U.S. patent application number 16/857596 was filed with the patent office on 2020-10-08 for optical system for beam shaping.
The applicant listed for this patent is TRUMPF Laser- und Systemtechnik GmbH. Invention is credited to Daniel Flamm, Daniel Grossmann, Myriam Kaiser, Jonas Kleiner, Malte Kumkar.
Application Number | 20200316711 16/857596 |
Document ID | / |
Family ID | 1000004914940 |
Filed Date | 2020-10-08 |
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United States Patent
Application |
20200316711 |
Kind Code |
A1 |
Kumkar; Malte ; et
al. |
October 8, 2020 |
OPTICAL SYSTEM FOR BEAM SHAPING
Abstract
An optical system for shaping a laser beam includes a beam
shaping element configured to receive the laser beam having a
transverse input intensity profile and to impose a beam shaping
phase distribution onto the laser beam. The optical system further
includes a near field optical element, arranged downstream of the
beam shaping element at a beam shaping distance and is configured
to focus the laser beam into the focus zone. The imposed phase
distribution results in a virtual optical image of the elongated
focus zone located before the beam shaping element. The beam
shaping distance corresponds to a propagation length of the laser
beam within which the imposed phase distribution transforms the
transverse input intensity profile into a transverse output
intensity profile at the near field optical element.
Inventors: |
Kumkar; Malte; (Weimar,
DE) ; Kleiner; Jonas; (Stuttgart, DE) ;
Grossmann; Daniel; (Suessen, DE) ; Flamm; Daniel;
(Stuttgart, DE) ; Kaiser; Myriam; (Ditzingen,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TRUMPF Laser- und Systemtechnik GmbH |
Ditzingen |
|
DE |
|
|
Family ID: |
1000004914940 |
Appl. No.: |
16/857596 |
Filed: |
April 24, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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15598816 |
May 18, 2017 |
10661384 |
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16857596 |
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PCT/EP2015/076707 |
Nov 16, 2015 |
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15598816 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B 27/0944 20130101;
G02B 27/0927 20130101; G02B 5/1871 20130101; B23K 26/064 20151001;
B23K 26/53 20151001; B23K 2103/54 20180801 |
International
Class: |
B23K 26/064 20060101
B23K026/064; G02B 5/18 20060101 G02B005/18; G02B 27/09 20060101
G02B027/09; B23K 26/53 20060101 B23K026/53 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 19, 2014 |
DE |
102014116957.3 |
Claims
1.-19. (canceled)
20. An optical system for modifying a shape of a laser beam that
processes a material in a focus zone elongated in a propagation
direction, the optical system comprising: a beam shaping element
arranged to receive the laser beam having a transverse input
intensity profile, wherein the beam shaping element is configured
to impose a beam shaping phase distribution over the transverse
input intensity profile of the laser beam; and a near field optic
arranged downstream of the beam shaping element at a beam shaping
distance, wherein the near field optic is configured to focus the
laser beam into the focus zone so that a virtual optical image of
the elongated focus zone appears at a position before the beam
shaping element, and wherein the beam shaping distance corresponds
to a propagation length of the laser beam within which the imposed
phase distribution transforms the transverse input intensity
profile into a transverse output intensity profile in the region of
the near field optic, and the transverse output intensity profile
has, in comparison with the input intensity profile, a local
maximum lying outside of a beam axis.
21. The optical system of claim 20, wherein the optical system is
an imaging system comprising a demagnifying imaging ratio for
imaging the virtual optical image and for generating the elongated
focus zone.
22. The optical system of claim 21, wherein the beam shaping
element is configured to impose onto the laser beam a spherical
phase distribution with focusing action such that imaging of the
virtual optical image onto the elongated focus zone is performed by
both the imposed spherical phase distribution of the beam shaping
element and the focusing of the near field optic.
23. The optical system of claim 21, wherein the optical system
comprises a far field optic arranged adjacent to the beam shaping
element such that imaging of the virtual optical image onto the
elongated focus zone is performed by both focusing of the far field
optic and focusing of the near field optic.
24. The optical system of claim 20, wherein a transverse beam
profile of the laser beam is present at the beam shaping element in
an image plane downstream of a longitudinal center of the image of
the virtual optical image.
25. The optical system of claim 24, wherein there is in the region
of the image plane a change, which changes fast in longitudinal
direction, from a lateral beam profile, which is given in the focus
zone, to a lateral beam profile having a dark center, the latter
for an lateral Gaussian beam profile of the laser beam and with
respect to beam portions of the incident laser beam, which generate
a divergent beam area that is attributed to the virtual optical
image.
26. The optical system of claim 20, wherein the optical system is
configured such that only a central area of the laser beam
contributes to a downstream end of the virtual optical image of the
focus zone, so that a change of the beam diameter of the laser beam
does not result in an longitudinal displacement of the downstream
end of the focus zone.
27. The optical system of claim 20, wherein the laser beam
comprises a Gaussian intensity distribution, and wherein the beam
shaping phase distribution is configured to generate, for a portion
of the laser beam, a divergent beam area comprising, downstream of
the optical beam shaping element, a transverse intensity
distribution decreasing from an inner region to outer region.
28. The optical system of claim 28, comprising a far field optic,
wherein the transverse intensity distribution is present upstream
of a downstream focal plane of the far field optic, and wherein a
phase imposed beam area comprises a lateral intensity distribution
having a section of a step-shaped increase of intensity, which
comprises a radially inward facing flank in the region between the
beam shaping element and a focal plane, which is attributed to at
least one of the near field optic, the far field optic and the beam
shaping element.
29. The optical system of claim 28, wherein the lateral intensity
distribution comprises a radially outward facing flank in the
region between the focal plane and the focus zone.
30. The optical system of claim 28, wherein the phase distribution
is such that at least one of an inverse quasi-Bessel beam-like beam
profile or an inverse quasi-Airy beam-like beam profile is formed,
for which only a central area of the laser beam provides
contributions to a downstream end of the elongated focus zone.
31. The optical system of claim 20, wherein the beam shaping
element is configured to induce phase-modulation without amplitude
modulation.
32. The optical system of claim 20, wherein the beam shaping
element comprises a hollow cone axicon-lens or mirror system, a
reflective axicon-lens or mirror system, or a programmable
diffractive optical element.
33. The optical system of claim 20, wherein the beam shaping
element is configured to impose a linear phase distribution, so
that a spatial separation of a usable beam portion from a
disturbing beam portion is achieved by a lateral beam deflection of
the usable beam portion.
34. The optical system of claim 20, wherein the elongated focus
zone comprises an aspect ratio of at least 10:1.
35. The optical system of claim 20, wherein the elongated focus
zone is an inverse Bessel beam-like beam focus zone, an inverse
Airy beam-like beam focus zone, or a combination therefore.
36. The optical system of claim 20, comprising a scan unit
configured to scan the elongated focus zone with respect to the
material.
37. The optical system of claim 20, comprising a beam preparation
unit configured to adapt at least one of the transverse input
intensity profile of the laser beam, an input divergence of the
laser beam, and a polarization of the laser beam.
38. An optical system for modifying a shape of a laser beam that
processes a material, the optical system comprising: a beam shaping
element arranged to impose a beam shaping profile onto the laser
beam, wherein the beam shaping profile comprises an inverse
quasi-Bessel beam-like beam profile, an inverse quasi-Airy
beam-like beam profile, or a combination thereof; and a near field
optic arranged downstream of the beam shaping element, wherein the
near field optic focuses the laser beam onto which the beam shaping
profile is imposed into a focus zone elongated in propagation
direction of the laser beam, wherein only a central region of the
laser beam makes contributions to a downstream end of the elongated
focus zone.
39. A laser processing machine comprising: a laser beam source; an
optical system according to claim 20; and a workpiece positioning
unit for positioning the material as a workpiece.
40. The laser processing machine of claim 39, comprising: a control
unit configured perform a control operation, wherein the control
operation comprises: i) setting a downstream end of the elongated
focus zone with respect to a workpiece positioning unit; ii)
setting an elongation parameter of the laser beam and the optical
system, wherein the elongation parameter specifies an elongation of
the elongated focus zone in an upstream direction, wherein at the
same time the position of the downstream end of the elongated focus
zone is maintained with respect to the workpiece positioning unit
without a follow-up correction of a distance of a near field optics
to the workpiece positioning unit; or iii) a combination of i) and
ii).
41. The laser processing machine of claim 39, wherein the laser
beam source is configured to generate a laser beam that modifies
the material by nonlinear absorption.
42. The laser processing machine of claim 41, wherein the laser
beam source is further configured to focus laser pulses to a
fluence of 2 J/cm.sup.2 within the elongated focus zone.
43. A method for beam shaping a laser beam, the method comprising:
imposing a beam shaping phase distribution onto a transverse input
intensity profile of a laser beam, wherein the imposed phase
distribution is such that a virtual optical image of the elongated
focus zone is produced; propagating the laser beam over a beam
shaping distance after which the imposed phase distribution has
transferred the transverse input intensity profile into a
transverse output intensity profile, so that the transverse output
intensity profile, in comparison to the input intensity profile,
comprises a local maximum located outside of a beam axis of the
laser beam; and focusing the laser beam into the focus zone to form
a near field based on the transverse output intensity profile.
44. The method of claim 43, wherein imposing the beam shaping phase
distribution onto the transverse input intensity profile is
performed together with at least one other step, wherein the at
least one other step comprises: imposing a spherical phase
distribution onto the laser beam; imposing a linear phase
distribution onto the laser beam; filtering out a beam portion of
the laser beam; filtering out a central non-modulated beam portion
of the laser beam; or filtering out beam portions of higher
diffraction order from the laser beam.
45. A method for processing a material with a laser beam, the
method comprising: applying a phase modulation to the laser beam to
generate at least one of an inverse quasi-Bessel beam-like laser
beam profile or an inverse quasi-Airy beam-like laser beam profile
with a focus zone, wherein the focus zone is elongated in a
propagation direction; and positioning the focus zone at least
partly in the material to be processed.
46. A method for laser material processing of a material with a
laser beam, the method comprising: applying a phase modulation to
the laser beam to generate at least one of an inverse quasi-Bessel
beam-like laser beam profile or an inverse quasi-Airy beam-like
laser beam profile with a focus zone, wherein the focus zone is
elongated in a propagation direction, wherein applying the phase
modulation comprises: passing the laser beam through a beam shaping
element configured to impose a beam shaping phase distribution over
a transverse input intensity profile of the laser beam;
subsequently focusing the laser beam into a focus zone with a near
field optic arranged downstream of the beam shaping element at a
beam shaping distance, wherein the imposed beam shaping phase
distribution is such that a virtual optical image of the focus zone
is located before the beam shaping element, and wherein the beam
shaping distance corresponds to a propagation length of the laser
beam within which the beam shaping phase distribution transforms
the transverse input intensity profile into a transverse output
intensity profile in a region adjacent to the near field optic, and
the transverse output intensity profile has, in comparison with the
input intensity profile, a local maximum lying outside of a beam
axis; positioning the focus zone at least partly in the material to
be processed.
