This document provides an introduction to lasers, including their key characteristics and components. It discusses how lasers produce coherent, monochromatic, and directional light through stimulated emission of radiation. The essential components of all lasers are described as the active medium, excitation mechanism, high reflectance mirror, and partially transmissive mirror. Examples of different types of active media and pump sources are given. The document also explains the function of the optical resonator formed by two mirrors around the active medium.

1. MODULE-2
Lasers

2. Lecture-1
• Introduction
• Laser Characteristics

3. S

4. What is Laser?
Light Amplification by Stimulated Emission of
Radiation
• A device produces a coherent beam of optical
radiation by stimulating electronic, ionic, or
molecular transitions to higher energy levels
• When they return to lower energy levels by
stimulated emission, they emit energy.

5. 5
Incandescent vs. Laser Light
1. Many wavelengths
2. Multidirectional
3. Incoherent
1. Monochromatic
2. Directional
3. Coherent

6. Characteristics of Laser
• Monochromatic
Concentrate in a narrow range of wavelengths (one specific
colour). 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.
• Coherent
All the emitted photons bear a constant phase relationship
with each other in both time and phase.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.

7. Characteristics of Laser
• Directional
A very tight beam which is very strong and
concentrated. 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.
High Intensity. It has been that a 1-mW He-Ne laser is
hundreds of times “brighter” than the sun. As difficult
as this may be to imagine, calculations for luminance
or visual brightness of a typical laser, compared to the
sun, substantiate these claims.

8. 8
Essential 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.

9. Components of laser

10. Pump Source
• Provides energy to the laser system
• Pumps can be optical, electrical, chemical or thermal in
nature. Examples: electrical discharges, flashlamps, arc
lamps and chemical reactions.
• The type of pump source used depends on the gain
medium. The important parameters governing this
type of pumping are the electron excitation cross-
sections and the lifetimes of the energy levels.
• →A helium-neon (HeNe) laser uses an electrical
discharge in the helium-neon gas mixture.
→Excimer lasers use a chemical reaction.

11. Active Medium: The amplifying medium or laser
medium is an important part of the laser device
• Major determining factor of the wavelength of
operation of the laser.
• Excited by the pump source to produce a population
inversion.
• Where spontaneous and stimulated emission of
photons takes place.
The most important requirement of the amplifying medium
is its ability to support a population inversion between
two energy levels of the laser atoms.
• Example:
solid, liquid, gas and semiconductor.

12. Optical Resonator
• Two parallel mirrors placed around the active
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.