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LASER & HOLOGRAPHY
MODULE 6
SEMICONDUCTOR LASER
or
DIODE LASER
• Semiconductor laser is a specially fabricated
p-n junction device.
• Emits coherent light when it is forward biased.
• Diode lasers are remarkably small in size
(.1mm long), and low power requirement.
• Direct band gap semiconductors are used to
make semiconductor laser.
• Modulating biasing current easily modulates
the laser output.
Direct & Indirect bandgap
Common materials for semiconductor
lasers
DIODE LASER - CONSTRUCTION
DIODE LASER - CONSTRUCTION
• Heavily doped GaAs is the n-type material.
• P-type is formed on the top by diffusing Zinc
atoms to it, heavily doped Zinc constitute p layer.
• The top and bottom faces are metalized.
• Metal contact are provided to pass current
through the diode.
• The front faces are polished, polished faces
constitute the RESONATOR.
• The remaining sides are roughened to eliminate
the lasing action in that direction.
• The entire structure is packaged in a small case,
which is looks like a metal case.
APPLICATIONS
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Used in communication systems.
Used as barcode reader.
Natural transmitter of digital data.
Laser beam printers & scanners.
Advantages
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Minimal size.
Highly efficient .
Easy to handle.
Very simple and portable.
Require low power to operate.
Laser output can be simply modulated by
controlling junction current.
Disadvantages
• Output is in the form of wide beam.
• Purity and monochromaticity are poorer than
other type solid state lasers.
Laser Applications
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Scientific
Industrial
Medical
Communication
Military and welfare
Metrology
Others
Scientific applications
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Spectrum analysis.
Raman spectroscopy.
Laser isotope research.
Atmospheric remote sensing
Holographic techniques employing lasers also
contribute to a number of measurement
techniques.
• Laser based LIDAR (Light RADAR) technology has
application in, remote sensing.
• Used to study the structure of molecules.
Industrial applications
• Laser welding
• Laser drilling
• Laser cutting
Laser welding
• When a coherent laser
beam is focused precisely
to a spot, heat energy is
generated.
• This heat melt the
material rapidly and weld
joints very efficiently.
• The process is frequently
used in high volume
applications, such as in
the automotive industry.
Laser drilling
• Laser heat the surface
of the materials and
produce vaporization.
• The resultant gas and
pressure blow away the
vaporized materials.
• High precision holes can
be drilled.
• Hard materials also can
be easily drilled.
• There is no tear and
breakage.
Laser cutting
• Melt and blow or fusion
cutting uses highpressure gas to blow
molten material from
the cutting area.
• First the material is
heated to melting point
then a gas jet blows the
molten material.
• Materials cut with this
process are usually
metals.
Medical applications
• Cosmetic surgery (scars, stretch marks, sunspots,
wrinkles, birthmarks, and hairs)
• Eye surgery
• Soft tissue surgery.
• laser therapy
• "No-Touch" removal of tumors, especially of the
brain and spinal cord.
• Tooth whitening and oral surgery.
• Used to control bleeding.
Communication
• Optical fiber communication
• Large number of phone conversations can be
easily sent through optical communication
with the help of diode laser.
• Under water communication.
Military applications
• Disorientation: Some weapons simply use a
laser to disorient a person.
• Guidance : Laser guidance is a technique of
guiding a missile or other projectile or vehicle
to a target by means of a laser beam.
• Another military use of lasers is as a laser
target designator. This is a low-power laser
pointer used to indicate a target typically
launched from an aircraft.
HOLOGRAPHY
• Holography = holos ( whole)
+
graphein (to write)
• Holography means writing the complete
image.
• It is actually a recording of interference
pattern formed between two beams of
coherent light coming from the same source.
• Both amplitude & phase components of light
waves are recorded in a light sensitive
medium such as photographic plate.
The recorded image is known as hologram.
PRINCIPLE OF HOLOGRAPHY
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Holography is a two step process.
RECORDING HOLOGRAM
RECONSTRUCTION
Its entirely different from conventional
photography, no lens is needed.
• A hologram is a result of interference
occurring between two waves, an object beam
and reference beam.
