1. LASERS
A laser is a device that produce a very narrow intense beam of monochromatic coherent light. The emitted beam is a nearly perfect plane wave. The action of a laser is based on quantum theory.

2. STIMULATED EMISSION See Fig1. (a) this is a resonant situation.
we have seen that energy hf
corresponds to the energy
difference between an occupied
energy level of the atom and an
available excited state it may of
course jump spontaneously to the
Lower state with the emission of a hf
Fig.1

3. Photon. However ,if a photon with This same energy strikes the excited
atom ,it can stimulate the atom to
make the transition sooner to the
lower state see Fig. (b)
This phenomenon is called
Stimulated emission, and
it can be seen that not
only do we still have the original
photon, but also a second one of the hf
Fig.1

4. Same frequency as a result of the
atom’s transition And these two
photons are exactly in phase ,and
they are moving in the same
direction ,this is how coherent
light is produced in a laser. Hence
the name “laser”, which is an
acronym for light amplification by
stimulated emission of radiation .

5. Normally ,most atoms are in the
lower state so incident photons will
mostly be absorbed in order to
obtain the coherent light from
stimulate emission, two conditions
must be satisfied. First: the atoms
must be excited to the higher state.
an inverted population is needed one
in which more atoms are in the upper
state than in the lower state .

6. Second: The higher state must be a
Metastable state -- a state in which
the electrons remain longer than
usual so that the transition to the
lower state occurs by stimulated
emission rather than spontaneously.
For now, we assume that the atoms
have been exited to an upper state.
See Fig2.is a schematic diagram of a
laser: the “lasting” material is a

7. Long narrow tube at the end of which are two mirrors ,one of which is
partially transparent (perhaps 1 or 2 percent ).some of these is the
atom shown on the far left in Fig 2.
Mirror
Partially transparent mirror
Fig.2

8. If the emitted photons strikes
another atom in the exited state, it
stimulates this atom to emit a photon
of the same frequency, moving in the
same direction ,and in phase with it.
these two photons then move on to
strike other atoms causing more
stimulated emission. As the process
continues, the number of photons
multiplies. When The photons strike
the end mirrors, most are reflected

9. Back ,and as they move in the
opposite direction ,they continue to
stimulate more atoms to emit photons.
As the photons move back and forth
between the mirrors a small
percentage passes through the
partially transparent mirror at one
end. These photons make up the
narrow coherent external laser beam.

10. In a ruby laser, the lasing material is a ruby rod consisting of with
THE RUBY LASER
In a ruby laser, the lasing material
is a ruby rod consisting of with
a small percentage of aluminum ( )
atoms replaced by chromium ( )atoms.
The atoms are the ones involved in
lasing. The atoms are excited by
strong flashes of light of
wavelength 550nm, which corresponds
to a photon energy of 2.2ev.shown as
Fig.3 , the atoms are excited from

11. State to state . this process
is called optical pumping. The atoms
quickly decay either back to or
to the intermediate state ,which
is metastable with a lifetime of
about s atoms can be forced
into the state than are in the
state. Thus we have the inverted
population needed for lasing . As
soon as a few atoms in the state

12. jump down to , they emit photons that produce stimulated emission
of the other atoms and the lasing
action begins. A ruby laser thus
emits a beam whose photons have
energy 1.8ev and a wavelength of
694.3nm(for
“ruby-red”
light)
Fig.3
Fig.3

13. Helium-Neon LASER In a helium-neon laser, the lasing
material is a gas, a mixture of
about 15 percent He and 85 percent
Ne. In this laser, the atoms are
excited by applying a high voltage
to the tube so that an electric
discharge takes place within the gas.
In the process ,some of the He atoms

14. are raised to the metastable state shown in Fig.4 which corresponds to
a jump of 20.61ev.Now Ne atoms have
an excited state that is almost
exactly The same energy above the
ground State, 20.66ev. The atoms do
collision
Helium
Neon
20.61ev
20.66ev
1. 96ev
Fig.4

15. Not quickly return to the ground
state by spontaneous emission, but
instead often give their energy to a
Neon atom when they collide. In such
a collision, the He drops to the
ground state and the Neon atom is
excited to the state .In this
manner, the state in Neon-which is
Metastable- becomes more populated
Than the level.

