optical instrumentation u2

7/31/2019 Optical Instrumentation u2

http://slidepdf.com/reader/full/optical-instrumentation-u2 1/94

OPTICAL

INSTRUMENTATION

ANUJ BHARDWAJ (UNIT-2) 1



OPTICAL INSTRUMENTATION

UNIT-2( Opto-Electronic devices and optical components)

Photo diode, PIN, Photo- Conductors, solar cells, Photo transistors, Materials used to fabricateLEDs and Lasers Design of LED for optical communication, Response times of LEDs , LED drivecircuitry, Lasers classification: Ruby lasers, Neodymium Lasers, He-Ne lasers, Dye lasers,Semiconductors lasers, Lasers applications



Types of photodiode

Photodiode

ConventionalPN junction

diodePIN photodiode

Avalanchephotodiode



PN junction photodiode

This photodiode is a PN junction

operated in the reverse biased

condition.Here photon current

flows as if a reverse current

flows in the case of the normal

reverse biased diode.



The VI chracteristics of a PN junction

photodiode is shown in figure:

• As the light is incident on the

photodiode photocurrent is developed

• in dark- no current flows



PIN Photodiode

• PIN Photodiode: the PIN

photodetector has a wide

intrinsic layer between the two

highly dopped regions.It gives

better performance compared to

the conventional photodiode.

p-i-n Photodiode



p-i-n Photodiode

Thick intrinsic layer for high absorption and low capacitance

Low bias required to deplete intrinsic layer

No-gain = linear response



VI characteristics:

a)In photoconductive mode

the photocurrent is slightly dependent on the

reverse bias.For a constant reverse bias the

current is linear.This is called current modeof operation of the photodiode.

b) In photovoltaic mode

when no reverse bias is provided thon for

incident optical power it results in a

forward voltage.This is called voltage mode

of operation of the photodiode.



Avalanche Photodiode – APD

APDs show an internal current gain effect (around 100) due

to impact ionization (avalanche effect)

http://en.wikipedia.org/wiki/Impact_ionization

http://en.wikipedia.org/wiki/Avalanche_breakdown

http://en.wikipedia.org/wiki/Avalanche_breakdown

http://en.wikipedia.org/wiki/Impact_ionization



Avalanche Photodiode (APD)

APDs internally multiply the

primary photocurrent before it

enters to following circuitry.

In order to carrier multiplication

take place, the photogenerated

carriers must traverse along a

high field region. In this region,photogenerated electrons and

holes gain enough energy to

ionize bound electrons in VB

upon colliding with them. This

multiplication is known asimpact ionization. The newly

created carriers in the presence of

high electric field result in more

ionization called avalanche

effect.

Reach-Through APD structure (RAPD)

showing the electric fields in depletion

region and multiplication region.

Optical radiation



Responsivity of APD

• The multiplication factor (current gain) M for all carriers generated in thephotodiode is defined as:

•

Where is the average value of the total multiplied output current &is the primary photocurrent.

• The responsivity of APD can be calculated by considering the current gain

as:

p

M

I

I M [6-6]

M I

P I

M M h

q0APD

[6-7]



Current gain ( M ) vs. Voltage for different optical

wavelengths



š p+

SiO2Electrode

net

x

x

E ( x)

R

E

h > E g

p

I ph

e – h+

Absorption

region

Avalanche

region

(a)

(b)

(c)

Electrode

n+



APD

APD gain varies strongly with the applied reverse bias and temperature,

it is necessary to control the reverse voltage to keep a stable gain



Advantages of photodiode

• Very low reverse bias is necessary

• High quantum efficiency

•

Larger bandwidths can be obtained• Lower noise level

The only disadvantage is that the PIN

photodiode does not give any amplificationaction.



• Solar cells:the solar cellis a device which convertsthe incident light radiationinto he electric power .

The most commonly usedtypes of solar cells are:

Silicon solar cellheterojuncion solar cell

Homojunction solar cell

amorphous silicon cellsGaAs solar cells



Schematic

representation of asilicon p-n junction

solar cell.



