U.S. patent application number 09/900238 was filed with the patent office on 2002-01-10 for laser assembly for material processing.
Invention is credited to Schluter, Holger.
Application Number | 20020003131 09/900238 |
Document ID | / |
Family ID | 7648136 |
Filed Date | 2002-01-10 |
United States Patent
Application |
20020003131 |
Kind Code |
A1 |
Schluter, Holger |
January 10, 2002 |
Laser assembly for material processing
Abstract
A laser beam generating assembly for materials processing
includes a resonator to generate a linearly polarized laser beam,
and a beam forming assembly which includes mirrors to orient the
polarization plane of the laser beam in a prescribed orientation
relative to the vertical and a delay plate which has its reflective
surface oriented so that the polarization plane of the beam
impinges thereon is at an angle of 45.degree. . The delay plate
serves to produce a polarized laser beam with a rotating
polarization vector, and in particular an elliptical or circular
polarization of the laser beam.
Inventors: |
Schluter, Holger; (West
Hartford, CT) |
Correspondence
Address: |
PEPE & HAZARD, LLP
GOODWIN SQUARE
225 ASYLUM ST.
HARTFORD
CT
06103
US
|
Family ID: |
7648136 |
Appl. No.: |
09/900238 |
Filed: |
July 6, 2001 |
Current U.S.
Class: |
219/121.73 ;
219/121.74 |
Current CPC
Class: |
B23K 26/064 20151001;
B23K 26/0643 20130101; B23K 26/0665 20130101 |
Class at
Publication: |
219/121.73 ;
219/121.74 |
International
Class: |
B23K 026/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 7, 2000 |
DE |
100 33 071.1 |
Claims
1. A laser beam generating assembly for producing a laser beam with
a rotating polarization vector comprising: (a) a laser resonator
for generating a linearly polarized laser beam; and (b) a laser
beam forming device including (i) a delay plate having the plane of
its reflective surface oriented at an angle of 45.degree. to the
vertical; and (ii) a plurality of beam-forming optical elements in
the beam path prior to said delay plate and having reflective
surfaces angularly oriented to orient the polarization plane of the
laser beam at an angle of 450 to the plane of incidence defined by
the normal vector to the plane of the reflective surface of the
delay plate and the oriented laser beam incident thereon, said
delay plate redirecting the beam in a vertical direction and
imparting a rotating polarization vector thereto
2. A laser beam generating assembly in accordance with claim 1
wherein said beam-forming optical elements preceding the delay
plate and aligning the polarization plane of the linearly polarized
laser beam relative to the delay plate in defined orientation
comprise two mirrors positioned in proximity to each other in the
path of the laser beam, whereby the linearly polarized laser beam
(13) impinging on the first optical element in the light path is
redirected into a U-shaped path section, said Ushaped section of
the laser beam path extending along a plane which together with the
vertical line perpendicular to the polarization plane of the laser
beam forms a 22.5.degree. angle in front of the first optical
element, and the second optical element the polarization plane of
the laser beam directed onto the delay plate by the second optical
element in the light path is inclined at a 45.degree. angle
relative to the reflective surface of said delay plate.
3. The laser beam generating assembly in accordance with claim 2
wherein said mirrors are spherical.
Description
BACKGROUND OF INVENION
[0001] The present invention relates to a laser system for
materials processing, with a device serving to produce a linearly
polarized laser beam and with at least one delay plate for
producing a polarized laser beam that has a rotating polarization
vector and in particular an elliptically or circularly polarized
laser beam.
[0002] In the processing of materials, for instance in cutting or
welding with a linearly polarized laser beam, the processing result
obtained depends on the beam vector. To minimize or eliminate this
dependency, prior art laser systems are equipped with devices
serving to produce a polarized laser beam with a rotating
polarization vector, and preferably an elliptical or circular
polarization of the laser beam. A laser system of that type,
incorporating a .lambda.4 plate as a delay surface for the circular
polarization of a linearly polarized laser beam has been described
in EP-B-0 591 541. In that design, the polarization plane of the
linearly polarized laser beam impinging on the .lambda.4 delay
plate is inclined at an angle of 45.degree. relative to the
reflection plane of the delay plate. This 45.degree. angle is
obtained by means of a mirror assembly preceding the .lambda.4
plate in the light path of the laser beam inside the prior-art
laser resonator.
