The document discusses the principles and properties of lasers. It begins by defining what a laser is, explaining that it stands for "Light Amplification by Stimulated Emission of Radiation". It then covers the basic principle of how atoms absorb and emit photons at specific wavelengths, and the three processes that can occur in a laser medium: stimulated absorption, spontaneous emission, and stimulated emission. The document emphasizes that population inversion is needed to favor stimulated emission over absorption, and that pumping is required to create this inversion. It also outlines the key components of a laser and some common laser types and their wavelengths.

1. AlMughtaribeenUniversity
LASER
LASER is an acronym for:
L : Light
A : Amplification (by)
S : Stimulated
E : Emission (of)
R : Radiation
Term coined by Gordon Gould.
Laser means to absorb energy in one form and to emit a new form of light energy
which is more useful.
Lec 5
1Feb 16, 2016

2. Principle of laser
 Atom will absorb and
emit light photons at
particular
wavelength
corresponding to the
energy differences
between orbits. The
wavelength l of
emitted or absorbed
photon can be
obtained by the
formula
 where ∆E is the
change in energy
between the initial
and final orbits.
hf
hc
E ==∆
λ
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3. Principle of laser (cont...)
 “Stimulated emission” of radiation
 Atoms or molecules are characterized by a set of
discrete allowed energy states
 An atom can move from one energy state to
another when it receives or releases an amount of
energy equal to the energy difference between the
two states
Three distinct process may occur in the medium
1. Stimulated absorption
2. Spontaneous emission
3. Stimulated emission
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4. Principle of stimulated emission
of radiation
 Two energy levels E1 and E2 (E1 < E2) whose
atoms can interact with light of frequency
 The group E1-E2 is called radiative transition
if atoms can only pass from E1 to E2 (or from
E2 to E1 ) by interacting with light
 E1 is called the lower energy level
 E2the upper energy level
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5. The emission-absorption principle
 Absorption:
 An atom in a lower level absorbs a photon of frequency hν and moves to an
upper level
 Spontaneous emission
 An atom in an upper level can decay spontaneously to the lower level and emit
a photon of frequency hν if the transition between E2 and E1 is radiative
 This photon has a random direction and phase
 Stimulated emission
 An incident photon causes an upper level atom to decay, emitting a
“stimulated” photon whose properties are identical to those of the incident
photon
 The term “stimulated” underlines the fact that this kind of radiation only
occurs if an incident photon is present
 The amplification arises due to the similarities between the incident and
emitted photons
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6. Absorption
. A photon of the
“right” energy
can be absorbed
and “bump” an
electron into a
higher energy
level.
E2
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7. Spontaneous Emission
An excited
electron falls
back to its lower
energy level,
releasing a
photon in a
random
direction.
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8. Stimulated Emission
A photon strikes an
excited electron. The
electron falls to its
lower energy level,
releasing a photon
that is going in the
same direction as,
and is in exact phase
with, the original
photon. Note that
whereas only one
photon strikes the
atom, two photons
leave it—the original
photon plus the
emitted photon.
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9. Population inversion and pumping
 To favour stimulated emission
over absorption, there need to
be more excited-state atoms
than ground-state atoms
 Spontaneous emission naturally
tends to empty the upper level
so this level has to be emptied
faster by stimulated emission
 It has been proved that
stimulated emission is much
more likely to happen if the
medium used is flooded with
light (i.e. with a large number of
photons)
+
 A good way to do this is to
confine the photons in an
optical cavity
 The confinement of the light
increases the probability of
stimulated emission rather than
spontaneous emission occurring
•If there are more atoms in the
upper level (N2) than in the
lower level (N1), the system is
not at equilibrium
•A situation not at equilibrium
must be created by adding
energy via a process known as
“pumping” light in order to
raise enough atoms to the upper
level
•This is known as population
inversion and is given by
N2 - N1 = Δ
•Light is amplified when the
population inversion is positive
•Pumping may be electrical,
optical or chemical
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11. Feb 16, 2016 11
1- Active medium – Gas, liquid or solid , Contained in glass
or ceramic tubes
2- Excitation Mechanism or Pumping
Excitation mechanisms pump energy into the active medium by one or
more of three basic methods; optical, electrical or chemical.
3- High Reflectance Mirror
A mirror which reflects essentially 100% of the laser light.
4- Partially Transmissive Mirror
A mirror which reflects less than 100% of the laser light and transmits the
remainder.
Mirrors are added to each end to increase the back and forth
movement of photons .Thus increasing the stimulation of
emission of radiation

12. Feb 16, 2016 12
properties of laser
1- Monochromatic
 The light emitted from a laser is monochromatic, that is, it is of one
color/wavelength. In contrast, ordinary white light is a combination of
many colors (or wavelengths) of light.
2- directional
Lasers emit light that is highly directional, that is, laser light is emitted
as a relatively narrow beam in a specific direction. Ordinary light, such
as from a light bulb, is emitted in many directions away from the
source.

