How Lasers Work

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How Helium Neon (HeNe) tube lasers work:

Tube lasers are glass tubes filled (to a partial vacuum) with a combination of gasses that will emit photons when energized by electrodes at either end of the tube. Both Neon and Florescent tubes work on this principal.

When the gas is energized by electrical current it applies enough energy to the HeNe to cause the electron in the molecules to drop to a lower vallence shell. When the electron drops down one level part of the energy exchange causes it to emit a photon. When the photons (squiggily arrows in diagram) emitted from the molecule hit a second molecule the additional energy from the collision causes the second molecule to emit an additional photon. This causes an avalanche effect of an increasing number of photons.

Lasers have partially transparent mirrors on either end of the tube. If the photons are traveling toward the side of the tube they pass through the side as normal light. If the photo avalanche is traveling parallel to the tube it will bounce off the mirrors on either end of the tube further amplifying the avalanche effect. When the photons reach a sufficient intensity they will pass through the mirror as laser light.

The photons that HeNe emits are of very narrow red wavelength. Because the photons are traveling in parallel they do not diverge as they do in "normal" light. This is what keeps a laser beam narrow over long distances. Because the light is all of the same wavelength and parallel all of the peaks and troughs of the light wave are in sync with each other. This causes the effect known as "coherent light˛.

You can see an effect of the coherence when you look at the laser light reflected off an object. You will notice a slight grain know as "speckle" that looks similar to photographic film grain. The speckle appears to be slightly in front or behind the surface the laser light is projected against. You see the speckle because the lightwave peaks and troughs are in sync at approximately the same wavelength are only a small number of nanometers wide. At this scale the surface you are projecting against is rough so each photon is traveling a slightly different distance before being reflected back producing null points where the waves cancel each other out and bright points where the waves are reinforcing each other, this looks like a fine grain or specklel pattern.

If you have question, comments or criticisms please email me. greg@gregdowning.com

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