[0003] Prior art laser systems for materials processing are also
equipped with devices for beam forming and in particular for
expanding the laser beam produced; these are in the form of
so-called "beam telescopes" . Their purpose is to keep the angle of
divergence of the laser beam reasonably small for a relatively long
laser beam path. In this fashion it is possible, with minimal
losses, to allow the laser beam to impinge on the optical
beam-focussing element typically positioned near the processing
point on the workpiece and to focus it on the latter. A laser
system employing a beam telescope is described for instance in
EP-A-0 428 734.
[0004] With respect to the above-mentioned prior-art designs, it is
the object of this invention to minimize the number of optical
elements needed in laser systems configured for materials
processing and incorporating a laser beam forming device.
[0005] A specific object is to provide a laser resonator in
combination with a novel beam forming device which shapes the laser
beam and imports a rotating polarization vector thereto.
SUMMARY OF TIE INVENTION
[0006] It has now been found that the foregoing and related objects
can be readily attained by a laser beam generating assembly for
producing a linearly polarized laser beam with a rotating
polarization vector comprising a laser resonator for generating a
linearly polarized laser beam, and a novel laser beam forming
device. The beam forming device includes a delay plane having the
plane of its reflective surface oriented at an angle of 45.degree.
to the vertical, and a plurality of beam-forming optical elements
in the laser beam path prior to the delay plate. The optical
elements have reflective surfaces angularly oriented to orient the
polarization plane of the laser beam at an angle of 45.degree. to
the plane of incidence defined by the normal vector to the plane of
the reflective surface of the delay plate and the oriented laser
beam incident thereon, whereby the laser plate redirects the beam
in a vertical direction and imparts a rotating polarization vector
thereto.
[0007] In the present invention, at least one beam-forming optical
element also doubles as a delay plate and at least one beam-forming
optical element is utilized for the defined alignment of the
polarization plane of the linearly polarized laser beam relative to
the delay plate. Thus, only a relatively small number of optical
elements need to be interpositioned in the light path of this type
of laser system. This fact offers a number of advantages. Laser
systems using this invention can be relatively small in design and
their correspondingly simple construction enhances their cost
effectiveness. Given the fact that laser beam forming or polarizing
optical elements inherently cause light-energy losses and that
optical elements of the type in question always constitute
potential sources of error in the proper path alignment of the
laser beam, the reduced number of optical elements in the design of
this invention also entails reduced attendant energy losses and
error sources in the beam alignment. The dual-purpose optical
elements offer the same functional properties as those conventional
optical elements whose functionalities they combine.
[0008] The characteristic features of patent claims 3 and 5 take
into account the fact that, in the case of materials-processing
laser systems according to this invention, it is typically laser
beams with a relatively high output energy that must reach the
workpiece.
BRIEF DESCRIPTION OF THIE ATTACHED DRAWINGS
[0009] The appended drawing explain this invention in more detail
with the aid of schematic illustrations of design examples in
which
[0010] FIG. 1 shows a first embodiment of a laser system embodying
the present invention with a beam telescope and a delay plate;
[0011] FIG. 2 is a schematic illustration of the novel beam forming
device of the laser system FIG. 1 as viewed in the direction of the
arrow II in FIG. 1;
[0012] FIG. 3 is a similar schematic illustration of the laser
system of FIG. 1 viewed in the direction of the arrow III in FIG.
1;
[0013] FIG. 4 is a similar schematic illustration of the laser
system of FIG. 1 viewed in the direction of the arrow IV in FIG.
