Passive Q-switching is a device to realize Q-switching by utilizing the phenomenon that the transmittance of optical materials at a certain laser wavelength depends on the laser intensity. Laser-crylink mainly produces saturable absorption crystals in passive Q-switching crystals. The absorption coefficient of saturated absorption crystal decreases with the increase of incident light intensity. When the saturation value is reached, the laser is transparent. The loss (Q value) in the laser cavity is modulated and the pulse is emitted. The pulse width is in the order of several nanoseconds or even tens of picoseconds.