Note: Descriptions are shown in the official language in which they were submitted.
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LASER MATE~IAL PROCESSING I'HROUGH A FIBER OPTIC
Background of the Invention
This invention relates to a laser beam delivery
method and system and more particularly to transmission of
laser energy through a fiber optic at power levels high enough
for manufacturing purposes.
Typically, laser beam delivery for material
processing is accomplished through the use of an ensemble
of mirrors and prisms for beam steering. An increase in
beam steering flexibility is possible when a laser beam is
passed through a fiber optic. This flexibility enhances
the access to difficult locations on a workpiece during
manufact~re. ~uch material processing as drilling, cutting,
welding, and selective heat treating and laser surfacing
is possible with the laser remote from the workstation.
Laser energy has been transferred along a fiber
optic for the purpose of laser communications and laser
surgery in the medical field. In both cases, the laser
beam is a continuous wave (CW) and average power levels of
100 watts have not been exceeded. As much as 20 watts
of CW power from a CO2 laser, which has a 10~6 micrometer
wavelength in the far infrared, have been transmitted through
a fiber optic. The 100 watt CW power level was achieved from
a laser that has a 1.06 micrometer wavelength in the near
infrared. Only the CO2 laser has been used with a fiber
optic for material processing with applica-tions such as
engraving and cloth cutting. The average or pea~ powers
are not sufficient for welding, cutting, drilling, and heat
treating metals at cost effective rates. The CO~ laser
fiber optic which is composed of thallous bromide and thallous
iodide is capable of 55 percent transmittance at 10O6
micrometers, and because of this level of transmissivity
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requires cooling. The neodymiumyttrium alumium garnet
laser, a source of 1O06 micrometer wavelength energy, has
provided the 100 wat-t CW average power for surgical applications.
Such power levels are adequate for limited metal processing
but have not been applied. Peak powers in excess of 1000 watts
would be more desirable for metal processing.
Summary of the Invention
Laser energy is coupled into a single fiber optic
which is used as a lightguide to deliver sufficient pulse
energy to a workpiece for material processing. A laser beam
generated by a solid state neodymium-YAG laser or other
laser which is operated in pulsed mode and has a wavelength
in the near infrared and visible spectrum, is focusd onto
one end of the fiber optic core, preferably made of quartz.
Energy with a peak power in the kilowatt range is passed
through the fiber to the output end. The emerging laser
beam is focused onto the workpiece at a power density high
enough for manufacturing processes such as drilling, cutting,
welding, heat treating, and laser surfacing.
The system has a lens that focuses the laser's beam
to a small spot whose diameter is less than the fiber core
diameter; the numerical aperture is such that the included angle
cf the focused beam is less than about 24. In a specific embodi-
ment, coupling takes place through a holding fixture made of
copper or gold which reflects laser energy and prevents stray
energy from entering the fiber optic cladding and melting it.
The cladding at the fiber end is removed and the fiber isreceived
in a hole in the fixture. A second embodiment for average power
levels up to 250 watts has another input coupler. The fiber end
is stripped of cladding and shielding and only the shielding is
removed in the nxt section to allow beam coupling through core-
air and core-cladding zones, and the prepared fiber end is mounted in a
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glass holder. At the output is a lens system -to recollimate
and refocus the laser beam onto the workpiece.
This is a flexible laser beam delivery system with
minimum optical losses, and increases the degrees of freedom
of laser beam manipulation. It is especially attractive Eor
metal processing with robotic control.
Brief Description of the Drawing
Fig. 1 is a diagram of the laser coupled fiber
optic system used to apply laser energy to a metal workpiece.
Fig. 2 is a longitudinal cross section through the
fiber optic and shows passage of the laser beam along the core.
Fig. 3 shows an improved input mechanism to transmi-t
higher amounts of average power into the fiber optic.
Detailed Description of the Invention
Metal processing and the processing of other
materials is accomplished by the laser energy delivery system
in the figures. Average power levels on the order of 250
watts and peak powers of se~eral kilowatts have been transmitted
through an individual fiber optic. A neodymium-yttrium
aluminum garnet laser~ which has a wavelength in the near-
infrared, is operated in pulsed mode. Other suitable solid
state lasers are the ruby laser with a waveleng~h of 680
nanometers, and the alexandrite laser with a wavelength of
~30-730 nanometers, both in the visible spectrum. All the
near infrared and visible wavelengths are transmitted in a
quartz fiber optic without melting the quartz. This kind
of fiber optic is preferred because the fiber is flexible
and quartz can be drawn into long ~ibers and is a pure
material; impurities tend to absorb energy. The system
includes provision for coupling the laser energy in-to the
fiber and focusing th~ beam leaving the fiber to a power density
sufficient for material processing.
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In Fig. 1, a Nd-YAG laser 10 being used in a pulsed
mode is coupled to a 1000 micrometer diameter fused quartz
fiber optic 11 by focusing the laser beam 12 down on the end
of the fiber with a lens 13. In order for the laser energy
to enter the fiber, two conditions are needed. Flrst, the
small spot at the focal plane has a diameter less than the
diameter of quartz core 14. Second, the numerical aperture
of the fiber optic is such that the included angle of the
focused beam (like a cone angle) is smaller than 22-24.
