The use of 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 dependant effects e.g. time resolved spectroscopy. In order to achieve extremely high peak power up to the Gigawatt range, laser systems are applied, which possess long lived excited states able to store energy and to emit it in an extremely short time. One of such lasers is e.g. the Nd:YAG laser. With q-switching in so called active or passive mode, it is possible to generate such short pulses. Here, in a first step the theory of laser operation with Nd:YAG is discussed and the steady state as well as time dependent solution of the four level rate equation is analysed.
A two level rate equation model is introduced to explain the saturation behaviour of an optical absorber. A saturable absorber for passive q-switching is introduced. The dynamics of the pulse generation, like repetition rate, pulse width and peak power are determined. The experiment consists of the laser diode pumped Nd:YAG - laser as basic version with an additional passive q-switch (Cr:YAG) crystal. The time dependant signals are displayed and evaluated using an optional oscilloscope. Beside the generation of short pulses, the behaviour 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 including the high voltage driver active q-switch can be performed and explored.
Free running q-switch pulse train
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.