To obtain a powerful laser pulse instead of irregular sequence of spikes, the technique of Q switching is used. In Q - switching, the cavity losses are kept high till some time to so that the population inversion can build up. If by opening a switch the losses are suddenly lowered, a giant laser pulse can be made to emanate from the laser cavity. The intensity profiles are shown in Fig 31.12.
Figure 31.12 Q - switching. a) Intensity profiles of Pp (t), PL(t) and N(t), b) Nonresonant cavity and c) Resonant cavity.
One way to keep the cavity non - resonant is to mount one of the mirrors on a spinning motor shaft. Only when the mirror is parallel to the resonator axis, high Q - value (quality of laser pulse) is attained . Another way to impair the resonance characteristics when the pump pulse is active is to include a saturable dye in the medium. When most of the dye molecules become excited by the pump pulse, it gets saturated with energy and can not absorb any more and it becomes transparent allowing the cavity to become resonant. Nanosecond pulses can be obtained by Q - switching.
N(t), b) Nonresonant cavity and c) Resonant cavity. 
