Page 251 - High Power Laser Handbook
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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.
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