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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