*1.2.2. Passive Q-switching*

In passive Q-switching, the losses are modulated or controlled by optical cavity light, rather than some external electrical source. A saturable absorber device is normally used as a Q-switcher in this technique. The transmission of this device increases when the intensity of light exceeds some threshold. The material may be an ion-doped crystal like Cr:YAG (chromium-doped Yttrium-Aluminum garnet), which is used for Q-switching of Nd:YAG (neodymium-doped Yttrium-Aluminum garnet) lasers, a bleachable dye, graphene mechanical exfoliation and PVA thin film, semiconductor saturable absorber mirrors (SESAM), and carbon nanotubes embedded in PVA thin films. Initially, the loss of the absorber is high, once a large amount of energy is stored in the gain medium, the laser power increases, and it saturates the absorber, and light can pass through as there are no electrons in the ground state to absorb pumped energy. As soon as the energy is depleted in the resonator, 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. In this way, it works as an on-off optical switch to generate pulses. The pulse repetition rate can only be controlled indirectly by varying the laser's pump power and the amount of saturable absorber in the cavity. For direct control of the repetition rate, a pulsed pump source is needed.

Passive Q switching is simpler and cost effective as compared to the active one. It eliminates the modulator and its electronics. Moreover, it is suitable for very high pulse repetition rates, but with lower pulse energies. External triggering of the pulses is not possible (except with an optical pulse from another source), and also pulse energy and duration are often more or less independent of the pump power, which only determines the pulse repetition rate [11, 12].
