How lasers work

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I know you take light and bounce it somehow but how do they get the intensity so high

In: Physics

Imagine you have a tube. At one end is a mirror that reflects 100% of light that hits it. At the other end is a mirror that is slightly transparent. Inside this tube we put a material called a *gain medium*. There are all sorts of gain mediums: crystals, gases, glass, etc. What’s important is that the material of the gain medium can be put into a state where it creates more ~~energy~~ **light** than it absorbs. The gain medium is energized so that the majority of its electrons go into an excited state.

As the light travels back and forth between the mirrors, it continually passes through the gain medium. When light passes through the gain medium, the medium *radiates* more light traveling in the same direction, which amplifies the light. This is called *stimulated emission*.

Eventually the light is amplified enough that the small portion of the light that escapes through the semi-transparent mirror is a powerful beam. This beam is made of **l**ight **a**mplified by **s**timulated **e**mission of **r**adiation, or a laser.

*Edit: In this house we obey the laws of thermodynamics!*

Let’s consider the acronym: Light Amplification by Stimulated Emission of Radiation. The part that’s relevant here is: what is stimulated emission? In your run-of-the-mill case, you have an atom that, by hook or crook, has an electron that is excited to a higher energy level. In general, what happens is that at a random time (though a well-defined average), that electron returns to its ground state and emits an photon in a random direction. The wavelength (think color) of this photon is determined by the energy of that return to ground. This is *spontaneous* emission.

There is a similar process, where one has the same excited atom, but a photon of the same frequency as the light emitted in the spontaneous emission happens to be passing by. What then happens is that the incoming photon causes the excited atom to immediately return to the ground state. The photon from this ground return transition emerges in phase with incoming photon, and goes in the same direction as the incoming photon. This is *stimulated* emission: on an atomic level, start with one photon, end up with two (nearly identical) photons (i.e., amplification).

As /u/bcustead describes, what is usually done is to place a gain media (a material containing lots of the atoms discussed previously) between two mirrors. On every round trip between the mirrors, the number of photons essentially doubles. The two kickers to focus on is on each pass, all the generated photons go in the same direction as the previous generation (thus giving the laser exceptional directionality) and those photons are in phase with one another (giving the laser an extremely small spread of frequencies, and a strong phase relationship (coherence) ).