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218   So l i d - S t at e   La s e r s     Nd:YAG Ceramic ThinZag® High-Power Laser Development     219


                         The TZ-3 was designed to operate at 100 kW as six modules in a
                      series power oscillator. Each TZ-3 module was designed to be low
                      gain, so that ASE loss along the length of the slab would be minimal
                      when operated at full power. To increase the gain of the modules for
                      more efficient operation, an alternative mode of operation—namely,
                      low  duty  cycle  (LDC)—was  tested.  In  LDC  mode,  the  current  is
                      pulsed on for, say, 30 ms and off for 30 ms. The on times and off times
                      are somewhat arbitrary, though the pulse time should be long com-
                      pared with the kinetic lifetime of Nd:YAG (0.25 ms) and short com-
                      pared with the slab’s thermalization time (~1 s).
                         During the on time, the current is set much higher than the nomi-
                      nal  80-A  full  pump  current  used  for  CW  operation;  therefore,  the
                      instantaneous gain is much higher. In this case, the laser operates at
                      the maximum allowed current of the Osram diodes. At the higher-
                      diode pump currents, the laser operates at higher gain, with more
                      efficient  extraction  and  consequently  lower  thermal  heating  of  the
                      slab. The LDC current is chosen such that the laser output essentially
                      doubles for the on time compared with the current for CW operation.
                      For LDC mode, the average laser output is the same as for CW cur-
                      rent; however, because of the more efficient extraction, the overall
                      heating of the slabs is reduced, as are thermal distortions. Figure 9.15
                      shows the calculated improved extraction for lower-output coupling
                      when operating in the LDC mode.
                         A series of measurements, shown in Fig. 9.16, was performed to
                      test LDC mode operation. Measurements made with stable (S) and
                      unstable (U) optical cavities gave essentially the same average out-
                      put,  as  one  would  expect.  The  individual  30-ms  pulses,  measured
                      using a fast-response Labsphere, were twice the average power, as
                      predicted.  Kinetic  code  calculations  of  the  expected  output  also
                      agreed. The stable optical cavity output power measured was 44 kW,
                      compared with code calculations that predicted 42 kW average power




                                                                      Labsphere
                               U     100            S                 Ophir
                   80                 80
                  Power (kW)  60      Power (kW)  60

                   40
                                      40
                   20                 20
                    0                  0
                      −1   0   1    2       0     1    2     3     4     5     6
                             Time (s)                      Time (s)
                 Figure 9.16  Three TZ-3 modules operating with unstable (U) and stable (S) optical
                 cavities. Note that the instantaneous power output is essentially double the average
                 power. Output from stable and unstable cavities is comparable.
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