1;
[0014] FIG. 5 shows a second embodiment of the novel laser system
with a beam telescope and delay plate; and
[0015] FIG. 6 is a schematic illustration of the beam forming
elements of the laser system of FIG. 5 viewed in the direction of
the arrow VI in FIG. 5.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS
[0016] As shown in FIG. 1, a laser system 1 incorporates as its
main components a laser resonator 2 for generating a laser beam 3
and a device for forming the laser beam 3 which is comprised of a
beam telescope 4 and a delay plate 5. The beam telescope 4 is a
collimating telescope with spherical mirrors 6, 7; either a Kepler
or a Galilean telescope is suitable. The spherical mirrors 6, 7
permit proper functional adjustment of both the angle of divergence
and the radius of the laser beam 3.
[0017] The laser resonator 2 is of a conventional design, and the
laser beam emitted by it features a radially symmetrical intensity
distribution and is linearly polarized. As indicated by a double
arrow 8 in FIG. 2, the polarization plane of the laser beam 3
extends in the horizontal direction in the direction of expansion
of the latter as viewed from the front of the spherical mirror
6.
[0018] In the manner depicted in FIG. 1, the laser beam 3 is
redirected twice by the spherical mirrors 6, 7. Along this double
deflection path, the laser beam 3 is oriented in a polarization
plane which is inclined by 22.5.degree. relative to the vertical
line on the polarization plane of the laser beam 3 prior to the
spherical mirror 6 as seen in FIG. 2. As a result of the
illustrated double redirection of the laser beam 3 and the
aforementioned inclination of the plane of the twice redirected
laser beam 3 relative to its polarization plane prior to the
spherical mirror 6, the originally horizontal polarization plane of
the laser beam 3 is rotated by 45.degree. into the position shown
in FIGS. 3 and 4. With its polarization plane oriented as shown in
FIGS. 3 and 4, the laser beam 3 is directed by the spherical mirror
7 onto the delay plate 5. Prior to that point, the laser beam 3 is
expanded by the spherical mirrors 6, 7 in conventional fashion so
as to reduce its angle of divergence. Accordingly, in the design
example illustrated, the spherical mirrors 6, 7 serve as
beam-forining, i.e. beam-expanding, optical elements and at the
same time as optical elements for the defined alignment of the
polarization plane of the linearly polarized laser beam 3 relative
to the surfaces of the delay plate 5.
[0019] The delay plate 5 is a conventional birefringent
quarter-wave plate, a so-called ".lambda.4 plate" . This delay
plate 5 deflects the incident laser beam 3 vertically downward by
90.degree. .Accordingly, the reflection plane of the delay plate 5
extends in the vertical direction.
[0020] As depicted in FIG. 4, the polarization plane of the laser
beam 3 is inclined between the spherical mirror 7 and the delay
plate 5 by 45.degree. relative to the reflection plane of the
latter. As a result, the previously linearly polarized laser beam 3
is circularly polarized by the delay plate 5. FIG. 4 also indicates
the circular polarization of the laser beam 3 reflected by the
delay plate. FIGS. 1 to 4 identify the laser beam 3 only by its
beam axis which is why they do not illustrate the expansion of the
laser beam 3.
[0021] In traditional fashion, the expanded and circularly
polarized laser beam 3 is directed to a processing station
positioned downstream in the light path of the laser system 1 where
it is then focussed by a focussing device onto the processing point
on the object workpiece (not shown).
[0022] A laser system 11 as illustrated in FIGS. 5 and 6
encompasses a laser resonator 12 of a conventional coaxial design
serving to generate a laser beam 13, and a beam telescope 14 as the
laser beam forming device. The individual constituents of the beam
telescope 14, i.e. the cylindrical mirrors 16, 17, 18, 19, serve as
the beam-forming optical elements.
[0023] The structural design of the beam telescope 14 is dictated
by the fact that the divergence, the extension and the intensity
distribution of the laser beam 13 emitted by the laser resonator 12
differ in two mutually perpendicular axial directions to such an
extent that the physical conditions along the two axial directions
mentioned must be accommodated independently from one another. The
conditions in one of the two axial directions are occasioned by an
unstable resonator, those in the other axial direction by a stable
resonator.
