Page 38 - High Power Laser Handbook

10 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s Carbon Dioxide Lasers 11

w I

g

Figure 1.6 Typical geometries for high-power diffusion-cooled lasers: Slab-
based design (left) and coaxial design (right). w: width; l: length; g: gap.

Typically, diffusion-cooled lasers with a slab geometry are also
referred to as waveguide lasers. The interelectrode gap d is about 2 mm,
which enables efficient cooling but also adds waveguide losses to the
resonator losses. To minimize waveguide losses, the surface finish and
positioning of the electrodes must be accurately controlled. Because
coaxial designs have a π times larger surface area in the same footprint
as a slab design, the interelectrode gap d can be increased by a factor
of π without losing cooling capacity. The larger interelectrode gap size
enables free space propagation and reduces resonator’s internal losses.
Because the electrodes are not part of the optical system of a resonator
with free space propagation, the electrode surface finish and the posi-
tioning of the electrodes in relation to one another are less critical.
The stable-unstable hybrid resonators (Fig. 1.7a and b) generate
beams that are not rotationally symmetric and that are therefore astig-
matic and not usable for any application. A beam-shaping telescope is
used to transform the astigmatic beam into a round, stigmatic beam.
The beam quality post-telescope is about M = 1.1
2
Excellent beam quality makes these lasers ideal for cutting sheet
metal up to about a half inch thick. Cutting speeds in thin sheet metal

Figure 1.7a Stable-unstable hybrid resonator for a diffusion-cooled laser
with a planar electrode structure.

Figure 1.7b Stable-unstable hybrid resonator for a diffusion-cooled laser
with a coaxial electrode structure.