1. Rayat Shikshan Sanstha’s Veer Wajekar A.S.C. College,Phunde
Department of Physics
Prof. Ghorpade U. T.
Topic – “LEASERS”

2. INTRODUCTION
LASER stands for ‘Light Amplification by Stimulated Emission of Radiation’.
According to Einstein’s theory, when a atom is in excited state, it can make transition to a lower energy state through the emission of an electromagnetic radiation.
It is a monochromatic beam.
It is unique source of light which is highly coherent and almost perfectly parallel.

3. The interaction of an atom with radiation occurs in three fundamental distinct ways. They are :
Absorption
Spontaneous emission
Stimulated (forced emission)

4. Absorption
Before
After E2
Incoming
photon →
hν = ∆E
An atom is in ground state E1 initially. This atom can absorb an amount of energy hν & move to the higher(excited state) E2. According to principle of conservation of energy,
hν = E2-E1
Where h- Planck's constant
ν- Frequency of incident radiation
This energy hν is provided by an incident photon and is absorbed. Thus an incident photon is lost on this process.
← ∆E = E2-E1 E1
Matter

5. SPONTANEOUS EMISSION
Before
After E2
Emitted photon radiation → hν = E2 - E1 E1
Matter
Spontaneous emission is the process in which an excited atom returns to ground state very soon.
In this process an excited atom radiates spontaneously an electromagnetic radiation to de-excite itself.
As shown in fig. initially atom is in excited state of energy E2. It drops into a lower state of energy E1 by emitting a photon of energy, hν = (E2 - E1).

6. The process is spontaneous because no external triggering is required.
The frequency of emitted radiation is directly proportional to the energy difference between two levels.
Also ν = c/λ. Hence smaller the energy difference greater will be the wavelength emitted.
Probability of stimulated emission increases rapidly with the energy difference between two states.
The process is spontaneous because no external triggering is required.
The light from ordinary bulbs or from filament is produced by this process.

7. STIMULATED EMISSION
Before
After
Stimulated emission is the process in which one uses an external triggering radiations of same frequency so that the transition from a higher to a lower state occurs at that frequency.
As shown in fig. a photon of frequency hν=E2-E1 when collides with an atom in excited state E2, the atom is forced to drop into lower energy state E1. A photon of frequency ν is released during this transition.
Thus along with an incident triggering photon, one more photon is also produced.
If these photons are further used to trigger more and more excited atoms one can have identical photons 4,8,……..etc. E2
Radiation →
Incoming photon hν →
Emitted photon → E1
Matter

8. Thus incident photons get coherently amplified in this process.
This process of amplification is called as ‘Amplification of light by stimulated emission of radiation’.
Rate of stimulated emission depends on:
a.) Intensity of an external triggering radiation
b.) Number of atoms in upper state.

9. PROPERTIES OF LASER
Monochromaticity or wavelength purity
Coherence
Intensity and power
Directionality

10. MONOCHROMATICITY :
Laser light is highly monochromatic than any other monochromatic source.
Light from conventional source is less sharp.
It spreads over wide frequency range(thousands of megacycles/sec).
But the light emitted by the laser is sharp and has a very little spread(100nm).
COHERENCE :
Laser light is highly coherent.
In stimulated emission, emitted photons are in same phase.
Wave train emitted by an individual laser can be several thousand kilometers long.
But a wave train emitted by a light bulb is typically less than a meter.

11. INTENSITY AND POWER :
The intensity of a laser beam is given by,
I = (10/λ)² P
where P- power produced by laser
I- intensity in w/m²
Laser light is highly intense.
It is much more brighter than any other source of light.
It can generate extremely high power.
High intensity laser can vaporize even the hardest metal.
It can produce temperature of the order of 10⁴ ˚C at a focused point.
DIRECTIONALITY :
Laser light is highly directional.
A laser beam is in the form of a pencil of rays that propagates to long distance.
Divergence of laser beam is by very small magnitude (at the exit aperture of the laser).
It can be focused sharply on small regions.

12. CLASSIFICATION
On the basis of active medium used, lasers are broadly classified into four different types :
Solid state
Gas lasers
Liquid lasers
Semiconductor lasers

13. HELIUM NEON LASER

14. Construction of He-Ne laser
The setup consists of a discharge tube of length 80 cm and bore diameter of 1.5cm.
The main medium of the laser, as suggested by its name, is a mixture of helium and neon gases, in a 5:1 to 20:1 ratio, contained at low pressure (an average 50 Pa per cm of cavity length ) in a glass envelope.
The energy or pump source of the laser is provided by an electrical discharge of around 1000 volts through an anode and cathode at each end of the glass tube. A current of 5 to 100 mA is typical for CW operation.
The optical cavity of the laser typically consists of a plane, high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end.
He-Ne lasers are normally small, with cavity lengths of around 15 cm up to 0.5 m, and optical output powers ranging from 1 mW to 100 mW.

15. Working of He-Ne laser
A description of the rather complex He-Ne excitation process can be given in terms of the following four steps.
(a) When the power is switched on, An energetic electron collisionally excites a He atom to the excited state. A He atom in this excited state is often written He*,where the asterisk means that the He atom is in an excited state.
(b) The excited He* atom collides with an unexcited Ne atom and the atoms exchange internal energy, with an unexcited He atom and excited Ne atom, written Ne*, resulting. This energy exchange process occurs with high probability only because of the accidental near equality of the two excitation energies of the two levels in these atoms. Thus, the purpose of population inversion is fulfilled.

16. When the excited Ne atom passes from metastable state to lower level, it emits photon of wavelength 632 nm.
This photon travels through the gas mixture parallel to the axis of tube, it is reflected back and forth by the mirror ends until it stimulates an excited Ne atom and causes it to emit a photon of 632nm with the stimulating photon.
The stimulated transition from level to level is laser transition.
This process is continued and when a beam of coherent radiation becomes sufficiently strong, a portion of it escape through partially silvered end.
The Ne atom passes to lower level emitting spontaneous emission. and finally the Ne atom comes to ground state through collision with tube wall and undergoes radiation less transition.

17. APPLICATIONS OF LASERS
INDUSTRIAL USES :
Lasers have very wide applications in industries. Lasers are used in industries for welding, for drilling, cutting of metals and non metals etc. Since lasers have very high power, they can be focused to a very small area of the order of 10 to 100μm.
2. The Narrow red beam of He-Ne laser is used in supermarkets to read bar codes.
3. APPLICATIONS IN THE FIELD OF MEDICINE :
Lasers have wide applications in the field of medicine. Lasers are used not only in diagnostics but also in surgery and other medical treatments.
E.g.
a) He-Ne laser is used in ulcer treatment and preparations of holograms.
b) CO₂ laser has applications in brain and spinal surgeries.
c) Argon laser is used in angioplasty, brain and eye surgery.
d) Nitrogen laser is used in DNA analysis and genetic engineering.