Lecture 8: Laser amplifiers

Two-level systems
Take the upward pumping rate W 12p = W p and the downward
pumping rate to be W 21P = p W p , where p is a constant that depends
on the detailed characteristics of the two-level atomic system and
the pump source.
In the steady state when dN 2 /dt = dN 1 /dt = 0,
N 2 e –N 1 a = [W p 2 ( e -p a )- a ]N t [1+(1+p)W p 2 + (I 2 /h)( e + a )] -1
For optical pumping
p = ep / ap = e ( p )/ a ( p ),
where ap and ep are the absorption and emission cross sections at
the pump wavelength.
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