Page 215 - High Power Laser Handbook
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184    So l i d - S t at e   La s e r s                                                                Intr oduction to  h igh-Power Solid-State Lasers      185


                      is used to derive a set of control signals to change the DM’s shape to
                      impose the conjugate wavefront aberration on the beam. The new
                      and (hopefully) reduced aberration wavefront is then sensed to close
                      the feedback loop.
                         Integration of an AO system with a high-power SSL can be com-
                      plex. A key consideration is to ensure that the loop rate and control
                      bandwidth are sufficient to keep up with dynamic changes imposed
                      by the laser. These changes can be due either to warm-up transients
                      of the gain modules or optics during cycled operation or to turbu-
                      lence driven by hot optics or mechanical parts near the beam path.
                      Another  consideration  is  to  ensure  that  the  number  and  stroke  of
                      actuators can appropriately compensate the spatial frequencies and
                      amplitudes  of  the  incident  OPD.  Finally,  some  high-power  SSL
                      designs integrate AO inside a resonant cavity—typically, an unstable
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                      resonator.  This can couple the AO system to resonator modes and
                      extraction  dynamics,  which  often  require  a  complex  control  algo-
                      rithm to generate stable output.


                 7.6  Conclusion and Future Directions
                      In  this  chapter,  we  introduced  the  underlying  concepts  and  most
                      widely used methods for achieving high power in SSLs. The selection
                      of  SSL  material,  pump  source,  heat  removal  and  laser  extraction
                      geometries, and overall system architecture plays a critical role in the
                      scalability of a design to high power. The following chapters provide
                      design details for some of the most successful SSLs to date.
                         Much  work  is  underway  to  continue  developing  SSLs  to  even
                      higher  power  levels.  Laser-pump  diodes  are  rapidly  becoming
                      cheaper, brighter, and more reliable, which enables more controlled
                      beam shaping and deterministic heat deposition profiles. High-power
                      diodes are also being developed with line-narrowed and stabilized
                      spectra,  enabling  pumping  on  low-quantum-defect  spectral  lines
                      such as 885-nm for Nd:YAG, which reduces waste heat. Improved
                      ceramic fabrication methods are yielding structures with gradient or
                      heterogeneous doping profiles for improved pumping uniformity or
                                  35
                      reduced ASE.  Ceramic fabrication methods are also enabling pro-
                      duction of new host materials with improved spectral and thermal
                                                                         36
                      characteristics for high-average-power ultrafast-pulse lasers.
                         Laser  damage  resistance  of  optical  elements  is  a  key  issue  for
                      HAP SSL reliability and usability. Historically, damage  has  been  a
                      concern  mainly  for  pulsed  lasers.  However,  with  the  advent  of
                      multikilowatt average powers, CW damage is emerging as a major
                                                     37
                      engineering and operational issue.  Finally, as will be discussed
                      in Chap. 19, beam combining of multiple HAP SSLs (or other HAP
                      lasers,  such  as  fibers  or  diodes)  is  a  very  active  field,  due  to  its
                      promise of ultimate scalability by bypassing the limits of any specific
                      laser architecture.
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