Lecture 8: Laser amplifiers

Two-level systems
While a pumping mechanism excites atoms from the lower
laser level to the upper laser level, the same pump also
stimulates atoms in the upper laser level to relax to the lower
laser level.
R 2 = -R 1 = W 12p N 1 -W 21p N 2 ,
where W 12p and W 21p are the pumping rates from 1 to 2 and
from 2 to 1.
Under these conditions, dN 2 /dt and dN 1 /dt are equivalent to
each other (N 1 + N 2 = N t = constant).
The upward (W 12p ) and downward (W 21p ) pumping rates are
not independent of each other but are directly proportional to
each other because both are associated with the interaction of
the same pump source with a given set of energy levels.
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