Divergence compensation, optimization of the optical-to-optical efficiency, and high beam quality of signal and idler beams of a high-energy mid-infrared ZnGeP (ZGP) optical parametric oscillator (OPO) have been demonstrated by use of a Galilean telescope inside the nonplanar fractional-image-rotation enhancement (FIRE) ring resonator. With a small variation of the distance between the lenses of the telescope, the divergences of signal and idler beams could be adjusted. Up to 36 mJ of mid-infrared pulse energy in the 3-5 µm wavelength range is obtained with 92 mJ of pump energy on crystal. The beam quality factors M are < 1.5 for the resonant signal beam and the non-resonant idler beam, respectively. Actually, this is an improvement of the beam quality by a factor 3 for the signal and ~2.7 for the idler beam compared without using a telescope inside the FIRE ring resonator.
A significant reduction of the influence of the thermal lens in thin-disk lasers in high power laser operation mode could be achieved, using dynamically stable resonators. For designing the resonator, investigations of thermally induced phase distortions of thin-disks as well as numerical simulations of the field distribution in the resonator were performed. This characterization was combined with thermo-mechanical computations. On the basis of these studies, about 500 W output power with an averaged M 2 = 1.55 could be demonstrated, using one disk. Almost 1 kW output power with good beam quality could be extracted, using two disks. For the purpose of further power scaling in nearly fundamental mode operation, experiments using more than two disks are in preparation.
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