Laser machining apparatus and method

A pulse laser beam of energy distribution having a plurality of peaks is generated (e.g. having a TEM.sub.01 mode or greater) and is directed onto an optical system. The system combines beam-condensing optical elements and multi-beam-splitting prism elements so that the peaks of energy distribution in the laser beam mode fall correspondingly on the cut surfaces of each of the multi-beam-splitting prism elements. Each of the condensing and splitting elements may either be of transmission or reflection type, and further they may be of separate or integral type. Fresnel lenses or meniscus lenses may be used as beam-condensing optical elements. As a result, a plurality of pulse laser beam-condensed spots are produced.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for machining holes in a 
workpiece by using plural condensed laser beams. 
2. Description of the Background 
A conventional apparatus employs condensed pulsed laser beams to make very 
small air circulation holes in filter wrapping paper. Such apparatus is 
disclosed in Japanese Patent Disclosure Publication No. 42200 of 1980, 
Japanese Patent Disclosure Publication No. 9784 of 1983 and Japanese 
Patent Disclosure Publication No. 180290 of 1991. Also well known is an 
apparatus which uses condensed pulsed laser beams to make very small holes 
in order to make the packing materials of fabrics or food, seasonings, 
etc., breathable and ensure ease of unpacking. 
Such apparatuses known in the art generally make a string of holes in a 
band-shaped material by condensing and applying pulsed laser beams to the 
material as the material moves at high speed. 
FIG. 12 is a diagram showing the arrangement of a laser cutting apparatus 
known in the art. FIG. 13 illustrates the changes of pulsed laser beam 
output in relation to time, and FIGS. 14a and 14b comprise a laser beam 
condensing status diagram, showing top and side views. 
Referring to these drawings, the numeral 1 indicates laser oscillator, 2 
denotes a pulsed laser beam, 3 represents a beam splitter, 4 indicates 
bend mirrors, 5 designates condenser lenses, 6 is a band-shaped material, 
7 denotes transfer rollers, 8 represents beam-condensed spots, and 9 shows 
holes made in the material 6. In FIG. 14b, 10 indicates a space energy 
distribution of the pulsed laser beam. FIG. 14b shows energy distribution 
in a single mode, i.e., Gaussian distribution having a single peak. 
The operation of the conventional apparatus will now be described with 
reference to these-drawings. The output of the pulsed laser beam 2 from 
the laser oscillator 1 is an intermittent, approximately rectangular wave 
as shown in FIG. 13. There are a wide variety of pulsing methods that are 
well known in the art, e.g., a method wherein the exciting action itself 
of laser oscillation is pulsed and a method wherein continuous oscillation 
is mechanically chopped to create pulses, and therefore these methods will 
not be detailed here. The pulsed laser beam 2 is divided by the beam 
splitter 3 and directed along separate paths. In a case as shown in FIG. 
12, beam 2 is divided into four quarters and the four pulsed outputs are 
deflected by the corresponding bend mirrors 4 and fall on the 
corresponding lenses 5. The pulsed laser beams impinging on the lenses 5 
are condensed and form beam-condensed spots 8 on the bend-shaped material 
6. Meanwhile, the bend-shaped material 6 or a workpiece is transferred by 
the rollers 7 in the direction of the arrow and the strings of holes are 
made in the band-shaped material 6 at intervals determined by the laser 
beam pulse frequency and the moving speed of the band-shaped material 6. 
Where higher productivity is desired, the moving speed can be increased but 
the pulsed light must operate at a higher frequency if closely spaced 
holes are desired. One suggested approach is to divide the beam after it 
passes through the condenser lens 5, by using a beam-splitting prism lens 
(not shown). FIG. 15 illustrates output contour lines that are generated 
when such a beam-splitting prism lens is employed under the lens 5 in FIG. 
12 to split the pulsed laser beam having single-mode energy distribution 
in the laser cutting apparatus shown in FIG. 12. As illustrated, when the 
single-mode laser beam is split by the prism lens, fringes 13a, 13b are 
produced on the workpiece by diffracted light, in addition to the two 
beam-condensed spots indicated by 8a, 8b. Since the fringes do not 
contribute to hole making, output loss occurs and hole making speed 
reduces substantially. 
