A @L03459@ in which the state of the device introducing important losses in the resonant cavity and preventing @L03479@ operation is suddenly switched to a state where the device introduces very low losses. This increases rapidly the Quality factor of the cavity, allowing the build-up of a short and very intense @L03459@ pulse. Typical pulse durations are in the ns range. The Q-switching may be active (a rotating mirror or electro-optic device) or passive (a saturable @A00034@).