Page 371 - High Power Laser Handbook

340 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 341

effects and prevents the laser from stable mode locking. As already
mentioned in Sec. 13.3, good thermal control and one-dimensional
heat removal in the disk geometry allow for very high output power
in TEM operation. However, it has also been shown that in VEC-
00
SELs, the general scaling law for TDLs is no longer valid when the
pump spot reaches a size at which the major part of the thermal
impedance is caused by the heat sink and no longer by the semicon-
21
ductor device itself. At a certain point, the increased temperature
will affect the threshold intensity as well as the slope efficiency; thus,
the total loss in efficiency will dominate the benefit of a larger gain
mode size. Furthermore, as mentioned in Sec. 13.3, the onset of ASE
can also be a challenge for further scaling the mode areas and thus the
25
output power. So far, the highest TEM output power that has been
00
achieved with a VECSEL is 20.2 W. In contrast, the maximum fun-
10
damental mode output power that has been obtained with solid-state
TDLs is in the order of several 100 W—for example, 360 W were dem-
57
onstrated by Killi et al. Even in this power regime, ASE was not a
84
limiting factor.
In addition, a much higher average power was obtained in mode-
locked operation with solid-state TDLs. The first SESAM mode-
locked solid-state TDL, which was demonstrated in 2000, already
delivered an average output power of 16.2 W in 730-fs pulses. This
64
laser used Yb:YAG as the gain material, which was also the first gain
material for solid-state TDLs. The pump spot had a diameter of 1.2 mm.
Power scaling to 60 W was later demonstrated by increasing the pump
spot diameter to 2.8 mm and adapting the mode areas on the disk and
85
the SESAM by an appropriate resonator design. The pulse dura-
tion of this laser was 810 fs at a repetition rate of 34 MHz. Further
power scaling of SESAM mode-locked TDLs was demonstrated up to
43
80 W average power using Yb:YAG. Recently a 100-W breakthrough
result was achieved with the new and more efficient gain material
Yb:Lu O . 12,13 This laser generated a maximum average output power
2
3
of 141 W with a pulse duration of 738 fs and a high efficiency of 37
percent. At a repetition rate of 58 MHz, the pulse energy is 2.1 mJ. At
a slightly lower average output power of 103 W, the laser operated at
an even higher optical-to-optical efficiency of 42 percent, 12,13 nearly
doubling the typical efficiency of Yb:YAG.
Today, mode-locked output powers in the kilowatt range seem
feasible, even though several additional issues must first be over-
come. Further scaling of the pump spot diameter to the necessary
centimeter range is challenging, because it requires a uniform ther-
mal lensing effect in the disk. A uniform thermal lens can be compen-
sated for by a standard resonator design without the need for
complicated adaptive optics. However, this compensation becomes
more difficult at larger pump spot diameters, because stability range
narrows with an increasing pump spot diameter. 86