47. The method of claim 46, comprising: setting a position of a
downstream end of the focus zone with respect to a workpiece
positioning unit.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of and claims priority
under 35 U.S.C. .sctn. 120 to U.S. patent application Ser. No.
15/598,816, filed on May 18, 2017, which is a continuation of PCT
Application No. PCT/EP2015/076707, filed on Nov. 16, 2015, which
claims priority to German Application No. 10 2014 116 957.3, filed
on Nov. 19, 2014. The entire contents of these priority
applications are incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to an optical system for beam
shaping a laser beam and in particular for beam shaping a laser
beam for processing materials that are essentially transparent for
the laser beam. Moreover, the invention relates to a method for
beam shaping.
BACKGROUND
[0003] There are many possibilities for using absorption of light
for processing a workpiece, in particular by introducing localized
modifications into the workpiece. The so-called volume absorption,
i.e., an absorption that is not limited to the surface, opens the
possibility to process brittle-hard materials that are essentially
transparent for the laser beam. Generally, volume absorption
benefits from a kind of nonlinear absorption, at which an
interaction with the material takes place only at a material
dependent (threshold) intensity.
SUMMARY
[0004] Herein, a nonlinear absorption is understood as an intensity
dependent absorption of light, that is not primarily based on the
direct absorption of the light. Instead, it is based on an increase
of the absorption during interaction with the incident light, often
a temporally limited laser pulse. Thereby, electrons can absorb
that much energy by inverse bremsstrahlung that further electrons
are set free by impacts, so that the rate of generating electrons
overcomes that rate of recombination. Under specific conditions,
those initial electrons, which are required for the avalanche-like
absorption, may already be present from the start or may be
generated by an existing rest-absorption by linear absorption. For
example, for ns-laser pulses, an initial ionization may result in
an increase in temperature that causes an increase of the number of
free electrons and therefore of the following absorption. Under
other conditions, such initial electrons may be generated by
multi-photon ionization or tunnel ionization as examples of
well-known nonlinear absorption mechanisms. For ultrashort laser
pulses with, for example, sub-ns-pulse durations such an
avalanche-like generation of electrons can be utilized.
[0005] A volume absorption may be used for materials, which are
essentially transparent for the laser beam (herein in short
referred to as transparent materials), for forming a modification
of the material in an elongated focus zone. Such modifications may
allow separating, drilling, or structuring of the material. For
separating, for example, rows of modifications may be generated
that cause a breaking within or along the modifications. Moreover,
it is known to generate modifications for separating, drilling, and
structuring that allow a selective etching of the modified areas
(SLE: selective laser etching).
[0006] The generation of an elongated focus zone can be affected
with the help of apodized Bessel beams (herein also referred to as
quasi-Bessel beam). Such beam profiles may be formed, for example,
with an axicon or a spatial light modulator (SLM: spatial light
modulator) and an incident light beam having a Gaussian beam
profile. A subsequent imaging into a transparent workpiece results
in the intensities required for volume absorption. Quasi-Bessel
beams--like Bessel beams--usually have a ring-shaped intensity
distribution in the far field of the beam profile existing within
the workpiece. Calculating phase distributions for beam shaping
quasi-Bessel beams, e.g., with an SLM is disclosed in Leach et al.,
"Generation of achromatic Bessel beams using a compensated spatial
light modulator," Opt. Express 14, 5581-5587 (2006), incorporated
herein by reference in its entirety.
[0007] Moreover, systems are known for forming a line of intensity
enhancements, e.g., with the help of multifocal lenses. Thereby, a
phase modification of the laser beam to be focused is per-formed in
the far field, i.e. during focusing, whereby the phase modification
results in the formation of longitudinally displaced focus
zones.
[0008] An aspect of the present disclosure has the objective to
provide an optical system that enables beam shaping for a tailored
volume absorption. In particular, the objective is, for laser
processing applications, to provide in beam propagation direction
elongated, slender beam profiles with a high aspect ratio for
processing transparent materials.
[0009] At least one of the objectives is solved by an optical
system of claim 1, a laser processing ma-chine of claim 12, a
method for beam shaping a laser beam of claim 15, and a method for
laser material processing of claim 17. Further developments are
given in the dependent claims.
[0010] In an aspect, there is disclosed an optical system for beam
shaping of a laser beam for processing an in particular transparent
material by modifying the material in a focus zone being elongated
in propagation direction. The optical system includes a beam
shaping element that is configured to receive the laser beam having
a transverse input intensity profile and to impose a beam shaping
phase distribution over the transverse input intensity profile onto
the laser beam. In addition, the optical system includes a near
field optics located downstream of the beam shaping element at a
beam shaping distance and configured to focus the laser beam into
the focus zone. Thereby, that imposed phase distribution is such
that a virtual optical image of the elongated focus zone is
attributed to the laser beam, the optical image being before the
beam shaping element, and the beam shaping distance corresponds to
a propagation length of the laser beam within which the imposed
phase distribution transforms the transverse input intensity
profile into a transverse output intensity profile in the region of
the near field optics, wherein the output intensity profile has, in
comparison with the input intensity profile, a local maximum
positioned outside of the beam axis.
[0011] In a further aspect, an optical system is disclosed for beam
shaping a laser beam for processing an in particular transparent
material by modifying the material. The optical system includes a
beam shaping element for imposing a phase distribution of an
inverse quasi-Bessel beam (e.g., inverse quasi-Bessel like beam)
profile and/or of an inverse quasi-Airy beam (e.g., inverse
quasi-Airy like beam) profile onto the laser beam, and a near field
optics for focusing the phase imposed beam. The phase distribution
is selected such that the focusing of the phase imposed beam forms
an inverse quasi-Bessel beam profile and/or an inverse quasi-Airy
beam profile having an, in propagation direction of the laser beam
elongated, focus zone, at which only a central region of the
incident laser beam makes contributions to a downstream end of the
elongated focus zone.
[0012] In a further aspect, a laser processing machine for
processing a transparent material with a laser beam by modifying
the material within a focus zone, which is elongated in the
propagation direction of the laser beam, includes a laser beam
source, such an optical system, and a workpiece positioning unit
for positioning the material as the workpiece to be processed.
[0013] In a further aspect, a method is disclosed for beam shaping
of a laser beam with a transverse input intensity profile for
processing of an in particular transparent material by modifying
the material in an, in propagation direction elongated, focus zone.
The method includes the step of imposing a beam shaping phase
distribution onto the transverse input intensity profile, wherein
the imposed phase distribution is such that a virtual optical image
of the elongated focus zone is attributed to the laser beam.
Moreover, the method includes the step of propagating the laser
beam over a beam shaping distance, after which the imposed phase
distribution has transferred the transverse input intensity profile
into a transverse output intensity profile, so that the transverse
output intensity profile in comparison to the input intensity
profile includes a local maximum positioned outside of the beam
axis. Moreover, the method includes the step of focusing the laser
beam into the focus zone for forming a near field based on the
output intensity profile.
[0014] In a further aspect, a method is disclosed for laser
material processing of an in particular trans-parent material by
modifying the material with a laser beam, wherein the method
includes the following steps: generating an inverse quasi-Bessel
laser beam profile and/or a laser beam profile of an inverse
accelerated beam, herein also referred to as a quasi-Airy beam-like
laser beam profile, with an in propagation direction elongated
focus zone by phase-modulation of the laser beam, and positioning
the elongated focus zone at least partly in the material to be
processed.
[0015] In a further aspect, the use of an inverse quasi-Bessel beam
profile and/or of an inverse quasi-Airy beam profile for laser
material processing of an in particular trans-parent material by
modifying the material within an elongated focus zone of the
inverse quasi-Bessel beam profile and/or of the inverse quasi-Airy
beam profile is dis-closed. Thereby, an inverse quasi-Bessel beam
profile and/or an inverse quasi-Airy beam profile can be
characterized, for example, by one or more of those features, which
are disclosed herein as characterizing, in particular by the
attribution of a virtual image before the beam shaping element, by
the, in comparison with respective conventional beams inverted,
radial distributions of amplitude/intensity, and by the in general
fixed position of the end of the focus zone.
