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198   So l i d - S t at e   La s e r s                   Zigzag Slab Lasers    199


                                       Coupling
                          Cu microchannel  optics           CCEPS slab design
                             coolers
                  High-power CW                               Nd: YAG  Undoped
                                                                    YAG
                   commercial
                   diode arrays
                                                                         Laser
                                                                         beam
                                                           Evanescent
                                                          wave coating  Pump
                             Conduction-cooled end-                   light
                               pumped thin slab
                 Figure 8.13  Conduction-cooled, end-pumped slab (CCEPS) laser concept.


                         Figure 8.13 shows the main features of a CCEPS gain module.
                                                                               8
                      A thin slab is sandwiched between two microchannel coolers with
                      a low thermal impedance interface for efficient conduction cool-
                      ing. The ends of the slab are cut at 45° angles, and diode light is
                      coupled into the slab through its side edge. The diode light makes
                      a total internal reflection from the slab’s input face and propagates
                      axially  down  the  slab.  The  diode  light  is  coupled  into  the  slab
                      either by a set of lenses or by a lens duct. Unlike the side-pumped
                      geometry, the diode arrays must have high brightness along the
                      thin direction of the slab to allow efficient coupling into the slab
                      aperture. This high brightness is achieved by using microlenses on
                      the diode arrays to collimate the fast axis of the diode emitters.
                      Because  the  slab  ends  protrude  beyond  the  coolers,  they  have
                      undoped, diffusion-bonded end caps that do not absorb light and
                      therefore remain cool. Finally, a 2 to 3 µm SiO  evanescent wave
                                                               2
                      coating on the slab’s TIR faces ensures near lossless zigzag propa-
                      gation  of  the  high-power  beam  down  the  slab.  The  high-power
                      beam is injected into the slab at angles that are 20° to 30° from the
                      normal to the 45°-cut input face; this ensures that TIR occurs at the
                      YAG-evanescent wave coating interface.
                         Although the CCEPS architecture overcomes some of the scaling
                      limitations of the side-pumped approach, it does have issues common
                      to all zigzag slabs. Foremost among these is the temperature unifor-
                      mity in the non-zigzag direction. Temperature uniformity in the non-
                      zigzag direction requires both uniform pumping and uniform cooling
                      in this direction. For uniform pumping, a beam homogenizer, such as a
                      lens duct, may be used; alternatively, a set of lenses can provide ade-
                      quate uniformity by imaging the bars onto the slab aperture. For uni-
                      form cooling, the slab coolers should be designed to minimize internal
                      temperature gradients, and the thermal interface between the slab and
                      the cooler must be uniformly thin with low thermal impedance.
                         A  similar  architecture  that  shares  with  CCEPS  the  common
                      feature  of  decoupling  the  slab  thickness  and  pump  absorption
                                                  9
                      length is the edge-pumped slab.  In this architecture, pump light is
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