getting my lasers on

So I finally allotted some time to find out about lasers. What they are, how they work, and what they actually do. I think alot of people take alot of "common" knowledge for granted. I think lasers are an example of this. Everyone "knows" what a laser is, but I doubt if anyone really knows how they function and what they really are. So, here is my (someone else's) best explanation. There is much to be read on the subject, but I feel I am getting close to understanding those wonderful lasers.

A laser is composed of an active laser medium, or gain medium, and a resonant optical cavity. The gain medium transfers external energy into the laser beam. It is a material of controlled purity, size, and shape, which amplifies the beam by the quantum mechanical process of stimulated emission, discovered by Albert Einstein while researching the photoelectric effect. The gain medium is energized, or pumped, by an external energy source. Examples of pump sources include electricity and light, for example from a flash lamp or from another laser. The pump energy is absorbed by the laser medium, putting some of its particles into high-energy ("excited") quantum states. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved. In this condition, an optical beam passing through the medium produces more stimulated emission than the stimulated absorption, so the beam is amplified. An excited laser medium can also function as an optical amplifier.

The light generated by stimulated emission is very similar to the input signal in terms of wavelength, phase, and polarization. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and monochromaticity established by the optical cavity design.

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