What produces pulses of light in lasers?
Stimulated emission paired with a population inversion is required to produce the pulses of light in lasers.
First the atoms of the gas in the laser are excited. The electrons spontaneously emit photons and drop down to lower energy levels.
In some cases electrons will collect in a state that takes a relatively long time to drop from. When this happens there can be more electrons in this excited state than in the lower states. This is called a population inversion.
If light has a wavelength such that a photon has the same energy as the energy difference between this long-lived excited state and a lower state, it can stimulate the electron to emit a photon and drop down from the excited state.
When the electron is stimulated to fall to a lower state, a photon is released with the same frequency, phase, polarization, and direction of travel as the photon that stimulated it. This is known as stimulated emission. There are now two photons that can stimulate other atoms. Other atoms are stimulated and then Bam, there are a lot of in phase photons with the same wavelength.
This is how lasers produce coherent light.
When a photon with the right energy (energy equal to difference between excited and lower state) strikes an atom, it can stimulate emission, but it can also be absorbed (stimulated absorption). The likelyhood of being absorbed is equal to the likelyhood of stimulating emission.
Stimulated emission is proportional to the number of electrons in the excited state.
Stimulated absorption is proportional to the number of electrons in the lower state.
A population inversion is necessary. If there were more electrons in the lower state stimulated absorption would happen more often than stimulated emission and we would run out of photons.
If you are curious, the Einstien A and B coefficients are used to describe the likelyhood of a spontaneous emission and stimulated emission/absorption respectively.