RECORDING OF THE HOLOGRAM
RECORDING OF THE HOLOGRAM
• Broad laser beam is divided into two beams by
a beam splitter.
• REFERENCE BEAM: this beam goes directly to
the photographic plate.
• OBJECT BEAM: The beam of light directed
onto the object to be photographed.
RECORDING OF THE HOLOGRAM
• Each point of the object scatters the incident
light and act as a source of spherical wave.
• Part of light scattered by the object is travels
towards the photographic plate.
• At the photographic plate all the wave fronts
from the object combine with the reference
beam.
• This forms interference fringes on the
photographic plate.
RECORDING OF THE HOLOGRAM
• The developed negative
of these interference
fringes pattern is a
hologram.
• Hologram doesn’t
contain the distinct
image of the object.
• It carries the intensity
and the recorded phase
of the light waves.
RECONSTRUCTION
• The hologram is illuminated with a parallel
beam of light from the laser source.
• This beam is identical to reference beam used
in construction of hologram.
• Most of the light passes through the complex
fine fringes, act as an diffraction grating.
• This reconstruction beam will undergo
phenomenon of diffraction during passage
through the hologram.
RECONSTRUCTION
• The reconstruction beam after passing through
the hologram produces a real as well as virtual
image of the object.
• Virtual image is formed behind the hologram at
the original site of the object and real image in
front of the hologram.
• virtual image exhibits all the true 3 dimensional
characteristics.
• If the observer moves round the virtual image
then other sides of the object which were not
noticed earlier would be observed.
• The real image can be recorded on a
photographic plate.
RECONSTRUCTION
APPLICATIONS
• Holographic interferometry is used in numerous
laboratories for non-destructive testing. It
visually reveals structural faults without
damaging the specimen.
• Holographic storage has the potential to become
the next generation of popular storage media.
• HOLOSTORE is a holographic computer memory
system being manufactured to replace your disc
drive. It will have thousands time more memory
capacity and no mechanical movements.
APPLICATIONS
• In some European countries, credit cards for
telephone calls use erasable holograms.
• High resolution spectrometers use
holographic gratings.
• Fog droplet camera: hologram camera used to
record and study fog droplets.
• Grocery store scanners use spinning
holograms.
Photonics
• Photonics is the science of light generation,
detection, and manipulation.
• photonic applications are in the range of visible
and near- IR light.
SOLID STATE LIGHTING - SSL
• The conventional light sources are replaced by
semiconductor diodes, organic light emitting
diodes etc..
• SSL creates visible light with reduced heat
generation and less energy dissipation.
CONTENTS
LED
PHOTO DETECTORS
PHOTODIODE
AVALANCHE PHOTODIODE
PHOTO TRANSISTORS
THERMAL DETECTORS
SOLAR CELL
LED
• LED – LIGHT EMITTING DIODE.
• LED is a P-N junction diode gives off visible
light when forward biased.
• When an electron recombine with hole in a
semiconductor energy is released.
• In Ge and Si like semiconductors it take place
only with the help of traps. When it takes
place, the released energy goes into the
crystal as heat energy.
LED
• Semiconductors like GaAs, considerable amount
of direct recombination takes place without the
help of traps.
• LED consisting of chip of semiconducting material
doped with impurities to create p-n junction.
• When an LED is forward biased properly e- from
the n-region and holes from the p-region
combine to produce an energy and that will be
emitted as radiation.
LED
LED
• The e- exist in the conduction band of n-type
high energy state.
• Holes exist in the valence band of p-type, low
energy state.
• Wavelength of the light emitted depends on
the band gap energy of the material forming
P-N junction.
• Most of the LED making materials have a very
high refractive index.
LED
• Common LED colors are amber, red, green and
blue.
• To get LED with white light, different color
LED’s are mixed or covered with a phosphor
material that convert the color of light.
• LED don’t have filaments which will burn out,
so they last much longer.
• Their plastic bulb make it lot more durable.
• Its luminous efficiency is one of the major
advantage of LED.
SEMICONDUCTOR MATERIALS AND ITS
COLOUR
LED
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LED output frequency, ν=
In the case of GaAsP,
= 1.930eV
E= ? Joule
E= hν
ν=?
