PRESENTING

CHARACTERSTICS OF LASER LIGHT

Presented by-

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INTRODUCTION TO LASER
LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.

CONSTRUCTION OF LASER
The three main components of any laser device are the active medium,
the pumping source, and the optical resonator.
Active Medium
Major determining factor of the wavelength of operation and other properties of laser. Hundreds of
different gain media in which laser operation has been achieved. The gain medium could be solid crystals such
as ruby or Nd:YAG, liquid dyes, gases like CO2 or Helium-Neon, and semiconductors such as GaAs.
Pumping Mechanism
The pump source is the part that provides energy to produce a population inversion. Pump
sources include electrical discharges, flash lamps, light from another laser, chemical reactions. The type of
pump source used principally depends on the gain medium.
Optical Resonator
Its simplest form is two parallel mirrors placed around the gain medium. Light from the
medium produced by the spontaneous emission is reflected by the mirrors back into the medium where it
may be amplified by stimulated emission. One of the mirrors reflects essentially 100% of the laser light
while the other reflects less than 100% of the laser light and transmits the remainder.
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 BASIC PRINCIPLE
Whenever electromagnetic radiation interacts with matter the most common process is
Absorption or Stimulated Absorption , in which an atom makes a transition from lower
state to upper state by absorbing a photon from radiation. The rate of stimulated
absorption depends upon both the energy density of radiation and the number of atoms in
lower state.
Г12 = -B12ρ(υ)N1
N2,E2
B12 Einstein’s co-efficient for stimulated
absorption.
N1,E1 ρ(υ) energy density of radiation within
frequency range υ and υ +dυ.
Г12 Rate of change of population of lower level
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 When the atom is in the excited state, it can make a transition to a lower energy state
through the emission of electromagnetic radiation, the emission process can occur in
two different ways;
1. The first is referred to as Spontaneous Emission in which an atom in the excited
state emits radiation even in the absence of any incident radiation. The rate of
spontaneous emissions is proportional to the number of atoms in the excited state.

Г21 = -A21N2
N2,E2.

A21 Einstein’s co-efficient for spontaneous

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 The second is referred to as Stimulated Emission, in which an incident signal of appropriate frequency
triggers an atom in an excited state to emit radiation. The rate of stimulated emission depends both on
the intensity of the em radiation and also on the number of atoms in the upper state. There is a
fundamental difference between the spontaneous and stimulated emission processes. In the case of
spontaneous emission, the atoms emits an em wave that has no definite phase relation with that
emitted by another atom. Furthermore, the wave can be emitted in any direction. In the case of
stimulated emission, since the process is forced by the incident em wave, the emission of any atom
adds in phase to that of the incoming wave and along the same direction

N2 ,E2 Г21 = -B21 ρ(υ) N2

B21 Einstein’s co-efficient of stimulated emission

N1 ,E1

The stimulated emission of light is the crucial quantum process necessary for the operation of a laser.
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 The lasing threshold is the lowest excitation level at which a laser’s output is dominated
by stimulated emission rather than by spontaneous emission.
The lasing threshold is reached when the optical gain of the laser medium is exactly balanced by
the sum of all the losses experienced by light in one round trip of the laser's optical cavity.
gain >loss
The achievement of a significant population inversion in atomic or molecular energy states is a
precondition for laser action
N2 > N1

To achieve population inversion condition rate of stimulated absorption should exceed the rate of
stimulated emission. And for lasing action to occur the rate of stimulated emission should exceed
the rate of spontaneous emission.

Metastable state is important to achieve population inversion having life time of about 10 -3 sec to
10-6 sec giving enough time span to stimulated emission to dominate over spontaneous emission.
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CHARACTERSTICS OF LASER LIGHT
Light from the laser arises primarily from stimulated emission and the resonator cavity
within which the amplifying medium is kept leads to the following special properties:

1. Directionality
2. Focus ability
3. Brightness
4. Coherence
5. Monochromatic
6. Ultra Short Pulse Generator
7. Non Classical Light Generator.

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 DIRECTIONALITY
The light from a typical laser emerges in an extremely thin beam with very little divergence.
Another way of saying this is that the beam is highly directional. This is a direct consequence of
the fact that laser beam comes from the resonant cavity, and only waves propagating along the
optical axis can be sustained in the cavity. The cavity of the laser has very nearly parallel front
and back mirrors which constrain the final laser beam to a path which is perpendicular to those
mirrors. The back mirror is made almost perfectly reflecting while the front mirror is about 99%
reflecting, letting out about 1% of the beam. This 1% is the output beam which we see. If the
photon is slightest bit off axis, it will be lost from the beam .
.

High Partial
reflectivity reflectivity

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 The wave nature of light imparts an intrinsic divergence to the beam due to the
phenomenon of diffraction the divergence of the laser beam is limited by diffraction.
The directionality is described by the light beam divergence angle. For perfect
spatial coherent light, a beam of aperture diameter D will have unavoidable
divergence because of diffraction. From diffraction theory, the divergence
angle θd is:

θd= βλ/D

Where λ and D are the wavelength and the diameter of the beam respectively, β is a
coefficient whose value is around unity and depends on the type of light amplitude
distribution and the definition of beam diameter. θd is called diffraction limited
divergence.

