Lecture 8: Laser amplifiers

Quasi-two-level systems
By pumping such a quasi-two-level system properly, it is
possible to reach the needed population inversion in the steady
state for an optical gain at a particular laser frequency .
Now the ratio p = ep / ap at the pump frequency p can be
made different from the ratio e / a at the laser frequency due
to the uneven population distribution among the sublevels
within an energy level.
The pumping requirements for a steady-state optical gain from
a quasi-two-level system (see p.53)
p = ep / ap < e / a ; W p > (1/ 2 ) a /( e –p a )
Because the absorption spectrum is generally shifted to the
short-wavelength side of the emission spectrum, these
conditions can be satisfied by pumping sufficiently strongly at a
higher transition energy than the photon energy corresponding
to the peak of the emission spectrum.
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