Page 321 - High Power Laser Handbook

290 So l i d - S t at e La s e r s Heat-Capacity Lasers 291

range from about 10 percent at t = 0.25 s to 100 percent at t = 5 s, with a
substantial amount of the aperture depolarized.

Beam Steering
A contour plot of the horizontal and vertical beam steering is shown
in Fig. 11.26 for t = 5.0 s. The steering angle is given in microradians
(mrad), with a positive value indicating that the beam is steered
toward the positive horizontal or vertical axis (the origin of the axes
is in the center of the aperture). Cross-sectional views along the verti-
cal midplane (for horizontal steering) and horizontal midplane (for
vertical steering) are given in Fig. 11.27. After 1 s, the maximum steer-
ing angle is about 200 mrad (four slabs, single pass) for both horizon-
tal and vertical steering. A double pass through the slabs would result
in a maximum steering of 400 mrad. This value could then be used to
determine the actual linear displacement of the beam on the DM,
given the path length in the cavity.

11.4 Current State of the Art

11.4.1 Power Extraction
In January 2006, the heat-capacity laser at LLNL achieved 67 kW of
average output laser power for short-fire durations consisting of
4
335 J/pulse at a 200-Hz pulse repetition rate, setting a world record
for pulsed, diode-pumped, solid-state lasers. The pulsed HCL had a
500-ms pulse width and used up to a 20 percent duty cycle from the
high-powered diode arrays. This power level was accomplished by
3+
pumping five transparent ceramic YAG:Nd slabs in series, each having
an active lasing region of 10 × 10 × 2 cm in thickness. Figure 11.28 shows
an end-view and side-view photograph of this HCL system.

11.4.2 Wavefront Control
To control the amount of wavefront distortion in the HCL, a number
of techniques were used. Figure 11.29 shows an optical layout sche-
matic of the HCL. One of the turning mirrors—and the main method
of controlling wavefront—is the intracavity deformable mirror (DM).
Tip-tilt corrections are applied to the high reflector, and a quartz rota-
tor midway through the optical chain acts as a birefringence compen-
sator. Not shown in the schematic is the beam sampling plate (placed
before the output coupler) and the Hartmann sensor which provides
the measurement of the wavefront as well as the signals necessary to
control the DM.
As mentioned earlier, the output beam quality depends very
strongly on phase distortions in the resonator. Some of the sources of
these distortions include (1) pump-induced thermal gradients in the
gain medium, (2) heating of resonator optics by absorbing some of