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3- coherent
The light from a laser is said to be coherent, which means that the
wavelengths of the laser light are in phase in space and time. Ordinary
light can be a mixture of many wavelengths.

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TYPES OF LASER
1.Based on state of active medium used
(i) Gas Laser
(ii) Solid state Laser
(iii) Semiconductor Laser
(iv) Tunable dye Laser

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16. WAVELENGTHS OF MOST COMMON
LASERS
Argon fluoride (Excimer-UV)
Krypton chloride (Excimer-UV)
Krypton fluoride (Excimer-UV)
Xenon chloride (Excimer-UV)
Xenon fluoride (Excimer-UV)
Helium cadmium (UV)
Nitrogen (UV)
Helium cadmium (violet)
Krypton (blue)
Argon (blue)
Copper vapor (green)
Argon (green)
Krypton (green)
Frequency doubled
Nd YAG (green)
Helium neon (green)
Krypton (yellow)
Copper vapor (yellow)
0.193
0.222
0.248
0.308
0.351
0.325
0.337
0.441
0.476
0.488
0.510
0.514
0.528
0.532
0.543
0.568
0.570
Helium neon (yellow)
Helium neon (orange)
Gold vapor (red)
Helium neon (red)
Krypton (red)
Rohodamine 6G dye (tunable)
Ruby (CrAlO3
) (red)
Gallium arsenide (diode-NIR)
Nd:YAG (NIR)
Helium neon (NIR)
Erbium (NIR)
Helium neon (NIR)
Hydrogen fluoride (NIR)
Carbon dioxide (FIR)
Carbon dioxide (FIR)
0.594
0.610
0.627
0.633
0.647
0.570-0.650
0.694
0.840
1.064
1.15
1.504
3.39
2.70
9.6
10.6
Key: UV = ultraviolet (0.200-0.400 µm)
VIS = visible (0.400-0.700 µm)
NIR = near infrared (0.700-1.400 µm)
Wavelength (µm)Laser Type
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17. Feb 16, 2016 17
(i) Continuous wave Laser systems
Continuous-wave (cw) operation of a laser means that the laser
is continuously pumped and continuously emits light.
(iii) Pulsed Laser systems
Pulsed lasers are switched on and off rapidly and may appear to
be continuously emitting a beam of light.
2- Based on the mode of
operation

18. 1- Wavelength
Laser Parameters :
2- Power : Power is ratio between energy (Usully measured in joules ) and time (in
second )
Power = Energy / Time
3- power Density : The ratio between the emitted light power and illuminated area is
called power density (irradiance )
Power density = Power / A is area in cm 2
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Size of beam (or Spot size ) of laser beam depend on several variables
, including the focal of lens and the wavelength of laser .
The focal length of lens determine the size of the beam spot. How ?
Lens with a short focal length can provide smaller spot size for
example when the laser is used as cutting tool, the spot size must be
small to concentrate the power into tiny area .
The wavelength of laser also limits the spot size and beam focusing ,
shorter wavelength can generate smaller spots size .
4- Dose : the energy delivered by the area cross section is called
Flounce or Dose in the number of instance , Flunce is most important
of laser therapy
Fluence = (Watts × time ) / Spot size (cm2)

20. Fluence, F is defined as the total energy delivered
by a laser on an unit area during an expose time TE,
F(J/cm2)=I(watts/cm2) x TE(s)
The advantage of directionality of a laser : we can
focus or defocus a laser beam using a lens. This
can be used to vary the intensity of the laser.
Incoming
parallel ray
f
Focused spot
Diverged
beamFeb 16, 2016 20

21. continuous wave (CW) lasers versus pulsed
lasers
• CW lasers has a constant power output during whole operation time.
• pulsed lasers emits light in strong bursts periodically with no light between pulses
usually T>>Tw

22. Pulse Laser :
I hope if my teacher knows ??
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