IV UNIT.pptx "Exploring Artificial Intelligence in Modern Business Applications"

UNIT IV
• Optical sources- LEDs, Structures, Materials, Quantum efficiency,
Power, Modulation, Power bandwidth product. Injection Laser
Diodes- Modes, Threshold conditions, External quantum efficiency,
Laser diode rate equations, Resonant frequencies, Reliability of
LED&ILD, Optical detectors- Physical principles of PIN and APD,
Detector response time, Temperature effect on Avalanche gain,
Comparison of Photo detectors, Related problems.

• Optical Sources : Optical transmitter coverts electrical input signal
into corresponding optical signal. The optical signal is then launched
into the fiber. Optical source is the major component in an optical
transmitter.
• Popularly used optical transmitters are Light Emitting Diode (LED) and
semiconductor Laser Diodes (LD).

Characteristics of Light Source of Communication
To be useful in an optical link, a light source needs the following characteristics:
• It must be possible to operate the device continuously at a variety of temperatures for
many years.
• It must be possible to modulate the light output over a wide range of modulating
frequencies.
• For fiber links, the wavelength of the output should coincide with one of transmission
windows for the fiber type used.
• To couple large amount of power into an optical fiber, the emitting area should be small.
• To reduce material dispersion in an optical fiber link, the output spectrum should be narrow.
• The power requirement for its operation must be low.
• The light source must be compatible with the modern solid state devices.
• The optical output power must be directly modulated by varying the input current to the
device. Better linearity of prevent harmonics and intermodulation distortion.
• High coupling efficiency.
• High optical output power.
• High reliability.
• Low weight and low cost

Light emitting diodes(LEDs)
• p-n Junction
• Conventional p-n junction is called as homojunction as same semiconductor material is used on
both sides junction. The electron-hole recombination occurs in relatively widelayer = 10 µm. As the
carriers are not confined to the immediate vicinity of junction, hence high current densities can not
be realized.
 The carrier confinement problem can be resolved by sandwiching a thin layer ( = 0.1 µm) between
p-type and n-type layers. The middle layer may or may not be doped. The carrier confinement
occurs due to bandgap discontinuity of the junction. Such a junction is call heterojunction and the
device is called double heterostructure
In any optical communication system when the requirements is –
Bit rate of 100-200b/sec.
Optical power in tens of micro watts. LEDs are best suitable optical source.

LED structures
• Hetero junctions
 A heterojunction is an interface between two adjoining single crystal
semiconductors with different bandgap.
 Hetero junctions are of two types, Isotype (n-n or p-p) or Antisotype (p-n).
• Double Heterojunctions (DH)
In order to achieve efficient confinement of emitted radiation double heterojunctions are used in LED
structure. A hetero junction is a junction formed by dissimilar semiconductors. Double heterojunction
(DH) is formed by two different semiconductors on each side of active region. Fig. 3.1.1 shows double
heterojunction (DH) light emitter.

 The crosshatched regions represent the energy levels of free charge. Recombination occurs only in active
InGaAsP layer. The two materials have different bandgap energies and different refractive indices. The
changes in bandgap energies create potential barrier for both holes and electrons. The free charges can
recombine only in narrow, well defined active layer side.
 A double heterojunction (DH) structure will confine both hole and electrons to a narrow active layer. Under
forward bias, there will be a large number of carriers injected into active region where they are efficiently
confined. Carrier recombination occurs in small active region so leading to an efficient device. Another
advantage of DH structure is that the active region has a higher refractive index than the materials on either
side, hence light emission occurs in an optical waveguide, which serves to narrow the output beam.
• LED configurations
At present there are two main types of LED used in optical fiber links –
 Surface emitting LED.
 Edge emitting LED.
Both devices used a DH structure to constrain the carriers and the light to an active
layer.

Surface emitting LEDs
In surface emitting LEDs the plane of active light emitting region is oriented
perpendicularly to the axis of the fiber. A DH diode is grown on an N-type substrate at the
top of the diode as shown in Fig. 3.1.2. A circular well is etched through the substrate of
the device. A fiber is then connected to accept the emitted
At the back of device is a gold heat sink. The current flows through the p-type material and forms the
small circular active region resulting in the intense beam of light.
Diameter of circular active area = 50 µm
Thickness of circular active area = 2.5 µm
Current density = 2000 A/cm2 ,half-power Emission

