FOCUS SHIFT DUE TO THERMAL LENSING IN ULTRAFAST LASER OPTICS AT KILOWATT-LEVEL AVERAGE POWERS

摘要

Thermal lensing at high average laser powers is a well-known issue for cw-Laser applications, when the average power exceeds about 1 kW for fundamental mode, and about 10 kW for multimode lasers. The laser-material interaction significantly changes during laser processing, if the focus shift exceeds about one Rayleigh length, as the incident laser intensity drops by a factor of two. Today, ps- and fs-lasers with average powers exceeding 2 kW were demonstrated in the laboratories. In addition to the high average power, pulsed lasers often feature very high peak powers exceeding Gigawatt. In this work, focal shift measurements were made for different combinations of different types of substrates and coatings, from different manufacturers. This was done using the IFSW-prototype ps-laser, which provides 8 ps pulses at a repetition rate of 300 kHz up to the aver-age power of about 520 W. Fused silica substrates with good wavelength-coatings caused a shift of less than half a Rayleigh-length at the maximum power. In contrast, the worst combination of a broadband coating on a BK7 substrate caused a shift of more than seven Rayleigh-lengths. In the talk, a brief overview over the theory will be given. In addition, the latest results of time resolved focus shift measurements using a novel matrix beam splitter will be presented and discussed.