Solar Cell

• Definition

• Principle of Solar Cell

• Current generation

•I-V characteristic of an illuminated p-n junction

• Physical process of Solar cell

• I-V characteristic of solar cell

• Solar cell parameter

• Applications

• reference



Definition:

• Device that converts optical energy into

electrical energy.

– It supplies a voltage and a current to a resistive load (light,

battery, motor). – Power = Current x Voltage=Current2 x R= Voltage2/R

• The voltage supplied by the cell changes with

changes in the resistance of the load.• It supplies DC power.



Principle of Solar Cells

• Most familiar solar cells are based on theeffect of photo voltaic.

• In this effect, light falling on a semi-conductor

device of two layers, produces a potentialdifference or photo voltage between thelayers.

•

The voltage thus produced can drive a currentthrough an external circuit producing usefulwork.



The Photovoltaic Effect in a Solar Cell



Principle of solar cell

• A solar cell is a very large diode.

– When Si that is doped p-type is next to a region of Sidoped n-type, the holes from the p-type side diffuse to then-type side. The electrons diffuse to the p-type side.

– This creates an electric field. – This electric field makes it easy for current to flow in one

direction, but hard to flow in the other.

– This electric field also separates electrons and holes that

have been created by the absorption of sun light. Whenthe electrons and holes are separated electric power canbe extracted from the circuit.



Current generation

• When light is incident onp-n junction-

Photon absorbed if h ν >

Eg No big change in majority

carriers in conduction

band.But a significant change in

minority carriers invalance band .Band diagram when light is

incident



Current generation

• A current is identified due to drift of minority carriersacross a junction as a generation current.

• Generation rate gop (EHP/cm3) participate in totalcurrent.

• So the total current is depend on current due to usualdiode and current due to optically generation.

I= Ith(eqV/KT

– 1) - Iop



I-V characteristic of an illuminated p-n

junction

• In I quadrant current and junction voltage both are+ve.(forward bias)

• In III quadrant current and junction voltage both are –ve.(reverse bias)

So power is delivered to thedevice from the externalcircuit



Physical process of solar cell

• But in IV quadrant junction voltage is +ve and

current is –ve.

• So power is delivered from junction to

external circuit.

• Solar cell work in this IV quadrant region.

The device delivers

power to the load



I-V characteristics of solar cell

• This figure gives many parameter of solar cell.Forward bias characteristics of solar cell

ISC =The Short circuit

current

VOC=The Open Circuit

Voltage

V MP

=The Voltage at the

maximum power point

I MP = The Current at the

maximum power point



Solar cell parameter

•Fill Factor- is essentially a measure of quality of the solarcell.

• It’s a figure of merit for a solar design.

FF= V MP IMP / ISCVOC

Pmax

= maximum power

PT = theoretical power

Efficiency (η) - is the ratio of the electrical power output Pout ,compared to the solar power input, Pin, into the solar cell

η = Pout / Pin = PMAX / Pin

η = V MP IMP / IT As

IT = radiation flux

As = surface area of p-n junction



Advantages of solar cell

• Solar energy is free and inexhaustible

• Solar energy is clean and non-pollution energy.

• Solar cell can be considered to be long life devices as they

have useful lifetimes in excess of 20 years.

• Maintenance is simple and automation and unmannes

operation is feasible.



Applications

Professional applications

• Ocean navigation aids

• Telecommunication systems

•

Remote monitoring and control• Cathodic protection

Electric power generation in space

Grid-connected systems

• PV Power Stations• PV in buildings



Applications

Rural electrification

• Water pumping

•

Domestic supply• Lighting

• Health care



Phototransistors

• Phototransistors:the

phototransistor is an opto-

electronic device like the

avalanche photodiode where the

current flow from aPN junction

detector is internally amplified.



Working of phototransistor

• When light falls on phototransistor, electronhole pairs are generaed in the collector baseregion

• The applied potential separates theseelectrons and holes.

• Multiplication of charge takes place.

•

These electrons enter into the collector region.

• Thus photocurrent flows.



