basic defintions of laser, fundamentals of laser, types of laser, He-Ne laser,merits and demerits of laser

1. T YPES OF
PREPARED BY
V.REVATHIAMBIKA
LECTURER IN PHYSICS

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INTRODUCTION OF LASER
 L – LIGHT
 A – AMPLIFICATION
 S – STIMULATED
 E – EMISSION
 R - REDIATION
 A. L. SCHAWLOW and C. H. TOWNES IN 1958
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 RUBY LASER by T. H. MAIMANN IN 1960

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BASIC IDEA
 Consider a group of atoms exposed stream
of photons, each with energy h . Let us
assume two energy levels E1
and E2 of an atom.
 During transition from one energy state to another, the
light is absorbed (or) emitted by particles. Under this
action, 3 processes can occur.
 They are,
 Stimulated absorption
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 Spontaneous emission
 Stimulated emission

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MECHANISMS OF LIGHT EMISSION
For atomic systems in thermal equilibrium with their
surrounding, the emission of light is the result of:
Absorption
And subsequently, spontaneous emission of energy
There is another process whereby the atom in an upper energy
level can be triggered or stimulated in phase with the an
incoming photon. This process is:
Stimulated emission
It is an important process for laser action
Therefore 3 process 1. Absorption
of light emission:
2. Spontaneous Emission 4
3. Stimulated Emission

5. LASER FUNDAMENTALS
7/28/2012
 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.
 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.
 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.
These three properties of laser light are what can make it more
hazardous than ordinary light. Laser light can deposit a lot of energy
within a small area.
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INCANDESCENT VS. LASER LIGHT
1. Many wavelengths 1. Monochromatic
2. Multidirectional 2. Directional
3. Incoherent 3. Coherent
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COMMON COMPONENTS OF ALL LASERS
1. Active Medium
The active medium may be solid crystals such as ruby or Nd:YAG, liquid dyes, gases like CO2 or
Helium/Neon, or semiconductors such as GaAs. Active mediums contain atoms whose electrons may be
excited to a metastable energy level by an energy source.
2. Excitation Mechanism
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.
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8. LASER COMPONENTS
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Gas lasers consist of a gas filled tube placed in the laser cavity. A voltage (the external pump
source) is applied to the tube to excite the atoms in the gas to a population inversion. The light
emitted from this type of laser is normally continuous wave (CW).
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9. LASING ACTION
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1. Energy is applied to a medium raising electrons to an unstable energy level.
2. These atoms spontaneously decay to a relatively long-lived, lower energy, metastable state.
3. A population inversion is achieved when the majority of atoms have reached this metastable
state.
4. Lasing action occurs when an electron spontaneously returns to its ground state and produces
a photon.
5. If the energy from this photon is of the precise wavelength, it will stimulate the production of
another photon of the same wavelength and resulting in a cascading effect.
6. The highly reflective mirror and partially reflective mirror continue the reaction by directing
photons back through the medium along the long axis of the laser.
7. The partially reflective mirror allows the transmission of a small amount of coherent radiation
that we observe as the “beam”.
8. Laser radiation will continue as long as energy is applied to the lasing medium.
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LASING ACTION DIAGRAM
Excited State
Spontaneous
Energy Emission
Metastable State
Introduction
Stimulated Emission
of Radiation
Energy
Ground State
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PRINCIPLE OF LASER ACTION
 Due to stimulated emission the photons
multiply in each step giving rise to an
intense beam of photons that are
coherent and moving in the same
direction . hence the Light Is Amplified
By Stimulated Emission Of Radiation
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PUMPING METHODS
 OPTICAL PUMPING
 DIRECT ELECTRON EXCITATION
 INELASTIC ATOM – ATOM
COLLISION
 DIRECT CONVERSION
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 CHEMICAL PROCESS

13. PROPERTIES OF LASER
 Monochromatic
Concentrate in a narrow range of wavelengths
(one specific colour).
 Coherent
All the emitted photons bear a constant phase
relationship with each other in both time and
phase
 Directional
A very tight beam which is very strong and
concentrated.

