laser class lectures for students

8/14/2019 LASER Class Lectures for Students

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Laser cavity

Laser medium

Energy source

Optical cavity

Irradiance: The power of the laser per unit area


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Construction


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Various active mediums

Atoms: helium-neon (HeNe) laser; heliumcadmium (HeCd) laser, copper

vapor lasers (CVL)

Molecules: carbon dioxide (CO2) laser, ArF and KrF excimer lasers, N2

laser

Liquids: organic dye molecules dilutely dissolved in various solvent

solutions

Dielectric solids: neodymium atoms doped in YAG or glass to make the

crystalline Nd:YAG or Nd:glass lasers

Semiconductor materials: gallium arsenide, indium phosphide crystals.


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Reflectors

Stable cavity

Unstable cavityStable cavity


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He-Ne laser : A 4 level laser

A helium-neon laser, usually called a He-Ne laser, is a type ofsmall gas laser. HeNe lasers have many industrial and scientific

uses, and are often used in laboratory demonstrations of optics

He-Ne laser is a four-level laser

Its usual operation wavelength is 632.8 nm, in the red portion of

the visible spectrum

It operates in Continuous Working (CW) mode


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Construction of He-Ne laser

The setup consists of a discharge tube (Pyrex glass) of length 80 cm and

bore diameter of 1.5 cm

The gain medium of the laser, as suggested by its name, is a mixture of

helium and neon gases, in a 5:1 to 20:1 ratio, contained at low pressure (an

average 50 Pa per cm of cavity length ) in a glass envelope

The energy or pump source of the laser is provided by an electrical

discharge of around 1000 volts through an anode and cathode at each endof the glass tube. A current of 5 to 100 mA is typical for CW operation

The optical cavity of the laser typically consists of a plane, high-reflecting

mirror at one end of the laser tube, and a concave output coupler mirror of

approximately 1% transmission at the other end. Brewster windows may beused at the end of the tube to minimize reflection losses

He-Ne lasers are normally small, with cavity lengths of around 15 cm up to

0.5 m, and optical output powers ranging from 1 mW to 100 mW

Diagram


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Brewster windows are used in laser cavities to reject the s-polarized

beam and to allow only linearly p-polarized light to be emitted. They

are designed for use at the Brewster angle where the reflectivepolarization effect is most efficient.

Brewster windows are windows designed for use at Brewster's angle

(usually uncoated). This is the angle where only incident p-polarized

light has 0% transmission loss.


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Energy levels and working principle

The left side of the representation shows the lower levels of the helium

atoms.The energy scale is interrupted and that there is a larger difference in

energy in the recombination process than is evident in the diagram

A characteristic of helium is that its first states to be excited, 21S1 and 21S0 are

metastable, i.e. optical transitions to the ground state 11S0 are not allowed,

because this would violate the selection rules for optical transitions. As a result

of gas discharge, these states are populated by electron collisions. These twolevels are considered as long-lived

A collision is called a collision of the second type (inelastic) if one of the

colliding bodies transfers energy to the other so that a transition from the

previous energy state to the next higher or lower takes place. Apart from theelectron collision of the second type there is also the atomic collision of the

second type. In the latter, an excited helium atom reaches the initial state

because its energy has been used in the excitation of a Ne atom. Both these

processes form the basis for the production of a population inversion in the Ne

system.


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19.

81eV

2061eV

16.7 eV

1e-7 sec

1e-8 sec

2s22p6


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When the excited Ne atom passes from metastable state(3s) to lower level(2p),

it emits photon of wavelength 632 nm

This photon travels through the gas mixture parallel to the axis of tube, it is

reflected back and forth by the mirror ends until it stimulates an excited Ne

atom and causes it to emit a photon of 632 nm with the stimulating photon

The stimulated transition from (3s) level to (2p) level is laser transition

This process is continued and when a beam of coherent radiation becomes

sufficiently strong, a portion of it escape through partially silvered end

The Ne atom passes to lower level 1s emitting spontaneous emission. and

finally the Ne atom comes to ground state through collision with tube wall and

undergoes radiationless transition


Applications: (a) Laser printer, (b) Bar-code reader, (c) Interferometry


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Gas laser: CO2 laser

It is a three level molecular gas laser and operates in continuous mode

Unlike the other lasers producing visible or short near-IR light, the output of a CO2

laser is medium-IR radiation at 10.6 mm and therefore, it is invisible

This laser is ideal for cutting, engraving, welding, heat treating, and other

industrial processing of many types of materials

The energy of this laser can go up to kW level for specific application

Instead of electronic energy levels, the vibrational levels take part in the laser

transition process. The levels are symmetric (0.163eV), asymmetric (0.276 eV)

and bending (0.078eV) modes of vibrations all of which are quantized associated

with several closely spaced rotational levels

DC excitation produces the pumping energy. Electron colliding with gas transfer

the energy.


