Laser and its applications

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Laser and its applications
By

• Prof. Dr. Taha Zaki Sokker

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Laser and its applications
Contents
page

• Chapter (1) Theory of Lasing
  (2) Chapter (2) Characteristics of
  laser beam ( )Chapter (3) Types of laser
  sources ( ) Chapter (4) Laser
  applications ( )

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Chapter (1) Theory of Lasing

• 1.Introduction (Brief history of laser)
• The laser is perhaps the most important
  optical device to be developed in the past 50
  years. Since its arrival in the 1960s, rather
  quiet and unheralded outside the scientific
  community, it has provided the stimulus to make
  optics one of the most rapidly growing fields in
  science and technology today.

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• The laser is essentially an optical
  amplifier. The word laser is an acronym that
  stands for light amplification by the stimulated
  emission of radiation. The theoretical
  background of laser action as the basis for an
  optical amplifier was made possible by Albert
  Einstein, as early as 1917, when he first
  predicted the existence of a new irradiative
  process called stimulated emission. His
  theoretical work, however, remained largely
  unexploited until 1954, when C.H. Townes and
  Co-workers developed a microwave amplifier based
  on stimulated emission radiation. It was called a
  maser.

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In 1960, T.H.Maiman built the first laser
device (ruby laser). Within months of the arrival
of Maimans ruby laser, which emitted deep red
light at a wavelength of 694.3 nm, A. Javan and
associates developed the first gas laser (He-Ne
laser), which emitted light in both the infrared
(at 1.15mm) and visible (at 632.8 nm) spectral
regions..

• Following the birth of the ruby and
  He-Ne lasers, others devices followed in rapid
  succession, each with a different laser medium
  and a different wavelength emission. For the
  greater part of the 1960s, the laser was viewed
  by the world of industry and technology as
  scientific curiosity.

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1.Einsteins quantum theory of radiation

• In 1916, according to Einstein, the
  interaction of radiation with matter could be
  explained in terms of three basic processes
  spontaneous emission, absorption and stimulated
  emission. The three processes are illustrated and
  discussed in the following

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Before
After

(i) Stimulated absorption
ii) Spontaneous emission )
(iii) Stimulated emission
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)ii) Spontaneous emission

• Consider an atom (or molecule) of the
  material is existed initially in an excited state
  E2 No external radiation is required to initiate
  the emission. Since E2gtE1, the atom will tend to
  spontaneously decay to the ground state E1, a
  photon of energy h? E2-E1 is released in a
  random direction as shown in (Fig. 1-ii). This
  process is called spontaneous emission
• Note that when the release energy
  difference (E2-E1) is delivered in the form of an
  e.m wave, the process called "radiative emission"
  which is one of the two possible ways
  non-radiative decay is occurred when the energy
  difference (E2-E1) is delivered in some form
  other than e.m radiation (e.g. it may transfer to
  kinetic energy of the surrounding)

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(iii) Stimulated emission

• Quite by contrast stimulated emission
  (Fig. 1-iii) requires the presence of external
  radiation when an incident photon of energy h?
  E2-E1 passes by an atom in an excited state E2,
  it stimulates the atom to drop or decay to the
  lower state E1. In this process, the atom
  releases a photon of the same energy, direction,
  phase and polarization as that of the photon
  passing by, the net effect is two identical
  photons (2h?) in the place of one, or an increase
  in the intensity of the incident beam. It is
  precisely this processes of stimulated emission
  that makes possible the amplification of light in
  lasers.

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Growth of Laser Beam
The theory of lasing

• Atoms exist most of the time in one of a
  number of certain characteristic energy levels.
  The energy level or energy state of an atom is a
  result of the energy level of the individual
  electrons of that particular atom. In any group
  of atoms, thermal motion or agitation causes a
  constant motion of the atoms between low and high
  energy levels. In the absence of any applied
  electromagnetic radiation the distribution of the
  atoms in their various allowed states is governed
  by Boltzmans law which states that

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• if an assemblage of atoms is in state of
  thermal equilibrium at an absolute temp. T, the
  number of atoms N2 in one energy level E2 is
  related to the number N1 in another energy level
  E1 by the equation.

Where E2gtE1 clearly N2ltN1 K Boltzmanns
constant 1.38x10-16 erg / degree
1.38x10-23 j/K T
the absolute temp. in degrees Kelvin
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• At absolute zero all atoms will be in the
  ground state. There is such a lack of thermal
  motion among the electrons that there are no
  atoms in higher energy levels. As the temperature
  increases atoms change randomly from low to the
  height energy states and back again. The atoms
  are raised to high energy states by chance
  electron collision and they return to the low
  energy state by their natural tendency to seek
  the lowest energy level. When they return to the
  lower energy state electromagnetic radiation is
  emitted. This is spontaneous emission of
  radiation and because of its random nature, it is
  incoherent

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• As indicated by the equation, the number
  of atoms decreases as the energy level increases.
  As the temp increases, more atoms will attain
  higher energy levels. However, the lower energy
  levels will be still more populated.
• Einstein in 1917 first introduced the
  concept of stimulated or induced emission of
  radiation by atomic systems. He showed that in
  order to describe completely the interaction of
  matter and radiative, it is necessary to include
  that process in which an excited atom may be
  induced by the presence of radiation emit a
  photon and decay to lower energy state.

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• An atom in level E2 can decay to level
  E1 by emission of photon. Let us call A21 the
  transition probability per unit time for
  spontaneous emission from level E2 to level E1.
  Then the number of spontaneous decays per second
  is N2A21, i.e. the number of spontaneous decays
  per secondN2A21.
• In addition to these spontaneous
  transitions, there will induced or stimulated
  transitions. The total rate to these induced
  transitions between level 2 and level 1 is
  proportional to the density (U?) of radiation of
  frequency ?, where
• ? ( E2-E1 )/h , h
  Planck's const.