λ=?
PHOTODETECTORS
• Photo detectors are used primarily as an optical
receiver to convert light into electricity.
• The principle that applies to photo detectors is
the photoelectric effect, which is the effect on a
circuit due to light.
• The photo electric effect is the effect of light on a
surface of metal in a vacuum, the result is
electrons being ejected from the surface.
• A photo detector operates by converting light
signals that hit the junction to a voltage or
current. The junction uses an illumination
window with an anti-reflect coating to absorb the
light photons.
PHOTODETECTORS
Commonly used photo detectors are,
• Photovoltaic cell
• Photoconductivity cell
• Photodiode
• Avalanche photodiode.
• Phototransistor
Also there are optical devices similar to photo detector
like solar cell.
Photovoltaic cell
Photovoltaic cell
• Pair of electrodes immersed in an electrolyte
and light is allowed to fall on one of the
electrode.
• Potential difference is established between
the electrodes.
• This phenomena is called PHOTOVOLTAIC
EFFECT.
• Device based on this effect is known as
photovoltaic cell.
Photovoltaic cell
• Photovoltaic cells are the devices in which
light energy is used to create potential
difference.
• This potential diff. developed is proportional
to the frequency and intensity of incident
light.
• It consist of a semiconducting material
bonded on a metal plate.
• Metals like selenium and silicon are most used
Photovoltaic cell
• If light is made to fall on the semiconducting
material, valence electrons and holes get
liberated from it.
• The electrons liberated move towards the
metal plate while holes flow in opposite
direction.
• A potential diff. created between the
semiconductor and the metal plate.
• So current flows in the external circuit.
Photovoltaic cell
• The strength of the current is proportional to
the intensity of incident radiation.
• APPLICATIONS
Used in relay circuit
Light intensity meter
exposure meter in photography
PHOTOCONDUCTIVITY CELL
• Photoconductivity cell is based on the principle
that the electrical resistance of semiconducting
materials are decreases when they are exposed
to radiation.
• Materials like lead sulphide, selenium etc.
showing this property.
• A photon striking the surface of such photosensitive material posses energy E=hν greater
than the energy band gap between valence band
and conduction band.
PHOTOCONDUCTIVITY CELL
• Absorbing energy electron will raise to the
conduction level, Hole left in the valence band.
• This electron hole pairs is free to serve as current
carriers and hence the conductivity increases.
• So the resistance decreases.
• Used as street light control.
• Used in voltage regulators.
• Burglar alarm.
• In camera for light setting.
PHOTOCONDUCTIVITY CELL
PHOTODIODE
• Photodiode is a reverse biased p-n junction.
Which when exposed to light, gives current
that varies linearly with luminous flux
produced by light.
PHOTODIODE
• Photodiode consist of a
P-N junction mounted
on an insulating
substrate and sealed
inside a metal case.
• A glass window is
provided at the top for
allowing light to strike
the junction.
• Two terminals act as
anode and cathode.
PHOTODIODE
• A photodiode is a p–n junction .
• When a photon of sufficient energy strikes the diode,
it creates an electron-hole pair.
• If the absorption occurs in the junction's depletion
region, these carriers are swept from the junction by
the built-in electric field of the depletion region.
• Thus holes move toward the anode, and electrons
toward the cathode, and a photocurrent is produced.
• When a small reverse voltage is applied across the
diode, a very small current flow through the diode
due to minority carriers- reverse saturation current.
PHOTODIODE
• The current flowing through the reverse biased
photodiode when no light is incident on the
junction is called DARK CURRENT.
• The corresponding resistance is called DARK
RESISTANCE.
• The total current through the photodiode is the
sum of the dark current (current that is generated
in the absence of light) and the photocurrent.
• so the dark current must be minimized to
maximize the sensitivity of the device.
PHOTODIODE
Avalanche photodiode
• An avalanche
photodiode (APD) is a
highly sensitive
semiconductor
electronic device that
exploits
the photoelectric
effect to convert light to
electricity.
Avalanche multiplication
• Materials conduct electricity if they contain mobile
charge carriers.
