The laser begins at the atomic level, and more specifically with the electron. When an electron becomes energized, it jumps to a higher level within the atom's structure. In a normal atom, all of the electrons fill the lowest energy states possible, and thus want to drop from an energized state to a base state. When an electron drops from a higher, energized state, it emits a photon as a byproduct. Lasers themselves are large groups of photons focused into a beam, but getting from a randomly moving photon generated from an electron to a nice beam is no easy task. From here, stimulated emission comes in.

An example of stimulated emission.
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Stimulated emission works like an amplifier. If we take a system with a large amount of atoms that have electrons in high energy states, the electrons will want to come down from those states into a lower one. Doing so emits a photon of a certain wavelength, depending on which energy state the electron began in and which state it dropped to. In order to get an electron to drop from a high state to a low one, we can make it interact with an electromagnetic wave, or another photon! Under the hood is a quantum process that allows us to essentially put in one photon and get two out of the exact same wavelength. This is our "stimulation" portion of Einstein's theory. When another photon interacts with the electron with a high energy atom, the electron will drop and emit two photons. This is our "emission". The entire process gives us an amplification of photons that can be used for our laser.

Diagram illustrating how photons are "aligned" and allowed through.
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But wait, how do we get our electrons to get energized in the first place? The answer: Flash Tubes. These tubes act as a "pump" to start driving the atoms in our lasing medium. They emit extremely intense bursts of light in very short amounts of time, and this full spectrum light energizes the laser medium. As stated before, now that we have energized electrons, we can get photons! Once the laser medium begins emitting photons, they need to be focused and lined up in order to establish our beam. To do this, we look to the physical structure of our laser.

A flash tube wrapped around a laser medium
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Most simple lasers have the same basic structure; the inside of a laser consists of a cylindrical tube of some medium that can be energized (such as a gas like helium or neon in a gas laser, or a crystal like ruby). This tube is surrounded by a flash tube, and has a 100% reflectivity mirror at one end to keep the photons inside the tube. The other end of the tube, the end that is actually emitting our beam, is capped with a semi-reflective mirror. This mirror allows photons to travel through it if they are moving in the right direction, but reflects the photons if they aren't. In some cases, a photon may travel back and forth billions of times before it leaves the medium.

Thomas Edwards - Physics 212x - F05 - 2011