Abstract
Thermal lensing at high average laser powers is a wellknown
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.
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