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• SEMICONDUCTOR LASERS

• EXCIMER LASER

• FREE ELECTRON LASER

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Laser heterostructure (a) Schematic projection (b) Refractive index profile

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Heterostructures• The heterostructure laser is a laser diode with more than single P and N layers.

•GaAs/AlGaAs is a heterojunction laser.

•The notations P+ and N+ and P- and N- indicate heavy doping and light doping respectively. The P-N structure consists of the two double layers, P+ - P- and N+ - N- .• A thin layer of GaAs is placed at the junction-the active region.

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The P and N regions are lightly doped regions that have an index of refraction n2 less than n1 of the active region. These three layers, n2-n1-n2, form a light waveguide much like the optical fiber, so that the light generated is confined to the active region.

Applications:

1. Communications

2. CD players

3. Pumping solid state lasers.

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Excimer laser

• Some diatomic molecules are unstable in the ground state

• They form excited “Dimers” which are stable.

• Eg. Xe2*,Kr2*,Ar2*

• “Excited Dimers”-Excimer

• “Excited Complex”- Exciplex

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• The excited compound can give up its excess energy by undergoing spontaneous or stimulated emission, resulting in a strongly-repulsive ground state molecule which very quickly (on the order of a picosecond) disassociates back into two unbound atoms.

•This forms a population inversion between the two states.

• Most "excimer" lasers are of the noble gas halide type • The wavelength of an excimer laser depends on the molecules used, and is usually in the ultraviolet region

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Excimer Wavelength

ArF 193 nm

KrF 248 nm

XeBr 282 nm

XeCl 308 nm

XeF 351 nm

CaF2 193 nm

KrCl 222 nm

Cl2 259 nm

N2 337 nm

Excimer Wavelength

Excimer lasers are usually operated with a pulse rate of around 100 Hz -8 kHz and a pulse duration of ~10-30 nsEnergy from 0.1-1J

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Construction:

•SS structure-Halogens are corrosive

•Discharge tube to excite gas to form excited dimer

•Highly reflecting mirror and quartz output coupler

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Typical excimer laser configuration

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Applications •Material processing

•Medical applications Laser surgery Corneal sculpting Dermatology

•Lithography- Microchip as small as 0.18-0.25 m

•Pumping other lasers

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Free electron laserIntroduction•Invented by J.M.J. Madey in 1971.

•Combination of particle-accelerator and laser physics .

•Electrons moving at relativistic velocity acts as a laser source

•Provide wide tunability - microwave,UV,X-ray

•High output power

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Free electron laser

• Electrons accelerated to relativistic speeds.

• Passes through undulator/Wiggler which produces ocillating field

• e assume a sinusoidal path

• Accelerated electron emits radiation

• The radiation emitted is in phase

• Mirrors used for feedback

• Frequency of laser depends on the energy of e and period of alternating magnetic field

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Applications

Medical

Military

Photochemical

Material processing

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Industrial applications

1. Drilling

•Use single or multiple pulses of the laser-percussion drilling

•Laser focussed to a small spot

•Holes of sized 10m to 1mm can be made

•Inert gas is blown to remove vaporized material

•Used to make holes in plastic, diamonds and aircraft engines

• High power pulsed or CW lasers are used.• CO2, Nd:YAG or Ruby lasers

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Industrial applications

2. Cutting

•Uses CW or pulsed CO2 laser

•A gas jet used to assist in cutting

•SS of 5mm thick with 500 W lasers

•Advantages- Precision,speed,special materials, lack of tool contact

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Industrial applications

3. Welding

•Uses CW CO2 or pulsed YAG laser

•High speed and low distortion

•Seam weld using CO2 and spot weld using Nd:YAG

•Inert gas used for cooling and prevent oxidation

4. Heat treatment: Surface hardnening of steel

5. Cladding Coating layers of different material on the surface

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Medical applications

1. Photodynamic therapy

•Destroys malignant tumors

•Chemical agents accumulate at the tumor cells

•Laser triggers photochemical reaction destroying the cells

2. Dermatology (low power IR)

3. Dentistry- root canal treatment, remove cavities

4. Gastric tumors- Nd:YAG

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Medical applications

5. Ophthamology

•Reattach tissues in the eye-Ar ion laser

•Treatment of cataract

•Excimer laser used for cutting the cornea

6. Surgery-cardiovascular, remove lesions or clots

Communications

Optical frequency= 1015 Hz

Larger bandwidth

Used in space communication