• If there is a voltage gradient in the semiconductor, the
electron will move towards the positive voltage while
the hole will "move" towards the negative voltage.
• Under the right circumstances, however, (i.e. when the
voltage is high enough) the free electron may move
fast enough to knock other electrons free, creating
more free-electron-hole pairs, increasing the current.
• Fast-"moving" holes may also result in more electronhole pairs being formed. In a fraction of a nanosecond,
the whole crystal begins to conduct.
Avalanche photodiode
• The operation principle
of an APD is based on
the conversion of the
energy of into free
charge carriers in the
semiconductor bulk.
• Their further
multiplication via the
process of impact
ionization.
Avalanche photodiode
• The basic element of the structure is the p-n – junction.
• APD consists of three regionsa) p- region,
b) intrinsic region I: is the depletion/ absorption
region.
c) n-region.
• Light enter through the p region which is connected to
a cathode.
• I region absorb light and generate electron hole pairs.
• Due to electric field in the depletion region +ve holes
drift towards cathode and –ve electrons drift towards
anode.
• No avalanche region on the p so no avalanche process
happens to holes.
Avalanche photodiode
• On the other side
electron enters into the
p region which is
accelerated by the high
electric field.
• HIGH E FIELD in the p-n
junction due to most of
the applied reverse bias
is across this region.
• And impact all the other
atoms as shown in the
picture.
Avalanche photodiode
• In this region the field
accelerate the electron to
such a high speed that
they create more hole
electron pairs through
collisions.
• This process is called
IMPACT IONOSATION/
AVALANCHE
MULTIPLICATION.
• These current pulses are
then detected in an
external circuit.
Phototransistor
• Transistor is a device
which produce amplified
output of applied signal.
• A phototransistor is a
light-sensitive transistor.
• Convert light into electric
signal with an internal
gain.
• Operated in common
emitter configuration
with the base opencircuited
Symbolic representation
Phototransistor
• The base is unconnected.
• Base collector junction is
photosensitive to act as
light gathering element.
Solar cell
• Solar cell is a photovoltaic
cell.
• Sun light absorbed by the
p-n junction interface
excite electrons into the
conduction band of the ntype from the valence
band of p-type.
• Electric field at the
junction pulls the
electrons towards the n
region and holes towards
p-region.
• As a result current flow
from p terminal to n
terminal.
Solar cell
• Each photon carry energy, E=hν joules.
• If this photon exceeds the band gap energy of
the semiconducting material, photon can
break the covalent bonds and produce
electron hole pairs.
• The resulting carriers can produce
photocurrent.
• Longest wavelength which can produce a
photocurrent in silicon is 1.1μm.
Solar panel
• Solar Photovoltaic
panels constitute the
solar array of
a photovoltaic
system that generates
and supplies solar
electricity in
commercial and
residential applications
Solar cell experiment
• Characteristics
V-I graph ??
• Fill factor :
• Efficiency :
=
=
Efficiency varies from
10-20%
=
Applications
• Concentrating Solar Power (CSP): Concentrating solar
power (CSP) plants are utility-scale generators that produce
electricity using mirrors or lenses to efficiently concentrate
the sun’s energy.
• Solar Thermal Electric Power Plants: Solar thermal energy
involves harnessing solar power for practical applications
from solar heating to electrical power generation.
• Photovoltaics: Photovoltaic or PV technology employs solar
cells or solar photovoltaic arrays to convert energy from the
sun into electricity.
• Solar Heating Systems: Solar hot water systems use
sunlight to heat water
• Solar Cars: A solar car is an electric vehicle powered by
energy obtained from solar panels on the surface of the car
which convert the sun’s energy directly into electrical
energy.
THERMAL DETECTORS
• Thermal detectors are those devices that
absorb the incident radiation and increase its
own temperature and produce resultant
electric signal.
• The classification of the thermal detectors are
based on the physical mechanism of
conversion.
• Bolometer
• Thermocouple / thermopile.
• Golay cell
Thermocouple
• Thermocouple is an electronic
device that converts thermal
energy into electrical energy.
• Two dissimilar metal wires are
joined together to form two
junctions.
• Two junction kept at 2 different
temperature.
