LASER

( Light Amplification by Stimulated Emission Radiation )

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IntroductionThe first theoretical foundation of LASER and MASER was given by Einstein in 1917 using Plank’s law of radiation that was based on probability coefficients (Einstein coefficients) for absorption and spontaneous and stimulated emission of electromagnetic radiation. Theodore Maiman was the first to demonstrate the earliest practical laser in 1960after the reports by several scientists, including the first theoretical description of R.W. Ladenburg on stimulated emission and negative absorption in 1928 and its experimental demonstration by W.C. Lamb and R.C. Rutherford in 1947 and the proposal of Alfred Kastler on optical pumping in 1950 and its demonstration by Brossel, Kastler, and Winter two years later. Maiman’s first laser was based on optical pumping of synthetic ruby crystal using a flash lamp that generated pulsed red laser radiation at 694 nm. Iranian scientists Javan and Bennett made the first gas laser using a mixture of He and Ne gases in the ratio of 1 : 10 in the 1960. R. N. Hall demonstrated the first diode laser made of gallium arsenide (GaAs) in 1962, which emitted radiation at 850 nm, and later in the same year Nick Holonyak developed the first semiconductor visible-light-emitting laser.

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LASER COMPONENTS

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ACTIVE MEDIUM

Solid (Crystal)GasSemiconductor (Diode)Liquid (Dye)

EXCITATION MECHANISM

Optical ElectricalChemical

OPTICAL RESONATOR

HR Mirror andOutput Coupler

• The Active Medium contains atoms which can emit light by

stimulated emission.

• The Excitation Mechanism is a source of energy to excite the

atoms to the proper energy state.

• The Optical Resonator reflects the laser beam through the active

medium for amplification.

Nd (Neodymium) – YAG (Yttrium Aluminium Garnet) LASER

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Principle :

Doped Insulator laser refers to yttrium aluminium garnet doped with neodymium. The Nd ion has many energy levels and due to optical pumping these ions are raised to excited levels. During the transition from the metastable state to E1, the laser beam of wavelength 1.064μm is emitted.

Type : Doped Insulator Laser

Active Medium : Yttrium Aluminium Garnet

Active Centre : Neodymium

Pumping Method : Optical Pumping (Xenon Flash Pump)

Optical Resonator : Ends of rods silver coated

Two mirrors partially and totally reflecting

Power Output : 20 Kilowatts

Nature of Output : Pulsed

Wavelength Emitted : 1.064 μm

Characteristics :

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Nd : YAG LASER Diagram

Non radioactive decay

Laser

1.064μm

Non radioactive decay

E3

E2

E0

E1

E

4

Nd

Energy Level Diagram of Nd : YAG laser

E1, E2, E3 – Energy levels of Nd

E4 – Meta Stable State

E0 – ground State Energy Level

Application of Nd : YAG Laser

These lasers are used in many scientific applicationswhich involve generation of other wavelengths of light.

The important industrial uses of YAG and glass lasers havebeen in materials processing such as welding, cutting,drilling.

Since 1.06 m wavelength radiation passes throughoptical fibre without absorption, fibre optic endoscopeswith YAG lasers are used to treat gastrointestinal bleeding.

YAG beams penetrate the lens of the eye to performintracular procedures.

YAG lasers are used in military as range finders and targetdesignators.

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CO2 ( Carbon dioxide ) LASERPrinciple :

The transition between the rotational and vibrational energy levels lends to the construction of a molecular gas laser. Nitrogen atoms are raised to the excited state which in turn deliver energy to the CO2 atoms whose energy levels are close to it. Transition takes place between the energy levels of CO2 atoms and the laser beam is emitted.

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Type : Molecular gas laser

Active Medium : Mixture of CO2, N2, He or H2O vapour

Active Centre : CO2

Pumping Method : Electric Discharge Method

Optical Resonator : Gold mirror or Si mirror coated with Al

Power Output : 10 kW

Nature of Output : Continuous or pulsed

Wavelength Emitted : 9.6 μm or 10.6 μm

Characteristics :

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Symmetric C - stationary

O - vibrates simultaneously

along molecular axis

Bending C & O vibrate perpendicular to

molecular axis

Asymmetric Stretching C & O atoms vibrate in

opposite directions along

molecular axis

A carbon dioxide (CO2) laser can produce a continuous laser beam with a power output ofseveral kilowatts while, at the same time, can maintain high degree of spectral purity and spatialcoherence.

In comparison with atoms and ions, the energy level structure of molecules is morecomplicated and originates from three sources: electronic motions, vibrational motions androtational motions. Modes of vibration in CO2

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The energy level diagram of vibrational – rotational

energy levels with which the main physical processestaking place in this laser.

As the electric discharge is passed through the tube,

which contains a mixture of carbon dioxide, nitrogen

and helium gases, the electrons striking nitrogen

molecules impart sufficient energy to raise them to

their first excited vibrational-rotational energy level.

This energy level corresponds to one of the vibrational - rotational level of CO2 molecules, designated as level 4.

Collision with N2 molecules, the CO2 molecules are raised to level 4.

The lifetime of CO2 molecules in level 4 is quiet significant to serve practically as a metastable state.

Diagram

Energy Level Diagram of CO2

Hence, population inversion of CO2 molecules is establishedbetween levels 4 and 3, and between levels 4 and 2.

The transition of CO2 molecules between levels 4 and 3 producelasers of wavelength 10.6 microns and that between levels 4 and 2produce lasers of wavelength 9.6 microns.

The He molecules increase the population of level 4, and also help inemptying the lower laser levels.

The molecules that arrive at the levels 3 and 2 decay to the groundstate through radiative and collision induced transitions to the lowerlevel 1, which in turn decays to the ground state.

The power output of a CO2 laser increases linearly with length. Lowpower (upto 50W) continuous wave CO2 lasers are available in sealedtube configurations.

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Application of CO2 Laser• Because of the high power levels available (combined with reasonable cost for the laser),

CO2 lasers are frequently used in industrial applications for cutting and welding, while lower power level lasers are used for engraving.

• They are also very useful in surgical procedures because water (which makes up most biological tissue) absorbs this frequency of light very well. Some examples of medical uses are laser surgery and skin resurfacing ("laser facelifts", which essentially consist of vaporizing the skin to promote collagen formation). Also, it could be used to treat certain skin conditions such as hirsuties papillaris genitalis by removing embarrassing or annoying bumps, podules, etc. Researchers in Israel are experimenting with using CO2 lasers to weld human tissue, as an alternative to traditional sutures.

• The common plastic poly (methyl methacrylate) (PMMA) absorbs IR light in the 2.8–25 µm wavelength band, so CO2 lasers have been used in recent years for fabricating microfluidic devices from it, with channel widths of a few hundred micrometers.

• Because the atmosphere is quite transparent to infrared light, CO2 lasers are also used for military rangefinding using LIDAR techniques.

• CO2 lasers are used in the Silex process to enrich uranium.

• The Soviet Polyus was designed to use a megawatt carbon-dioxide laser as an orbit to orbit weapon to destroy SDI satellites.

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