Editing Q-switching (section)

===Passive Q-switching===
In this case, the Q-switch is a [[saturable absorber]], a material whose transmission increases when the intensity of light exceeds some threshold. The material may be an ion-doped crystal like [[Cr-YAG|Cr:YAG]], which is used for Q-switching of [[Nd-YAG laser|Nd:YAG lasers]], a bleachable dye, or a passive [[semiconductor]] device. Initially, the loss of the absorber is high, but still low enough to permit some lasing once a large amount of energy is stored in the gain medium. As the laser power increases, it saturates the absorber, i.e., rapidly reduces the resonator loss, so that the power can increase even faster. Ideally, this brings the absorber into a state with low losses to allow efficient extraction of the stored energy by the laser pulse. After the pulse, the absorber recovers to its high-loss state before the gain recovers, so that the next pulse is delayed until the energy in the gain medium is fully replenished. The pulse repetition rate can only indirectly be controlled, e.g. by varying the laser's [[laser pumping|pump]] power and the amount of saturable absorber in the cavity. Direct control of the repetition rate can be achieved by using a pulsed pump source as well as passive Q-switching.