Polarization selection Two types of polarization selection were observed. Polarization selective elements exhibited diverse losses for different polarizations and generated a pre-ferred field direction. Experiments showed an OC transmission independent PER of more than 100:1 with a 1 mm thin fused silica window, limited only by the noise of the power meters. The intrinsic polarization significantly depended on the OC transmission. For low transmissions up to 10−2 the PER was between 1:1 and 10:1 and originated presumably from stress induced deviations of the isotropic structure. The responsible stress can come from the crystal growth, the mounting procedure on the heatsink, or from the coating. This yields to significant losses for an additional polarization selective element. Measurements determined the depolarization losses caused by the reflection of power at a Brewster plate in the order of 10−3for a pump spot diameter of 1.2 mm.
Single-transverse-mode operation Single-transverse-mode operation was achieved by a suitable laser/pump mode overlap with an appropriate resonator design for a pump spot diameter of 1.2 mm. The small pump spot decreased the influence of optical defects and the predominant astigmatism, which was confirmed with an SHWFS. In the experiments with a short resonator anM2 below 1.05 was measured for internal losses of 8.9×10−3. Intracavity powers higher than 100 kW Intracavity powers in the order of 200 kW and 150 kW were obtained with a pump spot diameter of 3.6 mm in multi-transverse-mode operation with Yb:Lu2O3 and Yb:Y3Al5O12. The pump power was 270 W in the case of Yb:Lu2O3 and 150 W in the case of Yb:Y3Al5O12. With a pump spot diameter of 1.2 mm, an intracavity power of 135 kW was measured for Yb:Y3Al5O12. This corresponded to an enhancement by a factor of 2500 with respect to the incident pump power of 54 W, enabled by the low losses of state-of-the-art processed gain disks. In all experiments further scaling of the intracavity power was limited by optical damage at disks and mirrors. Generally, the observed optical damage of laser disks could be divided into two groups. The first group of disks showed transverse cracks through the entire disk, which can be explained by thermo-mechanical stress. The second group exhibited small nodular defects with diameter up to several µm. Most defects were found in the center close to the area of the highest beam intensity. Defects in the HR and AR coatings which absorb light and finally melt are plausible. Another reason for optical damage are high field-seeking dirt or dust molecules, which can be attracted to optical surfaces, where they can absorb photons.
Electric field intensity The required intensities of 1010W cm−2 to 1012W cm−2were not achieved in the presented linear resonators. Those intensities require an intracavity focus for the available pump powers, which could not be achieved with the available laser disks in a linear resonator. Here, the highest intracavity intensities of approx. 11 MW cm−2were obtained on the pump spot, which probably already caused several optical defects.
Discussion of the results with linear resonators 77 Resonator roundtrip losses The resonator’s internal roundtrip losses were determined in a short and efficient multi-transverse-mode resonator for several disks. The losses were in the order of 3×10−4 for a pump spot diameter of 1.2 mm. No significant dependence of the resonator’s internal losses on the known disk parameters could be determined. The independence on the disk’s thickness suggests that the losses were caused at surfaces rather than in volumes. However, this could not be confirmed in experiments with a pump spot of 3.6 mm, which showed a comparable performance for low OC transmissions.
Measurements in a single-transverse-mode resonator with a pump spot of 1.2 mm exhibited losses of 9×10−3and with an additional optical window in the Brewster’s angle in the order of 1.5×10−2. They could not be assigned to specific loss channels due to their comparable magnitudes. A combination of scattering, transmission, absorption, diffraction at the disk and the mirror, additional losses at the Brewster window and nodular defects is plausible.
The losses at the Brewster window originate from depolarization and the angular difference of the outer parts of a Gaussian beam from the Brewster’s angle. Finally, the determined resonator’s internal roundtrip losses of 1.5×10−2with a linear field polarization in single-transverse-mode operation might require pump powers of more than 200 W to achieve intracavity powers in the order of 100 kW.
Choice of the gain material Under consideration of the estimated inaccuracy of the loss determination in the order of 5 %–10 %, no gain material stands out from measurements of the internal losses or laser performance. The most relevant difference between the gain materials can be seen in their thermal conductivity. Spectral properties, such as gain cross-sections, have less importance and should not influence the resonator’s internal losses, nor have a significant importance on the laser performance at low OC transmissions. In parallel, in respect to optical damage, no distinct advantage of one specific material was observed.
Absorption Scattering Angle missmatch Wavelength missmatch Mirror substrate
HR coating
AR coating Gain material
HR coating AR coating
Brewster window
Absorption Scattering Transmission
Diffraction
Absorption Scattering Resonator medium
Absorption Scattering Transmission
Diffraction Depolarization
Inversion dependent losses Figure 5.25: Overview about several loss mechanisms which can occur in a linear resonator with an optical window in the Brewster angle. Optical defects are not enlisted and can occur at all surfaces.
78 Laser experiments and discussion
10 µm Yb:YAG (D357)
Figure 5.26: Left: microscopic picture of a small optical damage of a thin disk, presumably induced by point defects in the HR coating. Right: Transverse cracks through nearly the entire disk.
This points towards gain material independent losses and thus towards causes which they have in common, such as coatings and contacting. The monitoring of optical damage is difficult, as it would require a continuous image of the disk’s surface in high resolution, to detect optical defects of µm size. In parallel, no specific range of the intracavity intensity and power could be determined yet, which would significantly reduce the risk of optical damage. The reason for the occurrence of optical damage in these experiments could not be finally assigned and needs further investigation.
A choice of the best suited host material can be made under consideration of the availability and commercial use of the gain materials. Yb:Lu2O3 bulk crystals nowadays are not com-mercially available in a constant optical quality, due to high requirements on the growth. In contrast, Yb:Y3Al5O12is industrially available since decades. This indicates, that production lines, such as polishing, coating or soldering, are adapted and optimized to the host mate-rial, which is not the case for Yb:Lu2O3. The same advantages are valid for Yb:Lu3Al5O12, which can probably be found in commercial high power TDL, because of its higher thermal conductivity in comparison to Yb:Y3Al5O12. Thus, considering the previous statements, at the moment, the suggested optical candidate for continuative experiments is Yb:Lu3Al5O12. Due to the planned purpose of laser operation at potentially low OC transmissions, there is room for further optimizations. A decrease of the disk’s thickness to reduce its temperature, or a reduction of the ytterbium doping concentration, which could improve the optical quality of the gain material, would be feasible.
Design of the folded resonators for thin-disk lasers 79