Page 257 - High Power Laser Handbook

226 So l i d - S t at e La s e r s Thin-Disc Lasers 227

In November 1991, during the Lasers and Electro-Optics Society
(LEOS) conference, Adolf Giesen listened to a talk about diode
pumped Yb:YAG lasers given by T. Y. Fan of the Massachusetts Insti-
tute of Technology (MIT). Fan explained the advantages of Yb:YAG
for diode laser pumping in detail, but he also stated that it would be
very difficult to build a high-power Yb:YAG laser using classical
designs due to Yb:YAG’s quasi-three-level nature. At that time, the
laser output power was only a few watts.
After this talk, Giesen made some initial calculations, which
showed that it would be possible to power scale Yb:YAG if the mate-
rial were simply a very thin sheet of material cooled from one or two
sides, so that the heat flux length to the cooling device were mini-
mum. At the University of Stuttgart in Germany, Giesen convinced
his colleagues of his idea. In January 1992, a small group (Uwe Brauch
from the German Aerospace Center [DLR], Adolf Giesen, Klaus Wit-
tig and Andreas Voss from the University of Stuttgart [IFSW]) started
to develop the details of such a laser design, using a thin sheet of laser
active material. The primary thin-disc laser design was developed at
the end of March 1992, and in late spring 1993, the first demonstra-
3, 4
tion was realized—first with 2 W output power and later with 4 W.
Also in 1993, the group applied for the first patent for this design,
which has since been successfully licensed to more than 20 compa-
nies. During the following years, Giesen’s group demonstrated power
scaling of thin-disc lasers, pulsed operation also with subpicosecond
pulse duration, and the applicability of this design to many other
laser active materials.
Fortunately, during the 1980s the German federal ministry of
Research and Technology (BMFT) identified laser technology as a
key technology for materials processing. Consequently, during these
years, many projects were initiated and funded between research
institutes and companies, which led to increased funds for thin-disc
laser work. Later, companies like Trumpf Laser, Rofin-Sinar and
Jenoptik started working on thin-disc lasers, generously supporting
the institute’s work. As a result, within just one decade, German
companies became very strong in the field of laser technology for
materials processing, eventually taking the leadership role in this
industrial area.

10.3 Principles of Thin-Disc Lasers

The core concept of the principle behind thin-disc lasers is the use of
a thin, disc-shaped active medium that is cooled through one of the
flat faces of the disc; simultaneously, the cooled face is used as a fold-
ing or end mirror of the resonator. This face cooling minimizes the
transversal temperature gradient, as well as the phase distortion
transversal to the direction of the beam propagation, and it accounts