Specifications

Diamond™ GEM, G, & K, Integrator’s Guide
26
Where the beam path is fixed, aluminum tubing makes a good beam
guard. It is available in many diameters and has a smooth outer
surface that seals well. In most systems, beam guard tubes are black
anodized although this has no effect on their performance.
Mirages
Nitrogen has a extremely small but finite absorption of 10.6 µm
light. At high average laser powers, the gas in the beam delivery
system will consequently be heated by the laser beam. If there is no
gas flow, the warmer gas will rise. In a long horizontal path, this
creates a gradient index lens that causes mode degradation. In severe
cases, an initially circular mode can be deformed into a semicircle.
The remedies for this are to increase the tube diameter (which
decreases the gradient for any given amount of heating) or turn up
the purge gas flow.
Seals for Moving
Beams
It is more difficult to seal moving elements than static ones. Where
the travel is less than half the distance between a pair of moving
elements, two telescoping rigid tubes with an oil seal work well.
Telescoping tubes do not work so well when there are more than two
of them because they tend to bind. Bellows can have a large expan-
sion ratio, but they are usually made of organic materials which can
be decomposed by reflected laser light. Spiral lead screw covers
(available from Centryco) resist light reflections, but they do not seal
well and can deform into the laser beam. Putting a bellows (for a gas
seal) over a spiral cover (for laser light resistance) is a reasonably
robust solution. For long runs, the bellows must be guided on rods
to keep it on the beam line.
Many other types of beam delivery optic protection for moving
beams have been tried. Some success has been achieved with open
beam paths by having the individual optics protected by ejectors
(Figure 10). This arrangement does not seem to protect
upward-facing optics, and works best if it is always on, even when
the system is shut down.
Pumping
A properly sealed moving axis acts as a pump. When the axis
extends, the volume of the beam delivery system increases by the
product of the distance traveled and the cross-sectional area of the
beam guard. At a sufficiently high travel rate, this can exceed the
purge gas flow and cause a negative pressure inside the beam path.
In most cases, this can be solved by calculating the maximum
volume increase rate with all moving axes extending and supplying
more purge gas than that value. It is helpful for such systems to have