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64. LASER CUTTING

64.1 LASERS

•Light Amplification by Stimulated Emission of Radiation.

•When are they best used?

-when highly focussed energy is required (light or heat)

-when contact forces must be eliminated

-when a small geometry is required

•What do LASERs do?

-Produce collimated light - all of the light rays are (nearly) parallel. This means the light doesn’t diffuse quickly like normal light.

collimated light

diffuse light

-Monochromatic - because the light is generated using specific gases, the frequency (wavelength) has a specific value. Normal white light tends to contain a wide mixture of different frequencies (a wide spectrum), but laser light is very specific.

-The light has significantly less power than a normal light bulb, but it is highly focussed, thus delivering a significantly higher light intensity.

•The principle behind lasers are

1. Excitation of light emission by electrical discharge.

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1. electrical charge moves an electron

to a higher energy orbit

2. electron is unstable in higher orbit and falls back to lower obit,

3.

As the electron falls, a photon is emitted

2. Resonance - the laser chamber has reflecting ends separated by a multiple of half wavelengths one end is completely reflecting, and the other end is partially reflecting. The result is a reflection that leads to resonance.

light is emitted by atoms

light is trapped in resonance between mirrors

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- light reflecting between mirrored ends

over time

•The height of the orbit the electron is in determines the wavelength of the photon. Larger atoms have higher orbits, therefore longer wavelengths (infrared). Smaller atoms have shorter falls, therefore shorter wavelengths (Ultraviolet).

•The electrons are caused to jump by a discharge of electrons with a potential charge in the range of KV.

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AN AXIAL FLOW CO2 LASER WITH COOLING

• Various gases are used in Lasers. The contents of a laser can be a single gas, or a combination of gases.

- e.g. in a CO2 laser, CO2 is used to produce light with a 10.6 micrometer wavelength. Nitrogen is used to maintain electron populations in the upper valence shell of the CO2 molecules. Helium is used as an intracavity cooling agent.

•Lasers are very inefficient and build up excessive heat. If this heat becomes high enough it will effect the performance, and eventually damage the laser. To counteract this, heatsinks, water, and other forms of heat dissipation are used.

•The lasers often have sensors to shutdown when the temperatures become too high.

•1 Angstrom A = 10-10m

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•

transition energy levels for electrons

E3

E3-1

E2-1 E1

E0

• Energy of a photon

c E = hf = h --λ

h = 6.6× 10–34Js = Planks constant

f = frequency of light

λ= wavelength of light

•Absorption is when energy causes an electron to accept enough energy to jump up one or more energy levels.

•Spontaneous emission is the drop of the electron to a lower energy orbit, and the release of the energy change as a photon.

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E1

• Absorption can be caused by energy sources, such as light, but it is also caused by the heat of an object. (as with incandescent lights)

T= temperature (K)

ε= emissivity factor dependant on frequency [0, 1], table lookup

Wein’s law predicts the maximum emitted frequency.

λ maxT = 2.9× 10–3mK

e.g. normal light sources have,

• We can draw out a spectrum for frequencies emitted.

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•How a laser works,

1.The electrical/light discharges are used to cause electron population inversion and cause a few spontaneous emissions of photons.

2.The new photons travel in all directions, but some travel toward the mirrors, where they are reflected back and forth between the mirrors.

3.As the photons travel, they cause the generation of other photons travelling in the same direction.

4.This builds until the laser has a high intensity output.

5.The output beam escapes through one end of the laser that has a half silvered mirror.

•Laser light is polarized

x

y

direction of propagation

two different photons with the same phase, but polarized 90 degree to one another

• Various lasers are suited to different applications.