www.industrial-lasers.com MAY/JUNE 2015 Industrial Laser Solutions 19
crystal and their damage threshold. To get the best out, the AOMs,
the laser source, and the beam path have to be well designed.
All state-of-the-art lasers are tested, especially concerning pulse
behavior, power stability, pointing stability, and mode. The rise and
fall time determines the pulse behavior and, thus, the engraving
speed. The nitrogen in the gas mix slows down the pulse frequency
to around 10kHz. This was sufficient for many applications in the
past, but it is not enough for the future. A typical laser power vs.
time diagram shows deviation values between ± 5 and 10%. This is
absolutely not suitable for a controlled 3D engraving into the depth
of the material. Laser pointing stability is surprisingly good for various tested lasers and has a direct impact on the use of AOMs,
which are sensitive on incident angles.
Near the power limit of the AOM, the germanium crystal is very
sensitive to bad laser modes. Hot spots cause a distortion of the outgoing beam and can easily destroy the crystal. A behavior of many
CO2 lasers is the bad mode in the near-field. Typically, the distance
between the output coupler and the AOM should be around 2 m or
more, which results in a much better mode (FIGURE 3). This is sometimes difficult to realize, especially in a compact engraving machine.
The new CO2 laser project
An obvious choice was to achieve a highly stable resonator and a
FIGURE 4. A carbon-fiber CO2 laser set up with two IR cameras
close-to-perfect beam mode, using a “classic” folded CO2 laser
with modern materi-
als like carbon fiber
for the resonator
fiber tubes can have
a very small thermal
ficient (less than
1µm per meter and
when well designed.
ment method (FEM)
calculations to opti-
mize thermodynamical behavior [ 1].
Beam path optimization. A custom-built carbon fiber optics table
is used for high precision of the laser resonator and the entire setup
for the beam path with the AOM and the infrared (IR) cameras
(PyroCams) to visualize the beam mode online. A precise mea-
surement of the influence of the germanium crystal—especially
regarding distortions—can be made with two PyroCams before
and after the AOM (FIGURE 4).
Hexapod. The AOM with a germanium crystal offers a compara-
tively good performance with up to or even more than 600 W of CO2
laser power, provided that the mode of the laser beam is close to
the Gaussian shape. If the power gets too high and especially if hot
spots on the crystal surface occur, it is easily damaged.
Optimizing an AOM means shaping the laser beam to a bal-
ance between a small beam spot and an intensity that is still suit-
able for the crystal. The smaller the beam spots, the higher the
pulsing frequency. Lateral and rotational optimization has to be
achieved, which is quite an effort unless the pivot point for the
two translational and three rotational movements can be shifted to
the centroid of the incident beam on the crystal surface. A hexa-
pod provides this feature in a perfect way and allows movement
FIGURE 3. A CO2 laser at full power at a distance of 1 m (a) and a
distance of 6 m (b), represented in 2D (left) and 3D (right).
(blue) before and after the AOM (yellow).
FIGURE 5. A CO2 laser with a carbon-fiber
resonator (background) and hexapod for
optimizing the AOM (left) on top.