Lecture 8: Laser amplifiers

Four-level pumping
An external source of energy pumps atoms from level 0 to
level 3 at a rate R.
If the decay from level 3 to level 2 is sufficiently fast, it may
be taken to be instantaneous, in which case pumping to level
3 is equivalent to pumping level 2 at the rate R 2 = R.
However, in this case, atoms are neither pumped into nor out
of level 1, s.t. R 1 = 0.
Thus, in the absence of amplifier radiation (W i = = 0), the
steady-state population difference is given by (see p.86)
N 0
R 2
1 1
21
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