 The isotropic emission pattern from surface emitting LED is of Lambartian pattern. In
Lambartian pattern, the emitting surface is uniformly bright, but its projected area
diminishes as cos θ, where θ is the angle between the viewing direction and the normal to
the surface as shown in Fig. 3.1.3. The beam intensity is maximum along the normal.
The power is reduced to 50% of its peak when θ = 60o
, therefore the total half-power beamwidth is 120o
. The
radiation pattern decides the coupling efficiency ofLED

Edge Emitting LEDS (ELEDs)
 In order to reduce the losses caused by absorption in the active layer and to make the beam
more directional, the light is collected from the edge of the LED. Such a device is known
as edge emitting LED or ELED.
It consists of an active junction region which is the source of incoherent light and two
guiding layers. The refractive index of guiding layers is lower than active region but higher than outer
surrounding material. Thus a waveguide channel is form and optical radiation is directed into the fiber. Fig.
3.1.4 shows structure of LED

QUESTIONS
1. State and explain the advantages and disadvantages of fiber optic
communication systems.
2. State and explain in brief the principle of light propagation.
3. Define following terms with respect to optical laws –
A ) Reflection
B ) Refraction
C ) Refractive index
D ) Snell’s law
E) Critical angle
F) Total internal reflection (TIR)
4. Explain the important conditions for TIR to exit in fiber.
5. Derive an expression for maximum acceptance angle of a fiber.
6. Explain the acceptance come of a fiber.
7. Define numerical aperture and state its significance also.
8. Explain the different types of rays in fiber optic.
9. Explain the
A ) Step index fiber
B ) Graded index fiber
10. What is mean by mode of a fiber?
11. Write short notes on following –
A ) Single mode step index fiber
B ) Multimode step index fiber
C ) Multimode graded index fiber.

Unit 2 assignment
• 1.explain briefly different materials used for manufacturing of optical
fibers.
• 2.Define attenuation? explain briefly different losses which causes
attenuation in fibers
• 3.define dispersion?explain different types dispersions in fiber optics
• 4.explain pulse broadening in graded index fiber?
• 5.define group delay and derive group delay equation?

Unit 3 assignment
• 1.define connector? Explain briefly types of connectors ?
• 2.define splicing? Explain different splicing methods
• 3.explain fiber alignment and joint losses
• 4.explain connector return losses?

Features of ELED:
Linear relationship between optical output and
current. Spectral width is 25 to 400 nm for λ = 0.8 –
0.9 µm.
Modulation bandwidth is much large.
Not affected by catastrophic gradation mechanisms hence are more reliable.
ELEDs have better coupling efficiency than surface emitter.
ELEDs are temperature sensitive.
Usage :
7. LEDs are suited for short range narrow and medium bandwidth links.
8. Suitable for digital systems up to 140 Mb/sec.
Long distance analog links

Light source materials
• The spontaneous emission due to carrier recombination is called electro luminescence. To encourage electro
luminescence it is necessary to select as appropriate semiconductor material. The semiconductors depending
on energy bandgap can be categorized into
• 1.Direct band gap semiconductors
• 2.Indirect band gap semiconductors
Some commonly used band gap semiconductors are
Semiconductor Energy bandgap (eV) Recombination Br (cm3 / sec)
GaAs Direct : 1.43 7.21 x 10-10
GaSb Direct : 0.73 2.39 x 10-10
InAs Direct : 0.35 8.5 x 10-11
InSb Direct : 0.18 4.58 x 10-11
Si Indirect : 1.12 1.79 x 10-15
Ge Indirect : 0.67 5.25 x 10-14
GaP Indirect : 2.26 5.37 x 10-14

• In direct bandgap semiconductors the electrons and holes on either side of bandgap have same
value of crystal momentum. Hence direct recombination is possible. The recombination occurs
within 10-8 to 10-10 sec.
• In indirect bandgap semiconductors, the maximum and minimum energies occur at different
values of crystal momentum. The recombination in these semiconductors is quite slow i.e. 10-2
and 10-3 sec.
• The active layer semiconductor material must have a direct bandgap. In direct bandgap
semiconductor, electrons and holes can recombine directly without need of third particle to
conserve momentum. In these materials the optical radiation is sufficiently high. These materials
are compounds of group III elements (Al, Ga, In) and group V element (P, As,Sb). Some tertiary
alloys Ga1-x Alx As are also used.

• Emission spectrum of Ga1-x AlxAs LED is shown in Fig. 3.1.6
• The fundamental quantum mechanical relationship between gap energy E and frequency v is given as –

• The bandgap energy (Eg) can be controlled by two compositional
parameters x and y, within direct bandgap region. The quaternary
alloy In1-x Gax Asy P1-y is the principal material sued in such LEDs.
Two expression relating Eg and x,y are.