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CO2 laser

Discharge tube of 2.5 cm dia and 5 meter length

DC excitation source as pumping energy

Gas mixture: CO2:N2:He = 1:4:5 with corresponding pressures 0.33 torr, 1.2 torr

and 7 Torr respectively

In the cavity, NaCl Brewster windows are used to allow only linearly p-polarized

light with 0% transmission loss

The confocal silicon mirrors with Al coating are used as reflectors in the cavity

Construction

Working principle

N2 plays the same role as He in He-Ne laser

High voltage plasma causes electrons to collide with N2. As a result, it vibrates insymmetric mode. Hence can not radiate their vibrational energy away effectively

Vibrational frequency of N2 molecules coincides very closely (within 2 meV) with

ground to upper laser level transition frequency of CO2. Hence N2 molecules

transfer very large fraction of their energy to pumping CO2 molecules by

resonant collision

The asymetric mode with levels (001) and (002) get the energy in such process

Diagram


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Carbon dioxide laser

Why helium?

Addition of He is effective in increasing laser power. Its low mass makes it effective

in trading its K.E. with CO2 molecules. Salient features are the following

Cooling the CO2 rotational temperature (but not the vibrational temperature)

Increasing the thermal conduction to the wall (thus keeping the translational

temperature low, the Doppler width small, and the gain high)

Increasing the depopulation rate of the 010 level of CO2 which in turn

depopulates the 100 lower level. This is because the 010, 020, and 100 levels

are all strongly coupled together through resonant collisions.


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Nd-YAG Laser

It is a solid state laser operating as four energy levels

It became popular in 1960`s

Nd:YAG lasers operate in both pulsed and continuous mode

The lasing medium is the colorless, isotropic crystal Y2Al

5O

12

(Yttrium-Aluminum Garnet - YAG).

When used in a laser, about 1% of the Yttrium is replaced by

Neodymium. The energy levels of the Nd3+ ion are responsible for

the lasing action. The rare earth ion does not affect the crystal

structuredue to similar size

Ytterbium, erbium, chromium etc. are also used as dopant in YAG

for laser operation


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1064mm

Nd-YAG Laser Energy Levels

The most common Nd:YAG

emission wavelength is 1064 nm.

Starting with that wavelength,

outputs at 532, 355 and 266nm

can be generated by frequency

doubling, frequency tripling and

frequency quadruplingrespectively.

Other emission lines are at 946,

1123, 1319, 1338 and 1444nm.

Crystal materials lacking

inversion symmetry


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Construction

Population inversion results from shining light on this crystal. If the

light is intense enough, atoms within the crystal that absorb this light

transition from ground state into the absorption bands This is oftendone with a krypton flash lamp.

The Nd-YAG laser is normally shaped as cylindrical crystal. The crystal

forms the laser cavity and has reflective ends - one coated so that it is

100% reflective, and the other is either sufficiently reflective, or is

coated to allow only part of the amplified light to pass - enough feed-back so that oscillation may occur. The following diagram may help

visualize this apparatus.


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ApplicationsMedicine

In oncology, it is used to remove skin cancer

It is used in opthalmology to correct posterior capsular opacification, a

complication of cataract operation

Cosmetic medicine

Dentistry: It is used for soft tissue surgeries in the oral cavity

Manufacturing

For engraving, etching, or marking a variety of metals and plastics.

Cutting and welding steel and various alloys.

For automotive applications (cutting and welding steel) the power levels

are typically 1-5 kW

Military

Military surplus Nd:YAG laser fires through a collimator, focusing the

beam, which blasts a hole through a rubber block, releasing a burst of

plasma.

The Nd:YAG laser is the most common laser used in military as laser

rangefinders

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