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• Let B21 and B12 denote the proportionality
  constants for stimulated emission and absorption.
  Then number of stimulated downward transition in
  stimulated emission per second N2 B21 U?
• similarly , the number of stimulated upward
  transitions per second N1 B12 U?
• The proportionality constants A and B are
  known as the Einstein A and B coefficients. Under
  equilibrium conditions we have

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SP ST
N2 A21 N2 B21 U? N1 B12 U?
A b
by solving for U? (density of the radiation) we
obtain U? N1 B12- N2 B21 A21 N2
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)1)
According to Plancks formula of radiation
)2)
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• from equations 1 and 2 we have
• B12B21
  (3)

)4(
equation 3 and 4 are Einsteins relations. Thus
for atoms in equilibrium with thermal radiation.
from equation 2 and 4
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(5)
Accordingly, the rate of induced emission is
extremely small in the visible region of the
spectrum with ordinary optical sources ( T?10 3
?K .(
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• Hence in such sources, most of the
  radiation is emitted through spontaneous
  transitions. Since these transitions occur in a
  random manner, ordinary sources of visible
  radiation are incoherent.
• On the other hand, in a laser the induced
  transitions become completely dominant. One
  result is that the emitted radiation is highly
  coherent. Another is that the spectral intensity
  at the operating frequency of the laser is much
  greater than the spectral intensities of ordinary
  light sources.

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Amplification in a Medium

• Consider an
  optical medium through which radiation is
  passing. Suppose that the medium contains atoms
  in various energy levels E1, E2, E3,.let us fitt
  our attention to two levels E1 E2 where E2gtE1 we
  have already seen that the rate of stimulated
  emission and absorption involving these two
  levels are proportional to N2B21N1B12
  respectively. Since B21B12, the rate of
  stimulated downward transitions will exceed that
  of the upward transitions when N2gtN1,.i.e the
  population of the upper state is greater than
  that of the lower state such a condition is
  condrary to the thermal equilibrium distribution
  given by Boltzmanns low. It is termed a
  population inversion. If a population inversion
  exist, then a light beam will increase in
  intensity i.e. it will be amplified as it passes
  through the medium. This is because the gain due
  to the induced emission exceeds the loss due to
  absorption.

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gives the rate of growth of the beam intensity in
the direction of propagation, an is the gain
constant at frequency ?
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Quantitative Amplification of light

• In order to determine quantitatively the
  amount of amplification in a medium we consider a
  parallel beam of light that propagate through a
  medium enjoying population inversion. For a
  collimated beam, the spectral energy density U?
  is related to the intensity ?? in the frequency
  interval ? to ? ?? by the formula.

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Due to the Doppler effect and other
line-broadening effects not all the atoms in a
given energy level are effective for emission or
absorption in a specified frequency interval.
Only a certain number ?N1 of the N1 atoms at
level 1 are available for absorption. Similarly
of the N2 atoms in level 2, the number ? N2 are
available for emission. Consequently, the rate of
upward transitions is given by
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and the rate of stimulated or induced downward
transitions is given by
Now each upward transition subtracts a quantum
energy h? from the beam. Similarly, each downward
transition adds the same amount therefore the net
time rate of change of the spectral energy
density in the interval ?? is given by
where (h? B? NU) the rate of transition of
quantum energy
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• In time dt the wave travels a distance dx c dt
  i.e

then
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• in which ?? is the gain constant at frequency
  ? it is given by

an approximate expression is
?? being the line width
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Doppler width

• This is one of the few causes seriously
  affecting equally both emission and absorption
  lines. Let all the atoms emit light of the same
  wavelength. The effective wavelength observed
  from those moving towards an observer is
  diminished and for those atoms moving away it is
  increased in accordance with Dopplers principle.
• When we have a moving source sending
  out waves continuously it moves. The velocity of
  the waves is often not changed but the wavelength
  and frequency as noted by stationary observed
  alter.

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• Thus consider a source of waves moving
  towards an observer with velocity v. Then since
  the source is moving the waves which are between
  the source and the observer will be crowded into
  a smaller distance than if the source had been at
  rest. If the frequency is ?o , then in time t the
  source emit ?ot waves. If the frequency had been
  at rest these waves would have occupied a length
  AB. But due to its motion the source has caused a
  distance vt, hence these ?ot waves are compressed
  into a length where

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thus
Observer
where nlc
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(No Transcript)
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Evaluation of Doppler half width

• According to Maxwelliam distribution of
  velocities, from
• the kinetic theory of gasses, the probability
  that the velocity will be between v and v?v is
  given by

So that the fraction of atoms whose their
velocities lie between v and v ?v is given by
the following equation
where B m molecular weight,
Kgas constant, Tabsolute temp
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• Substituting for v in the last equation from
  equation (1) and since the intensity emitted will
  depend on the number of atoms having the velocity
  in the region v and

then, i. e.
I(n
I(n) const .
) I
n n?
at
const
I) n) I max const
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• There for

I)n)I max
being the half width of the spectral line it is
the width at
, then
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Calculation of Doppler width

• 1- Calculate the Dopplers width for Hg198 .
  where
• K1.38x10-16 erg per degree at temp300k and
  5460Ao
• solution

molecular weight m const. ( atomic mass m\ )
const.1.668x10-24 gm

wave number

.015 cm-1
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• 2- Calculate the half-maximum line width (Doppler
  width) for He-Ne laser transition assuming a
  discharge temperature of about 400K and a neon
  atomic mass of 20 and wavelength of 632.8nm.
  
• (Ans., n1500MHz)