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EXAMPLE:
If an ordinary light travels a distance of 2 km, it
spreads to about 2 km in diameter.
On the other hand, if a laser light travels a
distance of 2 km, it spreads to a diameter less than
2 cm.
For laser light,
λ=1.06×10-3 mm,
D=3 mm,
β =1.1, then
θd=βλ/D
=1.1×1.06×10_3/3
=0.3887×10-3 radians
=0.022269° .
Compare this value with a normal flashlight, the
divergence is about 25° , a searchlight has a
divergence angle of 10°, the high directionality of
laser light is obvious.
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FOCUSSABILITY
Because of highly directional properties of laser beams, they can be focused to very small
areas of a few (μm)2. The limits to focusing are again determined by diffraction effects.
Smaller the wavelength, smaller the size of the focused spot. This property leads to
applications in surgery, material processing, compact discs, etc
If a truncated plane wave (of diameter 2a) is incident on a lens without any aberration of
focal length f, then the wave emerging from the lens will get focused to spot of radius ≈ λ f/a

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BRIGHTNESS
Lasers can generate extremely high powers, and since they can also be focused to
very small areas, it is possible to generate extremely high-intensity values. At
intensities such as 1021 W/m2, the electric fields are so high that electrons can get
accelerated to relativistic velocities. Apart from scientific investigations of extreme
conditions, continuous wave lasers having power levels ~ 105 W and pulsed lasers
having a total energy ~ 50,000 J have applications in welding, cutting, etc. A laser
beam of even moderate power (e.g. a few milliwatts) has a brightness that is several
orders of magnitude greater than that of the brightest conventional sources. This is
mainly due to the highly directional properties of the laser beam this means that the
peak intensity produced in the focal plane of a lens can be several order of
magnitude larger for a laser beam compared to that of a conventional source.

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EXAMPLE:
Intensity = power per unit area
i.e. I = P/A = E/(T×A)
= 103/[10-14 × (π/4) × (10-5)2]
where, E = 103 j
A = (π/4) × (10-5)2 m2
T = 10-14 s
I ≈ 1027 W/ m2 s
Brightness can be calculated by intensity per unit solid angle.

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COHERENCE
Coherence is one of the unique properties of laser light. Different parts of the laser beam are
related to each other in phase. These phase relationships are maintained over long enough
time so that interference effects may be seen or recorded photographically. It arises from the
stimulated emission process which provides the amplification. Since a common stimulus
triggers the emission events which provide the amplified light, the emitted photons are "in
step" and have a definite phase relation to each other. This coherence is described in terms
of temporal coherence and spatial coherence.
To understand spatial and temporal coherence we can find out the correlation of optical
fields at two space –time points i.e. let us think about a wave at two different points (r1, t1)
and (r2, t2) then ,
Spatial separation = | r1 - r2 |
Dimensionless spatial separation = | r1 - r2 | / λ ; λ is the wavelength of the radiation
Temporal coherence = | t1 - t2 |
Dimensionless temporal coherence = | t1 - t2 | / T ; T is the time period.
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In this case for temporal coherence to occur the following condition must be followed

and similarly for spatial coherency,

Coherence length of a typical thermal source comes out to be 300 cm. While that of a typical
laser light sources in labs comes out to be 300 m.

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 MONOCHROMATIC
Laser light consists of essentially one wavelength, having its origin in stimulated
emission from one set of atomic energy levels. The light from a laser typically comes
from one atomic transition with a single precise wavelength. So the laser light has a
single spectral color and is almost the purest monochromatic light available. We can
say that this property is due to the following two circumstances: (1) Only an em
wave of frequency υ can be amplified.
(2) Since the two-mirror arrangement forms a resonant cavity, oscillation can occur
only at the resonance frequencies of this cavity. The latter circumstance leads to the
laser line width being much narrower (by as much as to ten orders of magnitude!)
than the usual line width observed in spontaneous emission.
 λ/Δλ is the quantity which can tell us about the monochromaticity of the light source
also known as the line width.

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EXAMPLE:
For sodium light source ;
λ/Δλ = 6000 / 6
= 1000
= 103
For laser light source;
λ/Δλ = τcoh/T
= 102/[0.5×10-6/3×108 ]
≈ 1017
Which clearly explains high monochromaticity of lasers in comparison to a typical
thermal source.

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TYPES OF LASER

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ULTRA SHORT PULSE GENERATOR
Property of short time duration implies energy concentration in time. Lasers can produce ultra
short pulses using the technique of mode locking. The duration of the pulses will be roughly
equal to the inverse of line width of the stimulated transition frequency.
In case of gas lasers pulses of the order of 0.1~ 1 ns can be produced. While in a solid state
laser it can be of the order of femtosec. Using which femtosec laser are made.

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NON CLASSICAL LIGHT GENERATOR
Semiconductor diode laser is a non classical light generator.
It follows Sub - Poissonion distribution, which says a photon number distribution for
which the variance is less than the mean. i.e.
⟨(Δn)2⟩ < ⟨n⟩
And shows anti-bunching of photons. We can express anti- bunching by Mandel Q
parameter defined as,

For non classical light or anti bunching -1≤ Q ≤ 0.

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THANK YOU

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