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Phoenix666 [soylentnews.org] writes:

An accessible primer on photonics [rp-photonics.com]:

With this article, I want to show you a nice way to understand the basic operation principles of lasers – either for your own pleasure or when you try to explain it to beginners.

We begin by asking a seemingly unrelated question: How can we store light? Could we put some amount of light into some suitable kind of bucket, with which we could carry it around and use it later on?

An essential problem with storing light is that it moves away so fast. A relatively straightforward idea is to confine the light with some mirrors, which prevents it from escaping. In the simplest case, we would just use two highly reflecting mirrors in parallel, so that a light beam can be captured between those:
simple resonator

Such an optical arrangement is called an optical resonator. Ideally, the light would be perfectly reflected by the mirrors and circulate there forever.

One can easily imagine a number of problems with that approach, which however we will solve step by step.

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