[0016] Herein, concepts are disclosed that allow to at least partly
improve aspects of the prior art. In particular additional features
and their functionalisms result from the following description of
embodiments on the basis of the drawings. The drawings show:
DESCRIPTION OF DRAWINGS
[0017] FIG. 1 is a schematic illustration of an optical system for
beam shaping of a laser beam;
[0018] FIG. 2 is a schematic illustration of a laser processing
device with an optical system ac-cording to FIG. 1 for material
processing;
[0019] FIG. 3 is a schematic illustration of an optical system for
explaining the optical functioning;
[0020] FIG. 4 is an example of a longitudinal intensity
distribution in an elongated focus zone after imaging a virtual
optical image;
[0021] FIG. 5 is a ZR-section of the longitudinal intensity
distribution shown in FIG. 4;
[0022] FIG. 6 is an exemplary experimental study on the
modification of a transparent material in an elongated focus zone
according to FIGS. 4 and 5;
[0023] FIG. 7 is a schematic illustration for explaining the
generation and imaging of a real intensity enhancement,
[0024] FIG. 8 is an example of a longitudinal intensity
distribution in an elongated focus zone after imaging a real
intensity enhancement according to FIG. 7;
[0025] FIG. 9 is a schematic illustration of a first example of an
optical system based on a hollow cone axicon;
[0026] FIG. 10 is a schematic illustration of a second example of
an optical system based on a hollow cone axicon;
[0027] FIG. 11A and FIG. 11B are schematic illustrations of
examples for optical systems based on a reflective axicon;
[0028] FIG. 12 is a schematic illustration of an example of an
optical system based on a spatial light modulator;
[0029] FIG. 13 is a schematic illustration of an example of an
optical system based on a transmit-ting diffractive optical
element;
[0030] FIG. 14 is a schematic illustration of an example of a phase
distribution in a diffractive optical element in an optical system
according to FIG. 13;
[0031] FIG. 15 is an exemplary intensity cross-section of an output
intensity profile in an optical system according to FIG. 13;
[0032] FIG. 16 is an XY-view of the output intensity profile of the
intensity cross-section shown in FIG. 15;
[0033] FIG. 17 is a schematic illustration of an example of an
optical system with filtering non-phase-modulated beam
portions;
[0034] FIG. 18 is a schematic illustration of an example of an
optical system based on a diffractive optical element with a linear
phase contribution for separating a phase-modulated beam
portion;
[0035] FIG. 19 is a schematic illustration of an example of an
optical system with a scan device;
[0036] FIG. 20 is a schematic illustration for explaining the
imaging system of an optical system;
[0037] FIG. 21 is a schematic illustration for explaining an
optical system for the incidence of a converging laser beam;
[0038] FIG. 22 is a schematic illustration for explaining an
optical system with adaptation of the divergence;
[0039] FIG. 23 is an exemplary cross-section of the intensity of an
output intensity profile in an optical system for generation of a
flat-top intensity profile;
[0040] FIG. 24 is an XY-view of the output intensity profile of the
intensity cross-section shown in FIG. 23;
[0041] FIG. 25 is an example of a longitudinal intensity
distribution that results from the output intensity profile of
FIGS. 23 and 24;
[0042] FIG. 26 is an exemplary experimental study on the
modification of a transparent material in an elongated focus zone
according to FIG. 25;
[0043] FIG. 27 is an example of a longitudinal intensity
distribution when using a multifocal near field optics;
[0044] FIG. 28 is a schematic illustration of an example of a phase
distribution for generating an inverse Airy beam shape with a
diffractive optical element for use in an optical system according
to FIG. 13;
[0045] FIG. 29 is an exemplary intensity cross-section of an output
intensity profile for generating the inverse Airy beam shape
according to FIG. 28;
[0046] FIG. 30 is an XY-view of the output intensity profile of the
intensity cross-section shown in FIG. 29;
[0047] FIG. 31 is an example of a longitudinal intensity
distribution in an elongated focus zone for the inverse Airy beam
shape generated with the phase distribution according to FIG.
28;
[0048] FIG. 32 is a schematic illustration for explaining the
imaging of a virtual image in combination with the imaging of a
real intensity enhancement;
[0049] FIG. 33A to FIG. 33D are beam profiles for an inverse
quasi-Bessel beam at the propagation from the beam shaping element
to the near field optics; and
[0050] FIG. 34 is an amplitude distribution for a section along the
beam axis Z for illustration of the positions of the beam profiles
of FIGS. 33A through 33D.
DETAILED DESCRIPTION
[0051] Herein described aspects are based partly on the realization
that, due to the high intensities needed for laser processing,
intensities may be present already during the preparation of the
laser beam that result in damage of optical elements. In view
thereof, it was further realized that the generation of an
elongated focus zone within the workpiece may be based on the
imaging of a virtual beam profile. By this concept of imaging a
virtual beam profile, regions with intensity peaks can be reduced
or even avoided in the optical system. It was further realized that
a phase distribution attributed to the virtual beam profile may be
imposed onto the laser beam that causes the desired change of the
intensity distribution in the far field. In particular, it was
realized that by a far field distribution, which originates from
such a virtual beam profile, for example, inverse-Bessel beam-like
or inverse quasi-Airy beam-like intensity distributions,
specifically designed intensity distributions, and in particular
superpositions of the same in the focus zone can be created. For
such intensity distributions, a lateral energy entry into the focus
zone can take place, which in particular enables the processing of
transparent materials. It was further realized that, in comparison
to systems for imaging a real intensity enhancement, the concept of
the imaging of a virtual beam profile may lead to shorter
configurations of such optical systems.
[0052] An elongated focus zone relates herein to a
three-dimensional intensity distribution defined by the optical
system that determines the spatial extent of the interaction and
thereby the modification within the material to be processed. The
elongated focus zone determines thereby an elongated region in
which a fluence (energy per area)/intensity is present within the
material to be processed, which is beyond the threshold
fluence/intensity being relevant for the processing/modification.
Usually, one refers to elongated focus zones if the
three-dimensional intensity distribution with respect to a target
threshold intensity is characterized by an aspect ratio (extent in
propagation direction in relation to the lateral extent) of at
least 10:1, for example 20:1 and more, or 30:1 and more. Such an
elongated focus zone can result in a modification of the material
with a similar aspect ratio. In some embodiments, focus zones can
be formed that are, for example, also in propagation direction
parallel with respect to each other, wherein each of the focus
zones has a respective aspect ratio. In general, for such aspect
ratios, a maximal change of the lateral extent of the (effective)
intensity distribution over the focus zone can be in the range of
50% and less, for example 20% and less, for example in the range of
10% and less.
[0053] Thereby, the energy within an elongated focus zone can be
laterally supplied essentially over the complete length of the
created modification. As a consequence, a modification of the
material in the initial region of the modification does not have or
hardly has any shielding effects on the part of the laser beam that
causes a modification of the material downstream of the beam, for
example, in the end region of the modification zone. In that sense,
a Gaussian beam cannot generate a comparable elongated focus,
because the energy supply is performed essentially longitudinally
and not laterally.
[0054] The transparency of a material, which is essentially
transparent for a laser beam, relates herein to the linear
absorption. For light below the threshold fluence/intensity,
material, which is essentially transparent for a laser beam, may
absorb, for example, along a length up to the back end of the
modification, e.g., less than 20% or even less than 10% of the
incident light.
[0055] Herein described aspects further are partly based on the
realization that by a desired beam shaping, for example, with a
diffractive optical element (DOE), the density of free electrons,
which is created in the material by nonlinear absorption, may be
tailored. Along the thereby created modifications, a crack
formation may be specifically guided, which then results in the
separation of the workpiece.
[0056] Herein described aspects further are based partly on the
realization that, for a DOE, multiple phase distributions can be
provided in the phase distribution of a phase mask, for example, in
respective segments. Thereby, in particular the advantages of the
concept of a virtual optical image, for example, an inverse
quasi-Bessel beam shape, can be used at the superposition of the
imaging of multiple such virtual images (in longitudinal or lateral
direction), wherein also the interaction (e.g. interference) and
spatial constellation of multiple imaging may have effects onto the
formation of the common focus zone. In addition, it was recognized
that in this manner asymmetric `common` focus zones can be created.
For example, for material processing, asymmetric `common` focus
zones create a preference for a specific movement direction or a
specific separation direction. Moreover, it was recognized that,
during the laser processing, such preferred directions may be
adopted to desired processing trajectories by orienting/turning the
DOE within an optical system. For digital phase masks (SLMs etc.),
a direct controlling of the phase distribution may further be
performed to adapt the preferred direction.
[0057] Herein described aspects further are based in part on the
realization that, by the use of a DOE, additional phase
distributions may be imposed onto the beam, which, for example, may
simplify the setup of the underlying optical systems and/or the
isolation of a usable beam portion.
[0058] In other words, disadvantages of the prior art may in some
embodiments at least partly be overcome by an optic concept, in
which the beam profile, which is positioned in the region of the
workpiece and which is elongated in propagation direction, is
affected by an imaging of a created virtual beam profile. In some
embodiments, the optic concept further allows a filtering
possibility for undesired beam portions, for example, in a region
of the Fourier-plane of the beam profile and a separation of the
beam shaping from the focusing.
[0059] The systems and methods resulting from these realizations
can inter alia enable separating of transparent, brittle-hard
materials with high velocity and with good quality of the cutting
edge. Moreover, such systems and methods may further enable
separating without a taper angle as it is created in ablating
methods. In particular when separating based on non-ablating
modifications, there may be no or only a small removal, with the
consequence that the material has only a few particles on the
surface after the processing.
[0060] In the following, the underlying optical concept will be
generally explained with reference to FIGS. 1 to 8. Then, exemplary
embodiments of optical systems will be explained, which, on the one
side, implement the optical system by conventional optics such as
lenses and mirrors (see FIGS. 9 to 11B) and, on the other side, by
diffractive optical elements (see FIGS. 12 to 16). In connection
with FIGS. 17 to 22, the combinability of the optical system with
components and aspects for filtering and scanning as well as
general aspects of the beam development within the optical system
are explained. Finally, in connection with FIGS. 23 to 32,
exemplary embodiments of the elongated focus zones for material
processing are illustrated, which in particular can be realized
with diffractive optical elements. In FIGS. 33A to 33D and 34, beam
profiles and a longitudinal amplitude distribution are explained
for an inverse quasi-Bessel beam at the propagation from the beam
shaping element to the near field optics in the optical system.
[0061] FIG. 1 shows a schematic illustration of an optical system 1
for beam shaping a laser beam 3 with the aim to create a focus zone
7, which is elongated in a propagation direction 5, within a
material 9 to be processed. Generally, laser beam 3 is determined
by beam parameters such as wavelength, spectral width, temporal
pulse shape, formation of pulse groups, beam diameter, transverse
input intensity profile, transverse input phase profile, input
divergence, and/or polarization. According to FIG. 1, laser beam 3
is supplied to optical system 1 for beam shaping, i.e., for
transforming one or more of the beam parameters. Usually, for laser
material processing, laser beam 3 will be a collimated Gaussian
beam with a transverse Gaussian intensity profile, which is
generated by a laser beam source 11, for example a ultrashort pulse
high-intensity laser system. The transformation can be performed,
for example, into an inverse Bessel beam-like or inverse Airy beam
shape.
[0062] In the laser processing machine 21 shown in FIG. 2, optical
system 1 may, for example, be used for material processing. Laser
processing machine 21 includes a support system 23 and a workpiece
positioning unit 25. Support system 23 spans over workpiece
positioning unit 25 and carries laser system 11, which is
integrated in FIG. 2, for example, in an upper crossbeam 23A of
support system 23. In addition, optical system 1 is mounted at
crossbeam 23A to be displaceable in X direction, so that both
components are arranged close to each other. In alternative
embodiments, laser system 11 may be provided, for example, as a
separate external unit. Laser beam 3 of laser system 11 is guided
to optical system 1 by optical fibers or as a free propagating
beam.
[0063] Workpiece positioning unit 25 carries a workpiece that
extends in the X-Y-plane. The work-piece is the material 9 to be
processed. For example, the material to be processed includes a
glass plate or a plate in ceramic or crystalline embodiment such as
sapphire or silicon, that is essentially transparent for the laser
wave-length used. Workpiece positioning unit 25 allows displacing
the workpiece in Y direction relative to support system 23, so
that, in combination with the displaceability of optical system 1,
a processing area is provided, which extends within the
X-Y-plane.
