CO2
LENSES
Lenses are
used in most CO2 laser systems to focus all the
beam’s power into a very small spot. Small,
focused spot size is important for a couple of
reasons:
cutting, welding and heat treating generally call
for high-power density, and power density is a
strong
function of spot size.(Reduce the spot diameter
by 50% and the power density increases by 200%.)
Also, many jobs demand narrow kerf widths and heat
affected zones, which can only be achieved with
a tightly focused beam. The design variables that
will affect the performance of a CO2 laser lens
are: focal length, diameter, shape, material and
coating.
Focal Length
Focal length affects both spot size and depth
of focus. In general, a shorter focal length will
produce a smaller focused spot and a shorter depth
of focus. Usually, the specified focal length
is a compromise between desired spot size, penetration
depth and workpiece clearance.
Lens Diameter
Higher power lasers require larger diameter lenses
to prevent thermal overload. At any given focal
length, a larger diameter lens will yield a smaller
focused spot if the incoming beam is expanded
to fill the larger lens.
Lens Shapes
Plano-convex is the simplest and least expensive
lens shape. It is used in such applications as
welding where achieving the smallest spot size
is not critical, or at relatively long focal lengths
when more complex shapes would not be beneficial.
A plano-convex lens should be oriented with the
flat side toward the workpiece and the convex
side toward the laser.
Meniscus lenses have a concave curve on one side
and a convex curve on the other. At relatively
short focal lengths, a meniscus lens will yield
a smaller focused spot than a plano-convex lens.
However, meniscus lenses are more expensive to
manufacture than plano-convex lenses because both
sides are curved. A meniscus lens should be oriented
with the concave curve toward the workpiece.
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CO2
MIRRORS
In any CO2 system, mirrors are among the most
critical components. The design and manufacturing
quality of the laser’s mirrors are crucial
to generating, maintaining and delivering a high-quality
laser beam. There are two main categories of laser
mirrors: internal and external.
Internal mirrors are used to generate, maintain
and amplify the laser beam by forming a reflective
“resonator” around the excited CO2
gas mixture. Internal mirrors are sometimes called
resonator or cavity mirrors.
External mirrors are used to deliver, manipulate,
split and focus the laser beam. Most mirrors have
flat reflective surfaces, but some have curved
surfaces designed to reduce beam divergence. The
design of the substrate material and coating of
a CO2 laser mirror is primarily determined by
its intended function.
Windows are used in some systems as the primary
vacuum seal after the front mirror or to separate
an oscillator and amplifier. They are typically
simple, flat-parallel or flat-wedged optics made
from transmissive substrates with antireflective
coatings on both the inside and the outside surfaces.
Internal Mirrors
Output couplers, also known as front mirrors,
are designed to reflect a portion of the
beam
back into the laser resonator for continuous
amplification while transmitting a portion
of the beam to the
outside for use. Therefore, the substrate
material must be transmissive at the required
wavelength
of 10.6µ. Germanium and Gallium Arsenide
substrates are commonly used for low to medium
powered systems. The more expensive Zinc
Selenide
material is required for higher powered lasers
because of its lower absorption at 10.6 microns.
Rear mirrors are
designed to reflect all or nearly all of a laser
beam back through the laser gas mixture for amplification.
The inside surface is given a highly reflective
(99-100%) coating. In the 100% reflective case,
inexpensive silicon can be used as the substrate
material and the outside surface does not need
polishing or coating. Some rear mirrors, however,
are designed to transmit a small (0.5 - 1.0%)
percentage of the beam to a power detector for
real-time beam monitoring. These mirrors must
have a transmissive substrate (Ge is the most
common) and the outside surface usually has an
antireflective coating.
Substrates
For CO2 lasers, several substrate materials are
available. Silicon is used on low to medium power
systems when cost is a big consideration. Copper,
with its superior thermal conductivity is used
for very high-powered systems. Molybdenum is used
in dirty environments where durability and
repeated cleanings are required.
Coatings
Historically, coatings of enhanced and protected
gold or silver were the industry standards. In
recent years, however, enhanced multilayer dielectric
coatings have been developed offering higher reflectivities
and greater durability. These coatings are becoming
increasingly popular.
**For complete descriptions of this product,
please see our catalog pages 140-146. (click
here to view the catalog , *.pdf) |
