= Laser
{tag=Light source}
{wiki}
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: <Why can't you collimate incoherent light as well as a laser?>{full}
* can be phase and polarization coherent, though it is not always the case? TODO.
\Video[https://www.youtube.com/watch?v=_JOchLyNO_w]
{title=How Lasers Work by Scientized (2017)}
{description=
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.
* https://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
* https://youtu.be/_JOchLyNO_w?t=517 <stimulated emission>. This is the key to laser formation as it produces coherent photons.
* https://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.
* https://youtu.be/_JOchLyNO_w?t=832 the parallel mirrors select perpendicular photons preferentially
}
Bibliography:
* https://phys.libretexts.org/Courses/University_of_California_Davis/UCD%3A_Physics_9HE_-_Modern_Physics/06%3A_Emission_and_Absorption_of_Photons/6.3%3A_Lasers His <Rate My Professors> is amazing: https://www.ratemyprofessors.com/ShowRatings.jsp?tid=1783467