Population inversion is important factor you need to acheive . Consider gd , 2 , 3 and 4 . highest population need at level 3. since level 2 is spontaneous emission to level 1, the population of level 2 is always very low. hence easy to achieve populated level 3, a metastable state.
A 3 level laser , will have 1, 2 and 3 . populated at level 2, but level 1 is where it is quite populated as it is gd state .Hence harder to achieve population inversion.
4 level easier to operate than 3 level lasers. primarily due to ease of population inversion.
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