prepared notes for opto electronics

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.
ADVANCED ELECTRONICS
UNIT I -
OPTOELECTRONIC DEVICES
INTRODUCTION
The ancient Greeks speculated on the nature of light from about 500 BC. The practical
interest at that time centred, inevitably, on using the sun’s light for military purposes; and the
speculations, which were of an abstruse philosophical nature, were too far removed from the
practicalities for either to have much effect on the other.
In fact, the fundamental atomic processes of nature are not describable in these same terms
and it is only when we try to force them into our more familiar frame work.
Such as the wave-particle duality of Electrons and Photons arise.
Electrons and Photons are neither waves not particles but are entities whose true nature is
somewhat beyond our conceptual powers.
In Optoelectronics devices the following properties are very important:
The wave nature of light:
The frequency of the wave described by equation  F =
𝝎
𝟐𝝅
And wave length by  λ =
𝟐𝝅
𝒌
where
ω = angular frequency,
k = propagation constant
Velocity of light
C = fλ =
𝝎
𝒌
The wave nature of light will be analysed by Maxwell’s equations.
Polarization:
 It is customary to fix attention on the electric field for purposes of general
electromagnetic wave behaviour, primarily because the effect of the electric field on
the electrical charges within atoms tends to be more direct than that of the magnetic
field.
 The symmetry which exists between the E and H fields of the electromagnetic wave
means that conclusions arrived at for the electric field have close equivalence for the
magnetic field.
 It is simply convenient only to deal with one of them rather than two.
The electromagneticspectrum:
In practice, since electro – magnet wave sources cannot be markedly smaller than the
wave length of the radiation.

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Visible radiation lies in the range 400 – 700 nm (1 nm = 10-9 m)
The infrared region of the spectrum lies just beyond 700 nm to 300 000 nm
The ultraviolet region lies below 400 nm and begins at about 3 nm.
ClassificationofOpto Electronics Devices:
It is simplest form a p-n junction diode can be constructed as a homojunction using abrupt
change between p and n-type regions in a single piece of semiconductor.
The p and n-regions might initially be considered to be separated slabs of material when
these are placed in intimate contact. The difference in hole concentration between the p and
n-type regions should result in the diffusion of holes.
In the same way, electrons ought to diffuse in the reverse direction from the n-type
material into the p-type.
If the two carriers types meet near the interface, they will recombine. The effect should
then be that a layer known as depletion region.
The band diagram for a p-n junction:

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Some features will be expected.
 The relative spacing’s of the Conduction band.
 The Fermi level
 The valence band
Inside the depletion region, the band structure will clearly be modified.
Heterojunction Diodes:
Instead of metal-semiconductor junction, alternatively different semiconductors may be
used.
It must be feasible to grow the layered structure as a perfect crystal, which requires the
two materials to be lattice matched.
Despite these restrictions, it is entirely possible to fabricate diodes with different band-
gaps on either side of the Junction. In this case the structure is known as a heterojunction.
Features of Heterojunction diodes:
 The difference in refractive index between the two materials.
 The modification to the energy band diagram introduced by the variation may be used
to provide different potential barriers to the motion of electrons and holes.

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LASER DIODES:
Laser is acronym for Light Amplification by Stimulated emitted of radiation.
Require of light sources:
 Light output should be highly direction.
 It must emit light at required wavelengths.
 To couple large amount of power into an optical fibre, the emitting area should be
small.
 It must require very small power for its operation.
 The light source should have compact size and high efficiency.
 High optical output power and coupling efficiency.
 Ideal laser light is single-wavelength. This is related to molecular characteristics of
material being used in the Laser.
 For optical fibre systems the laser sources used for almost exclusively are
semiconductor Laser diodes.
 The output radiation is highly monochromatic and the light beam is very directional.
Principle of operation:
(i) Photon absorption
(ii) Spontaneous emission
(iii) Stimulated emission
(i) Photon absorption:
When photon with energy E2-E1 is incident on the atom. The atom is initially in E1.
The atom excited into the higher energy state E2 through absorption of photon.
(ii) Spontaneous emission:
Spontaneous emission, an atom returns to the lower energy state in random manner. It
gives incoherent radiation.

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(iii) Stimulated emission:
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. It gives the coherent radiation.
Coherent mean, when an atom is stimulated to emit light energy by an incident
wave, the liberated energy can be added to the wave.
LASER DIODE MODES:
Semiconductor Laser diodes are preferred over LED for the optical fibre
communication systems requiring BW greater than approximately 200 MHz
Laser diodes have
 Response time less than 1hs.
 Optical BW of 2nm or less
 High coupling efficiency
 Laser diodes multi-layered.
 Smaller temperature dependence.

