Page 227 - High Power Laser Handbook

196 So l i d - S t at e La s e r s Zigzag Slab Lasers 197

Figure 8.9
DP25 far-field
intensity
distribution.

array consists of 16 quasi–continuous wave (QCW) bars operating at
20 percent duty cycle and 50-W peak power per bar. Operating the
device in a QCW mode increased the gain and enabled the use of a
higher-magnification (M = 1.5) unstable resonator, which generated a
robust mode despite several waves of slab OPD.
Figure 8.9 shows the far-field intensity distribution of the beam at
a 5.4-kW power level. The measured beam quality based on the power
in the central lobe was 2.4 × DL, making this laser the highest-bright-
ness solid state laser for its time (year 2000).
Another interesting laser that used the traditional side-pumped
approach was the DARPA Diode-Array Pumped Kilowatt Laser
(DAPKL). A key feature of this laser was its simultaneous achieve-
ment of high pulse energy and high average power. The laser emit-
ted 10 J per pulse with 7-ns pulse duration, at an average power
level of 1 kW (100-Hz pulse repetition frequency) with 2 × DL beam
quality. This combination of high energy per pulse, coupled with
6
high brightness, was, at the time (1997) and even currently, a sig-
nificant challenge. The DAPKL laser used the master oscillator
power amplifier (MOPA) approach and phase conjugation via
stimulated Brillouin scattering (SBS) to provide good beam quality.
Figure 8.10 shows a schematic layout of the laser that used three
different sizes of amplifiers to achieve the required output. The
largest amplifier aperture was sized based on optical damage con-
siderations and had a cross-sectional area of 4 × 1.4 cm. Because
Nd:YAG crystal growth does not support a monolithic slab with
such an aperture, the slab was fabricated by diffusion bonding
3
three smaller 1.5 × 1.5 × 18 cm slabs (Fig. 8.11). Although diffusion
bonding of glasses was common at the time, diffusion bonding of
YAG was very rare. It has since become an important tool for laser
design and power scaling. 7
The DAPKL program also advanced the state-of-the-art of SBS
phase conjugation as an important tool for wavefront control in high-
power pulsed solid-state lasers. Energy scaling of greater than 1.5 J
with average powers greater than 150 W at the SBS cell was achieved
in a simple focus geometry, using liquid Freon 113 as the SBS medium
with good fidelity and without optical breakdown.