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*
An Introduction
The first step on the road to laser was the publication of paper by Albert
Einstein in 1916 –describing how atoms could interact with light.
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* What makes Laser light
different
* The unique properties of Laser
light
1.
Monochromaticity
2.
Directionality/ Collimation
3.
Coherence
*Monochromaticity
* Directionality
Radiation comes out of the laser in a certain direction, and
spreads at a defined divergence angle ()
This angular spreading of a laser beam is very small compared to
other sources of electromagnetic radiation, and described by a
small divergence angle (of the order of milli-radians)
*Coherence
Common light source
Light
beams of random phase
varying with time and position
Laser source
Light
of in step waves of
identical frequency, phase and
polarization
*
*
*
*
COMPONENTS OF LASER
1.Gain Medium(Something to amplify the light)
2.Pump source (something to create population inversion)
3.Resonator(something to provide suitable optical feedback)
* Gain Medium
*The
gain medium uses the pump energy to amplify or
increase gain of laser output.
*The
gain media should have the ability to absorb the
pump energy and store it in the form of excited
electrons and have at least one energy transition
capable of producing the laser emission at the desired
frequency- Major determining factor of wavelength of
the operation of laser.
*The
gain medium can be solid, liquid, gas,
semiconductor , free electrons , plasma etc. the gain
medium is material with properties that allow it to
amplify the light by stimulated emission.
*Pump Source
Provides energy to the laser system – a source of
energy to excite electrons in the gain medium to
high energy states.
Examples :electrical discharges ,flash lamps , arc
lamps and chemical reactions.
•The
type of pump source used depends on the
gain medium.
A helium –neon (He –Ne) laser uses an electrical
discharge in the helium –neon gas mixture.
*Optical Resonator
*Provides
an optical path which allows emitted photons
to oscillate and interfere constructively as energy is
added or pumped into the system .
*Two parallel mirrors placed around the gain medium.
*Light is reflected by the mirrors back into the medium
and is amplified .
*One
end of the resonator is highly reflecting mirror or
other reflecting devices to feed photons back into the
gain medium and the other end has a partially
reflecting mirror (i.e. output coupler ) to allow laser
radiation to come out.
*Laser Action
* If the gain of the medium is capable of storing energy in the form of
excited electrons long time before decaying back to the ground
state, and increase in proportion of electrons will assume a high
energy state as the medium is pumped , until a population
inversion occurs .
means more atoms in the upper state than the
ground state
Stimulated emission is the essence of Laser action
A population inversion must be
sustained for the laser action to
occur
The most common methods of pumping
commonly employed in laser systems are
a). optical pumping
b). Electrical pumping (Direct electron
excitation)
c). Collisional pumping (In elastic atom- atom
collisions)
d). Direct conversion (electrical pumping)
e). Chemical pumping ( chemical reactions)
* Energy Transitions leading to Laser action
*The simplest is the case of
a two level system .
*The
two level systems are impractical because they
strongly absorb the emitted photons and hence it is very
difficult to maintain population inversion to operate in a
continuous fashion.
*Most
of the laser operate as three level or four level
systems
*
Three-level Laser System
* Initially excited to a
short-lived highenergy state .
* Then quickly decay
to the intermediate
metastable level.
* Population inversion
is created between
lower ground state
and a higher-energy
metastable state.
*

Laser transition takes
place between the
third and second
excited states.

Rapid depopulation
of the lower laser
level.
*
*
*
*
* Semiconductor Diode Laser
* In
terms of numbers, the diode laser is the most common laser
today. The 2 common families of diode lasers are composed of:
* Ga AIAs (Gallium/Aluminum/Arsenide) with a wavelength output in
the 750 to 950 nanometers (used in CD &CD/ROM players),
* InGaAsP (Indium/Gallium/Arsenide/Phosphide) with a wavelength output in the
1100 – 1650 nm range (used in optical telecommunications).
* Another
family of diode lasers like AlGaInP (Aluminum/Gallium/Arsenide/Phosphide)
operates in the visible part of the spectrum, primarily red.
Semiconductor lasers are actually solid-state lasers, too, but because semiconductor lasers have a
different mode of laser operation, they have a different name.
HOLOGRAPHY
Two step process
2
1
Recording
Reconstruction
Recording Process
A laser beam with sufficient
coherence length is split into two
parts by a beam splitter.
One part of the wave illuminates the
object, scattered and reflected to
the recording medium.
The other one acting as the
reference wave illuminates the light
sensitive medium directly.
Both waves interfere. The resulting interference
pattern is recorded and chemically developed.
The developed photographic plate is
illuminated by the reference wave
Reconstruction
Process
Two images are seen ,one is due to
reflected light and other is due to
transmitted light
Image formed due to transmitted light
is real and some what distorted and
due to reflected light is virtual
Photographic plate
Reference beam
Transmitted beam
Virtual object
Real object
Thank
You