Q-switched alexandrite Cr³⁺:BeO.Al₂O₃ [1] solid state tunable laser system emitting a visible light in a region from 710 to 775 nm was developed and constructed. For Q-switching [2], three different optics shutters (passive, electro-optical and mechanical) were proved. It was a saturable NC-dye or LiF crystal, LiNbO₃ Pockels cell, and rotating prism.

For a passive Q-switching the saturable absorbers – the NC dye in an ethylalcohol or LiF:F^2- crystal were used. As a result Q-switched pulses with the maximum energy about 0.3 mJ and a pulse-width ranging between 110 and 250 ns were obtained for NC-dye. In the case of LiF:F^2- crystal single 20 mJ pulses of 130 ns (FWHM) pulse duration were generated.

For an electro-optical active Q-switching lithium niobate (LiNbO₃) Pockels cell was used. The Pockel’s cell switching voltage was 900 V, an optimal rising time of the Q-switched pulse was derived to be 125 µs. The function of this switch without and with a polarizer was investigated. Due to the polarized light output from the alexandrite crystal, the system can generate giant pulse neither any polarizer was inserted between a nonlinear (LiNbO₃) and an active (alexandrite) crystal. With the insertion of it, the generated output pulses were more stable and this polarizer was used also as a tuning element. The output radiation parameters were: the maximum energy 30 mJ, pulse-width 80 – 150 ns (FWHM), wavelength 745 – 750 nm, and bandwidth 5 nm.

A simple Q-switching technique, based on a rotating prism was also tested as an opto-mechanical shutter for alexandrite laser. A rotation speed was typically less than 10 000 rpm. The switching time followed the flash lamp trigger. The average output energy was equal to 50 mJ in the single pulse with the length of 35 nsec (FWHM). The generated radiation wavelength was 750 nm (bandwidth about 10 nm).

As a conclusion it can be summarized that three types of Q-switches were tested for the alexandrite laser: the saturable - NC dye or LiF:F^2- crystal, LiNbO₃ Pockels cell and rotating prism. The saturable Q-switch is characterised by the simplicity of it construction. The Pockels cell enables precise control of the giant pulse shape. The rotating prism gives the maximum output peak power. It was found that all Q-switches (except NC-dye) could be used in the applications.

References

[2] Koechner, W. : Solid state laser engeniering Springer-Verlag, Berlin, 1992

This research has been supported by the Grant Agency of the Czech Republic grant No. 102/99/1247.

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