Lecture 8: Laser amplifiers

Four-level pumping
This is not always the case because the population densities
of the ground state and level 3, N g and N 3 , are related to N 1
and N 2 by
N g
N 1
N 2
N 3
N a
where the total atomic density in the system, N a , is a constant.
If the pumping involves a transition between the ground state
and level 3 with transition probability W, then
R (N g
N 3
)W
If levels 1 and 3 are short-lived, then N 1 ≈ N 3 ≈ 0, N g + N 2 ≈
N a s.t. N g ≈ N a -N 2 ≈ N a - N
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