[0024] The beam telescope 14 converts the aforementioned intensity
distribution, differing in the two axial directions, into a nearly
rotationally symmetrical intensity distribution. In this process
the cylindrical mirrors 16, 19 handle the beam forming along the
unstable axis. In the case of the example shown, they make up a
Kepler telescope. By contrast, the stable axis is formed by a
Galilean telescope consisting of the cylindrical mirror pair 17,
18. A conventional spatial filter 20 positioned in the intermediate
focus of the Kepler telescope, i.e. the focus of the cylindrical
mirrors 16, 19, serves to remove secondary lobes along the unstable
axis.
[0025] As indicated in FIG. 6, the laser beam 13 exits the laser
resonator 12 in linearly polarized form and with a polarization
plane indicated by a double arrow that extends in the horizontal
direction. Thus polarized, the laser beam 13 impinges on the
cylindrical mirror 16 which reflects it at a 90.degree. angle onto
the cylindrical mirror 17. The cylindrical mirror 17 on its part
then reflects the laser beam 13 at a 90.degree. angle. In this
section of the beam path, the cylindrical mirrors 16, 17 this
provides a U-shaped path for the laser beam 13. This U-shaped
section of the laser beam 13 extends in one plane which forms a
22.5.degree. angle with the vertical line, i.e. with the line
perpendicular to the polarization plane of the laser beam 13 in
front of the cylindrical mirror 16.
[0026] By virtue of the double reflection of the laser beam 13 by
the cylindrical mirrors 16, 17, the polarization plane of the laser
beam, starting at its horizontal initial position in front of the
cylindrical mirror 16, is rotated by a total of 45.degree. . With
its polarization plane oriented in this manner, as shown in FIG. 6,
the laser beam 13 is reflected by the cylindrical mirror 17 onto
the cylindrical mirror 18. The axis of the cylindrical mirror 18
and the aperture of the preceding spatial filter 20 extend in
perpendicular fashion relative to each other and are inclined at a
45.degree. angle relative to the horizontal and, respectively,
vertical plane.
[0027] The laser beam 13 impinging on the cylindrical mirror 18 is
reflected onto the following cylindrical mirror 19. The reflection
plane of the cylindrical mirror 18 is indicated in FIG. 6 by a
dash-dot line and extends in the horizontal direction, so that it
is at a 45.degree. angle relative to the polarization plane of the
laser beam 13 redirected by the cylindrical mirror 17 onto the
cylindrical mirror 18.
[0028] The cylindrical mirror 18 is provided with a dielectric
coating. When the polarization plane of the laser beam 13 impinging
on the cylindrical mirror 18 relative to the reflection plane of
the cylindrical mirror is as shown in FIG. 6, this dielectric
coating enables the cylindrical mirror 18 to shift the phase
position of the s-polarized part of the laser beam 13 relative to
its p-polarized part by one quarter of a wavelength, thus
circularly polarizing the laser beam 13 which, before the
cylindrical mirror 18, was linearly polarized.
[0029] In its circularly polarized form, the laser beam 13 impinges
on the cylindrical mirror 19 whose axis as well is inclined at a
45.degree. angle relative to the horizontal and, respectively,
vertical plane. The cylindrical mirror 19 then directs the laser
beam 13 to a focussing device in a processing station associated
with the laser system 1. The laser beam 13 is ultimately focussed
by the focussing device, in conventional fashion, onto the
workpiece to be processed.
[0030] FIG. 5 clearly illustrates the expansion to which the laser
beam 13 is subjected on its way along the path defined by the
cylindrical mirrors 16, 17, 18, 19. In addition to serving as
telescope-type beam expanders, the cylindrical mirrors 16, 17 also
perform the defined alignment of the polarization plane of the
linearly polarized laser beam 13 relative to the cylindrical mirror
18, and the cylindrical mirror 18 performs the function of a delay
plate or phase shifter. The cylindrical mirror 19 of the beam
telescope 14 serves only to redirect and expand the beam.
Alternatively, the latter functions alone could be handled by the
cylindrical mirror 18 in which case the cylindrical mirror 19 would
have to be designed as the delay (or .lambda.4) plate.
* * * * *