Leroy's a nerd, Leroy's a nerd, Leroy's a nerd.
I know you are, but what am I?
Not bad Fork.
The key part about amplification, is the electron energy state POPULATION INVERSION. Everything about how a laser works is PI. Electrons live in their lowest energy states, that's just how the uni works. sometimes they get "bumped" into higher or excited energy states ( a different orbital shell), but they never stay there long (nano to microseconds) and drop back down releasing a photon of specific energy (i.e wavelength) as they do.. The key to lasers is that the outer electrons are kept in a state of excitation by the initial energy pump (usually electricity, but can be light too), and the "mirror" effect, which keeps the light inside, so those photons keep bumping the electrons back up into excited states. Thus a PI.
So you have electrical or light STIMULATION (input energy), then you have a cavity where all the electrons of some material can be excited and you have reflective surfaces ("mirrors"
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
to kept the resulting photons bouncing back and forth, to keep the electrons excited, so they keep dropping down and making more photons.
Now if i make one of the mirrors 0.1% less then perfect reflection, some light gets out. that's the LASER light we "see". 99.9% of the laser light is trapped inside the cavity and NEEDS to be or else the feedback loop fails and the lasing action stops. Lasers are god awful inefficient!!!
Now 2 things can happen to stop lasing:
1. open one of the mirrors and let ALL the light out (AMPLIFICATION stops)
or
2. turn off our input energy (STIMULATION), the photons bounce back and forth, but nature is NEVER without drag or resistance, so the photons scatter and deplete or don't perfectly hit the next electron, so it doesn't go to the right excitation level, etc, etc. Within microseconds the photons are lost and lasing stops.
Now some details about the "mirrors" and why I use quotes around that word. A "mirror" is anything that reflects light, right? When you look out a window direct on you see right through it right? but if you look at it at an ANGLE, it can be almost reflective...like a mirror! Like looking at a calm lake, you can see the whole horizon reflected in it! But is water a mirror?
So the word "mirror" is subjective when it comes to explaining lasers. It really just means "reflective to that wavelength of light".
Edit: i will explain more about cavities and mirrors if anyone is interested.