Before talking about laser mechanism about the stimulated radiation. There are three kinds of radiation process in optical radiation,

One is in the high energy particles spontaneously to the low-energy state transition, called the spontaneous radiation;

The second is in the high energy particles under the excitation of external light to the low-energy state transition, called the stimulated radiation;

Three is the particles in the low-energy state absorb foreign to the upper state transition is called stimulated absorption of light energy.

Spontaneous radiation, even the two at the same time from a high-energy state to the low-energy state transition of particle, phase, polarization state, they emit light emission direction may also be different, but stimulated radiation is different, when the upper state of particles under the excitation of external photon to the low-energy state transition, issued in terms of frequency, phase, polarization state and foreign same photons of light. In laser radiation is stimulated radiation, it emits laser in frequency, phase and polarization state are exactly the same. Any excited light system,

Laser diode

Which have stimulated radiation, also has stimulated absorption, only stimulated radiation advantage, can to enlarge foreign light and a laser. And are in general illuminant stimulated absorption edge, only the particle equilibrium is broken, the high energy particle number is greater than the low-energy state of particle number (it is called ion inversion), can a laser.