Lecture 8: Laser amplifiers

Population inversion
Population inversion in a medium is generally defined as
N 2 > (g 2 /g 1 ) N 1
(N 2 > N 1 for g 1 = g 2 )
However, this condition does not guarantee an optical gain at a particular
optical frequency when the population in each level, |1> or |2>, is
distributed unevenly among its sublevels.
A better condition for population inversion to guarantee an optical gain at
a given frequency
N 2 e () – N 1 a () > 0
The pumping requirement for the condition to be satisfied depends on the
properties of a medium. For atomic and molecular media, there are three
different basic systems. Each has a different pumping requirement to
reach effective population inversion for an optical gain. The pumping
requirement can be found by solving the coupled rate equations.
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