Quantum Efficiency and Power:
The internal quantum efficiency (ηint) is defined as the ratio of radiative recombination rate
to the total recombination rate.
• Rr is radiative recombination rate.
• Rnr is non-radiative recombination rate.
• If n are the excess carriers, then radiative life time,
And non-radiative life time,
The internal quantum efficiency is given
• The recombination time of carriers in active region is τ. It is also known as bulk recombination life
time.
• The optical output power emitted from LED is given as –

Advantages and Disadvantages of
LED Advantages of LED
Simple design.
Ease of manufacture.
Simple system
integration. Low cost.
High reliability.
Disadvantages of LED
Refraction of light at semiconductor/air interface.
The average life time of a radiative recombination is only a few nanoseconds, therefore
nodulation BW is limited to only few hundred megahertz.
Low coupling
efficiency. Large
chromatic dispersion.

Injection Laser Diode (ILD)
The laser is a device which amplifies the light, hence the LASER is an acronym for light
amplification by stimulated emission of radiation. The operation of the device may be
described by the formation of an electromagnetic standing wave within a cavity
(optical resonator) which provides an output of monochromatic highly coherent
radiation
Principle: Material absorb light than emitting. Three different fundamental process
occurs between the two energy states of an atom.
Absorption 2) Spontaneous emission 3) Stimulated emission.
• Laser action is the result of three process absorption of energy packets (photons)
spontaneous emission, and stimulated emission. (These processes are represented
by the simple two-energy-level diagrams).
• Where E1 is the lower state energy level.
• E2 is the higher state energy level.

 Quantum theory states that any atom exists only in certain discrete energy state,
absorption or emission of light causes them to make a transition from one state to
another. The frequency of the absorbed or emitted radiation f is related to the difference
in energy E between the two states.
• If E1 is lower state energy level. and E2 is higher state
energy level. E = (E2 – E1) = h.f.
• Where, h = 6.626 x 10-34 J/s (Plank’s constant).
• An atom is initially in the lower energy state, when the
photon with energy (E2 – E1) is
incident on the atom it will be excited into the higher energy state E2 through the absorption of
the photon

 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 E =
h.f. The emission process can occur in two ways.
• By spontaneous emission in which the atom returns to the lower energy state in random
manner.
• By stimulated emission when a photon having equal energy to the difference between
• the two states (E2 – E1) interacts with the atom causing it to the lower state with the
creation of the second photon

 Spontaneous emission gives incoherent radiation while stimulated emission gives coherent
radiation. Hence the light associated with emitted photon is of same frequency of incident
photon, and in same phase with same polarization.
 It means that when an atom is stimulated to emit light energy by an incident wave, the
liberated energy can add to the wave in constructive manner. The emitted light is bounced
back and forth internally between two reflecting surface. The bouncing back and forth of
light wave cause their intensity to reinforce and build-up. The result in a high brilliance,
single frequency light beam providing amplification.
Emission and Absorption Rates
 It N1 and N2 are the atomic densities in the ground and excited states.
Rate of spontaneous emission
Rspon = AN2 … 3.1.13
Rate of stimulated emission
Rstim = BN2 ρem … 3.1.14
Rate of absorption
Rabs = B’ N1 ρem … 3.1.15

Fabry – Perot Resonator
• Lasers are oscillators operating at frequency. The oscillator is formed by a resonant cavity providing a
selective feedback. The cavity is normally a Fabry-Perot resonator i.e. two parallel plane mirrors separated by
distance L,
• Light propagating along the axis of the interferometer is reflected by the mirrors back to the amplifying
medium providing optical gain. The dimensions of cavity are 25-500 µm longitudinal 5-15 µm lateral and
0.1-0.2 µm transverse. Fig. 3.1.10 shows Fabry-Perot resonator cavity for a laser diode.

Distributed Feedback (DFB) Laser
• In DFB laster the lasing action is obtained by periodic variations of refractive index along the longitudinal
dimension of the diode. Fig. 3.1.11 shows the structure of DFB laser diode
Lasing conditions and resonant Frequencies
 The electromagnetic wave propagating in longitudinal direction is expressed as –
E(z, t) = I(z) ej(ω t-β z)
… 3.1.23
where,
I(z) is optical field intensity.
is optical radian
frequency. β is
propagation constant.
The fundamental expression for lasing inFabry-Perot cavity is –

