Lecture 8: Laser amplifiers

Two-level systems
In a true two-level system, the energy levels |2> and |1> can
each be degenerate with degeneracies g 2 and g 1 , but the
population densities in both levels are evenly distributed
among the respective degenerate states.
In this situation, p = ep / ap = g 1 /g 2 = e / a
N 2 e –N 1 a = - a N t [1+( e + a )(I/hW p / a ) 2 ] -1 < 0
No matter how a true two-level system is pumped, it is clearly
not possible to attain population inversion for an optical gain
in the steady state.
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