Rami Arieli: "The Laser Adventure"

Rami Arieli: "The Laser Adventure" Chapter 2.9.1 page 1

2.9.1 Absorption Rate Equations
The incoming optical signal (photons) causes the atoms to "jump" from the lower energy level (E₁) to the upper energy level (E₂).

The absorption rate is proportional to the product of the density n(t) of the incoming photons (the number of photons in a unit volume) with the number of atoms (N₁(t)) in the lower energy level (E₁):

d N₂(t)/dt = K n(t) N₁(t)

Each photon excites one atom to the higher energy level.

K = Proportionality constant - A measure of the relative strength of the response of the atom to the incoming radiation in this specific transition.