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Are there natural processes we've discovered which lead to laser light of some distinct wavelength?

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    $\begingroup$ I've found a couple of papers suggesting the answer is yes. This seems to occur in stars and planetary atmospheres. Unfortunately this isn't my field so I can't really comment further: Here, here, and here are examples taken from the references of this paper. $\endgroup$ – or1426 Aug 22 '15 at 16:34
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    $\begingroup$ The division between "natural" and "artificial" is pretty artificial. Obviously, humans are the dominant natural process that leads to the occurence of lasers. $\endgroup$ – ACuriousMind Aug 22 '15 at 16:44
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    $\begingroup$ Also, there is the semantic distinction between "laser light" and "a laser" (the latter involving feedback). It seems as if the natural processes mentioned so far might be best described as amplified spontaneous emission, which is technically "laser light". But lacking the same coherence properties typically associated with lasers, these processes are perhaps not what the question intended? Is the question simply "does stimulated emission ever happen naturally"? $\endgroup$ – Gilbert Aug 30 '17 at 4:31
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    $\begingroup$ Lasers in nature discovered on Mars. Here's a reference. laserstars.org/history/mars.html $\endgroup$ – Godfrey Kwan Aug 30 '17 at 5:04
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    $\begingroup$ Do masers count? $\endgroup$ – honeste_vivere Sep 1 '17 at 14:35
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Yes there are. All you really need is for energy to be injected into a system, and for the particles to linger for relatively long periods of time in some excited state. For example, if you have states $A$, $B$, and $C$ in order of increasing energy, natural pumping might work well for $A \to C$ and poorly for $B \to C$. Suppose $C \to B$ occurs rapidly via spontaneous emission, but $B \to A$ takes a long time. You will end up with lots of state $B$ lying around, ripe for stimulated emission.

In particular, we see masers in all sorts of environments in space, including around active galactic nuclei powered by supermassive black holes and in star-forming regions. These tend to be microwave transitions between rotational/vibrational states in small molecules. Astrophysical sources are good places to look for natural population inversions, as explained well by that second link:

At low densities, being out of thermal equilibrium is more easily achieved because thermal equilibrium is maintained by collisions, meaning population inversion can occur. Long path lengths provide photons traveling through the medium many opportunities to stimulate emission, and produce amplification of a background source of radiation.

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