TYPES OF

PREPARED BY V.REVATHIAMBIKALECTURER IN PHYSICS

INTRODUCTION OF LASER

L – LIGHT

A – AMPLIFICATION

S – STIMULATED

E – EMISSION

R - REDIATION

A. L. SCHAWLOW and C. H. TOWNES IN 1958

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 Spontaneous emission Stimulated emission

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MECHANISMS OF LIGHT EMISSION

1. Absorption2. Spontaneous Emission3. Stimulated Emission

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For atomic systems in thermal equilibrium with their surrounding, the emission of light is the result of:

Absorption

And subsequently, spontaneous emission of energyThere 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 of light emission:

04/12/2023LASER FUNDAMENTALS

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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

2. Multidirectional

3. Incoherent

1. Monochromatic

2. Directional

3. Coherent

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COMMON COMPONENTS OF ALL LASERS

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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.

LASER COMPONENTS0

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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).

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.

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LASING ACTION DIAGRAM

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En

erg

y

Intr

od

ucti

on

Ground State

Excited State

Metastable State

Spontaneous Energy Emission

Stimulated Emission of Radiation

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

CHEMICAL PROCESS

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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.

04/12/2023PART 2:LASER HAZARDS

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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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LASER CLASS

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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

High ReflectanceMirror (HR)

Output CouplerMirror (OC)

ActiveMedium

Output Beam

Excitation Mechanism

Optical Resonator

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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.

COMPARISON CHART FOR ALL THE LASERS

Characteristics

Nd-YAG laser He-Ne laser CO2 laser Semiconductor (Ga-As) laser

Type Doped insulator laser(solid state laser)

Gas laser Molecular gas laser

Semiconductor laser

Active medium

Yttrium Aluminium Garnet (y3Al5O12)

Mixture of Helium and Neon in the ratio 10:1

Mixture of CO2, N2 and Helium (or) water vapour

P-N junction diode

Active centre

Neodymium(Nd3+ ions) Neon CO2 Recombination of electrons & holes

Pumping method

Optical pumping Electrical pumping

Electric disharge method

Direct pumping

Optical resonator

Ends of the rods polished with silver and two mirrors. One of them is to totally reflected and the other is partially reflecting

Pair of concave mirrors

Metallic mrror of gold (or) silicon mirrors coated with aluminium

Junction of diopdes-polished

Power output

2* 104 watts 0.5-50 mW 10 k W 1 m W

Nature of output

Pulsed Continuous waveform

Continuous (or) pulsed

Pulsed (or) continuous wave form

wavelength

1.064 µm 6328 A0 9.6 µm &10.6 µm

8400A0

- 8600A0

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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!

DIFFERENCE BETWEEN A PHOTOGRAPHY & HOLOGRAPHY

S.No photography Holography

1. Photography is a 2-dimensional recording process

Holography is a 3-dimensional recording process

2. Ordinary light can be used for recording

Only laser beam should be used for 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 needs printing

To get the positive of the image it 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 LASERWARNING LABELS

Symbol and Border: BlackBackground: Yellow

Legend and Border: BlackBackground: Yellow

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INVISIBLE LASER RADIATIONAVOID EYE OR SKIN EXPOSURE

TO DIRECT OR SCATTERED RADIATIONCLASS 4 LASER PRODUCT

WAVELENGTH 10,600 nmMAX LASER POWER 200 W

EN60825-1 1998

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CDRH CLASS WARNING LABELS

CLASS II LASER PRODUCT

Laser RadiationDo Not Stare Into Beam

Helium Neon Laser1 milliwatt max/cw

CLASS IV Laser Product

VISIBLE LASER RADIATION-AVOID EYE OR SKIN EXPOSURE TO DIRECT OR SCATTERED RADIATION

Argon IonWavelength: 488/514 nmOutput Power 5 W

Class IIClass IIIa with expanded beam

Class IIIa with small beamClass IIIbClass IV

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