• So a potential diff. developed
across them, it is a measure of
temp variations.
• Usually one junction at constant
temp. and the other with an
absorber.
• Incident radiation cause temp
variation.
• A thermopile is composed of
several thermocouples connected
usually in series.
Bolometer
• Bolometer is a device for
measuring the power of
incident radiation via the
heating of a material with
temperaturedependent electrical resistance.
• A bolometer consists of an
absorptive element, such as a
thin layer of metal, connected
to a thermal reservoir (a body
of constant temperature)
through a thermal link.
• Any radiation impinging on the
absorptive element raises its
temperature and hence the
resistance.
• Temp. measurement is done
using a resistance thermometer
Golay cell
• It consists of a gas-filled
enclosure with an
infrared absorbing material
and a flexible membrane.
• When infrared radiation is
absorbed, it heats the gas,
causing it to expand.
• The resulting increase
in pressure deforms the
membrane.
• Light reflected off the
membrane is detected by
a photodiode, and motion of
the membrane produces a
change in the signal on the
photodiode.
Fibre optics
• A technology that uses glass (or plastic)
threads (fibers) to transmit data.
• A fiber optic cable consists of a bundle of
glass threads, each of which is capable of
transmitting messages modulated onto light
waves.
Optic fiber
• Optical fibers are fine
transparent glass or plastic
fibers which can propagate
light.
• They work under the
principle of total internal
reflection from
diametrically opposite walls.
• In this way light can be
taken anywhere because
fibers have enough
flexibility.
• This property makes them
suitable for data
communication, design of
fine endoscopes, micro
sized microscopes etc.
Basic principles
TOTAL INTERNAL REFLECTION
• The field of fiber optics
depends upon the total
internal reflection of light
rays traveling through tiny
optical fibers.
• When a ray travelling fro
denser medium to rarer
medium at an angle of
incidence greater than
the critical angle total
internal reflection takes
place in the boundary
between two media.
Structure of optic fiber
• CORE : innermost
region made of glass or
transparent plastic.
Higher refractive index.
• CLADDING : smaller
refractive index region
• JACKET / SHEATH :
outermost layer. Made
of polythene or
polymers. Providing
mechanical strength.
LIGHT PROPAGATION
THROUGH OPTIC FIBER
• The acceptance angle of
an optical fiber is defined
based on a purely
geometrical consideration
(ray optics):
• The light rays which are
incident within a
particular angle with the
axis of core alone are
allowed to propagate
through the fiber and
undergo TIR.
• ACCEPTANCE ANGLE:(θa)
We can define acceptance angle as the maximum
angle or below which the light rays undergo total
internal reflection.
• ACCEPTANCE CONE:
he acceptance cone is derived by rotating
the Acceptance Angle about the fiber axis.
The maximum angle, represented in threedimensional view as a cone, at which an optical fiber
will accept incident light.
•
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•
•
sine of the acceptance angle of a fiber.
NA = Sin θa
DERIVATION
V number
MODES OF PROPAGATION
• The directions through which the light rays are
propagating through the optic fiber by total
internal reflection are called modes of
propagation.
• The number of paths of light rays in an optic fiber
along which they travel in same phase.
• The number of paths depends on the diameter of
the core and incident wavelength.
• There are two type fibers,
 Single mode fibers
 multimode fibers
Optic fiber communication
• Optic fiber communication consists of three sections
TRANSMITTER
INFORMATION CHANNEL
RECEIVER
• Modern fiber-optic communication systems generally include
an optical transmitter to convert an electrical signal into an
optical signal to send into the optical fiber.
• a cable containing bundles of multiple optical fibers
• An optical receiver to recover the signal as an electrical signal.
Optic fiber communication
Message origin: converting non electrical signal to
electrical signal.
Modulator: imposing a message on carrier wave for
propagation.
Modulations : Modulation is the addition of
information to an electronic or optical carrier signal. A
carrier signal is one with a steady waveform -- constant
height (amplitude) and frequency. Information can be
added to the carrier.
analog modulation : message transmitted in a
continuous manner.
digital modulation : message transmitted in a discrete
manner with the help of binary codes (ON-1 & OFF-0)
Modulations
analog modulation
digital modulation
Optic fiber communication
• Carrier source: produce carrier waves. LED or
LASER DIODES are used to generate stable
monochromatic waves. It behave like an optic
oscillator.
