Page 376 - High Power Laser Handbook
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344 So l i d - S t at e La s e r s Ultrafast Lasers in Thin-Disk Geometry 345
Possibilities for overcoming this problem and for achieving pulse
energies in the 10 to 100 mJ regime are to reduce the nonlinearity in
the resonator by operating it in vacuum or in helium or to reduce the
intracavity pulse energy by increasing the laser cavity’s output cou-
pler transmission. Under helium atmosphere, pulse energies of up to
60
11 mJ were obtained, with an intracavity pulse energy exceeding
100 mJ. The second approach requires an increased gain per cavity
roundtrip, which can be achieved in a cavity setup with multiple
passes through the gain medium. With this concept, pulse energies of
26 mJ and an output coupler of 78 percent, and thus an intracavity
pulse energy of only 34 mJ, were demonstrated in air atmosphere. 14
The available pulse energy for VECSELs is limited by the carrier
lifetime, which hinders operation at megahertz repetition rates for
CW pumping. As discussed in the previous section, the maximum
cavity length is limited by the onset of multiple pulsing instabilities
93
or harmonic mode locking. The highest pulse energies reported for
mode-locked VECSELs are only in the order of several 100 pJ, 38,68
which is orders of magnitude lower than in solid-state TDLs.
Pulse Duration
For both ultrafast TDLs and semiconductor disk lasers, the full poten-
tial to achieve extremely short pulse durations has not yet been
exploited. Currently, ultrafast TDLs are restricted to pulse durations
of more than 220 fs. To date, all ultrafast solid-state TDLs have been
3+
3+
based on Yb -doped gain materials. In the Yb ion, the so-called lan-
thanide contraction leads to a lowered distance of the 5s and 5d shells
from the atom core. Therefore, the 4f shell, in which the optical transi-
tions take place, is less shielded from the surrounding crystal field
than is the case in other rare earth ions. This situation leads to a stron-
ger coupling to the host’s phonons and thus to broad absorption and
emission spectra, which have been shown to support the generation
of pulse durations less than 60 fs in longitudinally pumped low-
100
power bulk lasers (e.g., in Yb:glass, Yb:LuVO , Yb:CaGdAlO ,
101
102
4
4
Yb:LaSc (BO ) , or Yb:NaY(WO ) 104 ). Such short pulses require a
103
3
4 2
3 4
gain bandwidth Df of about 20 nm in the spectral range of ~1 mm.
g
However, the most common gain material for the solid-state TDL is
Yb:YAG, which was chosen for its beneficial CW properties, even
3+
though its gain bandwidth is narrower than for many other Yb -
doped gain materials. Consequently, the shortest pulse durations
obtained with Yb:YAG TDLs were around 700 fs, 42,64 whereas
Yb:Lu O TDLs enabled the generation of 535-fs and 329-fs pulses at
3
2
67
63 W and 40 W of average output power, respectively. Investigation
of new gain materials with even broader emission bandwidths has
enabled the generation of pulses as short as 240 fs at 22 W of average
62
output power with Yb:KYW (Yb:KY(WO ) ) and 227 fs with an
4 2
average output power of 7.2 W with Yb:LuScO . The differences in
66
3