[0064] According to FIG. 2, in addition, optical system 1 or
cross-beam 23A is relocatable in Z direction, such that the
distance to the workpiece can be set. For a cut running in Z
direction, the laser beam is usually also directed in the Z
direction (i.e., normal to the workpiece) onto the workpiece.
However, additional processing axes may be provided as exemplarily
illustrated in FIG. 2 by the boom arrangement 27 and the additional
rotational axes 29. Accordingly, boom arrangement 27 is an optional
in the embodiment of FIG. 2. In addition, redundant add-on axes may
be provided for higher dynamics, as, for example, not the workpiece
or the optical system, but more compact and respectively designed
components are accelerated.
[0065] Laser processing machine 21 further includes a control unit
not explicitly shown in FIG. 1, which is, for example, integrated
within support system 23 and which in particular includes an
interface for inputting operation parameters by a user. In general,
the control unit includes elements for controlling electrical,
mechanical, or optical components of laser processing ma-chine 21,
for example, by controlling respective operation parameters such as
pump laser power, cooling power, direction and velocity of the
laser machine and/or the workpiece positioning unit, electrical
parameters for setting an optical element (for example, of an SLM)
and the spatial orientation of an optical element (for example, for
rotation of the same).
[0066] Additional arrangements for laser processing machines with
various degrees of freedom are disclosed, for example, in EP 1 688
807 A1, incorporated herein by reference in its entirety. In
general, for smaller workpieces often only the workpiece is moved,
and for larger workpieces only the laser beam or--as in FIG. 2--the
work-piece and the laser beam are moved. Moreover, two or more
optical systems and, thus, focus zones may be supplied by a single
laser system 11.
[0067] The modifications within the material, which are generated
by the laser processing machine, may be used, for example, for
drilling, separating by induced tensions, welding, creating a
modification of the refraction behavior, or for selective laser
etching. Accordingly, it is important to control the geometry as
well as the type of modification in a suitable manner. Besides
parameters such as laser wavelength, temporal pulse shape, number
of pulses, energy and temporal distance of the pulses within a
pulse group creating an individual modification, as well as pulse
energy or pulse group energy, the beam shape plays a decisive
role.
[0068] In particular, an elongated volume modification allows
processing of a, in beam propagation direction, volume region
within a single processing step. In particular, at one position in
feed direction, the processing can take place over a large extent
in only a single modification processing step. By the use of the
optical systems, beam shapes, and methods described herein, one can
achieve, on the one side, better work results (in comparison to
single modifications that are positioned next to each other at one
position in feed direction in succeeding modification processing
steps) and, on the other side, one can reduce the processing time
and the requirements for the system technology. Then, for single
modifications, multiple working steps are needed that increase the
time needed and that require a more involved ensuring of relative
positions of the single modifications.
[0069] In addition, an elongated focus zone can be helpful when
processing uneven materials, because essentially identical laser
processing conditions are given along the elongated focus zone such
that, in those embodiments, a respective readjusting in propagation
direction may not be necessary or only be necessary starting at a
larger deviation of the position of the material to be processed
than the lengths of the elongated focus area (in consideration of
the required processing/intrusion depth).
[0070] In general, it applies to the processing of transparent
materials by elongated volume absorption that, as soon as
absorption takes place, that absorption itself or the resulting
changes in the material properties can influence the propagation of
the laser beam. Therefore, it is advantageous, if beam portions,
which should cause a modification deeper within the workpiece,
i.e., in beam propagation direction downstream, essentially
propagate not through regions of considerable absorption.
[0071] In other words, it is favorable to lead those beam portions,
which contribute to the modification further downstream, under an
angle to the interaction zone. An example for this is the
quasi-Bessel beam, for which a ring-shaped far-field distribution
is given, the ring width of which is typically small in comparison
to the radius. Thereby, the beam portions of the interaction zone
are led in essentially with that angle in rotational symmetry. The
same applies for the inverse quasi-Bessel beam or for modifications
or extensions of the same such as the homogenized or modulated
inverse quasi-Bessel beam described herein. Another example is the
inverse accelerated `quasi-Airy beam-like` beam, for which the beam
portions are led into the modification under an offset angle, where
this is done clearly tangential and--not as for the pure
quasi-Bessel beam rotationally symmetric--to the curved
modification zone, e.g. as for a curved inverse quasi-Bessel
beam.
[0072] Moreover, it is desired to considerably pass the threshold
for the nonlinear absorption only within the desired volume region
and to choose the geometry of that volume area such that it is
suitable for the desired application, but that also the propagation
to further downstream positioned volume regions is not
significantly disturbed. For example, it may be advantageous to
keep secondary maxima of an apodized Bessel beam profile below a
threshold intensity needed for nonlinear absorption.
[0073] In view of modifications being subsequent in the feed
direction, the geometry of the modified volume may further be
selected such that, for a row of multiple modifications in the feed
direction, an earlier induced modification has only an
insignificant influence on the formation of the following
modifications.
[0074] As already mentioned, for fast processing, the generation of
a single modification can be performed with only a single laser
pulse/a single laser pulse group, so that a position on a
work-piece is approached only once in this case.
[0075] Ultrashort pulse lasers can make intensities (power
densities) available that allow causing a sufficiently strong
material modification in respective long interaction zones. The
geometric extent of the modification is thereby set with the help
of beam shaping such that a long extending, high density of free
electrons is created by nonlinear absorption in the material. The
supply of energy in deeper regions is performed laterally, so that
the shielding effect by an upstream interaction of the plasma can
be avoided in comparison to a Gaussian focusing. For example, an
electron density, which extends smoothly in longitudinal direction,
or an electron density, which is modulated spatially with a high
frequency, can be generated.
[0076] At the respective intensities, within regions with a
sufficiently high density of free electrons, an explosive expansion
of the material may be caused, whereby the resulting shock-wave can
create nanoscopic holes (nano-voids). Additional examples for
modifications (modification zones) are changes in the refractive
index, compressed and/or tensile stress induced regions,
micro-crystallites, and local changes in stoichiometry.
[0077] As explained, by accumulation of such modification zones in
feed direction, a course of a crack can be set. During processing,
the workpiece is accordingly separated along a respective modified
contour. The crack formation can then occur directly thereafter or
can be induced by another process. For example, for the separation
of non-pre-strained materials, ultrasound ramps or temperature
ramps may be used in order to cause a later separation along the
modified contour. A single modification usually does not lead to
crack formation.
[0078] With the help of a tailored beam shape, various tension
distributions within the material and between the modified regions
can be created in order to adapt the separation process to a given
material. In the process, strong spatial and temporal gradients can
favor the formation of a micro- or nano-explosion.
[0079] The modification geometry is thereby primarily determined by
the beam shaping (and not by the nonlinear propagation as, for
example, the filamentation). The generation of spatial gradients
can be achieved by the optical systems described herein, while the
generation of the temporal gradients can be achieved by pulse
trains or pulse shaping.
[0080] Generally, a scaling of the intensity distribution of a beam
shape can be achieved by the imaging ratio of the system, in
particular by the focal length and the numerical aperture of the
near field optics of the imaging system. Additional possibilities
for scaling result from the use of an additional lens as well as
the shifting of the beam shaping element and/or the far field
optics (see the description in connection with FIGS. 17 and 22).
Thus, the lateral and longitudinal extent of the beam profile
within the workpiece can be influenced. In addition, spatial
filters and apertures may be used within the beam path for beam
shaping, in order to prepare the beam.
[0081] Exemplary laser beam parameters for, for example, ultrashort
pulse laser systems and parameters of the optical system and the
elongated focal zone, which can be applied within the range of this
disclosure, are:
[0082] Pulse energy Ep: 1 .mu.J to 10 mJ (e.g. 20 .mu.J to 1000
.mu.J);
[0083] Energy of a pulse group Eg: 1 .mu.J to 10 mJ;
[0084] Ranges of wavelength: IR, VIS, UV (e.g. 2
.mu.m>.lamda.>200 nm; e.g. 1550 nm, 1064 nm, 1030 nm, 515 nm,
343 nm);
[0085] Pulse duration (FWHM): 10 fs to 50 ns (e.g. 200 fs to 20
ns);
[0086] Interaction duration (depending on the feed velocity):
smaller 100 ns (e.g. 5 ps-15 ns);
[0087] Duty cycle (interaction duration to repetition time of the
laser pulse/the pulse group): less than or equal to 5%, e.g. less
than or equal to 1%;
[0088] Raw beam diameter D (1/e2) when entering the optical system:
e.g. in the range from 1 mm to 25 mm;
[0089] Focal lengths of the near field optics: 3 mm to 100 mm (e.g.
10 mm to 20 mm);
[0090] Numerical aperture NA of the near field optics:
0.15.ltoreq.NA.ltoreq.0.5;
[0091] Length of beam profile within the material: larger 20
.mu.m;
[0092] Maximal lateral extent of the beam profile within the
material, where applicable in the short direction: smaller
20.lamda.;
[0093] Aspect ratio: larger 20;
[0094] Modulation in propagation direction: larger 10 periods over
the focus zone;
[0095] Feed dv between two neighboring modifications e.g. for
separating applications:
[0096] 100 nm<dv<10*lateral extent in feed direction;
[0097] Feed during interaction duration: e.g. smaller 5% of the
lateral extent in feed direction;
[0098] Thus, the pulse duration of the laser pulse and the
interaction duration relate to a temporal range, within which, for
example, a group of laser pulses interacts with the material for
the formation of a single modification at a location. Thereby, the
interaction duration is short regarding the present feed velocity,
so that all laser pulses of a group contribute to a modification at
one position.
[0099] If the workpiece is thinner than the focus zone is long, the
focus zone is positioned partially outside of the workpiece, so
that modifications may be caused that are shorter than the focus
zone. Such a situation may be advantageously used to make the
processing process robust also with respect to varying the distance
between the optics and the workpiece. In some embodiments, a
modification may be advantageous that does not reach through the
complete work-piece. In particular, the length of the focus zone
and/or its position within the workpiece may be adapted. In general
it is noted that, due to different thresholds for the nonlinear
absorption, a focus zone with assumed identical intensity may cause
differently large modifications in differing materials.