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LASER HAZARDS
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15. TYPES OF LASER HAZARDS
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1. Eye : Acute exposure of the eye to lasers of certain wavelengths and power can
cause corneal or retinal burns (or both). Chronic exposure to excessive levels may
cause corneal or lenticular opacities (cataracts) or retinal injury.
2. Skin : Acute exposure to high levels of optical radiation may cause skin burns; while
carcinogenesis may occur for ultraviolet wavelengths (290-320 nm).
3. Chemical : Some lasers require hazardous or toxic substances to operate (i.e.,
chemical dye, Excimer lasers).
4. Electrical : Most lasers utilize high voltages that can be lethal.
5. Fire : The solvents used in dye lasers are flammable. High voltage pulse or flash
lamps may cause ignition. Flammable materials may be ignited by direct beams or
specular reflections from high power continuous wave (CW) infrared lasers.
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LASER CLASS
The following criteria are used to classify lasers:
1. Wavelength. If the laser is designed to emit multiple wavelengths the
classification is based on the most hazardous wavelength.
2. For continuous wave (CW) or repetitively pulsed lasers the average power
output (Watts) and limiting exposure time inherent in the design are
considered.
3. For pulsed lasers the total energy per pulse (Joule), pulse duration,
pulse repetition frequency and emergent beam radiant exposure are
considered.
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LASER COMPONENTS
Optical Resonator
Output
Beam
Active
Medium
High Reflectance Output Coupler
Mirror (HR) Mirror (OC)
Excitation
Mechanism
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19. OPTICAL RESONATOR
 Two parallel mirrors placed around the
gain medium.
 Light is reflected by the mirrors back into
the medium and is amplified .
 The design and alignment of the mirrors
with respect to the medium is crucial.
 Spinning mirrors, modulators, filters and
absorbers may be added to produce a
variety of effects on the laser output.

20. COMPARISON CHART FOR ALL THE LASERS
Character Nd-YAG laser He-Ne laser CO2 laser Semiconduct
istics or (Ga-As)
laser
Type Doped insulator Gas laser Molecular Semiconductor
laser(solid state laser) gas laser laser
Active Yttrium Aluminium Garnet Mixture of Mixture of P-N junction
medium (y3Al5O12) Helium and CO2, N2 and diode
Neon in the Helium (or)
ratio 10:1 water vapour
Active Neodymium(Nd3+ ions) Neon CO2 Recombination
centre of electrons &
holes
Pumping Optical pumping Electrical Electric Direct pumping
method pumping disharge
method
Optical Ends of the rods polished Pair of Metallic Junction of
resonator with silver and two concave mrror of gold diopdes-
mirrors. One of them is to mirrors (or) silicon polished
totally reflected and the mirrors

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TYPICAL APPLICATION OF LASER
The detection of the binary data stored in the form of pits on the compact disc
is done with the use of a semiconductor laser. The laser is focused to a
diameter of about 0.8 mm at the bottom of the disc, but is further focused to
about 1.7 micrometers as it passes through the clear plastic substrate to
strike the reflective layer. The reflected laser will be detected by a
photodiode. Moral of the story: without optoelectronics there will no CD
player!
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22. DIFFERENCE BETWEEN A PHOTOGRAPHY & HOLOGRAPHY
S.No photography Holography
1. Photography is a 2-dimensional Holography is a 3-dimensional
recording process recording process
2. Ordinary light can be used for Only laser beam should be used for
recording recording (or) constucting a
hologram
3. It is based on lens systems It is a lensless systems
4. Amplitude alone can be recorded Both Amplitude and phase can be
recorded
5. Image is recorded totally Image is recorded bit by bit
6. Image has poor resolution Image has very high solution
7. To get the positive of the image it To get the positive of the image it
needs printing needs printing
8. No need of vibration less table Needs of vibration less table

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COMMON LASER SIGNS AND LABELS
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LASER SAFETY EYEWEAR
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INTERNATIONAL LASER
WARNING LABELS
INVISIBLE LASER RADIATION
AVOID EYE OR SKIN EXPOSURE
TO DIRECT OR SCATTERED RADIATION
CLASS 4 LASER PRODUCT
WAVELENGTH 10,600 nm
MAX LASER POWER 200 W
EN60825-1 1998
Symbol and Border: Black Legend and Border: Black
Background: Yellow Background: Yellow
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CDRH CLASS WARNING LABELS
Laser Radiation VISIBLE LASER RADIATION-
AVOID EYE OR SKIN EXPOSURE TO
Do Not Stare Into Beam DIRECT OR SCATTERED RADIATION
Argon Ion
Helium Neon Laser Wavelength: 488/514 nm
1 milliwatt max/cw Output Power 5 W
CLASS II LASER PRODUCT CLASS IV Laser Product
Class II Class IIIa with small beam
Class IIIa with expanded beam Class IIIb
Class IV
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