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We present the experimental investigations of different designs of resonant waveguide-grating mirrors (RWG) which
are used as intracavity folding mirror in an Yb:YAG thin-disk laser. The studied mirrors combine structured fused
silica substrates, a thin-layer waveguide (Ta2O5), a buffer layer (SiO2) and partial reflectors. The grating period was
chosen to be 510 nm to allow resonances at an angle of incidence of ~10° for TE polarization. The waveguide layer has
a thickness of 236 nm. It is followed by the buffer layer with a thickness of 580 nm and the subsequent alternating
Ta2O5/SiO2 layers. The exact coating sequence depends on the two design approaches which were investigated in this
work: either introducing different partial reflectors, i.e. stacks of quarter-wave layers on top of the waveguide while
keeping the groove depth of the grating constant, or increasing the grating depth, while keeping an identical partial
reflector. The investigation was focused on the rise of the surface temperature due to the coupling of the incident
radiation to a waveguide mode, as well as on the laser efficiency, polarization and wavelength selectivity. It is found
that, when compared to the simplest RWG design which consists of only a single Ta2O5 waveguide layer, damage
threshold as well as laser efficiency can be significantly increased, while the laser performances in terms of
polarization- and wavelength selectivity are maintained. So far, the presented RWG allow the generation of linear
polarization with a narrow spectral linewidth down to 25 pm FWHM in a fundamental mode Yb:YAG thin-disk laser.
Damage thresholds of 60kW/cm2 have been reached where only 63K of surface temperature increase was observed.
This shows that the improved mirrors are suitable for the generation of kW-class narrow linewidth, linearly polarized
Yb:YAG thin-disk lasers.
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