[0100] The aspect ratio relates to the geometry of the beam profile
(the focus zone) within the material to be processed as well as the
geometry of the modification created with a beam profile. For
asymmetric or in lateral direction modulated (for example,
non-rotationally symmetric or ring-shaped) beam profiles, the
aspect ratio is given by the ratio of the length of the
modification with respect to a maximum lateral extent in the
shortest direction that is present within that range of length. If
the beam profile includes a modulation in lateral direction, for
example, for ring-shaped beam profiles, then the aspect ratio
relates to the width of a maximum, for a ring-shaped beam profile,
for example, to the strength of the ring. When multiple
modification volumes, which are displaced in lateral direction, are
formed, the aspect ratio relates to the lateral extent of a single
modification. For a beam profile modulated in propagation direction
(e.g. due to interferences), the aspect ratio relates to the higher
ranking total length.
[0101] Assuming a distance d between the beam shaping element and
the focusing lens (near field optics), which is in particular
larger than the focal length fN of the near field optics, and an NA
of the near field optics with respect to air >0.15, the used
angular spectrum .alpha. of the beam shaping element can be in the
range tan(.alpha.)<f*NA/d<NA/2 and preferably
tan(.alpha.)>f*NA/(d*4).
[0102] The previously mentioned ranges for parameters may allow the
processing of a material thickness up to, for example, 5 mm and
more (typically 100 .mu.m to 1.1 mm) with roughness of the
cutting-edge Ra smaller than, for example, 1 .mu.m.
[0103] Optical system 1 may further include a beam processing unit
13 for adapting beam parameters such as beam diameter, input
intensity profile, input divergence, and/or polarization of laser
beam 3. For example, the laser beam of a pulsed laser system is
coupled into optical system 1 with, for example, a beam diameter of
5 mm, pulse duration of 6 ps at wavelengths around 1030 nm and is
led to processing unit 31.
[0104] FIG. 3 shows the schematic setup of optical system 1 for
explaining the functionality. Optical system 1 is based on a beam
shaping element 31 and an imaging system 33. Beam shaping element
31 is adapted to receive laser beam 3. Accordingly, it is adapted
to a transverse input intensity profile 41 of laser beam 3. In
addition, beam shaping element 31 is adapted to impose onto laser
beam 3 a beam shaping phase distribution 43 (schematically
indicated by dashes in FIG. 1) over transverse input intensity
profile 41. Imposed phase distribution 43 is such that a virtual
optical image 53 (essentially) of elongated focus zone 7 is
attributed to laser beam 53, with the virtual optical image 53
being located in front of beam shaping element 31. Beam shaping
element 31 creates in this manner a virtual beam profile that is
located upstream of beam shaping element 31, but does not
correspond to the real path of the beam being at that position.
[0105] Imaging system 33 is constructed such that the virtual beam
profile is imaged into the area of the laser processing machine, in
which the workpiece is positioned during the processing. In FIG. 3,
imaging system 33 includes for that purpose a, in beam direction,
first focusing element, which is referred to herein as far field
optics 33A, and a, in direction of the beam, second focusing
element, which is referred to herein as near field optics 33B.
[0106] Far field optics 33A is provided in the area of phase
imposing and is illustrated in FIG. 3 exemplarily by a lens shape
downstream of beam shaping element 31. As will be explained in the
following, far field optics 33A may also be arranged shortly before
beam shaping element 31, composed of components before and after
the beam shaping element 31, and/or completely or partially
integrated in the beam shaping element 31.
[0107] After the imposing of the phase within beam shaping element
31, laser beam 3 propagates in accordance with imaging system 33
over a beam shaping distance Dp to near field optics 33B. Beam
shaping distance Dp corresponds to a propagation length of the
laser beam 3, within which imposed phase distribution 43 transforms
the transverse input intensity profile 41 into a transverse output
intensity profile 51 at near field optics 33B. Herein, output
intensity profile 51 includes those transverse intensity profiles
in the optical system that are determined by the phase imposing.
This is usually completed at the latest in the area of the focal
length before the near field optics or within the area of the near
field optics.
[0108] For implementing the concept of a virtual beam profile,
there are the following considerations for the propagation length
(from beam shaping element 31 to near field optics 33B), which
laser beam 3 has to propagate within the optical system. In
general, the optical system forms an imaging system 33 with a far
field focusing action and a near field focusing action. The latter
is determined by near field optics 33B and thereby by near field
focal length fN. The former is determined by a far field focusing
action and a respective far field focal length fF. Far field focal
length fF can be realized by the separate far field optics 33A
and/or can be integrated into the beam shaping element. See in this
respect also FIG. 20. Imaging system 33 has an imaging ratio of X
to 1, whereby X for a demagnification of the virtual image usually
is larger than 1. For example, imaging ratios are implemented that
are larger than or equal to 1:1 such as larger than or equal to
5:1, 10:1, 20:1, or 40:1. In other words, with this definition of
the imaging, the factor X resembles the magnification of the
lateral size of the focus zone into the virtual profile. The angle
is respectively demagnified. Attention should be paid to the fact
that the imaging ratio goes quadratic into the length of the
profile. Accordingly, the longitudinal length of a virtual image
becomes smaller, for example, for an imaging ratio of 10:1 by a
factor of 100 and for an imaging ratio of 20:1 by a factor of
400.
[0109] At an imaging ratio of 1:1, there is fN=fF, an overlapping
alignment of the focal planes is assumed. In general, there is fF=X
fN. If the far field optics 33A is integrated into the beam shaping
element, it is positioned, e.g., at a distance fN+fF from the near
field optics, i.e., typically in the range of the sum of the focal
lengths of both optical elements. For a 1:1 or a de-magnifying
imaging system, the propagation length corresponds therefore at
least to twice the focal length of the near field optics.
[0110] Separating far field optics 33A and beam shaping element 31
and assuming, that the virtual optical image should not overlap (in
particular not within the intensity region being relevant for the
focus zone) with the beam shaping element, the beam shaping element
is arranged at at least a distance of I/2 downstream of the
longitudinal center of virtual beam profile 53. Here, the length I
is the longitudinal extent of virtual beam profile 53 with respect
to the relevant intensity area. The longitudinal center of virtual
beam profile 53 is located, e.g., at the entrance side focal plane
of far field optics 33A, which is located at a distance fN+fF from
near field optics 33B. In this case, the propagation length is
d=fN+2 fF-I/2=(1+2X) fN-I/2, therefore smaller than fN+2 fF=(1+2X)
fN, or, in other words, smaller than the distance between the
optical elements plus fF.
[0111] For the distance fN+2 fF=(1+2X) fN, also for increasing beam
enlargements a respectively increasing length I of virtual beam
profile 53 can be imaged, whereby--as explained later--a defined
end of the profile can be maintained.
[0112] In general, it is mentioned that, due to raw beam
divergences and convergences as well as for deviating adjustment of
the imaging system, deviations from the above considerations may
occur. In contrast to a comparable image of a real intensity
enhancement, i.e., images with comparable imaging ratios, the beam
shaping element is located closer (see the respective discussion on
FIGS. 7 and 8). A common distance therefore lies in a range (1+2X)
fN.gtoreq.d.gtoreq.2 fN.
[0113] Due to the imposed phase, transverse output intensity
profile 51 includes, in comparison to input intensity profile 41,
at least one local maximum 49 located outside of a beam axis 45.
Local maximum 49 being located outside beam axis 45 results in a
lateral energy entry into focus zone 7. Depending on beam shaping
element 31, local maximum 49 of transverse output intensity profile
51 can be made rotationally symmetric with respect to beam axis
45--as indicated in FIG. 3 in the cut view--or it can be formed in
an azimuthal angular range (see, e.g., FIGS. 29 and 30). Usually,
the beam axis is defined by the center of the lateral beam profile.
The optical system can usually be related to an optical axis, which
usually runs through a symmetry point of the beam shaping element
(e.g., through the center of the DOE or the tip of the reflective
hollow cone axicon). For rotationally symmetric beams and a
respective exact alignment, the beam axis may coincide with the
optical axis of the optical system at least in sections.
[0114] The local maximum can be considered a generic feature of
output intensity profile 51, where in particular for inverse
quasi-Bessel beam shapes, a typical substructure with a steep and
slowly falling flank can be formed. That substructure can invert
itself due to the focusing action of the beam forming element
and/or the far field optics in the range of an associated far field
focal plane. In particular, the output intensity profile can show
within the range of that far field focal plane the local maximum
particularly "sharp" or, for example, for inverse quasi-Bessel beam
shapes, the local maximum can form itself quite fast after the beam
forming element. However, the aspects of the substructure may vary
due to the various possibilities in the phase imposing.
[0115] The concept of a virtual beam profile can, on the one side,
reduce the constructional length of optical system 1 and, on the
other side, it can avoid the formation of an elongated beam profile
with significant intensity enhancement within optical system 1.
Imaging system 33 is configured such that, within optical system 1,
the far field of the virtual beam profile is formed and that the
focusing in the near field optics 33B can be done using a common
focusing component such as a lens, a mirror, a microscopic
objective, or a combination thereof. In that case, "common" is
understood herein in the sense of that the characteristic beam
shape is essentially imposed by beam shaping element 31 and not by
near field optics 33B.
[0116] In FIG. 3, a path of the beam is indicated for illustration
that corresponds to a beam herein referred to as an inverse
quasi-Bessel beam. For that purpose, the path of the beam is
illustrated downstream of beam shaping element 31 with solid lines.
Upstream of beam shaping element 31, instead of incident collimated
beam 3, the virtual beam profile is sketched in analogy to a real
quasi-Bessel beam by dashed lines.
[0117] Similar to a common quasi-Bessel beam, also the inverse
quasi-Bessel beam has a ring structure in the focal plane of far
field optics 33A. However, divergent beam areas 55A, 55B indicated
in the schematic cut view, which impinge on far field optics 33A,
do not result from a "real" quasi-Bessel beam profile, but they
result directly from the interaction of beam shaping element 31
with incident laser beam 3. Due to the direct interaction, beam
areas 55A, 55B are shaped in their lateral intensity distribution
by transverse beam profile 41 of laser beam 3. Accordingly, for a
Gaussian input beam, the intensity decreases in the radial
direction principally in beam areas 55A, 55B away from a beam
center. Due to the divergence of beam areas 55A, 55B, typically an
area of low (in the ideal case no) intensity is formed accordingly
on the beam axis for the phase-modulated beam portions. In that
case, the divergence of a beam portion, accordingly also a
divergent beam portion, relates herein to a beam portion that moves
away from the beam axis. However, in that area, a beam portion of a
phase unmodulated beam and/or also an additional, phase-modulated
beam portion may be superimposed. With respect to the development
of the beam within the optical system during the shaping of an
inverse Bessel like beam, it is referred to the description of
FIGS. 33 and 34. This intensity behavior is schematically indicated
in transverse intensity courses (e.g., transverse intensity beam
profile segments) 57A and 57B. It is noted that the intensity
courses along the propagation length can change due to imposed
phase distribution 43. At least, however, within the initial area
(i.e., beam areas 55A, 55B laying close to beam shaping element 31)
and due to the beam shaping element 31 acting in general as a pure
phase mask, the incident intensity profile of laser beam 3
dominates the divergent phase-modulated beam portions.
