Lecture 8: Laser amplifiers

Rate equations in the presence of amplifier radiation
dN 2 /dt = R 2 –N 2 / 2 –N 2 W i + N 1 W i
dN 1 /dt = -R 1 –N 1 / 1 + N 2 / 21 + N 2 W i -N 1 W i
The population density of level 2 is decreased by stimulated
emission from level 2 to level 1 and increased by absorption
from level 1 to level 2.
Under steady-state conditions (dN 1 /dt = dN 2 /dt = 0), the
population difference in the presence of amplifier radiation
(assuming g 1 = g 2 )
N = N 2 –N 1 = N 0 /(1 + s W i )
The characteristic time s (saturation time constant) is always
positive ( 2 ≤ 21 ) is given by
s = 2 + 1 (1 – 2 / 21 )
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