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 Lasers are oscillators operating at optical frequency the oscillator is formed by a
resonant Cavity providing Selective Feedback.
 This cavity is much smaller being approximately 250-500 um long. 5-15um wide, 0.1-
0.2um thick. These dimensions are referred to as the longitudinal lateral and
transverse dimensions of the cavity.
 The purpose of reflecting mirrors is to provide strong optical feedback in the
longitudinal direction.
 The two Heterojunctions provide carrier and optical confinement in a direction
normal to the junction. The current at which lasing starts is the threshold current.
Above this current the output power increases sharply.
Distributed feedback laser (DFB):
In DFB laser the lasing action is obtained by periodic variations of refractive index
which are incorporated into the multiplayer structure along the length of the diode.
Modes:
 Longitudinal modes
 Lateral modes
 Transverse modes

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Lasing Conditions
E(z,t) = I(z)ej(ωt-βz)
Where,
ω  is the optical radian frequency
β  is the propagation constant
The condition to just reach the lasing threshold is the point at which the optical gain is
equal to the total loss at in the Cavity.
Optical gain at threshold = Total loss in the Cavity (∝t)
gth = β Jth
Jth =
1
𝛽
( ∝ +
1
2𝐿
ln 1/𝑅1𝑅2 )
Resonant Frequency:
Ej2βL = 1 ---------------------------------------------(1)
Here 2βL = 2πm (m --- integer)----------------(2)
β =
2𝜋𝑛
𝜆
---------------------------------------------(3)
m =
𝛽𝐿
𝜋
--------------------------------------------(4)
sub (3) in (4)
m =
𝐿
𝜆/2𝑛
------------------------------------------(5)
λ =
𝑐
𝑣
-------------------------------------------(6)
m = 2L
𝑛𝑣
𝑐
-------------------------------------------(7)

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PHOTOCONDUCTIVE CELLS:
Applications:
Photoconductive cells are used in many different types of circuits and applications.
Analog Applications:
• Camera Exposure Control
• Auto Slide Focus - dual cell
• Photocopy Machines - density of toner
• Colorimetric Test Equipment
• Densitometer
• Electronic Scales - dual cell
• Automatic Gain Control - modulated light source
• Automated Rear View Mirror
Digital Applications:
• Automatic Headlight Dimmer
• Night Light Control
• Oil Burner Flame Out
• Street Light Control
• Absence / Presence (beam breaker)
• Position Sensor
Pin-Photodiode:
 The basic limitation of a p-n junction Photodiode – that the depletion layer is
so thin that radiation of long wavelength.
 A region of intrinsic or lightly doped material is introduced between two
heavily doped p and n-type regions.
 The effective depletion layer width may therefore be fixed at a value far
greater than the natural one approximately the width of the intrinsic region.
 The n- region is a mesa of GaInAs
 In this device, light enters through the InP Substrate, which has an energy gap
of 1.35ev and so is transparent to wavelength longer than about 0.92um.
 The quantum efficiency is then almost uniform between 1.0 and 1.6 um just
below GaInAs.

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Avalanche Photodiode:
 In the Avalanche photodiode impact ionisation is used as a method of
multiplying the photo current before it enters the circuit of any electrical
amplifier.
 RAPD is a device with p+ -i -p-n+ structure. Under reverse bias, most of the
applied voltage is dropped across p-n junction.
 As the voltage is increased, the depletion layer associated with this junction
widens, until it just reaches through the intrinsic region.
 In the depletion region, the field is extremely high. Normally the peak field is
held at 10% below the value at which avalanche breakdown occurs.
 The photo generated carriers drift in the moderate electric field, with electrons
travelling towards the p-n+ junction.
 Carrier multiplication then takes place inside the high field region by impact
ionization.
 However, since the avalanche process is a statistical one, there is a
corresponding increase in the noise level.

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Photovoltaic cell(or) Solarcell:
 Scientific base for solar pv electric power generation is solid state physics of
Semiconductor.
 Silicon is a popular candidate material for a solar pv cells because,
 It is a semiconductor material
 Technology is well developed to make silicon to be +ve (or) –ve charge carriers –
essential elements for an electric cell (or) battery.
 Silicon is abundant in supply and relatively inexpensive in production
 Micro and nano technologies have enhanced the opto-electricity conversion efficiency
of silicon solar pv cells.
Working principle of Silicon Solar p-v cells:
 Photovoltaic material of device converts light into electric energy.
 Silicon solar pv cells is a device made up of semiconductor materials that produce
electricity and light.
 A p-n junction is created in Silicon by a doping process.

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 The Photons from the exposed light prompted electrons flowing from n-junction to
the p-junction.
 The Solar cell is composed of a p type semiconductor and n-type semiconductor.
 Solar light hitting the cell produces two types of electrons, (+ve) and (-ve)
 When we connect loads such as light electric current will flow between the two
electrodes.