[0118] For a clear explanation of an inverse quasi-Bessel beam,
further intensity courses 57A' and 57B' are schematically indicated
in FIG. 3. Here, it is assumed that beam shaping element 31
influences only the phase and not the amplitude. One recognizes
that the focusing by far field optics 33A (or the respective far
field action of beam shaping element 31) reverses the intensity
course at the exit of optical system 1 such that, during the
formation of elongated focus zone 7 on beam axis 45, at first low
intensities are superposed, which originate from the de-creasing
flanks of the incident Gaussian beam profile. Thereafter, the
higher intensities superpose, which originate from the central area
of the incident Gaussian beam profile. In this context it is noted
that not only the intensity on the beam shaping element, but also
the contributing area has to be acknowledged. For rotationally
symmetry, the distance enters accordingly quadratic. As in
particular illustrated in connection with FIG. 4, the longitudinal
intensity profile ends exactly in that area, in which the beam
portions from the center of the input profile cross. In the center,
although the highest intensity is present, the area goes to zero.
Moreover, it is noted that, after the focus zone, a reversed
intensity course is present again, which corresponds to intensity
courses 57A, 57B after the beam shaping element (assuming no
interaction with a material).
[0119] Due to imaging with imaging system 33, there are incident
virtual intensity courses 57A'' and 57B'', which are accordingly
schematically indicated with respect to the virtual beam shaping in
FIG. 3. Those correspond in principle to intensity courses 57A' and
57B'.
[0120] Those intensity courses, which are inverted in comparison to
a quasi-Bessel beam, cause a specific longitudinal intensity course
for the inverse quasi-Bessel beam for focus zone 7 as well as in
the virtual beam profile, i.e., optical image 53, because here the
superposition of beam portions 55A, 55B is done virtually. For the
respective discussion of the intensity course for a conventional
quasi-Bessel beam, it is referred to FIGS. 7 and 8 and the
respective description.
[0121] FIG. 4 exemplarily illustrates a longitudinal intensity
distribution 61 within elongated focus zone 7 as it can be
calculated for the imaging of virtual optical image 53 of an
inverse quasi-Bessel beam shape. Depicted is a normed intensity I
in Z direction. It is noted that a propagation direction according
to a normal incidence (in Z direction) onto material 9 is not
required and, as illustrated in connection with FIG. 2, can take
place alternatively under an angle with respect to the Z
direction.
[0122] One recognizes in FIG. 4 an initially slow intensity
increase 61A over several 100 micrometer (initial superposition of
low (outer) intensities) up to an intensity maximum, followed by a
strong intensity decrease 61B (superposition of the high (central)
intensities). For an inverse Bessel beam shape, the result is
therefore a hard border of the longitudinal intensity distribution
in propagation direction (the Z direction in FIG. 4). As one can in
particular recognize in view of intensity courses 57A' and 57B'
shown in FIG. 3, the hard border is based on the fact that the end
of longitudinal intensity distribution 61 relies on the
contributions of the beam center of the incident laser beam having
admittedly a lot of intensity, but on a strongly reduced (going to
zero) area. In other words, the end relies on the imaging of a
virtual beam pro-file in which in the center for the inverse
quasi-Bessel beam a hole is created. The strong gradient at the
intensity decrease at the end relies on the high intensity in the
center of the input profile, limited, however, by the disappearing
area. For an ideal imaging system, the longitudinal extent of
intensity distribution 61 is defined by the position of the virtual
profile and the imaging scale. If in addition the workpiece
includes a large refractive index, the beam profile is accordingly
lengthened.
[0123] In this context it is added that the hard border has the
consequence in laser processing machines that the, in propagation
direction, front end of a modification is essentially stationary in
propagation direction also if the incident transverse beam profile
is increased. The modification changes its extent only in the back
part, i.e., it can lengthen in direction to the near field optics,
if the input beam diameter of the laser beam enlarges. A once set
position of the hard border with respect to the workpiece support
or the workpiece itself can thereby avoid high intensities
downstream of the modification. In contrast thereto, an enlargement
of the input beam diameter, when imaging a real intensity
enhancement, causes an elongation of the modification in
propagation direction, i.e., for example into a workpiece support,
which can result in damages of the same.
[0124] FIG. 5 shows an exemplary X-Y-cut 63 of the intensity within
focus zone 7 for the longitudinal intensity distribution 61 shown
in FIG. 4. It is noted that herein some grayscale illustrations
such as FIGS. 5, 30, 31 are based on a color illustration so that
maximum values of the intensity I amplitude can be illustrated
dark. For example, the center of focus zone 7 (highest intensity)
in FIG. 5 is shown dark and is surrounded by a brighter area of
lower intensity. The same applies to focus zone 707 in FIGS. 30 and
31. One recognizes the elongated formation of focus zone 7 over
several hundred micrometer at a transverse extent of some few
micrometer. Together with the threshold behavior of the nonlinear
absorption, such a beam profile can cause a clearly defined
elongated modification within the workpiece. The elongated shape of
focus zone 7 includes, for example, an aspect ratio, i.e., the
ratio of the length of the focus zone to a maximal extent in the
lateral shortest direction being present within that length--the
latter for non-rotationally symmetric profiles, in the range from
10:1 to 1000:1, e.g. 20:1 or more, for example 50:1 to 400:1.
[0125] If one frees oneself from the beam shape--shown in FIG.
4--of an inverse quasi-Bessel beam, which is not modified in
propagation direction with respect to amplitude, beam shaping
element 31 can additionally create an amplitude redistribution in
the far field, which e.g. can be used for an intensity modification
in propagation direction. However, the thereby created intensity
distribution in front of focus zone 7 can no longer be presented in
a very clear manner. Nevertheless, often initial stages of
inversions will show up in the beginning region or in the end
region of the longitudinal intensity profile, for example a slow
increase and a steep decrease. However, a (phase caused) amplitude
redistribution by the phase description of beam shaping element 31
may just exactly be set to an inverted intensity distribution, in
order to cause, for example, a form of a longitudinal flat top
intensity profile.
[0126] Additionally, the following feature for distinguishing from
a "real" beam shape may be maintained: For the case of a real
Gaussian input beam, there exists, e.g. for a real axicon, a plane
between near field optics and focus zone at which the demagnified
Gaussian transverse beam profile of the input beam is present and
can be made visible. A respective imaging exists for the virtual
optical image. However, in this case, the image plane, in which the
demagnified Gaussian transverse beam profile is present, lies
behind the focus zone. The transverse beam profile can accordingly
be made visible. This applies generally to phase masks for the
herein disclosed inverse beam shapes, if those are illuminated with
a Gaussian beam profile. Specifically, the demagnified Gaussian
transverse beam profile is positioned in the image plane of the
beam shaping element and therefore usually directly downstream of
the focus zone. Due to the already performed divergence,
demagnified Gaussian transverse beam profile is therefore
significantly larger than the transverse beam profile of the
inverse quasi-Bessel beam in the focus zone. Also, the demagnified
Gaussian transverse beam profile is much lower in intensity.
[0127] One can recognize the position of the imaged Gaussian
transverse beam profile of the input beam by a fast
flipping/inversion of the structure of the beam profile, i.e., a
strong change over a small lateral area. For example, the
transverse intensity profile of the inverse quasi-Bessel beam is
present in the focus zone. When passing through the image plane of
the beam shaping element, the dark spot in the center is formed
"quasi" immediately. For an inverse quasi-Bessel beam, this is
different at the beginning of the focus zone. There, due to the
increased superposition of the border areas of the Gaussian beam
profile, a slow transition is made from a dark center to the
transverse intensity profile of the inverse quasi-Bessel beam,
which is filled in the center. In other words, in longitudinal
direction, the intensity increases over a larger area then it
decreases at the end. At the end, that transition is accordingly
clearly sharply limited. It is added that, when imaging a real
Bessel beam-like intensity enhancement, the behavior at the end and
the behavior at the beginning are interchanged, i.e., at the end of
the Bessel beam profile, the dark spot forms more slowly.
[0128] As previously explained, the concept of using a virtual beam
profile therefore has an effect inter alia on the phase imposing to
be applied and the resulting intensity courses in focus zone 7.
[0129] FIG. 6 illustrates modification zones 65 that were created
in the context of an experimental study for examining the formation
of modifications in a material. Each modification zone 65 goes back
to the interaction with a group of laser pulses, for example two 6
ps pulses at a temporal separation of about 14 ns. The shape of the
modification zones correspond to the shape of elongated focus zone
7 as assumed in accordance with FIGS. 4 and 5. The maximal length
is limited by the geometry of elongated focus zone 7 at a required
intensity I fluence.
[0130] The upper four images illustrate the threshold behavior for
pulse group energies Eg from about 20 .mu.J to 40 .mu.J. The lower
four images illustrate the shaping of the elongated modification
zones 65 at pulse group energies Eg from about 30 .mu.J to 200
.mu.J. With increasing total energy Eg, the modification zone
lengthens in the direction of the beam entrance (near field
optics), because the threshold intensity for the nonlinear
absorption is reached within a longer area of focus zone 7. The end
of the modification in beam propagation direction is in its
position essentially stationary, and even in particular without
secondary correction of the distance of a near field optics (33B)
to the workpiece to be processed. At lower energies, an initial
walk in beam direction of the back end may occur due to the
existing gradient in longitudinal direction, in particular if the
modification threshold lies at small intensities within the beam
profile. However, the walk decreases at medium and high energies,
because the generation of the in-verse quasi-Bessel beam profile
includes in propagation direction an implicit maximal back end.
[0131] A similar behavior in the change of the longitudinal extent
of the modification is also created for a radially increasing beam
diameter of incident laser beam 3. Also in that case, the
modification zone is lengthening in direction of the beam entrance
(near field optics), because the intensity areas of incident laser
beam 3, which are added in a radial direction at the outside, guide
energy into the longitudinal intensity area in the area of slow
intensity increase 61A (i.e., intensity increase with slow
gradient). The maximum of the intensity distribution will
accordingly be shifted in direction of the beam entrance. The end
of the modification in beam propagation direction is in contrast in
its position essentially stationary, because that position is
sup-plied with energy by the center of the beam of incident laser
beam 3. In addition it is noted that this behavior can be observed
also for modified inverse quasi-Bessel beam shapes. For example,
for a flat top beam shape as discussed in connection with FIGS. 23
to 26, the position of the end of the modification would
essentially not change for a change in the beam diameter. For such
a changed incident intensity profile, the beam shaping element may
further eventually no longer result in an optimized flat top
structure so that this may result in modulations in the intensity
and eventually a variation of the beginning.
