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Introduction
LASER stands for light Amplification by Stimulated Emission of Radiation. The
theoretical basis for the development of laser was provided by Albert Einstein in 1917. In
1960, the first laser device was developed by T.H. Mainmann.
1. Definitions
Stimulated absorption (or) Absorption
Let E 1 and E 2 be the energies of ground and excited states of an atom. Suppose, if a
photon of energy E (^) 1 − E 2 = h υ interacts with an atom present in the ground state, the atom
gets excitation form ground state E 1 to excited state E 2. This process is called stimulated
absorption
Stimulated absorption rate depends upon the number of atoms available in the lowest energy state as well as the energy density photons. Stimulated absorption rate ᔃ number of atoms in the ground state ᔃ Density of photons
Spontaneous emission
Spontaneous emission was postulated by Bohr. Let E 1 and E (^) 2 be the energies of
ground and excited states of an atom. Suppose, if a photon of energy E (^) 1 − E 2 = h υ
interacts with an atom present in the ground state, the atom gets excitation form ground state E 1 to excited state E 2. The excited atom does not stay in a long time in the excited
state. The excited atom gets de-excitation after its life time by emitting a photon of energy E 1 (^) − E 2 = h υ. This process is called spontaneous emission.
The spontaneous emission rate depends up on the number of atoms present in the excited state. Spontaneous emission rate ᔃ number of atoms in the excited state
Dr. P. Venkata Ramana, AUCE(A)
Stimulated emission
Stimulated emission was postulated by Einstein. Let (^) E 1 and (^) E 2 be the energies of
ground and excited states of an atom. Suppose, if a photon of energy E (^) 1 − E 2 = h υ
interacts with an atom present in the ground state, the atom gets excitation form ground state E 1 to excited state E 2. Let, a photon of energy E (^) 1 − E 2 = h υinteracts with the excited
atom with in their life time; the atom gets de-excitation to ground state by emitting of another photon. These photons have same phase and it follows coherence. This phenomenon is called stimulated emission.
Stimulated emission rate depends upon the number of atoms available in the excited state as well as the energy density of photons. Stimulated emission rate ᔃ number of atoms in the excited state ᔃ Density of photons
Spontaneous and Stimulated emission
Spontaneous emission Stimulated emission
- The spontaneous emission was postulated by Bohr 1. The stimulated emission was postulated by Einstein
- Additional photons are not required in spontaneous emission 2. Additional photons are required in stimulated emission
- One photon is emitted in spontaneous emission 3. Two photons are emitted in stimulated emission
- The emitted radiation is poly- monochromatic 4. The emitted radiation is monochromatic
- The emitted radiation is Incoherent 5. The emitted radiation is Coherent
- The emitted radiation is less intense 6. The emitted radiation is high intense
- The emitted radiation have less directionality Example: light from sodium or mercury lamp 7. The emitted radiation have high directionality Example: light from laser source.
2. Characteristic of laser radiation
The laser light exhibits some peculiar properties than compare with the convectional light. Those are
- Highly directionality
- Highly monochromatic
- Highly intense
- Highly coherence 1. Highly directionality
Dr. P. Venkata Ramana
Dr. P.Sreenivasula Reddy (^) M.Sc, PhD Website: engineeringphysics.weebly.com Page 4
P 1
P 2
There are two types of coherence i) Temporal coherence ii) Spatial coherence.
Temporal coherence (or longitudinal coherence):-
The predictable correlation of amplitude and phase at one point on the wave train w .r. t another point on the same wave train, then the wave is said to be temporal coherence
To understand this, let us consider two points P 1 and P 2 on the same wave train,
which is continuous as in shown in figure (1). Suppose the phase and amplitude at any one point is known, then we can easily calculate the amplitude and phase for any other point on the same wave train by using the wave equation
ᡷ 㐄 ᡓ sin 䙸
Where ‘a’ is the amplitude of the wave and ‘x’ is the displacement of the wave at any instant of time‘t’.
Spatial coherence (or transverse coherence)
The predictable correlation of amplitude and phase at one point on the wave train w. r .t another point on a second wave, then the waves are said to be spatial coherence (or transverse coherence)
Two waves are said to be coherent, the waves must have
3. Population inversion
Definition
The number of atoms present in the excited (or higher) state is greater than the
number of atoms present in the ground state (or lower) state is called population inversion.
Or
The population present in the excited (or higher) state is greater than the population
present in the ground state (or lower) state is called population inversion.
Let us consider two level energy system of energies ᠱ⡩ and ᠱ⡰ as shown in figure.
