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I'm brushing up my concepts on LASER's and I was just curious about this. Does a three level pumping scheme necessarily imply that exactly three energy states have to be present? Or can more or or less energy states can be present as well? I know that a four level pumping scheme is more efficient but this is just a question that popped up in my mind. I'm sorry if I got my concepts wrong in some place and would really appreciate it if someone could explain the rationale behind this to clear my confusion. Sorry for taking up your valuable time!

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  • $\begingroup$ Four-level schemes are not necessarily more efficient. Lasers with efficiencies of >70% exist which are three-level systems. I might be wrong, but I think all high efficiency systems are three-level. This comes from the fact that the lasing wavelength is typically closer to the pump wavelength, meaning that the photon energy difference between the two can be quite low. i.e. when pumping a four level system you have to throw away energy 2 times, while in 3 level systems only 2. Zero-phonon line pumping of Yb doped materials is three level, but is quite close to 2 level even. $\endgroup$ Commented May 21, 2020 at 19:35
  • $\begingroup$ But isn't it extremely difficult to produce a continuous output from a Three-level Pumping Scheme? Doesn't that automatically make a Four-level pumping scheme more efficient(Since it produces a continuous output)? @JoséAndrade, when we're talking about efficiency, we have to talk about the output efficiency as well right? $\endgroup$ Commented May 22, 2020 at 18:26
  • $\begingroup$ Not really, no. The lasers I meant above are CW lasers with kW level output. They are also not technically pure three-level systems, but quasi-three. Also, I meant optical-optical efficiency, i.e. how much power output divided by how much optical power used to pump the medium. Check these two links: thin-disk lasers and three and four level systems. $\endgroup$ Commented May 23, 2020 at 16:59
  • $\begingroup$ Lastly, weirdly, Yb:YAG pumped at the zero-phonon line resembles the quasi-three picture (right most picture of the second link I sent you), except that instead of pumping to the highest state shown there, you pump directly to the upper laser level. There are three levels involved, where the ground state and lower laser level are from the same band just slightly separated via some energy shift. The depopulation of the lower laser level to the ground state happens fast simply due to re-thermalization within the band. It almost looks like a two-level system with a split ground-level $\endgroup$ Commented May 23, 2020 at 17:06
  • $\begingroup$ @JoséAndrade That's a really interesting thing to know. Thanks! I really didn't have a clue about this. $\endgroup$ Commented May 25, 2020 at 19:41

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You will need at least three states for lasing to occur (with reasonable probability) but that does not mean that other states cannot be present. If the pump frequencies are not tuned to reach these other states then they will not be occupied with any appreciable probability and hence will not influence the lasing process.

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  • $\begingroup$ Ok, I get your point @Lewis Miller, but what do you exactly mean by reasonable probability? $\endgroup$ Commented May 18, 2020 at 17:37
  • $\begingroup$ What I meant by that is that nonlinear optical effects might sometimes invalidate my statement but these will have such a low probability that they can be ignored. $\endgroup$ Commented May 18, 2020 at 20:34
  • $\begingroup$ Ok, I think I get what you're saying @LewisMiller. So, will the same thing be applicable to 4 - Level LASER's as well? Based on what you're saying, it should be applicable to all types of LASERs, so I'm just clarifying. Thanks for clearing my doubt! $\endgroup$ Commented May 19, 2020 at 9:31

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