Lecture 8: Laser amplifiers

Coherent light amplification
As seen earlier, stimulated emission allows a photon in a
given mode to induce an atom whose electron is in an upper
energy level to undergo a transition to a lower energy level
and, in the process, to emit a clone photon into the same
mode as the initial photon. A clone photon has the same
frequency, direction and polarization as the initial photon.
These two photons in turn serve to stimulate the emission of
two additional photons, and so on, while preserving these
properties.
The result is coherent light amplification. Because
stimulated emission occurs only when the photon energy is
nearly equal to the transition energy difference, the process is
restricted to a band of frequencies determined by the
transition linewidth.
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