Page 359 - High Power Laser Handbook

328 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 329

before its temperature increases from the absorbed energy. This non-
thermal “cold” ablation enables precise materials processing with
1–3
negligible secondary damage effects from heating and melting. To
date, however, ultrafast laser technology has not found widespread
use in industry. The main challenges have been low average power,
high costs, and limited reliability of typical femtosecond laser sys-
tems. The pico- to femtosecond materials processing application is a
representative example for many other industrial applications, for
which, in principle, excellent improvements and even new opportu-
nities have been demonstrated in research laboratories.
We believe that novel ultrafast lasers in the thin-disk geometry
based on either diode-pumped ytterbium (Yb)-doped solid-state
lasers or semiconductor lasers can offer a solution for many applica-
tions. Stable ultrafast pulses are obtained with semiconductor satu-
4,5
rable absorber mirrors (SESAMs). Such SESAM mode-locked
thin-disk lasers offer reduced complexity and cost, with improved
6,7
reliability and average power.
SESAM mode-locked ultrafast laser oscillators in the thin-disk
geometry are very promising. The gain material’s geometry is an
important factor for a laser’s efficient thermal management. For aver-
age power scaling, the gain medium must be efficiently cooled, which
is achieved through a large surface-to-volume ratio. Possible options
are fiber, slab, and thin-disk geometries. In thin-disk geometry, the
active medium has the shape of a thin-disk with an aperture much
larger than its thickness. Applying this concept to diode-pumped
8
solid-state lasers led to the development of the thin-disk laser (TDL),
which initially used the crystalline material Yb:Y Al O (Yb:YAG) as
3
12
5
the active medium. Today, multikilowatt continuous-wave (CW)
Yb:YAG TDLs have successfully been established in the automotive
industry and have demonstrated excellent reliability, high efficiency,
9
and good beam quality. In addition, CW semiconductor TDLs can
generate greater than 20 W of output power in fundamental trans-
10
verse mode, which is significantly higher than any other semicon-
ductor laser. Such lasers were initially referred to as vertical external
11
cavity surface-emitting lasers (VECSELs) or optically pumped semi-
conductor lasers (OPSLs); because of their similarity to solid state
thin-disk lasers, however, they are more recently also referred to as
semiconductor disk lasers (SDLs).
Because both VECSELs and TDLs use the same thin-disk geome-
try of the gain material, they share many common features. Both
lasers produce state-of-the-art performance and are ideally suited for
ultrafast passive mode locking with a SESAM. Even though both
4,5
rely on SESAM mode locking, it is important to realize that their basic
mode-locking mechanisms are significantly different. In addition,
their ideal operation parameters, with 10 to 100 mJ pulse energies at
megahertz repetition rates for TDLs and pico- to nanojoule pulse
energies at gigahertz repetition rates for VECSELs, are very different,