Page 379 - High Power Laser Handbook

348 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 349

power of 35 mW and the quality of the pulse train are not sufficient
for many applications, in particular because multiple pulses were cir-
culating in the cavity. An important aspect for improving the per-
110
formance will be the optimization of GDD in the cavity; this idea is
109
supported by recent experiments confirming the quasi-soliton-
111
theory, which predicts the shortest pulse durations for slightly pos-
112
itive cavity GDD. In any case, a careful control of the cavity GDD is
regarded as being crucial for achieving femtosecond high-average-
power operation of mode-locked VECSELs.
13.5 Conclusion and Outlook

Ultrafast SESAM mode-locked thin-disk lasers based on either
Yb-doped solid-state gain materials or semiconductors offer a robust
and power-scalable solution to the challenges of ultrashort pulse gen-
eration at high power levels. The key for this performance is efficient
heat removal, which minimizes thermal lensing and aberrations, thus
enabling high power levels in a fundamental transverse mode. The
SESAM is an ideal device for mode locking at high power levels due
to its large design flexibility. The concept of the SESAM mode-locked
thin-disk laser has the essential advantage of power scalability: The
output power can be scaled up by increasing pump power and mode
areas on both gain medium and SESAM. For high-power ion-doped
solid-state as well as for semiconductor lasers, this technology has
enabled new power records.
Femtosecond ion-doped solid-state TDLs achieved pulse ener-
gies beyond the 10-mJ level at megahertz repetition rates directly
from the oscillator. The average power level was increased to the
100-W level, which is particularly attractive for materials processing
applications at high throughput. The first thin disk gain material was
Yb:YAG, which until recently delivered the highest average output
powers and pulse energies. However, the impressive advances in the
research and development of new Yb-doped hosts and the availabil-
ity of suitable pump diodes operating in the 980-nm spectral region
have both led to new power records by applying Yb-doped sesquiox-
ides. In particular, Yb:Lu O is a promising material, achieving a
3
2
mode-locked average output power of 141 W in 738-fs pulses. 12,13
Further scaling toward several hundred watts of average power and
pulse energies of more than 50 mJ appear to be within reach. A critical
challenge will be the demonstration of similar power levels and pulse
energies from systems operating at pulse durations below 100 fs,
which will require gain materials with larger-emission bandwidth
than the dominant Yb:YAG gain material. Such systems will be useful
for numerous industrial and scientific applications—for example, in
the area of high-field science and high harmonic generation. 6
Ultrafast semiconductor disk lasers operate at multiwatt power
levels, which is higher than any other ultrafast semiconductor laser