Page 251 - High Power Laser Handbook
P. 251
220 So l i d - S t at e La s e r s Nd:YAG Ceramic ThinZag® High-Power Laser Development 221
for this case. Note that the instantaneous power of the pulses is close
to 88 kW, close to calculation.
Several issues must be addressed when using LDC mode. The
diode current’s rise time must be fast as compared with the several-
millisecond pulse on time. The Sorenson SFA-150 power supplies
used have current rise times less than 0.25 ms, which is significantly
less than the pulse on time. Diode wavelength measurements indicate
that the steady-state diode temperature is achieved on the scale of the
pulse rise time. A key issue that is yet to be addressed is optimized
control of the deformable mirrors in pulsed mode for good beam qual-
ity. Controlling deformable mirrors is more straightforward in CW
than in LDC mode. The downside is that thermal heating of the slabs
is greater in CW mode, so the intrinsic medium distortion is greater.
Figure 9.17 shows the three modules set up as a single-aperture
power oscillator. The design of the optical cavity has provisions for
operation with a deformable mirror coupled to each cell to correct
medium distortion. The distortions are sensed using a second color
probe laser; a Shack-Hartmann detector measures phase and controls
a deformable mirror to correct the phase of each module. Initial mea-
surements were made without the internal deformable mirrors in the
optical cavity. An external deformable mirror built by MZA Associ-
ates was used to correct beam quality.
A series of measurements was made at diode pump currents of
50, 60, and 70 A. The optical cavity used a GRM output coupler
with a nominal magnification of 1.4. The GRM feedback was still
Figure 9.17 Three TZ-3 modules operating as a single-aperture power
oscillator.