Editing Q-switching (section)

===Active Q-switching===
Here, the Q-switch is an externally controlled variable attenuator. This may be a mechanical device such as a shutter, chopper wheel, or spinning mirror/prism placed inside the cavity, or (more commonly) it may be some form of [[modulator]] such as an [[Acousto-optics|acousto–optic]] device, a [[magneto-optic effect]] device or an [[electro–optic effect|electro-optic]] device{{snd}} a [[Pockels cell]] or [[Kerr cell]]. The reduction of losses (increase of Q) is triggered by an external event, typically an electrical signal. The pulse repetition rate can therefore be externally controlled. Modulators generally allow a faster transition from low to high Q, and provide better control. An additional advantage of modulators is that the rejected light may be coupled out of the cavity and can be used for something else. Alternatively, when the modulator is in its low-Q state, an externally generated beam can be coupled ''into'' the cavity through the modulator. This can be used to "seed" the cavity with a beam that has desired characteristics (such as [[transverse mode]] or wavelength). When the Q is raised, lasing builds up from the initial seed, producing a Q-switched pulse that has characteristics inherited from the seed.