Hetero junction LED



Hetero-junction LED

Homo-junction LED has two

drawbacks

Narrow p-side encourages

surface recombination of the

injected carriers through surface

defects

If the recombination occurs over a large volume (longer

distance) due to the longer electron diffusion length, then the

chance of re-absorption of emitted photons becomes higher, asthe amount of re-absorption increases with material volume

The double hetero-structure LED increases the intensity of

output light

Surface Emitted LED &Edge Emitted LED



Surface Emitted LED &Edge Emitted LED

If the emitted radiation emerges

from an area in the plane of the

recombination layer, then the deviceis surface emitting LED

If the emitted radiation

emerges from an area on a

crystal perpendicular to the

active layer, then the device is

edge emitting LED

Surface Emitted LED



Surface Emitted LED

The thin SiO2 layer at the back isolates the contact layer

Photons are generated in the thin p-GaAs region

The photon emitted from the active region p-GaAs doesn't

absorbed by the neighboring layer (AlGaAs), which has wider

bandgap

Edge Emitted LED



Edge Emitted LED

Edge emitting LED provides a great intensity light and more

collimated beam than the surface emitted LED

Hetero-structure

InGaAsP/ InGaAs /InGaAsP



• Materials used to

fabricate LED:The following are the requirements

for a good quality LED material:

a) It must have energy gap of

appropriate widh.

b) Both P and N typpes must exist

,preferably with low

resistivities.c) Effective radiative paths should

be present.



Double Heterostructure LEDs



Isoelectronic doping – indirect bandgap



LED drive circuitry



LED analog transmission:

The analog signal is basically a time varying voltage waveform in which both

amplitude and phase are varying with time. Therefore LED shall provide a

linear region in which output power of LED responds linearly to the

variation of input voltage or current.Using the linear characteristics of LED

,it is possible to do analog communication from LED source using

transistor circuits when extremely low level distortions are not required.



• LED DIGITAL TRANSMISSION:LED digital transmitters

deal with high speed switching of on or off LED currents in

response to logic circuts coding.The switching speeds of

several kilohertz to six hundred megahertz are used whereas

current pulses may range from several tens of milliamperes to

several hundreds of milliamperes.



1. Definition of laser

• A laser is a device that generates light by a processcalled STIMULATED EMISSION.

• The acronym LASER stands for Light Amplification byStimulated Emission of Radiation

• Semiconducting lasers are multilayer semiconductordevices that generates a coherent beam of monochromatic light by laser action. A coherentbeam resulted which all of the photons are in phase.



3.2.2 The Basic Principles of Lasers

The word LASER is an acronym for Light Amplification by Stimulated

Emission of Radiation.

The essential elements of a laser device are as follows:

•An active medium, consisting of a collection of atoms, molecules, or

ions in a gaseous, liquid, or solid state, which generates and amplifieslight by means of appropriate transitions between its quantum energy

levels.

•An energy pumping mechanism, the energy pump which selectively

pumps energy into the active medium to populate selected energy levels

and to achieve population inversion•An optical resonator, which provides the optical feedback and

determines the emission modes.

The light-matter interaction process that takes place in the active medium



constitutes the essential key of laser radiation. On the basis of

phenomenological consideration, Einstein developed a theory that permits aqualitative understanding of the processes related to light absorption and

emission by atoms. Three basic light-matter interactions can be considered:

absorption, spontaneous emission, and stimulated emission of photons, as

shown in the figure. The basis of laser radiation lies in the last process:

stimulated emission. In the stimulated emission, an incident photon can lead



to generation of more photons with identical properties. Thus, the stimulated

emission in itself constitutes an optical amplification phenomenon.The quantum mechanical perturbation theory that allows us to calculate the

probabilities of the absorption and stimulated emission shows that both the

processes have equal transition probabilities, provided that the levels have

equal degeneracy.

3.2.3 The Threshold Condition: Population Inversion

It is clear that in order to get stimulated emission, a pumping process is

certainly required to excite the active medium to its excited state. Real

materials can be pumped in many ways, as will be mentioned later. For laseraction to occur, the pumping must produce not merely excited atoms, but the

condition of population inversion.



level lasers.

3.2.4 Pumping Techniques

In general, different pumping techniques are used in practical devices

depending on the types of active medium.

Gas discharges are among the most widely used pumping processes for gas

lasers. Direct electron impact with atoms or ions and transfer of energy bycollisions between different atoms are the two main mechanisms involved.