For best results the end of core 14 is ground optically flat
and has an antireflection coating 15. Coupling takes place
through a holding fixture 16 made of copper which has a hole
to receive the fiber 11. About one-quarter inch of the
transparent silicon cladding 17 is removed from the end of
the fiber. The copper fixture 16 helps to protect the fiber
cladding from any stray laser energy that does not enter the
end of the fiber, and prevents melting of the cladding.
Copper tends to reflect 1.06 micrometer laser energy at
moderate power levels. A better material would be gold,
a higher reflective material.
Referring to the cross section of fiber optic 11
in Fig. 2, the laser beam travels along quartz core 1~ in a
zig-~ag path and is reflected at the interface with silicon
cladding 17. The optical fiber has a nylon shielding or
jacket 18. If a fused quartz fiber optic with a glass
cladding is used, the flexibility of the fiber would be
decreased but the power carrying capability may increase,
since the 1.06 micrometer wave length is transparent to
glass, thus leading to lower risk in potential cladding
damage. The fiber has a diameter less than 1 millimeter;
fibers larger than this are less flexible.
Having transmitted laser energy through the fiber
optic 11 a lens assembly 19, 20 is used to collimate and focus
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the laser beam. The emerging beam at the output end of the
~iber optic tends to spread out. The beam is recollimated
by lens 19 and refocused by lens 20 onto the me-tal workpiece
21. The power density of the focused beam at the focal
plane is sufficient for various metal processes. The laser
beam may be passed through a glass plate 22 to protect the
lenses from any metal vapor. An antireflection coating on
the three lens elements increases the transmissivity.
Average power levels up to 155 watts were transmitted
into the fiber. With a pulse width (pulse length) of 0.6
milliseconds and a pulse rate of 30 pulses per second, a peak
power range of 4000-6000 watts was achieved. After focusing
this beam at the output of the fiber optic, power densities
of 106-107 watts/cm2, capable of drilling and cutting, were
achieved. The 155 watts of laser pulse energy were transmitted
through a 1 millimeter fiber optic without any detectable
attenuation with bend radii greater than 8 inches (200
millimeters). With fiber bend radii of 1.5 inches (37.5
millimeters), transmittance at 1.06 micrometers is 87 percent.
The laser beam output from the fiber optic was focused on a
0.30 inch (0.75 millimeters) thick Inconel 718 workpiece
resulting in both drilling and cutting the material.
The diameter o~ output lens 19, 20 may be much
smaller than illustrated, resulting in an output end that is
much easier to move around. The end of the fiber may be ground
to be a lens element or part of a lens, or a separate element
may be attached to the fiber.
The input mechanism in Fig. 1 allows only up to 155
watts of average laser energy, not enough for all processing
tasks; higher powers are prohibited due to thexmal limitatlons
at the input coupling. Up to 250 watts of average power were
transmitted into the fiber optic with the improved coupler
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in Fig. 3. The fiber tip is stripped of silicon cladding
17 and shielding 18 for 0.75 inches. In the next section
just the shielding is removed for an equal distance. This
prepared end is then placed in a Pyrex~ holder 23 and set
at the proper focal plane of the laser. The prepared end
allows beam coupling through two zones, core-air and core-
cladding. The first region permits the highly divergent
incident beam to enter the fiber ll through a greater acceptance
angle provided by the core-air interface. The second zone
will provide additional reflections to insure transmission
of the collected light energy. The third zone of core,
cladding and shielding provide a rugged housing for fiber
handling.
Average power levels up to 250 watts were transmitted
into a fiber of approximately five meters in length. With a
pulse width of 0.2 millisecond and a pulse rate of 200 pulses
per second, a peak power range of 5000-9000 watts was achieved.
After focusing the beam at the output of the fiber optic,
power densities (106-107 watts/cm2) capable of drilling and
cutting were achieved.
Up to 250 watts of Nd-YAG laser pulse energy can be
transmitted through a l millimeter fiber optic without
detectable attenuation with fiber bend radii greater than
4 inches (lO0 millimeters). At the 4 inch radii, transmittance
at 1.06 micrometers is 90 percent. A 0.060 inch (154 milli-
meter) thick titanium 6Al-4V workpiece was both drilled and
cut. With the ability to transmit the higher amounts of average
power the system is much more versatile to the materials
processing industry.
The main advantage of the fiber optic laser delivery
system is an increase in beam steering flexibility. The
degrees of freedom of laser beam manipulation are increased.
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With the fiber optic basically light in weight, the laser
beam is moved in almost any direction at a rapid speed.
Being able to loca-te the laser remotely from the workstation
is an additional advantage of transmit-ting a laser beam
through a lightguide such as a fiber optic. The flexibility
inherent in a fiber optic laser beam delivery system also
makes it very attractive for laser material processing with
robotic control.
While the invention has been particularly shown and
described with reference to preferred embodiments thereof,
it will be understood by those skilled in the art that
changes in form and details may be made therein without
departing from the spirit and scope of the invention.
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