[0132] FIG. 7 serves as an illustration of a beam guidance at which
a real intensity enhancement 71 is generated by a beam shaping
optics 73 such as an axicon. This corresponds to the known
formation of a quasi-Bessel beam. Intensity enhancement 71 is then
imaged by a telescope 75 into workpiece 9 by forming a focus zone
77. As shown in FIG. 7, in such a setup, there is the danger that
the real intensity enhancement 71 damages a far field optics 79 of
telescope system 75, in particular if a small setup length is to be
realized. The herein disclosed optical system (see, e.g., FIG. 3),
which implements the concept of a virtual image, bypasses that risk
of a damage to the beam guiding optics.
[0133] FIG. 8 illustrates for completeness a longitudinal intensity
distribution 81 in Z direction that results from the setup of FIG.
7. After an ab initio steep increase 81A, an intensity maximum is
reached, beginning at which the intensity decreases again. At lower
intensities, a slowly vanishing drop 81B (vanishing drop of small
gradient) begins. One sees the general reversal of the longitudinal
intensity distributions 61 and 68 of FIGS. 4 and 8, at which the
"hard border" at the end is replaced by a "hard beginning".
[0134] For such a quasi-Bessel beam, the passing through an axicon
with a laser beam having an incident Gaussian beam profile 83 will
result in superposed beam portions 85A, 85B, the intensity weights
of which result in real longitudinal intensity distribution 81 (at
first superposition of the intensities of the central area of
Gaussian beam profile 83, then superposition of lower (outer)
intensities of Gaussian beam profile 83). For explaining, again
schematic intensity courses 87A and 87B are indicated downstream of
far field optics 79, and intensity courses 87A' and 87B' are
indicated upstream of focus zone 77.
[0135] In the following, various exemplary configurations of
optical systems are explained that implement the concept of virtual
intensity enhancement. They comprise beam shaping elements in the
transmission and reflection, wherein the imposing of the phase
distribution is performed in particularly refractive, reflective,
or diffractive. It is referred to the preceding description with
respect to the already described components such as laser system
11.
[0136] In view of the distances of beam shaping optics 73 from the
near field optics, the following values can apply similar to the
considerations for the virtual image. For a real beam profile, one
would typically position the center of the to be imaged real beam
profile of length I in the entrance-side focal length of the far
field optics. A typical distance would then be at least
[0137] fN+2 fF+I/2=(1+2X) fN+I/2, thus larger than fN+2 fF, in
other words, larger than the distance between the optical elements
plus fF.
[0138] FIG. 9 shows a refractive beam shaping with the help of a
hollow cone axicon 131A. This creates a virtual inverse
quasi-Bessel beam profile 153A upward of hollow cone axicon 131A.
The same is indicated in FIG. 9 by dashed lines, a real intensity
enhancement is not present in that area. In addition, in the
embodiment of FIG. 9, the far field optics is configured in beam
propagation direction downstream of hollow cone axicon 131A as
plano-convex lens 133A. Near field optics 33B causes the focusing
of the laser beam into focus zone 7, so that the virtual inverse
quasi-Bessel beam profile 153A is related to the laser beam as
virtual optical image of focus zone 7.
[0139] FIG. 10 shows an embodiment with a hollow cone axicon lens
system 131B that is used as a refractive beam shaping element.
Here, the far field optics is integrated in the beam shaping
element as convex lens surface 133B, which is positioned at the
entrance side of the hollow cone axicon. This setup creates
similarly a virtual inverse quasi-Bessel beam profile 153B.
[0140] FIG. 11A illustrates an embodiment with a reflective beam
shaping element, in particular a reflective axicon mirror system
131C. A highly reflective surface of the beam shaping element is
shaped such that the beam shaping feature of a reflective axicon is
combined with the far field forming component of a focusing hollow
mirror. Accordingly, axicon mirror system 131C has the function of
beam shaping as well as of the far field optics. A virtual inverse
quasi-Bessel beam profile 153C is indicated at the backside of
axicon mirror system 131C, thus in an area that is not passed by
laser beam 3.
[0141] As is further shown in FIG. 11A, after beam adaptation unit
13, laser beam 3 of laser system 11 is coupled into optical system
1 by a deflection mirror 140. Deflection mirror 140 is, for
example, arranged on the optical axis between axicon mirror system
131C and near field optics 33B and guides the beam to beam shaping
element 131C. In some embodiments, the deflection mirror may, for
example, be centrally drilled through to guide as less as possible
light onto the central area of beam shaping element 131C, which
potentially has optical errors. In addition to those aspects of
filtering described in the following in connection with FIGS. 17
and 18, it is already noted at this stage that deflection mirror
140 at the same time blocks an undesired central beam portion such
that the same is not focused by near field optics 33B.
[0142] FIG. 11B shows a further embodiment of an optical system
based on a reflective beam shaping element. The beam shaping
element in form of reflective axicon mirror system 131C is
illuminated thereby with laser beam 3 through an opening 141 of a
drilled through deflection mirror 140'. That reflected and phase
imposed beam impinges then after the formation of a e.g.
ring-shaped far field onto deflection mirror 140'. The same guides
the beam onto near field optics 33B for focusing into the elongated
focus zone. The opening serves accordingly in addition as kind of a
filter/diaphragm of the central area of the reflected beam.
[0143] In another embodiment with a reflective beam shaping
element, the optical system includes a reflective axicon, a drilled
through off-axis-parabolic mirror, and the near field optics. That
reflective axicon includes for the beam shaping a conical grinded
based body, the conical surface of which is coated highly
reflective. The laser beam can be irradiated through the opening in
the off-axis-parabolic mirror onto the reflective axicon. The
reflected and beam shaped beam impinges then on the
off-axis-parabolic mirror that redirects the beam on near field
optics 33B and at the same time collimates the same.
[0144] FIGS. 12 and 13 show embodiments of the optical system with
digitalized beam shaping elements. Here, the digitalization can
relate to the use of discrete values for the phase shift and/or the
lateral structure (for example, pixel structure). The use of
spatial light modulators (SLMs) is one of many different
possibilities to realize the beam shaping with programmable or also
permanently written diffractive optical elements (DOE).
[0145] In addition to the simple generation of one or more virtual
beam profiles, e.g. according to the phase imposing of one or more
hollow cone axicons, diffractive optical elements allow the desired
modification, for example, for homogenizing of the longitudinal
intensity distribution. For this, deviations in the phase can
exemplarily be used in the range equal to or smaller than 50%, e.g.
equal to or smaller than 20% or equal to or smaller than 10% with
respect to, for ex-ample, the hollow cone axicon phase (and thereby
of an inverse quasi-Bessel beam). In general, SLMs allow very fine
phase changes at a lateral rough resolution, in contrast to, for
example, lithographically generated, permanently written DOEs.
Permanently written DOEs comprise e.g. plano-parallel steps, the
thickness of which determine the phase. So, the lithographic
manufacturing allows a large lateral resolution. Binary steps can
result in real and virtual beam pro-files. Only a number of more
than two phase steps can result in a differentiation in the sense
of a preferred direction for the virtual beam profile. For example,
four or eight or more phase steps allow an efficient beam shaping
with respect to the virtual beam profile. However, the
discretization can cause secondary orders that can, for example, be
filtered out. In general, several optical elements can be combined
within a DOE, by determining e.g. the transmission function of all
elements (e.g. hollow cone axicon(s) and lens(es); adding the
individual phase functions (exp(-1i (phi 1+phi2+ . . . )). In
addition or alternatively, some type of superposition of individual
transmission functions can be done. For the determination of the
phase distributions, it was initially referred to the publication
of Leach et al. Manufacturing methods for continuous
microstructures comprise, for example, the analog-lithography or
the nanoimprint-lithography.
[0146] Herein, the structural element of a diffractive optical beam
shaping element, which causes the phase imposing and is configured
in an areal shape, be it an adjustable SLM or a permanently written
DOE, is referred to as a phase mask. Depending on the type of
configuration of the DOE, it may be used in transmission or in
reflection to impose a phase distribution on a laser beam.
[0147] In FIG. 12, a spatial light modulator 31A is used in
reflection for phase imposing. For example, spatial light modulator
31A is based on a "liquid crystal on silicon" (LCOS) that enables a
phase shift that is programmable for the individual pixels. Spatial
light modulators can further be based on micro-systems (MEMS),
micro-opto-electro-mechanical systems (MOEMS), or
micro-mirror-matrix systems. In SLMs, the pixels can, for example,
be controlled electronically to cause a specific phase imposing
over the transverse input intensity profile. The electronical
controllability allows, for example, the online-setting of phases
and, thus, the adaptation of focus zone 7, e.g. in dependence of
the material to be processed or in reaction of fluctuations of the
laser. In the configuration of FIG. 12, the function of a
diffractive axicon for the generation of a virtual inverse
quasi-Bessel beam profile can be combined, for example, with the
far field forming action of a far field optics by the phase
shifting of the spatial light modulator 31A. Alternatively, a
permanently written reflective DOE can be used as beam shaping
element 31A.
[0148] FIG. 13 is a schematic view of an optical system based on a
DOE 31B, for which the phase imposing is permanently written in DOE
31B. DOE 31B is used in that case in transmission. As in FIG. 12,
the phase shift, which, for example, results in a virtual
quasi-Bessel beam profile, as well as the focusing property of far
field optics are combined within the DOE 31B.
[0149] The optical systems of FIGS. 9 to 13 can result in output
intensity profiles that correspond to inverse quasi-Bessel beam
profiles and that are attributed to virtual optical images.
[0150] FIG. 14 illustrates an example of a phase distribution 243
as it can be provided e.g. in DOE 31B. Phase distribution 243 is
rotationally symmetric. One recognizes ring-shaped phase
distributions, the frequency of which is modulated in radial
direction. The rings point to the generation of a rotationally
symmetric virtual quasi-Bessel beam profile. The frequency
modulation points to the integration of the phase component of the
far field optics in the phase distribution for beam shaping. In
FIG. 14, the phases are indicated in the range of .+-..pi.. In
alternative configurations, discrete phase distributions such as
binary phase distributions or multi-step (for example, 4 or more
levels in the range of the phase shift from 0 to 2.pi.) phase
distributions can be implemented in DOE phase masks.
