Q-switching

A Q-switch and Q - switch is an optical device for pulse laser, abruptly laser action can be allowed with the. Compared to mere pulsed excitation pulses are by incorporating a Q-switch in the resonator delayed, shorter and reach higher peak flows (gigawatt range). Active Q-switch also have lower jitter.

Compared to mode coupling, although not as short pulses and lower repetition rates, but achieved higher total pulse energy.

The concept was first demonstrated in 1962 by Robert Hellwarth and FJ McClung.

Operation

The operation can be explained by the time course of the energy content of the laser medium and the optical intensity in the resonator, see picture on the right. By optical pumping, population inversion and gain increase. Without the artificial loss of Q-switch the laser threshold would soon reached at which the gain exceeds the losses. Laser action would limit the use and further increase the energy content.

The Q-switch, however, initially produced high resonator losses (low quality of the resonator ) so that the laser threshold is not reached and the stored energy in the laser medium increases to the maximum. In this state of maximum gain, the resonator is increased abruptly. It follows a chain reaction with an exponential increase in the number of photons and thus the intensity until the associated reduction of the population inversion is felt. While the inversion falls below the laser threshold, the intensity goes through a sharp maximum and decays rapidly thereafter. In this way, pulse durations can be achieved in the range of nanoseconds.

Technically, this basic principle will be implemented as active or passive Q-switching.

Active Q-switch

An active Q-switch changed the resonator out to an external signal. Which can be realized in different ways. Approximately mechanically with a rotating mirror or a moving aperture (in the form of a "chopper wheel" ). Such devices, however, are relatively slow for Q-switch and react sluggishly to control inputs. Therefore, usually two other types are used by active Q-switches: for an electro- optical devices such as Pockels cell or Kerr cell, on the other acousto-optic modulators.

In the case of acousto-optic Q-switch, a part of the incident laser light is diffracted out of the resonator to the acoustic optical modulator. This proportion is therefore to be regarded as resonator, the quality is low. To switch, changing the modulation frequency so that the beam in the resonator remains. The losses on the acousto-optic modulator disappear and the quality is high. A useful side effect of such an arrangement is that the diffracted out of the resonator beam can be offset to a wide variety of purposes. Disadvantage is that, especially for large beam cross-sections, the necessary cooling of the modulator. In addition, with electro-optical effects even shorter switching times are possible. To this end, thin film polarizers are brought together with one of said cells in the resonator. Depends on the angle between the polarization directions of the polarizers, a more or less large part of the incident intensity is removed from the resonator. If the correct voltage to the Pockels (or Kerr ) cell created, this behaves like a λ/2-Plättchen and rotates the polarization so that the resonator is leaving no more intensity to the polarizers. The resonator is high.

Passive Q-Switches

Passive Q-switch respond ultimately to the increasing gain in the laser medium. Because with increasing population inversion increases, the background intensity of the light generated by spontaneous emission in the resonator. Passive Q-switches are usually implemented by saturable absorber. The pulse frequency in this case can only indirectly, for example by variation of the pumping power or the quantity of the absorbent material, can be influenced.