The laser cutting apparatus, which is arranged as described above, may 
increase the transfer speed of the band-shaped material to enhance 
production capability. However, such a conventional apparatus may require 
an increase in the output of the pulse laser beam to shorten the rise and 
fall times of the output. Consequently, a large laser oscillator is 
required to increase the laser output, resulting in high price. In 
addition, there are limitations based on the principles of laser 
oscillation and on the mechanical structure of the mechanical chopper 
which produces the pulsed beam that limit the output rise and fall times 
and the pulse frequency, preventing the improvement of productivity. 
SUMMARY OF THE INVENTION 
It is, accordingly, an object of the present invention to overcome the 
disadvantages in the conventional apparatus by providing a laser cutting 
apparatus and method which greatly enhances productivity without 
increasing the transfer speed of a band-shaped material. 
The present invention achieves a laser cutting apparatus for making holes 
in a traveling band-shaped material which allows more strings of holes and 
more holes to be made without increasing the transfer speed of the 
bend-shaped material, thereby improving production capability. 
Specifically, the apparatus and method causes a pulse laser beam of energy 
distribution having a plurality of peaks to impinge on an optical system. 
The system combines beam-condensing optical elements and 
multi-beam-splitting prism elements so that the peaks of energy 
distribution in the laser beam mode fall correspondingly on the cut 
surfaces of each of the multi-beam-splitting prism elements. As a result, 
a plurality of pulse laser beam-condensed spots are produced. 
Also, the present invention employs Fresnel lenses or meniscus lenses as 
beam-condensing optical elements, thereby improving the energy density of 
the beam-condensed spots. 
In addition, the peripheral edge on the prism cut side of each 
multi-beam-splitting prism lens in the apparatus may be designed to be a 
flat shape which is orthogonal to an optical axis. With this structure, 
the optical elements are held stably and reliably and a cooling effect is 
further improved. 
Furthermore, a carbon dioxide gas pulse laser oscillator in a discharge 
excitation method in the apparatus is designed to oscillate a pulse laser 
beam in a system wherein a discharge direction, a laser gas flowing 
direction and a laser oscillating direction are orthogonal to each other. 
As a result, the apparatus allows strings of holes to be made without 
increasing the transfer speed of the band-shaped material, and production 
capability can be enhanced greatly. 
Furthermore, the present invention employs a fast axial flow laser 
oscillator which generates a laser beam whose energy distribution of laser 
mode has a plurality of peaks.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the present invention will now be described with reference 
to FIGS. 1-3 in the appended drawings. 
FIG. 1 is a laser cutting apparatus arrangement according to the first 
embodiment of the present invention, FIG. 2 shows the condensed status of 
a laser beam, and FIG. 3 is a top view of FIG. 2. In these drawings, the 
numeral 2 indicates a laser beam, 3 indicates a beam splitter, 4 denotes 
bend mirrors, and 5 designates lenses which are general flat convex lenses 
in this case. A prism lens 11 is provided adjacent to the bottom of the 
lenses 5 and has a prism configuration wherein a disc-shaped bottom 
surface is cut into two mutually intersecting planes and is operative to 
produce two separate beams. FIG. 2 illustrates a TEM.sub.01 mode beam 
wherein the space energy distribution 12 of the pulsed laser beam 2 has 
two peaks. The generation of a pulsed laser beam with TEM.sub.01 or other 
multiple peak distributions is well known in the art. As in the 
conventional design illustrations, 6 indicates a band-shaped material, 7 
denotes transfer rollers, 8a and 8b represent beam-condensed spots, 9a and 
9b indicate holes made in the material 6. 
The operation of the apparatus according to the present embodiment will now 
be described with reference to the appended drawings. The pulse laser beam 
2 output from the laser oscillator 1 is divided by the beam splitter 3 to 
make plural beams for use in the simultaneous production of strings of 
holes. In a case such as shown in FIG. 1, the laser beam 2 may be split 
into four outputs, each of which is then deflected by a corresponding bend 
mirror 4 and impinges on a corresponding lens 5. Although only two of the 
four outputs and their corresponding structures are shown in FIG. 1, the 
preferred embodiment of the described apparatus uses four outputs and 
makes up to eight strings of holes simultaneously. 
