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The Er3+:YAG Solid State Heat-Capacity Laser (SSHCL) as a source for medium and high energy laser systems
in the “eye-safe” range is currently under investigation at ISL. The aim is to obtain a robust laser source with
low complexity, high beam quality (M2 < 3) and scalable to 100 kW and beyond. In a SSHCL the laser
medium is cooled only after the laser action has ended, resulting in low temperature gradients in the laser
medium itself during operation. Previous investigations demonstrated the scalability of the SSHCL and up to
4.65 kW and 440 J in less than 800 ms have been achieved. Optical-to-optical efficiencies of more than 41% and
slope efficiencies of over 51% has been obtained. The residual thermal gradients, due to non perfect pumping
homogeneity, negatively affect the performance in terms of laser pulse energy, duration and beam quality. Using
an intra-cavity adaptive optics system, beam aberrations were limited to less than 1/10 of the wavelength for
each of the considered Zernike polynomials, and the shot duration lengthened by about 50%. In this paper
we investigate how to further increase the SSHCL pulse duration. The influence of the crystal geometry on the
pump distribution homogeneity and the pulse duration are analysed. We consider the use of a mechanical crystal
changer for extending the laser pulse duration. By using a revolver with several crystals, we demonstrated that
crystals can be correctly positioned in less than 100 ms, allowing a quasi-cw operation that can largely exceed
the time constraints imposed by the heating of the crystal. Finally, we address the problem of measuring the
laser beam quality. Since the current standard techniques are suitable only for stable cw lasers, they cannot be
used for the SSHCL. A new kind of device, capable of measuring the M2 at intervals of less than 100 ms, is
presented.
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