Let ᡀ⡩ and ᡀ⡩ be the populations (means number of atoms per unit volume) of energy
levels ᠱ⡩ and ᠱ⡰.
According to Boltzmann’s distribution the population of an energy level E, at temperature T
is given by
ᡀ〶 㐄 ᡀ⡨ᡗ䙲
⡹ 〆㊄ 〒〡 䙳
Where ᡀ⡨ is the population of the lower level or ground state and k is the Boltzmann’s
constant. From the above relation, the population of energy levels ᠱ⡩ and ᠱ⡰ are
⡹〆ㄗ 〒〡 䙳
ᡀ⡰ 㐄 ᡀ⡨ᡗ䙲
⡹〆ㄘ 〒〡 䙳
At ordinary conditions ᡀ⡩ 㐈 ᡀ⡰, i.e., the population in the ground or lower state is always
greater than the population in the excited or higher states. The stage of making,
population of higher energy level is greater than the population of lower energy level is called population inversion i.e., ᡀ⡰ 㐈 ᡀ⡩.
Consider a three energy level system with energies ᠱ⡩, ᠱ⡰ ᡓᡦᡖ ᠱ⡱ of populations ᡀ⡩, ᡀ⡰, and
ᡀ⡱.At normal conditions, ᠱ⡩ 㐇 ᠱ⡰ 㐇 ᠱ⡱ and ᡀ⡩ 㐇 ᡀ⡰ 㐇 ᡀ⡱. In the ground state ᠱ⡩ the life
time of atom is more and the life time of atom in the excited state ᠱ⡱ is 10 ⡹⡶^ ᡱᡗᡕᡕᡧᡦᡖᡱ. But
in the intermediate state ᠱ⡰ the atom has more life time䙦10⡹⡱^ ᡱᡗᡕᡕᡧᡦᡖᡱ䙧. So it is called
metastable state.
When a suitable energy is supplied to the system, atoms get excited into ᠱ⡱. After their
lifetime the atoms are transit to ᠱ⡰. Due to more lifetime of an atom in state ᠱ⡰ , the atoms
stay for longer time than compare with the state ᠱ⡱. Due to the accumulation of atoms
in ᠱ⡰, the population inversion is established in between the ᠱ⡰ and ᠱ⡩ states.
Dr. P. Venkata Ramana
Dr. P.Sreenivasula Reddy (^) M.Sc, PhD Website: engineeringphysics.weebly.com Page 7
䙦やㄘ⡹やㄗ䙧 㐕〒〡
From Planck’s law, the radiation density
Comparing the two equations (2) and (3)
3
3
21
C
h
B
A π υ
= and 1
21
B
B
The above relations referred to as Einstein relations
From the above equation for non degenerate energy levels the stimulated emission rate is
equal to the stimulated absorption rate at the equilibrium condition.
B 21 = B 12
5. Pumping mechanisms (or techniques) of population inversion
A system in which population inversion is achieved is called as an active system. The method of raising the particles from lower energy state to higher energy state is called pumping. (Or the process of achieving of population inversion is called pumping). This can be done by number of ways. The most commonly used pumping methods are
I. Optical pumping II. Electrical discharge pumping III. Chemical pumping IV. Injection current pumping
Optical pumping
Optical pumping is used in solid laser. Xenon flash tubes are used for optical pumping. Since these materials have very broad band absorption, sufficient amount of energy is absorbed from the emission band of flash lamp and population inversion is
created. Examples of optically pumped lasers are ruby, Nd: YAG Laser ( Y 3 Al 5 G 12 )
(Neodymium: Yttrium Aluminum Garnet), Nd: Glass Laser
Electrical discharge pumping
Electrical discharge pumping is used in gas lasers. Since gas lasers have very narrow absorption band pumping them any flash lamp is not possible. Examples of Electrical discharge pumped lasers are He-Ne laser, CO 2 laser, argon-ion laser, etc
Chemical pumping
Chemical reaction may also result in excitation and hence creation of population inversion in few systems. Examples of such systems are HF and DF lasers.
Injection current pumping
In semiconductors, injection of current through the junction results in creates of population inversion among the minority charge carriers. Examples of such systems are InP and GaAs.
Different Types of lasers
On the basis of active medium used in the laser systems, lasers are classified into several types I. Solid lasers : Ruby laser, Nd;YAG laser, Nd;Glass II. Liquid lasers : Europium Chelate laser, SeOCl 2 III. Gas lasers : CO 2 , He-Ne, Argon-Ion Laser IV. Dye lasers : Rhodamine 6G V. Semiconductor lasers : InP, GaAs.
