The technical paper Control of pulse duration and shape in a 400-W Q-switched 532-nm laser present a 400 W Q-switched 532 nm laser using several techniques to control the pulse duration and shape. The laser consists of two separate intracavity frequency-doubled cavities, incoherently combined to give 50 mJ total pulse energy. By extending the cavity length the pulse duration can be increased from 66 ns to 140 ns at 8 kHz with no significant power reduction. Varying the repetition rate and the triggering delay for each cavity’s Q-switch allows further adjustment of the pulse duration. The pulse shape can be controlled by adjusting this delay and attenuating the individual cavities.

An introduction to Control of pulse duration and shape in a 400-W Q-switched 532-nm laser

Pulse duration is very often a critical parameter for nanosecond lasers used in drilling and surface modification applications. Many processes require a specific pulse duration in order to exceed or avoid various material thresholds, or as determined by the speed of thermal diffusion or the evolution of generated plasma and ejected material. Pulse duration is an important parameter in the hybrid water jet laser machining process developed by Synova S.A. that is the motivation for this work. For this application it is beneficial to have a pulse duration in the range of hundreds of ns, with tunability to allow the process to be optimised for each particular machining task.

While Q-switched diode-pumped solid state (DPSS) systems are particularly suited to applications such as these due to the high energy and average power that can be achieved, they are at a disadvantage when it comes to tuning the pulse duration. Since this is determined by the laser dynamics of the Q-switching process, controlling the pulse duration is not trivial. The pulse duration of a laser depends primarily on the net laser gain and the round trip time of the cavity; the higher the gain, the fewer round trips required for the pulse to build up and extract the energy stored in the amplifier. Laser gain is itself a function of many parameters such as pump power, choice of laser medium and pump geometry, most of which cannot be grossly changed without major alterations to the laser design. The pulse repetition period directly affects the amount of stored energy (and hence gain) that is present when the Q-switch pulse begins, so this provides a simple way to vary the pulse duration on the fly without physical changes to the laser. However, this is of limited utility as it also affects pulse energy, which is a critical parameter for most pulsed laser machining applications.

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