Without double Q before the switch, in order to effectively shut off the high power laser beam, typically using two c mode Q switching (such as QS27-4C-B), they are orthogonal to each other in a laser cavity. Because the mode c switch effectively shut off the laser beam, so first to Q switch effectively shut off the light in one direction, the second Q switch effectively shut off the light in the other direction, but the disadvantage is that the two Q-switch where you need more placed in the cavity space.