Power Current Characteristics
The output optic power versus forward input current characteristics is plotted in Fig.
3.1.12 for a typical laser diode. Below the threshold current (Ith) only spontaneous
emission is emitted hence there is small increase in optic power with drive current.
at threshold when lasing conditions are satisfied. The optical power increases
sharply after the lasing threshold because of stimulated emission.
 The lasing threshold optical gain (gth) is related by threshold current density
(Jth) for stimulated emission by expression –
g th = β Jth … 3.1.28
where, β is constant for device structure.
Fig. 3.1.12 Power current
characteristics

External Quantum Efficiency
 The external quantum efficiency is defined as the number of photons emitted per electron
hole pair recombination above threshold point. The external quantum efficiency
ηext is given by –
… 3.1.29
where,
ηi = Internal quantum efficiency (0.6-0.7). gth = Threshold
gain.
α = Absorption coefficient
 Typical value of ηext for standard semiconductor laser is ranging between 15-20 %.

Resonant Frequencies
 At threshold lasing
2β L = 2π m
where, (propagation constant)
m is an integer.
 m 
Since c = vλ

Substituting λ in 3.1.30
=z…
3.1.31
 Gain in any laser is a function of frequency. For a Gaussian output the gain
and frequency are related by expression –
… 3.1.32
where,
g(0) is maximum gain.
λ0 is center wavelength in spectrum.
is spectral width of the gain.The frequency spacing between the two successive modes is –
… 3.1.34

Comparison of LED and Laser Diode
Sr. No. Parameter LED LD (Laser Diode)
1. Principle of operation Spontaneous emission. Stimulated emission.
2. Output beam Non – coherent. Coherent.
3. Spectral width Board spectrum (20 nm – 100 nm) Much narrower (1-5 nm).
4. Data rate Low. Very high.
5. Transmission distance Smaller. Greater.
6. Temperature sensitivity Less sensitive. More temperature sensitive.
7. Coupling efficiency Very low. High.

Optical Detectors
• Principles of Optical Detectors
 The photodetector works on the principle of optical absorption. The main requirement of light detector or photodector is
its fast response. For fiber optic communication purpose most suited photodetectors are PIN (p-type- Instrinsic-n-type)
diodes and APD (Avalanche photodiodes)
 The performance parameters of a photodetector are responsivity, quantum efficiency, response time and dark current.
 Cut-off Wavelength (λc):
Any particular semiconductor can absorb photon over a limited wavelength range. The highest wavelength is known as
cut-off wavelength (λc). The cut-off wavelength is determined by bandgap energy Eg of material.
 where,
 Eg in electron volts (eV) and
 λc cut-off wavelength is in µm.
 Typical value of λc for silicon is 1.06 µm and for germanium it is 1.6 µm.

Quantum Efficiency (η)
 The quantum efficiency is define as the number of electron-hole carrier pair
generated per incident photon of energy h v and is given as –
… 3.2.2
where, Ip is average photocurrent.
Pin is average optical power incident on photo detectors.
 Absorption coefficient of material determines the quantum efficiency. Quantum
efficiency η < 1 as all the photons incident will not generate e-h pairs. It is normally
expressed in percentage.

Detector Responsivity
 The responsivity of a photodetector is the ratio of the current output in amperes
to the incident optical power in watts. Responsivity is denoted by
… 3.2.3
But
 …3.2.4
Therefore   

Principle of Photo diodes:
In order to convert the modulated light back into an electrical signal, photodiode or photodetectors are used. As
the intensity of optical signal at the receiver is very low, the detector has to meet high performance
specifications.
 The conversion efficiency must be high at the operating wavelength. The speed of response must be high enough
to ensure that signal distortion does not occur
 The detection process introduce the minimum amount of noise.
 It must be possible to operate continuously over a wide range of temperatures for many years.
 The detector size must be compatible with the fiber dimensions.
• At present, these requirements are met by reverse biased p-n photodiodes. In these devices, the semiconductor material
absorbs a photon of light, which excites an electron from the valence band to the conduction band (opposite of photon
emission). The photo generated electron leaves behind it a hole, and so each photon generates two charge carriers. The
increases the material conductivity so call photoconductivity resulting in an increase in the diode current. The diode equation
is modified as –
• where,
• Id is dark current i.e. current that flows when no signal is present.
• Is is photo generated current due to incident optical signal

Forward bias, region 1: A change in incident power causes a change in terminal voltage, it is called as photovoltaic
mode. If the diode is operated in this mode, the frequency response of the diode is poor and so photovoltaic operation is
rarely used in optical links
.Reverse bias, region 2 : A change in optical power produces a proportional change in diode current, it is called as
photoconductive mode of operation which most detectors use. Under these condition, the exponential term in equation
3.2.6 becomes insignificant and the reverse bias current is given by –
 Responsivity of photodiode is defined as the change in reverse bias current per
unit change in optical powr, and so efficient detectors need large responsivities.
Avalanche breakdown, region 3 : When biased in this region, a photo
generated electron-hole pair causes avalanche breakdown, resulting in large diode for a
single incident photon. Avalance photodiodes (APDs) operate in this region APDs exhibit
carrier multiplication. They are usually very sensitive detectors. Unfortunately V-I
characteristic is very steep in this region and so the bias voltage must be tightly controlled
to prevent spontaneous breakdown.

