Electro-optic solution to overcome the transient regime of a cavity dumped UV laser source?

Abstract

Laser marking is one of the main industrial applications of lasers. To mark many materials or to obtain patterns of small dimensions, triggered pulsed solid laser sources are increasingly used emitting in the fundamental in the infrared and to which two nonlinear (NL) stages are added to obtain an emission in the UV. The conversion efficiency as well as that of the marking then depends mainly on the peak power of the UV pulses generated. One of the solutions for obtaining a higher peak power is to reduce the duration of the laser pulses. To do this, it is possible to change the architecture of the laser cavity by switching from a Q-Switch operating mode (QS) to a cavity dumped mode (CD). With this architecture it is thus possible to reduce the duration of the pulses by an order of magnitude. This peak power gain is unfortunately achieved at the cost of a very long transient regime, up to a few tens of milliseconds, compared to the classic QS which generally only occurs for the first laser pulse. Unfortunately, laser marking is by nature based on chopped operation for the laser, which corresponds to working permanently in a transient state. Marking with a CD architecture laser is then often full of defects and therefore unacceptable in terms of quality. After a study of the problem specific to the use of a UV source with CD architecture in the context of marking, we propose an original solution to dissociate the chopped regime, specific to marking, from the transient regime of the laser. This solution is based on the use of an RTP (Rubidium Titanyle Phosphate RTiOPO4) electro-optical crystal placed between the laser output and the NL stages. This crystal is driven like a half-wave plate by high voltage square pulses of a duration corresponding to the duration of the marking laser pulse trains. This thus makes it possible to obtain laser operation in a stabilized regime, therefore outside the transient regime, and simultaneously chopped operation characteristic of laser marking. The new operating mode obtained is characterized in detail with the new marking performances which prove to be optimal and compatible with the requirements of a quality marking. In addition to making the CD architecture compatible with laser marking, this solution also makes it possible to adjust the average power, and therefore the energy of the laser pulses to adapt it according to the materials and this without changing the thermal behavior of the laser. Last advantage, by adjusting the extinction voltage applied to the RTP outside the cavity, it is also possible to keep a few milliwatts of the marking laser beam as the aiming beam, with the certainty of perfect alignment with the full power beam.