Monday, May 14, 2012

How lasers work

A laser is effectively a machine that makesbillions of atoms pump out trillions of photons (light particles) all at once so they line up to form a really concentrated light beam.
A red laser contains a long crystal made of ruby (shown here as a red bar) with a flash tube (yellow zig-zag lines) wrapped aroundit. The flash tube looks a bit like a fluorescent strip light, only it's c...oiled around the ruby crystal and it flashes everyso often like a camera's flash gun .
How do the flash tube and the crystal makelaser light?
1. A high-voltage electric supply makes the tube flash on and off.
2. Every time the tube flashes, it "pumps" energy into the ruby crystal. The flashes it makes inject energy into the crystal in the form of photons.
3. Atoms in the ruby crystal (large green blobs) soak up this energy in a process called absorption . When an atom absorbsa photon of energy, one of its electrons jumps from a low energy level to a higherone. This puts the atom into an excited state, but makes it unstable. Because the excited atom is unstable, the electron can stay in the higher energy level only for a few milliseconds. It falls back to its original level, giving off the energy it absorbed as a new photon of light radiation (small blue blob). This process iscalled spontaneous emission .
4. The photons that atoms give off zoom up and down inside the ruby crystal, traveling at the speed of light.
5. Every so often, one of these photons hits an already excited atom. When this happens, the excited atom gives off two photons of light instead of one. This is called stimulated emission . Now one photon of light has produced two, so the light has been amplified (increased in strength). In other words, " l ight a mplification" (an increase in the amount of light) has been caused by " s timulated e mission of r adiation" (hence the name"laser", because that's exactly how a laser works!)
6. A mirror at one end of the laser tube keeps the photons bouncing back and forth inside the crystal.
7. A partial mirror at the other end of the tube bounces some photons back into thecrystal but lets some escape.
8. The escaping photons form a very concentrated beam of powerful laser light.

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