Optical pumping techniques are also very common. The source of the

pumping light may be a continuous arclamp, a pulsed flash lamp, another

laser, or even focused sunlight.

Other pumping techniques include chemical reactions, high-voltage electron

beam bombard, and direct current injection across the junction of a

semiconductor laser.



LASER :LIGHT AMPLIFICATION BY STIMULAED EMISSION OF

RADIATION

CHRACTERISTICS:• DIRECTIONALITY

• HIGH INTENSITY

• EXTRAORDINARY MONOCHROMATICITY

• HIGH DEGREE OF COHERENCE



Three Mechanisms of Light Emission

For atomic systems in thermal equilibrium with their

surrounding, the emission of light is the result of:

Absorption

And subsequently, spontaneous emission of energy

There is another process whereby the atom in an upper energylevel can be triggered or stimulated in phase with the an

incoming photon. This process is:

Stimulated emission

It is an important process for laser action

1. Absorption

2. Spontaneous Emission

3. Stimulated Emission

Therefore 3 processof light emission:



Absorption

E1

E2



Spontaneous Emission



Stimulated Emission



Background Physics

• In 1917 Einstein predicted that:

under certain circumstances a photon

incident upon a material can generate a

second photon of Exactly the same energy (frequency)

Phase

Polarisation

Direction of propagation

In other word, a coherent beam resulted.



Background Physics

• Consider the ‘stimulated emission’ as shown

previously.

• Stimulated emission is the basis of the laser action.

• The two photons that have been produced can thengenerate more photons, and the 4 generated can

generate 16 etc… etc… which could result in a

cascade of intense monochromatic radiation.



E 1

E 2

h

(a) Absorption

h

(b) Spontaneous emission

h

(c) Stimulated emission

Inh

Out

h

E 2

E 2

E 1 E

1

Absorption, spontaneous (random photon) emission and stimulatedemission.

© 1999 S.O. Kasap,Optoelectronics (Prentice Hall)



Stimulated Emission



Population Inversion• Therefore we must have a mechanism where N2 > N1

• This is called POPULATION INVERSION• Population inversion can be created by introducing a so call metastable

centre where electrons can piled up to achieve a situation where more N2 than N1

• The process of attaining a population inversion is called pumping and the

objective is to obtain a non-thermal equilibrium.• It is not possible to achieve population inversion with a 2-state system.

• If the radiation flux is made very large the probability of stimulated emissionand absorption can be made far exceed the rate of spontaneous emission.

• But in 2-state system, the best we can get is N1

= N2

.

• To create population inversion, a 3-state system is required.

• The system is pumped with radiation of energy E31 then atoms in state 3relax to state 2 non radiatively.

• The electrons from E2 will now jump to E1 to give out radiation.



Typical Exam Question…

• Define the term population inversion for a

semiconducting laser (diode) explain what is

the condition of population inversion.

• Why is population inversion required for alasing action?

(40 marks)



Therefore in a laser….

Three key elements in a laser

•Pumping process prepares amplifying medium in suitable state

•Optical power increases on each pass through amplifying medium

•If gain exceeds loss, device will oscillate, generating a coherent output



3.3.3 Dye Lasers

In this type of lasers, theactive media are organic dye

molecules dissolved in a liquid,

which display strong

broadband fluorescence

spectra under excitation of

visible light or UV light.

A simplified energy-level

diagram for a dye in a liquid

solution is shown in the right

figure. The dye moleculeshave singlet as well as triplet

electronic states. The singlet

and triplet states refer to those

states that have a total electron



spin quantum number equal to 0 and 1, respectively. Each electronic state

comprises several vibrational states, and each of these in turn contains

several rotational levels. When the dye molecules are excited with photonsof appropriate energy, higher vibrational levels of the first or second excited

singlet states (S1 or S2) are populated from rotational-vibrational electronic

levels of the ground state S0. Then, induced by collisions with the solvent,

the excited molecules undergo a rapid nonradiative relaxation to the lower

vibronic level of the state S1. At a sufficiently high pump intensity,population inversion may be achieved between the level v0 in S1 and the

level vk of S0.

