Page 377 - High Power Laser Handbook

346 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 347

the gain bandwidths of some of these materials can be seen in Fig.
13.8a. Other Yb-doped materials may have the potential to push the
high-power TDLs into the sub-100-femtosecond regime. On the
105
other hand, longer pulse durations can easily be achieved by insert-
ing a spectral filter into the thin-disk laser cavity, thus limiting the
available gain bandwidth. 8
The pulse durations in high-power TDLs are significantly longer
than the pulse durations achievable by low-power SESAM mode-
locked lasers, which use a bulk crystal as gain material. For example,
pulses as short as 340 fs were obtained with Yb:YAG in such a
107
setup, 106 while a Yb:LuScO delivered 111-fs pulses. This differ-
3
ence occurs because the pulse duration is not only determined by the
gain bandwidth but also depends on other parameters. Detailed
investigations 52,53 on stable soliton mode locking with a SESAM
revealed that according to

/
 1  34  t  14 g 38
/
t ≈ 02 .    a  (13.3)
p  f D  D R Φ 18
/
 g    0
the pulse duration t is also strongly influenced by the gain satura-
p
tion g, even more so than by the SESAM parameters recovery time t
a
and modulation depth DR or the soliton phase shift Φ . High-power
0
solid-state TDLs usually use a larger output coupler transmission
than do low-power mode-locked lasers, because high intracavity
pulse energies lead to an unwanted SPM contribution of the ambient
atmosphere (compare Sec. 13.4.2) or even to damage of the optical
components. Thus, these lasers are operated at a significantly higher
saturated gain. Moreover, the short length of the active medium
requires a comparably high inversion level, which often narrows the
gain bandwidth that can be used for generating the pulses. Shorter
pulse durations may be achieved with the concept of the active mul-
tipass cell, with multiple passes through the gain material during one
14
resonator roundtrip (see Sec. 13.4.2). With a lower output coupler,
one would obtain low saturated gain and inversion, which may
enable the generation of shorter pulses in the future.
Typical VECSELs exhibit a broad gain spectrum that is compara-
3+
ble to that of broadband Yb -doped materials (see Fig. 13.8a). Fur-
thermore, the emission bandwidth can be easily engineered by
appropriate design of the gain structure. The overall gain spectrum
depends on the intrinsic emission properties of the QW layers, as
well as on the wavelength-dependent field strength at the position of
the QW layers. The latter can be influenced by the design of a reso-
nant or antiresonant structure for the standing wave pattern inside
the gain medium, which is referred to as field enhancement. Typically,
several QW layers are employed, and the overall bandwidth can even
be larger than the intrinsic bandwidth of one QW layer (the VECSEL