Generally, pulsed laser oscillates near the threshold. When the number density of the inversion particles reaches the threshold, the laser is emitted, and just after a part of the laser is emitted, the number density of the inversion particles drops below the threshold, and the oscillation stops. After that, with the suction, the number density of inverted particles rises above the threshold, and the previous process is repeated. Until the pump stops. Therefore, a general pulse laser emits laser light with a sharp waveform.

Q-switched laser oscillates when the inversion particle number density is much greater than the threshold. When the Q switch is closed, the threshold of the laser is very high, so the number density of the inversion particles can be large without oscillation. At the end of pumping, the number density of inversion particles reaches enough (at this time, the laser does not oscillate), the Q switch suddenly opens, and the threshold of the laser suddenly drops to a very low level. The laser starts to oscillate above the threshold. This forms a smooth giant pulse.