First, particles in a high-energy state spontaneously transition to a low-energy state, which is called spontaneous emission;
Second, particles in a high-energy state transition to a low-energy state under the excitation of external light, which is called stimulated radiation;
Third, the energy of the low-energy particles absorbing the external light to the high-energy state is called stimulated absorption.
Spontaneous radiation, even if two particles transition from a high energy state to a low energy state at the same time, the phase, polarization state, and emission direction of the emitted light may be different, but the stimulated radiation is different, when the particles in the high energy state are outside. Under the excitation of photons, it transitions to a low-energy state, emitting light that is identical to the external photons in terms of frequency, phase, and polarization state. In a laser, the radiation that occurs is stimulated radiation, and the laser it emits is exactly the same in terms of frequency, phase, and polarization state. Any excited light system,
That is, there is stimulated radiation and stimulated absorption. Only the stimulated radiation is dominant, and the external light can be amplified to emit laser light. In general light sources, the stimulated absorption is dominant, and only the equilibrium state of the particles is broken, so that the number of particles in the high energy state is larger than the number of particles in the low energy state (so called the ion number inversion), and the laser can be emitted.