Page 202 - High Power Laser Handbook

170 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 171

thermal conductivity an order of magnitude smaller than YAG;
though this does not affect the pulse energy, it typically limits pulse
repetition frequencies to millihertz or lower.
Ti:Sapphire
If the SSL’s purpose is to generate the highest peak power pulses, an
alternative to increasing the pulse energy is to decrease the pulse
duration. This is often much less expensive, because it reduces the
required pumping power; moreover, ultrashort pulses enable many
unique high-power applications (see Chap. 12). The ability of an SSL
material to generate short pulses directly is limited by its gain band-
width. Ti:sapphire is the most commonly used ultrashort-pulse mate-
rial, owing to its nearly 1 octave of spectral coverage from 650 to
1100 nm. Although the thermal properties of sapphire are even
15
better than those of YAG, Ti:sapphire is not particularly well suited to
energy storage, due to its short (∼3 µs) upper-state lifetime, which
typically requires pulse amplifiers to be pumped with short-pulse,
Q-switched, and frequency-doubled (515 or 532 nm) YAG lasers. The
large quantum defect between pump and emission wavelengths,
along with the lack of economical high-power pump sources in the
blue-green, typically limits average powers from Ti:sapphire to less
than 100 W.

Other Pulsed Materials
Although the previously discussed materials dominate most high
energy and peak power lasers, some less common materials warrant
3+
mention for their pulsed laser characteristics. Yb:SFAP [Yb :Sr (PO ) F]
5
4 3
has been investigated as a diode-pumped material that is suitable for
high-energy storage, due to its combination of large size, long upper-
state lifetime, and intermediate saturation fluence, which balances
energy storage against damage limits. 16,17 Yb-doped tungstates and
sesquioxides have recently been developed for directly diode-
pumped, short-pulse lasers. These materials exhibit microscopically
disordered structures that result in the broad gain bandwidths needed
to support ultrafast pulses. In particular, the Yb-doped sesquioxides
Yb:Sc O and Yb:Lu O exhibit thermal conductivity slightly greater
2
3
2
3
than YAG and have demonstrated potential for average power scal-
ing in thin-disk geometries. 18
7.3 Pumping, Cooling, and Thermal Effects
Careful management of thermal effects in the gain medium is the
overriding engineering imperative for any successful high-power
SSL design. There are two primary reasons temperature is so impor-
tant for SSLs. First, thermal gradients must not be allowed to become
large enough so as to pose a fracture risk to the laser material. Thermal