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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?

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  • $\begingroup$ I think (so not an answer) the linked answer is saying you need more atoms in E3 state than E2 to get lasing. It is easier to keep the E2 state empty, than to keep the ground state E1 empty. $\endgroup$ Oct 25, 2021 at 9:25
  • $\begingroup$ @user253751 Anyway the absorption (which is disruptive to amplification of the laser) is occurred by the electrons in the ground state, so I think that electrons in E2 state is not important. Furthermore, even if there exists some electrons in E2 state, it is relaxed to the ground state very fast. Then again absorption can be occurred. I can't understand the role of E2 state. $\endgroup$
    – Krang Lee
    Oct 25, 2021 at 9:29
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    $\begingroup$ Please take a look at my answer to this question and this question, will probably also give you some insights. I do not see, nowadays, 4-level systems being more efficient, mainly because we became too good at making laser diodes to pump laser systems and we can pump at the zero-phonon line which means almost no wasted energy. $\endgroup$ Oct 25, 2021 at 18:41

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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.

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  • $\begingroup$ In four-level laser system, once the electrons transition into lower metastable level, they readily relaxes to ground level. Then can't the absorption occur by these electrons? $\endgroup$
    – Krang Lee
    Oct 25, 2021 at 9:54
  • $\begingroup$ @KrangLee after they relax, their $\Delta E$ is no longer the same that they released while transitioning from upper working level to the lower. This is what makes absorption impossible. $\endgroup$
    – Ruslan
    Oct 25, 2021 at 10:00
  • $\begingroup$ @KrangLee Once in the ground level, the transitions from the ground level are different transitions that respond to different frequencies of radiation. So they won't absorb your laser light. $\endgroup$ Oct 25, 2021 at 10:00
  • $\begingroup$ @user253751 Then similary in the three-level laser system, energy difference between ground state and excited state is different with that between ground state and metastable state too, so absorption cannot occur like in the case of four-level system? Then I think the absorption cannot occur whether lower metastable state exists or not. $\endgroup$
    – Krang Lee
    Oct 25, 2021 at 10:34
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    $\begingroup$ @KrangLee electrons can be excited from the ground state into the metastable state by absorbing the laser radiation. $\endgroup$ Oct 25, 2021 at 10:35
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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.

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