Page 205 - High Power Laser Handbook

174 So l i d - S t at e La s e r s Intr oduction to h igh-Power Solid-State Lasers 175

structure to match any desired absorption feature in the near IR.
Selecting a diode wavelength close to the laser emission wavelength
reduces the quantum defect and thus minimizes waste heat. For
Nd:YAG, the most common band for diode pumping is the absorp-
tion feature centered at 808 nm, which boasts a high absorption cross
section and a fairly broad line width.
The narrow, engineerable emission spectrum of diodes has enabled
use of SSL gain materials that simply could not be pumped effectively
with lamps. The most prominent of these is Yb:YAG. Owing to
Yb:YAG’s simple energy-level structure, it is almost completely trans-
parent except in the 900 to 1100 nm range (Fig. 7.4). Yet this makes
Yb:YAG an ideal candidate for diode pumping, most commonly at the
∼10-nm broad absorption line near 940 nm. Yb-doped materials can
also be pumped at the narrower ∼980-nm, zero-phonon line to mini-
mize the quantum defect.
Another advantage of diode pumping is its highly directional (i.e.,
bright) emission. Although high-power diode emitters are multimode
lasers, their beam quality is nevertheless sufficient to enable beam
shaping using conventional optics or lens ducts. Hence, the pump
intensity distribution within the SSL gain medium can be tailored to
generate smooth excitation profiles to minimize thermal nonuniformi-
ties across the lasing aperture. Another advantage of high diode
brightness is that the pump light can be focused to very high intensi-
ties, easily surpassing the tens of kW/cm needed for efficient pump-
2
ing of the quasi-three-level Yb transition. Diode focusability also
eliminates the need for the SSL gain material to have a large surface
area devoted to receiving pump light. With the use of focused diodes,
pump light can be coupled into the gain medium through a relatively
thin edge or tip, allowing large-area faces to be devoted to heat removal
or laser extraction, with the corresponding thermal advantages.
Diodes have numerous other practical advantages as SSL pump
sources. They may be scaled nearly arbitrarily in power (although not
in brightness) by incoherently stacking multiple emitters or bars to
form large arrays, with multikilowatt modules commonplace. They
have benefited from years of investment in reliability engineering to
achieve lifetimes typically measured in tens of thousands of hours.
Finally, diodes are relatively compact and enable packaging of SSLs
for platforms and environments (such as space), where lamp-pumped
systems simply would not be feasible. In particular, fiber delivery of
diode light allows unprecedented design flexibility and packaging
convenience.

7.3.2 Laser Extraction and Heat Removal
Removal of heat through the laser material’s surface creates thermal
gradients that can aberrate the extracting laser beam and thus limit the
output BQ, or even lead to catastrophic failure due to stress fracture.