K. Kiedrowski
M. Jupé
M. Kennedy
H. Ehlers
A. Wienke
D. Ristau

High power laser-induced damage investigations of mirrors with several substrate materials in combination with heat transfer simulations

Proc. SPIE 12142, Fiber Lasers and Glass Photonics: Materials through Applications III
Type: Zeitschriftenaufsatz (non-reviewed)
Nowadays, continuous wave (cw) lasers have conquered a broad spectrum of applications in industrial laser processing and can be considered as the dominant tools in many manufacturing floors. This is reflected by the enormous average annual growth rates of 25-30 \% and the continuous research efforts dedicated to this laser type leading to ever increasing output power and beam quality. This development imposes ever increasing demands on the quality of the optical laser components, that have to withstand the usually harsh industrial environment and high power levels. In fact, the corresponding of the laser components is a key factor for the efficiency and economic success of an employed laser material process. This in turn requires a thorough assessment of the quality parameters ruling the stability of such components. Among many other quality parameters, the Laser Induced Damage Threshold (LIDT) is one of the leading parameters that has to be investigated in detail. The corresponding measurement facilities and protocols as well as the evaluation of the data have to be performed with high reproducibility and comparability among different testing laboratories. As a consequence, such qualifications can only be achieved on the basis of well-defined international standards defining the complete procedure for the determination of LIDT values. We investigated the laser induced damage threshold of different types of optics using a cw laser with a wavelength of 1030 nm and power up to 6 kW, applying beam diameters of approximately 200-300 μm on the surface. The samples were irradiated for at least 30s or until damage occurred. First, it was necessary to review the existing DIN EN ISO 21254 regarding cw-irradiation of mirrors with a 25 mm diameter. An important aspect is the number of possible irradiation spots on each optic with respect to the damage size as well as the emitted debris. Both effects limit the statistical accuracy, the ISO procedure needs to be adapted to the measurement conditions. Additionally, we investigated the influence of substrate materials and coating processes on the LIDT of high reflective coatings and their damage behavior, especially regarding their thermal conductivity. The results were then compared with simulations concerning the maximum temperature within the optical component.