# Would a laser with four possible energy levels be better than three?

I'm wondering about achieving population inversion for a laser. I learned that without an active medium, it's not possible to create a laser with only two energy levels, but it would be possible with three. What is the advantage of having four levels then?

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That is just some non-radiative transition (e.g. through collisions) that, if faster than decay from E$_3$ to E$_2$, guaranties that the population inversion will be achieved. –  Ondřej Černotík Feb 14 '13 at 8:14