Generally, the optical pulses output by solid-state pulsed lasers are not single to smooth pulses, but a series of small spike pulses of varying intensity in the microsecond range. With a duration of hundreds of microseconds or even milliseconds, and a peak power of only a few tens of kilowatts, this optical pulse train is far from meeting the requirements of many important practical applications such as lidar, laser ranging, laser guidance, high-speed photography, and laser nuclear fusion. For this reason, the concept of Q-switched laser was proposed shortly after the discovery of the laser, and the first Q-switched laser was made in 1962. With its appearance, the laser pulse output performance has been improved by several orders of magnitude, the pulse width is compressed to the nanosecond level, and the peak power is up to gigawatts. This has played an important role in the development of applications such as laser ranging, lidar, laser processing, and dynamic holography.
The Q-switched principle refers to the use of a certain method to make the resonator in a state of high loss and low Q value at the beginning of pumping, when the threshold of laser oscillation is very high, and the number of particle density reversals will not oscillate even if it accumulates to a high level; When the number of particle density inversions reaches its peak, the Q value of the cavity suddenly increases, which will cause the gain of the laser medium to greatly exceed the threshold, resulting in an extremely rapid oscillation. At this time, the ground energy of the ground particles stored in the metastable state will be quickly converted into the energy of photons, and the photons will grow at an extremely high rate like an avalanche, and the laser can output a laser giant pulse with high peak power and narrow width. The technique of adjusting the Q of the resonator to obtain a laser giant pulse is called laser Q-switched technology.
Because the losses of the resonator include reflection loss, absorption loss, diffraction loss, scattering loss and transmission loss, different Q-switched technologies are formed by controlling different types of losses in different methods. The technologies to control the reflection loss include the Q-switching technology of the mechanical rotating mirror and the electro-optical Q-switching technology, the technologies to control the absorption loss include the Q-switching technology of saturable absorption dyes, and the technologies to control the diffraction loss are the acousto-optic Q-switching technology. The principle of Q-switching technology is relatively intuitive, and it is rarely used at present.
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