Abstract

The short-pulse lasers enables the generation of high peak power pulses in a short time, which are useful for the investigation of non-linear effects and for the investigation of time dependent effects. With q-switching in so called active or passive mode, it is possible to generate such short pulses.

Beside the generation of short pulses, the behavior of the Nd:YAG laser can also be the subject of additional investigations, like measuring the threshold, slope efficiency and so on. By using the optional Pockels cell (AQ) including the high voltage driver (QD), active q-switch can be performed and explored.

Principle of passive Q-switch with saturable absorber

The q-switch crystal is a saturable absorber whose absorption depends on the intensity of the incident light, the higher it is, the less the absorption will be. Placing such a crystal into the Nd:YAG laser cavity will prevent the laser to oscillate. However, the stimulated and spontaneous emission increases and reduces the absorption of the crystal to such an extend, that the laser reaches the threshold and emits a giant pulse. Immediately after the pulse ends, the crystal’s absorption goes up again and prevents any laser action, until the crystal becomes transparent again under the influence of the strong fluorescence light. In this way a periodic pulse emission is created. Since the occurrence of the laser pulse depends on the systems parameter and its dynamics the pulse cannot predicted and thus this method is termed as passive q-switching opposed to the active q-switching where the operator controls the pulse release.

Principle of passive q-switch with an intra-cavity KTP frequency doubler crystal

The Figure 2 shows the combination of a passive q-switch and an intra-cavity KTP for frequency doubling. Due to the very high peak intensity of the fundamental laser at 1064 nm, the peak intensity of the “green” radiation is significantly higher.

Principle of passive q-switch with a pockels cell

The active q-switch consists of a DKDP crystal (potassium di-deuterium phosphate (KD*P = DKDP)). Applying a high voltage to it, a phase retardation results which value depends on the applied voltage. The Brewster plate forces the Nd:YAG laser to oscillate in a fixed polarization. If the retardation of the Pockels cell causes circular polarization, the losses at the Brewster plate prevent the laser oscillation.