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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.