The pulse laser beam 2 provides the TEM.sub.01 -mode energy distribution 12 
so that each of the peaks of the energy distribution impinge 
correspondingly on the two cut surfaces on the bottom of the 
two-beam-splitting prism lens 11 provided under the lens 5 as shown in 
FIG. 2. The laser beam is designed to be split into two beams at right 
angles with the arrow-indicated moving direction of the band-shaped 
material 6. The beam-condensed spots 8a and 8b are separated by a distance 
d and produce strings of spots 9a and 9b when the material is moved past 
the spots as the beams are pulsed. The lens 5 is a beam-condensing optical 
element and, the two-beam-splitting prism lens 11 is a beam-splitting 
optical element. 
String width d between the beam-condensed spots 8a and 8b and between the 
holes 9a and 9b produced by beam-condensing and beam-splitting is 
determined by the expression of d.apprxeq.2(n-1).alpha.f, where n is an 
index of refraction determined by the wavelength of the laser beam and the 
material of the elements, f is the focal length of the lens 5, and .alpha. 
is the cut surface angle of the beam-splitting prism lens 11. Since the 
band-shaped material 6 is transferred by the rollers 7 in the direction of 
the arrow in relation to the beam-condensed spots 8a, 8b, etc., of the 
pulse laser beam, the strings of holes 9a, 9b, etc., are made in the 
band-shaped material 6. 
Table 1 is a hole making performance-specifications comparison table 
between the conventional apparatus and the apparatus according to the 
present invention. As indicated in Table 1, the hole-making speed may be 
reduced by 20% but the number of bands produced may be doubled since the 
number strings of holes may be increased and the overall production 
capability may be improved 1.6 times as compared to that of the 
conventional apparatus. 
TABLE 1 
______________________________________ 
Conventional 
Apparatus of 
Apparatus Present Invention 
______________________________________ 
Hole-making size 
0.1-0.2 .times. 
.rarw. 
(shorter diameter .times. 
0.1-0.2 .times. 
longer diameter .times. 
0.5 
pitch, mm) 
Number of hole 
4 8 
strings 
Number of cutting 
4 4 
heads 
Lens Flat convex lens 
Flat convex lens + 
specifications prism lens 
Laser mode TEM.sub.00 TEM.sub.01 
(single mode) 
Laser output 37.5/head 44/head 
(average W) 
Pulse frequency 
5 4 
(kHz) 
Hole-making speed 
150 120 
(m/min.) 
Number of bands 
1 2 
produced 
Speed in terms of 
150 240 
productivity 
(m/min.) 
______________________________________ 
A further embodiment as illustrated in FIGS. 4a and 4b will now be 
described. The numeral 14 indicates a high-order laser beam mode wherein 
four peaks of energy distribution exist radially. A 
beam-splitting/condensing optical system 15 combines a four-beam-splitting 
prism lens 16 and a flat convex lens 5. The lenses 15 and 16 are shown as 
an integral part in the figure but may be separate. Also, the system's 
beam-splitting/condensing sequence and form may be those of the 
beam-condensing/splitting optical system as shown in FIG. 2. The 
high-order mode pulse beam 14 having four peaks is applied as shown in the 
figure so that the peaks impinge correspondingly on the four cut surfaces 
of the four-beam-splitting prism lens 16, thereby producing four 
beam-condensed spots 8a, 8b, 8c, 8d of the pulse laser beam. This 
hole-making apparatus, which allows more strings of holes to be made as 
compared to the laser beam mode shown in FIGS. 2 and 3, achieves further 
improvement in hole-making capability. 
An arrangement shown in FIG. 5 is similar to that in FIG. 2 but uses a 
Fresnel lens 17 as a beam-condensing optical element, in place of element 
5. The Fresnel lens 17, which has been machined by stages to reduce 
diffraction loss as it condenses a beam on the basis of the wavelength of 
the laser beam, improves the beam-condensing characteristic and further 
enhances the energy density of the beam-condensed spots 8a, 8b. Hence, a 
laser hole-making apparatus that is operative at higher workpiece transfer 
speeds can be obtained. 
It will be appreciated that the TEM.sub.01 -mode 12 and the 
two-beam-splitting prism lenses 11 described above may be replaced by a 
laser beam mode having a plurality of energy distribution peaks and 
multi-beam-splitting lenses to produce an identical effect. 
An arrangement shown in FIG. 6 is similar to that in FIG. 5 but uses a 
meniscus lens 18 as a beam-condensing optical element. The meniscus lens 
18, which has been machined to reduce spherical aberration at the time of 
its beam condensing, produces the same effect as in FIGS. 5 and 6, i.e., a 
Fresnel/meniscus lens, would produce a higher effect. 
