2. LASER
•LASER stands for LIGHT APLIFICATION by
STIMULATED EMISSION of RADITIONS
•First laser was constructed by Maiman
•Laser action has been obtained with atoms, ions &
molecules in gases, liquids, solids, semiconductors
at wavelength( λ ) ranging from ultraviolet to radio
frequency regions and with power ranging from a
few milli watts to mega watts.
4. Stimulated Absorption
• E1 & E0 are two energy levels such
that E1 is excited energy level & E0
is lower energy level.
• If hƒ =E1- E0 ,where ƒ is frequency
of incident radiation, then
radiation photon is absorbed &
electron jumps from E0 toE1.
5. Spontaneous Emission
•The excited atom does not
remain in higher energy state
for long.It gives out a photon
of energy hƒ= E1- E0 &
returns to ground state. As
this process does not require
stimulus from outside,so it is
termed as spontaneous
emission.
6. Stimulated Emission
• The excited atom in energy state E1
may not come to ground state at given
instant. So it requires external
stimulation. When a photon of energy
hƒ= E1- E0 strikes the atom in excited
state , it comes to ground state. The
photon emitted goes parallel to
incident photon & both of these
photons are in phase. For laser to
have Stimulated Emission, the
following two conditions must be
satisfied
1)The higher energy state should have
longer life time
2) The number of atoms in higher
energy state E1 must be more than
that in E0
7. Population Inversion
• The situation in which number of
atoms in higher energy state is
greater than in lower energy state is
called population inversion. It can be
obtained even at room temperature
by laser pumping.various pumping
methods are:optical
pumping,chemical pumping,
electrical pumping. when population
inversion is obtained, emitted
photons have same frequency and
phase. They travel in same direction.
Thus the number of photons goes on
multiplying by stimulated emission.
Hence we get a highly intense,
monochromatic, coherence and
unidirectional beam.
8. Active medium
Fully
Reflecting
mirror
Partially
reflecting
mirror
RESONANCE CAVITY
A medium with population inversion is capable of amplification but in order
to get a continous supply of light energy it is required to couple a part of
output back into the medium. It can be made by placing an active medium
between two mirrors such that one is fully reflecting and other is partially
reflecting.
Active medium: A medium which on excitation produces population
inversion and promotes stimulated emission
9. There are Severel Properties Of LASER which are defined as follows:-
MONOCHROMATICITY
COHERENCE
DIRECTIONALITY
BRIGHTNESS
DIVERGENCE
INTENSITY
These are several Properties about LASER which we will be going to
discuss in this Presentation.
10. MONOCHROMATICITY:-
The energy of a photon determines its wavelength through the relationship E = hc/λ, where
c is the speed of light, h is Planck's constant, and λ is wavelength. In an ideal case, the laser
emits all photons with the same energy, and thus the same wavelength, it is said to be
monochromatic. 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.
11. When an excited atom, depending on its lifetime at the higher energy level, comes down to
lower energy level, a photon is emitted, corresponding to the equation,
hv = E2 - E1
where h is the Planks constant, n is the frequency of the emitted photon and E2 and
E1 correspond to higher and lower energy levels respectively. This type of natural emission
occurs in different directions and is called spontaneous emissions. It is characterized by the
lifetime of the upper excited state after which it spontaneously returns to lower state and
radiates away the energy by emission.
12. One of the important properties of laser is its high directionality. The mirrors placed at opposite ends
of a laser cavity enables the beam to travel back and forth in order to gain intensity by the stimulated
emission of more photons at the same wavelength, which results in increased amplification due to the
longer path length through the medium. The multiple reflections also produce a well-collimated
beam, because only photons traveling parallel to the cavity walls will be reflected from both mirrors. If
the light is the slightest bit off axis, it will be lost from the beam.
13. It is defined as the power emitted per unit surface area per unit solid angle. The units are watts
per square meter per steradian. A steradian is the unit of solid angle, which is three-
dimensional analogue of conventional two-dimensional (planar) angle expressed in radians.
For small angles the relation between a planar angle and the solid angle of a cone with that
planar angle is to a good approximation is:
Ω = (π / 4) θ2
where θ is the planar angle and Ω is the solid angle as shown in the figure.
15. A solid-state laser is a laser that uses a gain medium that is a solid, rather than
a liquid such as in dye lasers or a gas as in gas lasers. Semiconductor-based lasers are
also in the solid state, but are generally considered as a separate class from solid-
state lasers.For eg. Ruby Lasers,Nd-YAG,etc
16. A gas laser is a laser in which an electric current is discharged through a gas to
produce coherent light. The gas laser was the first continuous-light laser and the first
laser to operate on the principle of converting electrical energy to a laser light output.
The first gas laser, the Helium–neon laser (HeNe), was co-invented by Iranian-
American physicist Ali Javan and American physicist William R. Bennett, Jr. in 1960.
It produced a coherent light beam in the infrared region of the spectrum at 1.15
micrometres.
17. A dye laser is a laser which uses an organic dye as the lasing medium, usually as
a liquid solution. Compared to gases and most solid state lasing media, a dye can usually
be used for a much wider range of wavelengths, often spanning 50 to 100 nanometers or
more. The wide bandwidth makes them particularly suitable for tunable lasers and
pulsed lasers. The dye rhodamine 6G, for example, can be tuned from 635 nm (orangish-
red) to 560 nm (greenish-yellow), and produce pulses as short as 16 femtoseconds.
18. Semiconductor lasers or laser diodes play an important part in our everyday lives by providing
cheap and compact-size lasers. They consist of complex multi-layer structures
requiring nanometer scale accuracy and an elaborate design. Their theoretical description is
important not only from a fundamental point of view, but also in order to generate new and
improved designs. It is common to all systems that the laser is an inverted carrier density
system. The carrier inversion results in an electromagnetic polarization which drives an electric
field E(t). In most cases, the electric field is confined in a resonator, the properties of which are
also important factors for laser performance.
Semiconductor Lasers(520nm,445nm,635nm)
