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Since it is way easier for a 4-level laser to get population inversion than 3-level laser, 3-level laser is still in use nowadays. So what's the advantage of 3-level laser over 4-level laser?

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I guess you should take the view of an experimental physicist:

  • Which atoms are available and where are their energy levels?
  • How can I excite the atoms in order to get inversion?
  • What are the properties which I need my laser to fulfill (e.g. line width, power, ...)?
  • How complicated and stable is the set-up? How much does it cost and how long will it take to get the laser running?
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  • $\begingroup$ +1 for first point. There are quite a few of the lanthanide ions (Er, Ho, Tm) working in three-level scheme. Of course 4-level would be better, but 3-level is what we have for these ions. And this is technology, so "good enough" is all we need. $\endgroup$ – Jarosław Komar Jul 2 '17 at 9:14

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