• Input channel coupler: directs the modulated
light waves into information channels.
• Information channel: path to transmit the
information from transmitter to receiver.
• Output channel coupler: modulated signal from
information to detector.
Optic fiber communication
• Detector: separate the message from the
modulated signal. Ie., demodulation takes place.
Light waves converted into electrical signal using
a photo detector.
• Signal processor: filter selects the required
frequency from the waves. Selected frequency is
amplified. Unwanted frequency is filtered out.
• Message output: original message is reproduced.
The electrical pulses converted into audio signal
or visuals.
Optic fiber communication
MESSAGE ORIGIN
MESSAGE
OUTPUT
MODULATOR
SIGNAL
PROCESSOR
CARRIER
SOURCE
DETECTOR
INPUT CHANNEL
COUPLER
INFORMATION
CHANNEL
OUTPUT CHANNEL
COUPLER
Advantages of communication with
optic fiber
• Bandwidth - Fiber optic cables have a much greater
bandwidth than metal cables.
The amount of information that can be transmitted per
unit time of fiber over other transmission media is its
most significant advantage.
• Low Power Loss - An optical fiber offers low power
loss. This allows for longer transmission distances. In
comparison to copper; in a network, the longest
recommended copper distance is 100m while with
fiber, it is 2000m.
• Size - In comparison to copper, a fiber optic cable has
nearly 4.5 times as much capacity as the wire cable has
and a cross sectional area that is 30 times less.
Advantages of communication with
optic fiber
• Security - Optical fibers are difficult to tap. As
they do not radiate electromagnetic energy,
emissions cannot be intercepted. As physically
tapping the fiber takes great skill to do
undetected, fiber is the most secure medium
available for carrying sensitive data.
• Safety - Since the fiber is a dielectric, it does not
present a spark hazard.
• Cost - The raw materials for glass are plentiful,
unlike copper. This means glass can be made
more cheaply than copper.
Advantages of communication with
optic fiber
• Flexibility - It is flexible, bends easily and resists
most corrosive elements that attack copper
cable.
• Immunity to electromagnetic interference - Fiber
optic cables are immune to electromagnetic
interference.
• Temperature resistant
• Easy to maintain
• Electrical isolation – raw materials are insulators.
• Lack of cross talk
Disadvantages
• Cost - Cables are expensive to install but last longer
than copper cables.
• Transmission - transmission on optical fiber requires
repeating at distance intervals.
• Fragile - Fibers can be broken or have transmission
loses when wrapped around curves of only a few
centimeters radius.
However by encasing fibers in a plastic sheath, it is
difficult to bend the cable into a small enough radius to
break the fiber.
• Protection - Optical fibers require more protection
around the cable compared to copper.
OPTIC FIBER SENSORS
• Optic fiber sensors are very sensitive devices
used to measure physical quantities like pressure,
displacement, liquid level etc.
• Sensor have 3 components,
SORCE OF LIGHT
OPTIC FIBER COIL
DETECTOR
• 2types of sensors, active & passive.
• Active sensors: modulation takes place inside.
• Passive sensors: modulation takes place outside.
Intensity modulated sensor
• Pressure sensor
an optic fiber mounted
between two pair of plates
containing parallel groove.
• In the absence of pressure
fiber will be straight,
transmitted light producing
maximum intensity.
• When the pressure is
applied a large number of
micro bending are form,
transmitted intensity is
minimum.
Liquid level sensor
• Light beam enter through the inlet fiber, it
undergo total internal reflection and reaches
the detector through outlet fiber.
• If the prism is below the liquid level?
Phase modulated sensor
• Gyroscope
• A gyroscope is a device used primarily for
navigation and measurement of angular velocity.
• their functionality depends only on the constancy
of the speed of light.
• If the source stays stationary, then both beams of
light require an equal amount of time to traverse
the circle and arrive back at the source.