6. Ruby laser
Ruby laser is a three level solid state laser and was constructed by Mainmann in
- Ruby (Al 2 O 3 +Cr 2 O 3 ) is a crystal of Aluminium oxide, in which 0.05% of Al+3^ ions are
replaced by the Cr+3^ ions. The colour of the rod is pink. The active medium in the ruby rod
is Cr+3^ ions.
Construction
In ruby laser 4cm length and 5mm diameter rod is generally used. Both the ends of
the rods are highly polished and made strictly parallel. The ends are silvered in such a
way, one becomes partially reflected and the other end fully reflected. The ruby rod is surrounded by xenon flash tube, which provides the pumping light to excite the chromium
ions in to upper energy levels.
Xenon flash tube emits thousands joules of energy in few milli seconds, but only a
part of that energy is utilized by the chromium ions while the rest energy heats up the
apparatus. A cooling arrangement is provided to keep the experimental set up at normal
temperatures Working
The energy level diagram of chromium ions is shown in figure.
The chromium ions get excitation into higher energy levels by absorbing of 5600A^0 of wave length radiation. The excited chromium ions stay in the level H for short interval of
time (10-8^ Sec). After their life time most of the chromium ions are de-excited from H to G
and a few chromium ions are de-excited from H to M.
Dr. P. Venkata Ramana, AUCE(A)
Construction In He-Ne gas laser, the He and Ne gases are taken in the ratio 10:1 in the
discharge tube. Two reflecting mirrors are fixed on either ends of the discharge tube, in
that, one is partially reflecting and the other is fully reflecting. In He-Ne laser 80cm length
and 1cm diameter discharge is generally used. The out power of these lasers depends on
the length of the discharge tube and pressure of the gas mixture.
Working
When the electric discharge is passing through the gas mixture, the electrons accelerated towards the positive electrode. During their passage, they collide with He atoms and excite them into higher levels. F 2 and F 3 form F1. In higher levels F 2 and F 3 , the life time of He atoms is more. So there is a maximum possibility of energy transfer between He and Ne atoms through atomic collisions. When He atoms present in the levels F 2 and F 3 collide with Ne atoms present ground state E 1 , the Ne atoms gets excitation into higher levels E 4 andE 6.
Due to the continuous excitation of Ne atoms, we can achieve the population
inversion between the higher levels E 4 (E 6 ) and lower levels E 3 (E 5 ). The various
transitions E 6 → E 5 , E 4 → E 3 and E 6 → E 3 leads to the emission of wavelengths
3. 39 μ m , 1. 15 μ mand 6328 A^0. The first two corresponding to the infrared region while the
last wavelength is corresponding to the visible region. The Ne atoms present in the E 3 level
are de-excited into E 2 level, by spontaneously emitting a photon of around wavelength
6000A^0. When a narrow discharge tube is used, the Ne atoms present in the level E 2 collide
with the walls of the tube and get de-excited to ground level E 1.
Dr. P. Venkata Ramana
Dr. P.Sreenivasula Reddy (^) M.Sc, PhD Website: engineeringphysics.weebly.com Page 11
8. Applications of lasers
Due to high intensity, high monocromacity and high directionality of lasers, they are widely used in various fields like
- communication
- computers
- chemistry
- photography
- industry
- medicine
- military
- scientific research 1. communication
In case of optical communication semiconductors laser diodes are used as optical sources and its band width is (10^14 Hz) is very high compared to the radio and microwave communications. More channels can be sent simultaneously Signal cannot be tapped As the band width is large, more data can be sent. A laser is highly directional and less divergence, hence it has greater potential use in space crafts and submarines.
2. Computers
In LAN (local area network), data can be transferred from memory storage of one computer to other computer using laser for short time. Lasers are used in CD-ROMS during recording and reading the data.
3. Chemistry
Lasers are used in molecular structure identification Lasers are also used to accelerate some chemical reactions. Using lasers, new chemical compounds can be created by breaking bonds between atoms are molecules.
4. Photography
Lasers can be used to get 3-D lens less photography. Lasers are also used in the construction of holograms.
5. Industry
Lasers can e used to blast holes in diamonds and hard steel Lasers are also used as a source of intense heat Carbon dioxide laser is used for cutting drilling of metals and nonmetals, such as ceramics plastics glass etc. High power lasers are used to weld or melt any material. Lasers are also used to cut teeth in saws and test the quality of fabric.
6. Medicine
Pulsed neodymium laser is employed in the treatment of liver cancer. Argon and carbon dioxide lasers are used in the treat men of liver and lungs. Lasers used in the treatment of Glaucoma.
