Rami Arieli: "The Laser Adventure" Chapter 2.9 page 1
2.9 Stimulated (Forced) Transitions(*)

The strength of an optical signal (The number of photons) is described by:

An Optical signal is an oscillating electromagnetic field, and an atom can be described as an electric dipole.

When an optical signal of the right frequency (hn equal to the energy difference between the energy levels E2-E1) is approaching an atom, both the atoms at the lower energy level (E1) and the atoms at upper energy level (E2) will start to oscillate.

That is why there are two forced processes: absorption and stimulated emission