FIGS. 7 and 8, which show embodiments different from those of the laser 
hole-making apparatus according to the present invention described above, 
will now be described. In FIG. 7, there is a transmission type 
beam-condensing optical element 19 that receives the TEM.sub.01 -mode beam 
2 with energy distribution 12 in two peaks and provides a condensed beam 
to a reflection type beam-splitting prism element 21. FIG. 8 shows a 
transmission type beam-splitting prism element 20 that receives the beam 
2, divides the beam and transmits each beam to a reflection type 
beam-condensing optical element 22. In each case, the laser pulse of 
TEM.sub.01 -mode energy distribution 12 is applied so that the peaks of 
the energy fall correspondingly on the cut surfaces of the 
multi-beam-splitting prism element, whereby the beam condensing operation 
of the beam-condensing optical element produces beam-condensed spots 8a, 
8b. 
While the TEM.sub.01 mode was employed as the laser beam mode in the 
present embodiment, corresponding multi-beam-splitting prism elements may 
be used in a mode having a plurality of energy distribution peaks. 
FIG. 9, which shows an embodiment of a beam-splitting prism lens used in 
the present invention, will now be described. 23 indicates a 
multi-beam-splitting prism lens of a disc shape which has been machined to 
a flat configuration wherein the prism cut surface-side peripheral edge 
23d on the bottom is made orthogonal to the optical axis of the lens, 
illustrated by the dotted line. The lens mount 24, annular 
heat-dissipating sheets 25 that are fitted to the top and bottom of the 
multi-beam-splitting lens 23, and the hold-down screw 26 form part of the 
assembly. If the cut surface-side peripheral edge is not flat, the 
disc-shaped prism lens may not be held securely or oriented precisely. 
Also, due to a small contact area of the lens and the lens mount 24, a 
lens cooling effect would be low. The present invention overcomes these 
disadvantages by providing an apparatus which holds the beam-splitting 
prism lenses securely and accurately and offers high lens cooling 
efficiency. 
As in the embodiment shown in FIGS. 7 and 8, beam condensing and beam 
splitting may be done in either sequence, each element may either be of 
transmission or reflection type, and further they may be of separate or 
integral type. 
FIG. 10 is an arrangement diagram of a pulsed laser oscillator which is an 
embodiment of the present invention, and FIG. 11 is a sectional view taken 
at a right angle to an optical axis in FIG. 10. Referring to these 
drawings, 27a, 27b, indicate a pair of discharge electrodes provided in a 
vertical direction, and 28 designates arrows which show a direction in 
which a laser gas including a CO.sub.2 gas flows between the pair of 
discharge electrodes 27a and 27b. 29 indicates a partial reflector and 30 
denotes a total reflector, between which the laser beam is excited and 
amplified and the pulse laser beam 2 is then output from the partial 
reflector 29. As is clear from the above description, the three axes in 
the discharge exciting direction (vertical) of pulse oscillation, the 
laser gas flowing direction and the laser oscillating direction are 
designed to be orthogonal to each other. It is known in the art that this 
system is advantageous to the increased output of the oscillator. 
Particular advantages of this system are that laser photons of high 
oscillation density are prone to exist in the vicinity of the discharge 
electrodes 27a, 27b which perform discharge and excitation, that two 
energy distribution peaks are liable to exist nearer to the electrodes 
than to the optical axis as shown in FIG. 11, and that the generation of 
energy distribution 12 in the TEM.sub.01 -mode improves the efficiency of 
oscillatory operation of the pulse laser beam. 
In a fast axial flow laser oscillator (not shown) whose laser gas flowing 
direction and laser oscillating direction are designed to be the same, a 
laser beam whose laser mode is axially symmetrical about an axis (to be 
radial) is easily generated and, it is especially advantageous to employ 
the distribution type as shown in FIG. 4. 
The present invention is not limited to the laser cutting apparatus which 
makes holes in a traveling band-shaped material with a pulse laser beam as 
described in the several embodiments, and may be utilized for welding, 
marking, etc. using a continuous laser beam. 
Also, it will be apparent that a workpiece is not limited to the 
band-shaped material and a stationary workpiece may be cut with 
beam-condensed spots moving or kept stationary. Further, the 
beam-splitting elements are not limited to the optical prisms as employed 
in said embodiments and beam splitting may be done by another method, 
e.g., optical fiber.