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Energy Payback Time:
 EPBT is the time necessary for a photovoltaic panel to generate the energy equivalent
to that used to produce it.
 A ratio of total energy used to manufacture a pv module to daily energy of a pv
system.
Common solar cell materials:
SINGLE CRYSTALLINE POLY CRYSTALLINE
 Silicon (Si)
GaAs  Cadmium telluride (CdTe)
 Copper indium
diselenide(CIs)

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Types of Solarpv cells:
 Flat plate systems:
On rigid flat surface
Usually from single wafers from 300 to 250 to 200 μm
Area : 170 cm2 approximately
Output power : 1-2w approximately
Output voltage: 0.5v approximately
 Concentratorsystems:
With optical components, eg: lenses to direct and concentrate sunlight on the
pv cells of small areas.
Involving tracking mechanisms for detecting the sunlight
Can increase power flux of sunlight hundred of times
Heat dissipation required.
LASER RANGE FINDER:
 A laser range finder is a range finder which uses a laser beam to determine the
distance to an object.
 The most common form of laser range finder operates on the time of light principle.
 Due to the high speed of light this technique is not appropriate for high precision sub
millimetre measurements.
 Some of the laser light might reflect of leaves (or) branches which are closer than the
object giving an early return and a reading which is too low.
Calculation:
D =
𝒄𝒕
𝟐
between A and B point
c  speed of light
t  amount of time for round trip.
t =
𝝋
𝝎
-----------------------(1)eq
  Phase delay
ω  angular frequency
D =
𝟏
𝟐
ct
Sub eq 1 here,
D =
𝟏
𝟐
𝒄𝝋
𝝎

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Technologies:
Time of light:
This measures the time taken for a light pulse to travel to the
target and back with the speed of light known and an accurate measurement of
the time taken.
Multiple frequency phase shift:
This measures the phase shift of multiple
frequencies on reflection then solves some simulations equations.
Interferometry:
The most accurate and most useful technique for measuring
changes in distance rather than absolute distances.
Applications:
 Military
 3-D modelling
 Forestry
 Sports
 Industrial production processes
 Laser measuring tools.
Light Activated SCR:
The Light Activated SCR is also known as Light triggered thyristor (LTT)
It may be triggered with a light source (or) with a gate signal. Sometimes a
combination of both light source and gate signal is used to trigger an SCR
The Light Intensity required to turn-on the SCR depends upon the voltage bias
given to the gate. Higher the voltage bias, lower the light intensity required.
These devices are available up to the 6kv and 3.5kv
The symbol for a light Activated SCR(LASCR) is shown below

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Basic operation of LASCR:
i. When no light is present, the LASCR is OFF, no current will flow through the load.
ii. However, when LASCR is illuminated, it turns ON, allowing current to flow through
the load.
iii. The resistor in this circuit is used to set the triggering level of the LASCR.
iv. Resistors can be added between the Gate(G) and Cathode(C) to reduce its
susceptibility to noise and
𝑑𝑣
𝑑𝑡
effects, but this degrades its sensitivity to light
triggering.
v. When photons from a light source collide with electrons within the p-type
semiconductor, they gain enough energy to jump across the p-n junction energy
barrier.
vi. Even when the photons are eliminated, the LASCR will remain ON until the polarity
of the anode and cathode are reversed or the power is cut.
Applications of LASCR:
 High voltage current applications.
 High voltage anode-cathode circuits.
 High voltage Direct Current (HVDC) Transmission.
Optical Isolator:
 An Optical isolator, or optical diode is an optical component which allows the
transmission of light in only one direction
 It is typically used to prevent unwanted feedback into an Optical oscillator.
 The operation of the device depends on the Faraday effect.
 The main component of the optical isolator is Faraday rotator.
 The magnetic field B, applied to the Faraday rotator causes a rotation in the
polarization of the light due to the Faraday effect.
 Angle of rotation  β = vBd
 Faraday isolator is made up of three parts, they are
input polarizer
Faraday rotator
O/P polarizer
it is also known as polarization dependent isolator.
 The most important optical element in an isolator is the Faraday rotator.
 The characteristics that one looks for in a Faraday rotator optic include a
 Verdict constant
 Low absorption coefficient
 Low non-linear refractive index
 For long distance fibre communication, typically at 1310nm (or) 1550nm yttrium iron
garnet (YIG) crystals are used.
 Optical isolators are different from ¼ wave plate, base isolators.

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 The polarization rotation due to the Faraday rotator is always in the same relative
direction.
 In the forward direction the rotation is +45°
 In the reverse direction the rotation is -45°.