Laser

The key advantages of lasers over other light sources are:

it emits a very narrow range of frequencies (small linewidth), which for many purposes can be considered a single frequency
It does however have a small range of frequencies. The smaller the range, the better the laser quality.
it can be efficient collimated, while still emitting a lot of output power: Section "Why can't you collimate incoherent light as well as a laser?"
can be phase and polarization coherent, though it is not always the case? TODO.

Video 20. How Lasers Work by Scientized (2017) Source.

An extremely good overview of how lasers work. Clearly explains the electron/photon exchange processes involved, notably spontaneous emission.

Talks about the importance of the metastable state to achieve population inversion.

Also briefly explains the imperfections that lead to the slightly imperfect non punctual spectrum seen in a real laser.

youtu.be/_JOchLyNO_w?t=188 says LED is "also monochromatic", but that is not strictly true, it has way way larger frequency band than a laser. Only shorter compared to
youtu.be/_JOchLyNO_w?t=517 stimulated emission. This is the key to laser formation as it produces coherent photons.
youtu.be/_JOchLyNO_w?t=581 spontaneous emission happens too fast (100 ns), which is not enough time for stimulated emission to happen. Metastable electrons to the rescue.
youtu.be/_JOchLyNO_w?t=832 the parallel mirrors select perpendicular photons preferentially

Bibliography:

 Ancestors