16. CONCAVE MIRRORS AND BREWSTER’S WINDOWS
Four commonly used configurations
Are shown in Fig.5.The hemispherical
arrangement at the Center, with a
concave mirror at one end only, has
its center of curvature at the
center of the reflecting surface of
the plan mirror . The spherical
Mirror arrangement has the two

17. Centers of curvature falling together at the center point C of
the configuration.
Hemispherical
Plane parallel
Fully silvered
Partly silvered
Spherical
Confocal C
Fig.5

18. The confocal arrangement has the two
centers at the centers of the opposite
mirror faces. One mirror is usually
fully silvered, and the other is
partially silvered or fully silvered
with a clear spot at its center. By
Titling these plates or beveling the
ends of a solid laser to the
polarizing angle ,the windows or
ends will have a 100 percent
transmission for light whose electric

19. Vector is parallel to the plane of incidence see Fig.6
gas
Brewster Window
Fully silvered
Polarized light
Partly silvered
(a)
solid
(b)
Fig. 6

20. The normal component is partially
reflected at each interface with
traversal of the laser. The laser beam
is thereby polarized, as with a pile
of plates. The polarizing angle is
given by where n is the
refractive index of the medium. .

21. FREQUENCY DOUBLING In 1961 four physicists at the
University of Michigan focused a beam
from a ruby laser emitting 3-kw pulses
of red light of wavelength 6943Ǻ on to
a quartz crystal, thereby producing an
observable number of photons of half
the wavelength, of Ǻ This new
wavelength, which lies in the
ultraviolet region of the spectrum,

22. is exactly double the frequency of the
laser’s red light. The possibility
that this was fluorescent light could
be ruled out since it was emitted in a
highly directional beam parallel to
the incident light. The classical
explanation of these phenomena
involves ionization of the loosely
bound valence electrons, which in many
Crystals are shared by other atoms in

23. The bonding of the structure. An atom giving up
an electron to its neighbor leaves it with a net
positive charge, and the neighbor with an extra
electron has a net negative charge. As light
waves pass through, these ions respond to the
associated electric and magnetic fields by being
set in to vibration with the source frequency.
When the incident light intensity is extremely high,
as it is in a laser beam, the induced atomic
vibrations are nonlinear in their response, just as
they are with loud sounds, and higher harmonics
are generated. The second harmonic is far more

24. intense than higher modes. From the point of
View of quantum theory, when two photons
interact with matter, both energy and momentum
are conserved in producing a single photon.

25. OTHER LASERS Other types laser include :chemical lasers,
in which the energy input comes from the
chemical reaction of highly reactive gases;
dye lasers ,whose frequency is tunable;
gas lasers, capable of high power
output in the infrared; rare-earth solid-
state lasers such as the high-power
neodymium: YAG laser; and the junction
laser in which the transitions occur
between the bottom of the conduction band
and the upper part of the valence band .

26. LASER SAFETY Laser light varies in intensity from a
milliwatt for an inexpensive He-Neon laser
to many kilowatts for a laser. Laser
injuries have been few, and their dangers
highly debatable. However, the greatest
danger is the inadvertent direction
of an undiverged laser beam directly in to
the eye. The weak beam from a continuous
He-Neon laser is probably of little danger,
since eyelids can close upon sudden
exposure. More intense beams, and
especially pulsed beams, can cause injury,

27. Due primarily to the ability of the eye to focus the
parallel beam onto a small area of the retina .
Good safety practice in the presence of high-
powered lasers involves the use of filtering
glasses and shields and awareness that a laser
beam incident upon a specular reflecting surface
can redirect the beam undiminished in intensity.

28. Home work
Questions
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