• However, if the source is rotating along the circle,
then it takes more time for the beam in front of
the source to complete its path.
gyroscope
gyroscope
•
•
•
•
•
Clock wise ray path = (2πr- rθ)
Anti clock wise ray = (2πr+ rθ)
Then the corresponding path diff. = 2rθ
Then, phase diff. =
Then rate of rotation = θ/t
LASER
• A laser is a device that
emits light through a process
of optical amplification based
on the stimulated
emission of electromagnetic
radiation.
• The term “LASER" originated
as an acronym for "light
amplification by stimulated
emission of radiation“
• A laser differs from other
sources of light in that it
emits light coherently.
STIMULATED EMISSION
• Atoms and molecules are normally present in the so-called
ground state. This is a stable condition - atoms in the ground
state cannot release energy. Atoms can exist in other states in
which there is more energy present than in the ground state a supply of energy is needed to raise the atoms to these
higher states.
• The atoms can return to lower energy levels from this
‘excited’ state by releasing energy.
• The surplus energy can be passed on to another particle a
photon which is then emitted
• the energy of the photon is equivalent to the difference in
energy between the higher and lower states.
• This de-excitation can be spontaneous or stimulated by other
photons.
• Stimulated emission is a basic requirement for lasing.
Population Inversion
• To achieve lasing action, the number of
stimulated emission >>> no of spontaneous
emission.
• ie., the no. of atoms in the higher energy state
to be made larger than that of the no of atoms
in the lower energy level. This condition is
called population inversion.
• ie., N2 > N1
COMPONENTS OF LASER
• Active medium (Lasing medium)
• Energy source ( Pumping)
• Optical Activity (Optical Resonator)
ACTIVE MEDIUM
• Laser light is generated in the active medium of the
laser.
• Active laser media are available in all aggregate
states:
• solid (crystalline or amorphous)
crystals (ytterbium, or erbium)
transition metal ions (titanium)
• liquid (dye solutions)
• Gaseous (CO2, He-Ne)
• plasma (A plasma is a gas that has been energized to the point that some of the
electrons break free from, but travel with, their nucleus.)
ENERGY PUMPING
• Laser pumping is the energy transfer from an
external source into the active medium.
• Optical Pumping:
Optical pumping is a process in which light is used
to raise electrons from a lower energy level in
an atom to a higher one.
• pumping is done in active laser medium so as to
achieve population inversion.
• Chemical Pumping:
Chemical reaction is used as a power source
in chemical lasers .
• Electron Beam Pumping
ENERGY PUMPING
• Electrical Pumping :
Electric glow discharge is common in gas lasers. For
example, in the helium–neon laser the electrons from
the discharge collide with the helium atoms, exciting
them. The excited helium atoms then collide
with neon atoms, transferring energy. This allows an
inverse population of neon atoms to build up.
• Nuclear Pumping:
Nuclear fission is used in exotic nuclear pumped
lasers (NPL), directly employing the energy of the fast
neutrons released in a nuclear reactor
Optical Activity
• An optical cavity, resonating
cavity or optical resonator is
an arrangement
of mirrors that forms
a standing wave cavity
resonator for light waves.
• Optical cavities are a major
component of lasers,
surrounding the gain
medium and
providing feedback of the
laser light.
• They are also used in optical
parametric oscillators and
some interferometers.
Optical Activity
• Light confined in the cavity
reflects multiple times
producing standing waves for
certain resonance frequencies
• A part of a laser, consisting of two
mirrors,
one highly reflective and one
partly reflective, placed on either s
ide of a laser pump. Amplified light
bounces back and forth between t
he mirrors, enhancing stimulated
emission within the pump, eventua
lly being emitted through the partl
y reflective mirror.
RUBY LASER
RUBY LASER
• Solid state LASER built by
T H Mainman.
• 3 level laser
• Operates in
discontinuous wave
mode.
Components of Ruby laser
• Active medium:
Ruby crystal,
Al2O3+0.05% Chromium atom
• Optical resonator:
a cross-sectional face are put
parallel and polished with a
highly reflecting material.
One face 100% reflecting
mirror and others reflectivity
is 94-98%
• Pump:
Xenon flash tube, wounded
helically over the ruby rod.
White light from the tube
serves as pumping.