Question

What is a simulated emission in a laser diode?

Answer 1

In a general diode, electron-hole pairs are generated and then recombined.

In general silicon diode, this recombination at the junction creates heat because silicon is indirect bandgap material.

In Light Emitting Diode, this recombination occurs by emission of radiation.BUT, the photon is emitted spontaneously, meaning that the photon wave has random phase and random direction.This is called spontaneous emission.
In LASERs, recombination time or the time interval between generation and recombination, is very high as compared to spontaneous emission. Therefore, as the material is excited the average number of electrons in the conduction band are more in number than in valence band.This is called population inversion. At this condition, when a light of proper frequency(i.e with energy equal to bandgap energy) is bombarded to an excited electron, the incident light STIMULATES the recombination (or de-excitation) of the excited electron, causing the release of the energy This is called stimulated emission.BUT, unlike photo diode, this generated electromagnetic wave or photon has same direction as the incident light and has phase same phase as the incident light.