LASERs: Why is population inversion required for amplification?

[TheOfficialAB]

Hi,

I'm doing a module on quantum optics and lasers. Most of the texts and online source I have read simply state that 'population inversion is essential for amplification in laser operation'.

My question is, why? Amplification/Gain is essentially the result of stimulated emission.
Can't stimulated emission occur without population inversion?

Wouldn't a population of atoms in a higher state that is equal to or less than 50% (but still sufficient) be enough to have a high enough chance of stimulated emission if an adequate photon comes along?
Theoretically, couldn't one atom in a higher state be enough to produce stimulated emission if the correct photon interacts with that atom?

I feel I might not be understanding something about the rate of absorption (for ground state atoms to reach a meta-stable higher energy level) and the rate of spontaneous decay back to ground state.

Could someone please clarify this?
Thanks for your time.

AB

 

[Khashishi]

Without population inversion, you have more absorption than stimulated emission, so your gain is less than one.

 

[blue_leaf77]

Theoretically, couldn't one atom in a higher state be enough to produce stimulated emission if the correct photon interacts with that atom?

Yes, it's enough. But how would you bring this atom to the excited state if not by absorption?

 

[Henryk]

There are actually a number of ways to produce a population inversion. The first laser, ruby laser used optical pumping and three levels. The dopant in ruby was pumped to a highest level, which decayed to a second level creating an inversion between the second level and the ground level.
He-Ne laser the energy comes from an electrical discharge which excites He atoms that transfer energy to Ne atoms creating inversion there.
Diode laser use carrier injection in a p-n junction to create inversion between electrons in the conduction and valence band. There are also chemical lasers a a few others.