Page 360 - 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

even though they share the high-average-power scaling benefits of
their respective operation regimes. Ultrafast TDLs can generate 141
W of average power in femtosecond pulses, 12,13 which is higher than
any other mode-locked laser oscillator. They also generate the high-
est pulse energies, with up to 25 mJ at a pulse repetition rate of 2.93
MHz, which is sufficient for high-speed micromachining applica-
tions. Ultrafast VECSELs access a different operation regime than
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TDLs, generating pulse energies in the pico- to nanojoule regime at
gigahertz pulse repetition rates but with relatively high average
power in the 100 mW to multiwatt regime, which is the highest in
comparison to any other gigahertz laser oscillator. Ultrafast VECSELs
have a number of compelling advantages, including compactness
and their ability to operate in wavelength regions that are not easily
accessible with established ion-doped, solid-state laser materials.
Furthermore, it is possible to combine gain and saturable absorber in
one semiconductor structure, enabling mode locking in a simple,
straight cavity. These devices are referred to as mode-locked inte-
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grated external-cavity surface emitting lasers (MIXSELs). Their
good mode-locking performance, in combination with the potential
for cost-efficient mass production, makes MIXSELs a promising alter-
native for many applications that currently rely on more bulky and
expensive laser systems.
This chapter describes the differences between and common fea-
tures of passively mode-locked high-power laser oscillators in the
thin-disk geometry using either diode-pumped solid-state lasers or
optically pumped semiconductor lasers. The chapter starts with a
brief introduction of the pump concepts of solid-state TDLs and VEC-
SELs, including a discussion of their thermal management. We then
explain why the fundamental laser material parameters lead to dif-
ferent pulse formation mechanisms and to different operation
regimes, though with the same power scaling benefits. The chapter
closes with a brief summary and an outlook toward further improve-
ment of the performance of passively mode-locked solid-state TDLs
and VECSELs.

13.2 Pump Geometry
In the thin-disk geometry (c.f., Chap. 10), the disk-shaped active
medium has a highly reflective (HR) coating on the back and an anti-
reflection (AR) coating on the front for both the pump and laser
wavelength. In the simplest case, the resonator can be formed by the
disk, which then acts as an end mirror, and only one additional out-
put coupler (Fig. 13.1a), which is why it is also known as the active
mirror concept. Especially for diode-pumped solid-state TDLs, the
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pump absorption length is significantly larger than the disk thick-
ness. Therefore, the pump light is launched onto the disk under a