This document provides an overview of lasers, including: 1. A definition of a laser as a device that generates light through stimulated emission. 2. Descriptions of the key components and processes that enable laser operation, including population inversion and optical feedback. 3. Examples of common laser applications like CD players, fiber optics, and medical devices. 4. Safety considerations regarding laser hazards and the importance of controls and personal protective equipment when working with lasers.

1. LASERS
Presented by :-
Sushil Mishra (05311502809)
Ketan Gupta (04211503809)
Yatin Jain (04811502809)
Harshit Jain (05911502809)

2. Seminar Contents
Definition of a laser
Emission and absorption of radiation
Population Inversion
Optical Feedback
Fundamentals of laser operation
Laser Hazards

3. 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!

4. 1. Definition of laser
A laser is a device that generates light by a
process called STIMULATED EMISSION.
The acronym LASER stands for Light
Amplification by Stimulated Emission of
Radiation
Semiconducting lasers are multilayer
semiconductor devices that generates a
coherent beam of monochromatic light by
laser action. A coherent beam resulted
which all of the photons are in phase.

5. Another Typical Application
of Laser – Fibre Optics
An example of application is for the light source for
fibre optics communication.
Light travels down a fibre optics glass at a speed,
= c/n, where n = refractive index.
Light carries with it information
Different wavelength travels at different speed.
This induce dispersion and at the receiving end
the light is observed to be spread. This is
associated with data or information lost.
The greater the spread of information, the more
loss
However, if we start with a more coherent beam
then loss can be greatly reduced.

6. Fibre Optics Communication

7. 3 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
3. Stimulated Emission

8. Absorption
E1
E2

9. Spontaneous Emission

10. Stimulated Emission

11. Background Physics
Consider the ‘stimulated emission’ as
shown previously.
Stimulated emission is the basis of the
laser action.
The two photons that have been produced
can then generate more photons, and the 4
generated can generate 16 etc… etc…
which could result in a cascade of intense
monochromatic radiation.

12. Population Inversion
Therefore we must have a mechanism where N2 > N1
This is called POPULATION INVERSION
Population inversion can be created by introducing a so call metastable
centre where electrons can piled up to achieve a situation where more N 2 than
N1
The process of attaining a population inversion is called pumping and the
objective is to obtain a non-thermal equilibrium.
It is not possible to achieve population inversion with a 2-state system.
If the radiation flux is made very large the probability of stimulated emission
and absorption can be made far exceed the rate of spontaneous emission.
But in 2-state system, the best we can get is N 1 = N2.
To create population inversion, a 3-state system is required.
The system is pumped with radiation of energy E31 then atoms in state 3 relax
to state 2 non radiatively.
The electrons from E2 will now jump to E1 to give out radiation.

13. 3 states system

14. Population Inversion
When a sizable population of electrons resides in upper levels,
this condition is called a "population inversion", and it sets the
stage for stimulated emission of multiple photons. This is the
precondition for the light amplification which occurs in a LASER
and since the emitted photons have a definite time and phase
relation to each other, the light has a high degree of coherence.

15. Optical Feedback
The probability of photon producing a
stimulated emission event can be
increased by reflecting back through
the medium several times.
A device is normally fashioned in
such a way that the 2 ends are made
higly reflective
This is term an oscillator cavity or
Fabry Perot cavity

16. Therefore in a laser….
Three key elements in a laser
•Pumping process prepares amplifying medium in suitable state
•Optical power increases on each pass through amplifying medium
•If gain exceeds loss, device will oscillate, generating a coherentoutput

17. Fundamentals of Laser
Operation
17

18. Laser Fundamentals
 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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19. Incandescent vs. Laser Light
1. Many wavelengths 1. Monochromatic
2. Multidirectional 2. Directional
3. Incoherent 3. Coherent
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20. 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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21. Laser Components
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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22. Lasing Action
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.
22

23. Lasing Action Diagram
Excited State
Spontaneous
Energy
Emission
Metastable State
Stimulated
Emission of
Radiation
ygr en E
cu dort nI
Ground State 23

24. Laser Output
Continuous Output (CW) Pulsed Output (P)
Energy (Joules)
Energy (Watts)
Time Time
watt (W) - Unit of power or radiant flux (1 watt = 1 joule per second).
Joule (J) - A unit of energy
Energy (Q) The capacity for doing work. Energy content is commonly used to characterize the output
from pulsed lasers and is generally expressed in Joules (J).
Irradiance (E) - Power per unit area, expressed in watts per square centimeter. 24

25. Laser Hazards
25

26. Types of Laser Hazards
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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27. Lasers and Eyes
What are the effects of laser energy on the eye?
Laser light in the visible to near infrared spectrum
(i.e., 400 - 1400 nm) can cause damage to the
retina resulting in scotoma (blind spot in the
fovea). This wave band is also know as the "retinal
hazard region".
Laser light in the ultraviolet (290 - 400 nm) or far
infrared (1400 - 10,600 nm) spectrum can cause
damage to the cornea and/or to the lens.
Photoacoustic retinal damage may be associated
with an audible "pop" at the time of exposure. Visual
disorientation due to retinal damage may not be
apparent to the operator until considerable thermal
damage has occurred.
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28. 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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29. Control Measures and Personal
Protective Equipment
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30. CONTROL MEASURES
Engineering Controls
 Interlocks
 Enclosed beam
Administrative Controls
 Standard Operating Procedures (SOPs)
 Training
Personnel Protective Equipment (PPE)
 Eye protection
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31. Common Laser Signs and Labels
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32. 1-9 sUSHIL
9-16 Yatin
17-24 Ketan
25-31 Harshit