Lecture 8: Laser amplifiers

Gain coefficient
The average density of absorbed photons (number of photons
per unit time per unit volume) is N 1 W i .
Similarly, the average density of clone photons generated as a
result of stimulated emission is N 2 W i .
The net number of photons gained per second per unit
volume is therefore NW i , where N = N 2 –N 1 is the
population density difference.
N is referred to as the population difference.
• If N > 0, a population inversion exists, in which case the
medium acts as an amplifier and the photon-flux density
can increase.
• If N < 0, the medium acts as an attenuator and the photonflux
density decreases.
• If N = 0, the medium is transparent.
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