Lasers don't need cavities. (This may not be a universally held opinion.) X-ray lasers, nitrogen lasers, and astrophysical masers are examples I've seen. http

Lasers don't need cavities. (This may not be a universally held opinion.) X-ray lasers, nitrogen lasers, and astrophysical masers are examples I've seen. https://en.wikipedia.org/wiki/X-ray_laser https://en.wikipedia.org/wiki/X-ray_laser https://en.wikipedia.org/wiki/Nitrogen_laser https://en.wikipedia.org/wiki/Nitrogen_laser https://en.wikipedia.org/wiki/Astrophysical_maser https://en.wikipedia.org/wiki/Astrophysical_maser