BASICS OFLASERS AND LASER

LIGHT

Jayant K. JogiAsst. Professor in Physics,L. E. College,MORBI.

L ight

A mplification by

S timulated

E mission of

R adiation

The word LASER is an acronym for

Depending upon the active medium used there are FOUR types of LASER.

1. Solid LASER

2. Liquid LASER

3. Gas LASER

4. Free Electron LASER

Types of LASER

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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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Incandescent v/s Laser Light

1. Many wavelengths

2. Multidirectional

3. Incoherent

1. Monochromatic

2. Directional

3. Coherent

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Common Components of all Lasers1. Active Medium

The active medium may be solid crystals such as ruby or Nd:YAG, liquid dyes, gases like CO2 or He - Ne, 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 Leaky Mirror

A mirror which reflects less than 100% of the laser light and transmits the remainder.

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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).

(Totally Reflecting)

(Partially Leaky/Partially Reflecting)

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• Before discussing the action of LASER below mentioned three processes are important to discuss.

1. Absorption

2. Spontaneous Emission

3. Stimulated Emission

Absorption of a Photon by an Atom

E0E0

EX EX EX - E0 = hf

Before After

Here, E0 and EX are energy of the Ground State and Excited State of an atom respectively.

Difference between

Spontaneous Emission and

Stimulated Emission

Spontaneous Emission

Stimulated Emission

E0 E0

E0E0

EX EX

EX EX

After

AfterBefore

Before

Stimulating Photon

These Photons are identical in

every way

EX - E0 = hfEX - E0 = hf

EX - E0 = hf

Metastable State Normally, the mean life of excited atom before spontaneous

emission occurs is about 10-8 sec. However, for some excited states, this mean life is perhaps as much as 105 times longer (i.e.,10-8 sec). These states are called Metastable States.

Population Inversion Population inversion is a state in which more number of atoms

are in excited state compared to the ground state. To achieve population inversion Optical Pumping is required.

To obtain continuous LASER beam Metastable State and Population Inversion are required.

Population Inversion Normal Population

Optical Pumping The process of exciting electrons from the ground to the

metastable state is called Optical Pumping.

Population Inversion and Normal Population are shown below.

E0 E0

EX EX

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Lasing Action1. 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 DiagramEn

erg

y

Intr

od

ucti

on

Ground State

Excited State

Metastable State

Spontaneous Emission

Stimulated Emission of Radiation

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ND:YAG Laser

Construction:

Working:

HELIUM-NEON GAS LASER

ND:YAG Laser

Ruby (Al2O3) Laser

Radio

Long WavelengthShort Wavelength

Gamma Ray X-ray Ultraviolet Infrared Microwaves

Visible

ELECTROMAGNETIC SPECTRUM

Lasers operate in the ultraviolet, visible, and infrared.

Radio

RedBlue YellowGreen

It is used as a convenient drill to bore holes in diamond. It is used in long distance surveying where great precision is

required. Light received from small laser (about 200 mW) is used in

optical fibers for communications. Large lasers are used in nuclear fusion and for military and

research purpose. Lasers are also used in bar code reading and in reading and

writing CDs and DVDs. Lasers are used in garment industries in cutting cloths and also

in automobile industries. In the field of medicine, laser is being used for the retina

detachment surgery, blood vessel cut etc. Lasers are also used in various types of alarm systems. Lasers are used in LIDAR (Light Detection and Ranging).

Some Applications

Laser Scanners

Laser Pointers

Many Thanks