laser's spectral characteristics and
producing a laser beam with a very narrow linewidth and stable single-
frequency output.
• single frequency solid-state lasers with Distributed Bragg Reflectors
(DBRs) offer precise wavelength control, narrow linewidths, and high
frequency stability. These lasers find applications in fields where high
spectral purity and coherence are essential for accurate measurements
and precise optical interactions.

• Single Frequency Operation:
– Single frequency lasers emit light at a very narrow
wavelength range, producing a well-defined spectral line.
This characteristic is essential for applications such as
precision metrology, interferometry, and high-resolution
spectroscopy.
• Distributed Bragg Reflector (DBR):
– A DBR is an optical device that consists of alternating layers
of high and low refractive index materials. These layers form
a periodic structure, acting as a wavelength-specific mirror
that reflects a narrow band of wavelengths while transmitting
others.

• Principle of Operation:
– In a single frequency solid-state laser with DBR, the DBR
structure is incorporated into the laser cavity. It serves as
one of the cavity mirrors and selects a single longitudinal
mode (wavelength) for laser oscillation.
• Wavelength Selectivity:
– The DBR structure reflects light within a very narrow
wavelength range, effectively limiting the laser emission to a
single longitudinal mode. This results in a laser beam with a
narrow linewidth and a well-defined frequency.
• Frequency Tuning:
– The wavelength of the single-frequency laser can be finely
tuned by adjusting the temperature of the DBR structure or
applying an external optical or electrical field, allowing for
precise control of the emitted frequency.

• Narrow Linewidth:
– The DBR-based cavity provides high spectral selectivity,
resulting in a laser beam with an exceptionally narrow linewidth.
This property is crucial for applications demanding high spectral
purity and stability.
• Stability and Coherence:
– Single frequency solid-state lasers with DBRs exhibit
exceptional frequency stability and coherence due to the strict
control of the oscillating mode. These characteristics are highly
desirable for applications in laser spectroscopy and other
precision measurements.
• Applications:
– DBR-based single frequency solid-state lasers are employed in
various fields, including telecommunications, high-resolution
spectroscopy, interferometric sensing, metrology, and laser-
based atomic and molecular physics experiments.

• Mode-Hop-Free Operation:
– The use of DBRs helps mitigate mode hopping, a
phenomenon where the laser abruptly switches between
longitudinal modes. DBRs provide stable single-frequency
operation and reduce mode hopping.
• Advancements and Research:
• Ongoing research aims to further improve the performance of
single frequency solid-state lasers with DBRs, including efforts
to reduce noise, increase output power, and expand their
wavelength coverage.

Optical Laser Amplifiers
• Optical laser amplifiers are devices that use the principle of
stimulated emission to amplify light signals, typically in the form of
optical beams or pulses. These amplifiers are essential
components in optical communication systems, high-power laser
systems, and various scientific and industrial applications. Here's
an overview of optical laser amplifiers:
• Stimulated Emission Amplification:
– Optical laser amplifiers amplify light through the process of
stimulated emission. Incoming photons stimulate excited
atoms or molecules to emit additional photons of the same
wavelength and phase, resulting in coherent amplification.
• Active Medium:
– Optical amplifiers have an active medium, often a gain
medium such as rare-earth-doped fibers (e.g., erbium-doped
fiber) or semiconductor materials (e.g., semiconductor optical
amplifiers).

• Continuous Wave (CW) and Pulsed Operation:
– Optical amplifiers can operate in continuous wave (CW)
mode or pulse amplification mode, depending on the
application. They can amplify both continuous and pulsed
light signals.
• Amplification Process:
– Light signals to be amplified are coupled into the active
medium. As they pass through the medium, they undergo
stimulated emission, leading to the amplification of the
signal.
• Gain and Gain Coefficient:
– The amplification achieved by the optical amplifier is
quantified by the gain, which is the ratio of output power to
input power. The gain coefficient characterizes the
amplification ability of the active medium.

1.Noise Considerations:
1. Optical amplifiers can introduce noise into the amplified signal.
Various techniques, such as signal pre-amplification, can be employed
to minimize noise effects.
2.Types of Optical Amplifiers:
1. Erbium-Doped Fiber Amplifiers (EDFAs): Amplify signals in the 1550
nm wavelength range, commonly used in long-haul optical
communication.
2. Semiconductor Optical Amplifiers (SOAs): Based on semiconductor
materials, used for fast signal amplification and switching.
3. Raman Amplifiers: Amplify signals through stimulated Raman
scattering, enabling amplification at various wavelengths
Gain Saturation:
Optical amplifiers have a maximum achievable gain due to the limited population
inversion within the active medium. At high input powers, gain saturation occurs,
limiting further amplification.

Temperature variation of the threshold
current
0
/
)
( T
T
z
th e
I
T
I 

DFB(Distributed FeedBack) Lasers
• In DFB lasers, the optical resonator structure is due to the incorporation
of Bragg grating or periodic variations of the refractive index into
multilayer structure along the length of the diode.
• To reduce the spectral width, we need to make a laser diode merely
radiate in only one longitudinal mode. The distributed feedback (DFB)
laser, which is a special type of edge-emitting lasers, is optimized for
single-mode (single-frequency) operation. The single-mode operation is
achieved by incorporating a periodic structure or a Bragg grating near
the active layer as depicted in the figure below.
• In DBF laser the lasing action is obtained by periodic variations of
refractive index along the longitudinal dimension of the diode.
•

Output spectrum symmetrically distributed around Bragg wavelength in an idealized DFB laser diode
)
2
1
(
2
2


 m
L
n e
e
B
B



[4-35]
DFB(Distributed FeedBack) Lasers

Advanced optoelectronics and optical communication

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