Why is four-level laser more efficient than three-level laser? In similar question ( Would a laser with four possible energy levels be better than three? ), one explains that it is because laser transition does not end in ground state in four-level laser system. But I can't understand why this is the reason. Our purpose is maintaining more electrons in excited level (metastable level in this case) than in ground level. In four-level laser level, after transition into extra metastable level which is close to ground level, transition from this level to the ground level is very fast. So it is negligible. This means the time consumed in transition from upper metastable level to the ground state is almost equal whether lower metastable level exists or not. I think that maintaining more electrons in upper metastable level than ground state, not lower metastable level, is important. What did I misunderstand?
 A: Energy level transitions work both ways. Electrons can drop down from the upper level to the lower level and release radiation, and they can go up from the lower level to the upper level by absorbing radiation.
To have a working laser, you only want the emission and not the absorption. That means you want to keep the upper level full and the lower level empty - i.e. you want to make sure there are lots of electrons in the upper state and not many electrons in the lower state. If absorption could happen, then every time one atom released some radiation, it would just be absorbed by a nearby atom and this wouldn't make a laser.
If the lower state is the ground level, you'll have to supply a lot of energy to keep as many electrons as possible out of the lower state.
If the lower state is somewhat above the ground level, and it's a state where electrons quickly transition to the ground level, it'll keep itself empty or nearly empty and therefore absorption of the laser radiation cannot happen.
A: There is no definite answer to your question.
The 1st would be: in terms of the inversion threshold (power or intensity), a 4-level lasing scheme might generally be more efficient in the sense that inversion is achieved at lower pump powers/intensities than in a 3-level lasing scheme.
The 2nd answer could be: in terms of power conversion (input to output power ratio), the 3-level scheme tends to be more efficient.
See my answer here for an heuristic explanation.
Obviously, exceptions will apply.