The bottom figure in the figure given in preceding slide shows the

absorption and emission of a commonly used dye, Rhodamine 6G.

The essential characteristic of dye lasers is their broad homogeneous gain

profile that results in broad emission band (tens of nanometers). Therefore,

with different dyes the overall spectral covered by these dyes can be

extended from around 400 nm to 1.1 μm, as shown in the figure in next



slide. Dye lasers are thus the wavelength tunable lasers. Due to their

flexibility in design and performance, dye lasers have been commonly used

in a great variety of spectroscopic techniques, including high-resolution

spectroscopy.



S



RUBY LASER

• Ruby consists of Cr+++ions doped into

crystalline Al203 at a

typical concentration of

around .05% by weight

RUBY LASER



RUBY LASER

Ruby has the strongabsorption bands in the

blue and green spectral

regions. These electrons

relax rapidly to the

upper laser level by

non-radiative

transitions in whichphotons are emitted.



Energy level diagram in He-Ne laser

Helium has two electrons .In the ground

state both electrons are in the 1s

level.The first excited state is the 1s2s

configuration.There are two possible

energies for this state because there

are two possible configurations of the

electron spin.

Nd YAG LASER



Nd:YAG LASER

• Neodymium ions formthe basis for series of

high power solid state

lasers.The main laser

transition is in the near

infrared at 1.06

micrometer



The diagram on the right shows the levelscheme for the Nd lasers, which are

four level lasers.Electrons are excited

to the pump bands by absorption of

photons from a powerful flashlamp

or from a diode laser operating

around 800 nm.



.73105.1

101.1

9

11

v

v N

Hz L

c

v

9

8

105.11.02

103

2

EXAMPLE: For a Nd:YAG laser crystal located in a cavity of length 0.1

m, determine (a) the frequency separation among the axial modes and (b)

the number of axial modes. Assume that the laser is operating in a laser line at 1.06 μm, whose full width, δv, is 1.1 x 1011 Hz.

(a) The frequency separation,Δv, between two adjacent modes is

since the axial modes (optical standing waves) in a cavity of length L

fulfill the condition v = nc/2L, where n is an integer and c is light speed

in vacuum.

(b) The number of axial modes, N , with the laser linewidth is given by

S i d t di d l



Semiconductor diode laser

These lasers are used inlasers printer.it consists

of p-n diode cleaved

into a small chip as

shown to the

right.electrons are

injected into the n-

region and holes intothe p region .



3.3.4 Semiconductor Lasers

In this section, we shall summarize the basic principles of the semiconductorlasers. Following figures illustrate the basis of a semiconductor laser diode.

The laser action is produced by electronic transitions between the

conduction and the valence bands at the p-n junction of a diode. When an

electric current is sent in the forward direction through a p-n semiconductor

diode, the electrons and holes can recombine within the p-n junction and



may emit the recombination energy as electromagnetic radiation. Above a

certain threshold current, the radiation field in the junction becomes

sufficiently intense to make the stimulated emission rate exceed thespontaneous processes.

The radiation can be amplified by an optical resonator, which, in the

simplest case, is constituted by the semiconductor itself, shaped in the

appropriate manner; for example, by cutting the crystal so that two end faces

are parallel to each other,

and exactly perpendicular to

the laser beam emitted by the

junction, as shown in the right

figure.

In semiconductor lasers, the

emission wavelength is

essentially determined by the

Diode Laser



Diode Laser

Applications of laser



Applications of laser

• In measurement of long distance• In making fine holes and cutting the thick metal sheet’

• In telecommunication.

• In photography

• In surgery

• In chemistry

• In space etc

Typical Application of Laser

http://www.semiconductor-technology.com/projects/iqe/index.html






Typical Application of Laser The detection of the binary data stored in the form of pits on the compact disc is done

with the use of a semiconductor laser. The laser is focused to a diameter of about 0.8mm at the bottom of the disc, but is further focused to about 1.7 micrometers as it

passes through the clear plastic substrate to strike the reflective layer. The reflected

laser will be detected by a photodiode. Moral of the story: without optoelectronics

there will no CD player!

LED VS Laser Output Characteristics















LED VS Laser Output Characteristics

optical instrumentation u2

Documents