Quality switch

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A quality switch , also known as a Q-Switch , is an optical component for pulsed lasers , with which laser activity can suddenly be permitted. Compared to simple pulsed excitation, the installation of a Q-switch in the resonator delays the pulses, makes them shorter and achieves higher peak outputs ( gigawatt range). Active Q-switches also have less jitter .

In comparison to mode coupling , pulses that are not so short and lower repetition rates are achieved, but higher total pulse energies are achieved.

The concept was first demonstrated in 1962 by Robert Hellwarth and FJ McClung . At that time at the Hughes Research Laboratories, where Theodore Maiman had developed his ruby laser two years earlier, they developed a method of using the laser for distance measurements, which required a better pulse shape and sequence than the original ruby ​​laser pumped by flashlight lamps.


Schematic time course

How it works can be explained based on 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 rise population inversion and gain. Without the artificial losses of the Q-switch, the laser threshold would soon be reached, from which the gain exceeds the losses. Laser activity would start and limit the further increase in energy content.

The Q-switch, on the other hand, initially generates high resonator losses (low quality of the resonator), so that the light emanating from the medium does not remain in the resonator. A population inversion is achieved in the material, but this cannot be reduced due to the low quality of the resonator. This causes the energy stored in the laser medium to increase to the maximum. In this state of maximum gain, the resonator quality is suddenly increased. The resonator now has a high quality, radiation is reflected at the end mirror of the resonator and thus runs back to the active medium. A chain reaction follows with an exponential increase in the number of photons and thus the intensity, until the associated decrease in the population inversion is noticeable. While the inversion falls below the laser threshold, the intensity passes through a sharp maximum and then quickly fades away. In this way, pulse durations in the nanosecond range can be achieved.

Technically, this basic principle can be implemented as an active or passive Q-switch.

Active quality switches

An active Q-switch changes the resonator quality in response to an external signal. This can be done in different ways. For example mechanically with a rotating mirror or a moving screen ( e.g. in the form of a chopper wheel ). Such devices, however, are relatively slow for Q-switches and only react slowly to control inputs. Two other types of active Q-switches are therefore usually used: on the one hand, electro-optical components, such as electro-optical modulators , Pockels cells or Kerr cells , and on the other hand, acousto-optical modulators .

In the case of the acousto-optical Q-switch, part of the incident laser light is diffracted out of the resonator at the acousto-optical modulator. This proportion is therefore to be regarded as a resonator loss, the quality is low. To switch, you change the modulation frequency so that the beam remains in the resonator. The losses at the acousto-optical modulator disappear and the quality becomes high. A useful side effect of such an arrangement is that the beam diffracted from the resonator can be collected for various purposes. The disadvantage is the need to cool the modulator, especially with large beam cross-sections. In addition, even shorter switching times are possible with electro-optical effects. For this purpose, thin-film polarizers are brought into the resonator together with one of the cells mentioned. Depending 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 is applied to the Pockels (or Kerr) cell, it behaves like a λ / 2 plate and rotates the polarization so that the resonator at the polarizers no longer leaves any intensity. The resonator quality becomes high.

Passive Q-switch

Passive Q-switches ultimately react to the increasing amplification in the laser medium. Because with increasing population inversion, the basic intensity of the light generated by spontaneous emission in the resonator also increases. Passive Q-switches are mostly implemented using saturable absorbers . The pulse rate can only be indirect, e.g. B. by varying the pump energy or the quantity of the absorber material.

Web links

Individual evidence

  1. FJ McClung, RW Hellwarth Giant optical pulsations from ruby, Journal of Applied Physics, Volume 33, 1962, 828-829.