PIN Photodiode
• PIN diode consists of an intrinsic semiconductor sandwiched between two heavily doped p-type and n-type
semiconductors . Sufficient reverse voltage is applied so as to keep intrinsic region free from carries, so its
resistance is high, most of diode voltage appears across it, and the electrical forces are strong within it. The
incident photons give up their energy and excite an electron from valance to conduction band. Thus a free
electron hole pair is generated, these are called as photocarriers. These carriers are collected across the
reverse biased junction resulting in rise in current in external circuit called photocurrent.

• In the absence of light, PIN photodiodes behave electrically just like an ordinary
rectifier diode. If forward biased, they conduct large amount of current.
• PIN detectors can be operated in two modes : Photovoltaic and photoconductive. In
photovoltaic mode, no bias is applied to the detector. In this case the detector works
very slow, and output is approximately logarithmic to the input light level. Real world
fiber optic receivers never use the photovoltaic mode.
• In photoconductive mode, the detector is reverse biased. The output in this case is a
current that is very linear with the input light power.
• The intrinsic region some what improves the sensitivity of the device. It does not
provide internal gain. The combination of different semiconductors operating at
different wavelengths allows the selection of material capable of responding to the
desired operating wavelength.

Detector response time
Depletion Layer Photocurrent
Consider a reverse biased PIN photodiode.
 The total current density through depletion layer is –
Jtot = Jdr + Jdiff … 3.2.7
Where,
Jdr is drift current densioty due to carriers generated in depletion region.
Jdiff is diffusion current density due to carriers generated outside depletion region.
 The drift current density is expressed as –
where,
A is photodiode area.
0 is incident photon flux per unit area.
 The diffusion current density is expressed as –
… 3.2.
where,
Dp is hole diffusion coefficient
Pn is hole concentration in n-type material.
Pn0 is equilibrium hole density.
Substituting in equation 3.2.7, total current density through reverse biased
depletion layer
is –

Response Time
Factors that determine the response time of a photodiode are
Transit time of photocarriers within the depletion region.
Diffusion time of photocarriers outside the depletion
region. RC time constant of diode and external circuit.
The transit time is given by –
The diffusion process is slow and diffusion times are less than carrier drift
time. By considering the photodiode response time the effect of diffusion
can be calculated. Fig. 3.2.4 shows the response time of photodiode which
is not fully depleted.
 The detector behaves as a simple low pass RC filter having passband of
where
RT, is combination input resistance of load and amplifier. CT is sum of photodiode and amplifier capacitance.

Avalanche Photodiode (APD)
• When a p-n junction diode is applied with high reverse bias breakdown can occur by two separate
mechanisms direct ionization of the lattice atoms, zener breakdown and high velocity carriers impact
ionization of the lattice atoms called avalanche breakdown. APDs uses the avalanche breakdown phenomena
for its operation. The APD has its internal gain which increases its responsivity.
• Below Fig.shows the schematic structure of an APD. By virtue of the doping concentration and physical
construction of the n+ p junction, the electric filed is high enough to cause impact ionization. Under normal
operating bias, the I-layer (the p‫־‬ region) is completely depleted. This is known as reach through condition,
hence APDs are also known as reach through APD or RAPDs
• Similar to PIN photodiode, light absorption in APDs is most efficient in I-layer. In this region, the E-field
separates the carriers and the electrons drift into the avalanche region where carrier multiplication occurs. If
the APD is biased close to breakdown, it will result in reverse leakage current. Thus APDs are usually biased
just below breakdown, with the bias voltage being tightly controlled.

 The multiplication for all carriers generated in the photodiode is given as –
…
where,
IM = Average value of total multiplied output
current. IP = Primary unmultiplied photocurrent.
 Responsivity of APD is given by –
…
where, 0 = Unity gain responsivity

IV UNIT.pptx "Exploring Artificial Intelligence in Modern Business Applications"

IV UNIT.pptx "Exploring Artificial Intelligence in Modern Business Applications"

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