[0151] FIGS. 15 and 16 illustrate exemplarily an output intensity
profile 251 within the intensity cross-section (FIG. 15) and in the
2D-top view (FIG. 16). One recognizes an intensity maximum 249
extending in a ring shape around beam axis 45. There is hardly any
intensity in the beam center.
[0152] In some embodiments, the transition into the inverse
quasi-Bessel beam will not be complete such that accordingly a
non-phase-modulated remaining beam, for example with a Gaussian
beam shape, is superposed to the ring-shaped intensity profile.
FIG. 15 indicates schematically such a non-phase-modulated beam
portion 252 by a dash-dotted line.
[0153] Maximum 249 of that intensity distribution in FIG. 15 is an
example of a local intensity maximum, with which an original input
intensity profile (e.g. a Gaussian beam profile) was modified in
the area of the transverse output intensity profile. The rotational
symmetry of the ring structure is due to the rotational symmetry of
the inverse quasi-Bessel beam profile. In alternative embodiments,
the local intensity maximum is limited to an azimuthal angular
range. In addition, a superposition of azimuthal limited and/or
ring-shaped local maxima may be given.
[0154] When using a refractive hollow cone axicon (see FIGS. 9 and
10) for the generation of an inverse quasi-Bessel beam-shaped
output intensity profile, undesired beam portions may be created
under undesired angles for a non-perfect tip of the axicon. Also
for diffractive beam shaping elements, non-desired beam portions
may appear. For example, a non-phase-modulated beam portion, which
cannot be neglected, or additional orders of diffraction in the far
field of the laser beam can be present.
[0155] The herein disclosed optical systems simplify, by using the
far field components, the insertion and the shape selection of
filters to filter out such disturbing beam portions. In particular
these undesired beam portions can be separated from the desired
beam portions (beam for use) in a simple manner in the area of the
Fourier plane.
[0156] Referring to the non-phase-modulated beam portion 252 of
FIG. 15, FIG. 17 shows an exemplary optical system that is based on
the optical system shown in FIG. 3. However, additionally a
filtering of non-phase-modulated portions is performed in the area
of the Fourier plane of imaging system 33. As an example, a spatial
filter unit 220 is positioned upstream of near field optics 33B in
FIG. 17.
[0157] Filter unit 220 includes a central area around beam axis 45
that blocks, for example, the Gaussian intensity
distribution--indicated in FIG. 15--of the non-phase-modulated beam
portion 252. Filter unit 220 can additionally include sections,
which are located radially further away from the beam axis 45, for
blocking higher orders of diffraction by the DOE or the SLM.
[0158] In general, filter unit 220 is provided for the suppression
of non-phase-modulated base modes and higher diffraction orders as
well as of scattered radiation of the various herein disclosed
refractive, reflective, or diffractive beam shaping elements. For
rotationally symmetric output intensity profiles, usually also the
filter unit is made rotationally symmetric. In some embodiments,
only some portions of filter unit 220 or no filtering at all is
provided.
[0159] Diffractive beam shaping elements allow a further approach
for suppressing the non-phase-modulated beam portions. For this, an
additional phase contribution is imposed to deflect the
phase-modulated beam portion.
[0160] FIG. 18 shows, for example, an optical system in which the
diffractive optical element 31 is additionally provided with a
linear phase contribution. The linear phase contribution results in
a deflection 230 of phase-modulated beam 203A. Non-phase-modulated
beam portion 203B is not deflected and impinges, for example, on a
filter unit 222.
[0161] FIG. 19 shows a further embodiment of an optical system that
utilizes the use of the far field component additionally for the
implementation of a scan approach. In general, a scan system allows
shifting focus zone 7 within a certain range. In general, it is
possible by the separation of the beam shape from the near field
focusing to provide favorable telecentric scan approaches, in
particular for the volume absorption. In some embodiments, in
addition the location as well as the angle can be set. Accordingly,
such scanner systems can allow writing fine contours into a
workpiece.
[0162] In the configuration of FIG. 19, a scanner mirror 310 is
positioned at the image side focal plane of a near field optics
333B. Scanner mirror 310 deflects the laser beam in the range of
the out-put intensity distribution onto near field optics 333B
positioned at the side. The deflection in the Fourier plane results
in that the propagation direction in the workpiece is preserved
despite the displacement in location. The scanning region itself is
determined by the size of near field optics 333B.
[0163] If scanner mirror 310 is not correctly positioned in the
focal plane of near field optics 333B or if it can be moved with
respect thereto, then an orientation of the elongated focus zone,
in particular an angular deviation from the Z direction in FIG. 2,
can be set.
[0164] With the help of a configuration in accordance with the
optical system shown in FIG. 13, FIG. 20 explains exemplarily the
underlying imaging features. The optical system includes a beam
shaping element 31 that operates also as a far field optics and is
therefore characterized by a focal length fF. In addition, the
optical system includes near field optics 33B that is characterized
by focal length fN. In FIG. 20, the focal planes of the far field
optics and the near field optics coincide. Accordingly, in FIG. 20
only one focal plane 340 is indicated by a dashed line. In that
configuration of overlapping focal planes, the imaging system
images for incidence of a plane wave front generally a virtual beam
shape 253 onto elongated focus zone 7, for example, an inverse
quasi-Bessel beam profile, inverse modulated or homogenized
quasi-Bessel beam profiles as examples for inverse
quasi-Bessel/Airy beam shapes.
[0165] Though the focal planes do not need to overlap always. For
example, the imaging system can be adapted to a given beam
divergence, but laser beam 3 may be incident with another
divergence. In those cases, still a virtual optical image being
positioned in front of the beam shaping element is attributed to
elongated focus zone 7, but it does not need to be a perfect
imaging. A similar situation may be given for an intended
misalignment of the imaging system, for example, in connection with
a scanner device.
[0166] FIG. 20 illustrates in addition the terms "far field optics"
and "near field optics". The far field optics creates the far field
of virtual beam path 253 in the range of far field focal length fF.
As previously already explained, the far field optics can be
distributed in its function, for example, be made of one or more
components, which are arranged before and/or after the beam shaping
element and displaced with respect to the same, and/or be
integrated into the beam shaping element. The near field optics
focuses the beam with the smaller focal length fN in the direction
of the workpiece and thereby forms the focus zone. Thus, the far
field of virtual beam profile 53 with respect to the far field
optics, as well as the far field of focus zone 7 with respect to
near field optics 33B is positioned in the area of focal plane
340.
[0167] Also for non-perfect imaging (e.g. non-overlapping focus
planes of far field optics and near field optics), essentially an
acceptable intensity distribution in the focus zone can be given,
because the intensity profile, which impinges onto the near field
optics, changes only a little.
[0168] For example, in the case of an inverse quasi-Bessel beam
shape, the first focusing by the far field optics within the
optical system causes an adaptation of the ring size on the near
field optics. In that manner, the far field optics has a focusing
action onto the ring diameter, which, as indicated in the figures,
decreases up to some type of intermediate focus.
[0169] FIG. 21 illustrates the beam path in an optical system for
the case that a convergent laser beam 3' impinges on beam shaping
element 31. Phase-modulated portion 303A of the laser beam is
focused onto elongated focus zone 7. Due to the convergence of
incident laser beam 3' (and eventually due to a separate focusing
far field optics or an integration into the phase distribution of
beam shaping element 31), the non-phase-modulated portion 303B
(dash dotted line) decreases further during the propagation length
Dp and impinges on a central area of near field optics 33B.
Thereby, a focus 350 for non-phase-modulated beam portion 303B is
formed that is closer to near field lens 33B than it is elongated
focus zone 7. The non-phase-modulated portion is strongly divergent
after focus 350, so that those intensities are no longer reached
within the workpiece with respect to the non-phase-modulated beam
portion 303B that result in nonlinear absorption. In such a
configuration, one can do without filtering non-phase-modulated
beam portions 303B.
[0170] Nevertheless, a spatially localized filter unit can be
provided in the area of focus 350 (or even between far field optics
and near field optics, if the beam is strongly focused) such that
non-phase-modulated beam portion 303B is kept out of the
interaction zone and the workpiece.
[0171] FIG. 22 shows an optical system that is equipped with an
additional lens 400 upstream of beam shaping element 31. Lens
400--as an example of an additional focusing component--is located
at a distance DA to beam shaping element 31.
[0172] Beam shaping element 31 has a phase distribution that is set
for a specific beam diameter. The illuminated part of that beam
shaping element, i.e. the beam diameter of the input intensity
profile at beam shaping element 31, can be adapted by the
translatability of lens 400 with respect to beam shaping unit
31.
[0173] In some embodiments, lens 400 can be compensated before beam
shaping element 31 within the phase mask of beam shaping element 31
so that the imaging does not change and only the 0th order, i.e.
the non-phase-modulated, portion is focused.
[0174] In general, lens 400 can also be understood as a component
of the far field optics. If the far field optics includes a
plurality of components, which can be translated with respect to
each other and with respect to the near field optics, then the
imaging scale can be changed by a suitable translation. In some
embodiments, lens 400, the beam shaping element, or both can be
translated together to adjust the imaging scale of optical system
1. In some embodiments, lens 400 can be used as a first
telescope-part-lens for adapting the beam diameter on the beam
shaping element, whereby a second telescope-part-lens is calculated
into the phase mask.
[0175] In some embodiments, lens 400 can be translated to perform a
fine adjustment of the raw beam in particular for a longitudinal
flat top beam shape or multi-spot formation.
[0176] If the input beam is selected such that a convergent or
divergent beam is present at beam shaping element 31, then one
can--also in this case in accordance with FIG. 21 under certain
conditions--do not use a filter unit for non-phase-modulated beam
portion 403B. I.e., intensities for the nonlinear absorption within
the workpiece are only reached by the phase-modulated beam portion
403A.
[0177] Diffractive optical elements allow a digitalized and e.g.
pixel based phase adaptation over the input intensity profile.
Starting from the intensity distribution of an inverse quasi-Bessel
beam shape, a longitudinal flat top intensity profile can, for
example, be generated in focus zone 7. For that purpose, the phase
distribution within the beam shaping element can be influenced such
that intensity contributions in the output intensity profile are
taken out of the area, which forms the intensity maximum and the
tails of the Bessel beam, and are radially redistributed by a phase
change such that, for the later focusing by near field optics 33B,
the increasing area 61A and the decreasing area 61B are magnified
or far extending tails are avoided to the most part (e.g. by
pushing power from the tails into the homogenized area).
[0178] A respective output intensity profile 551 is shown in FIG.
23 (intensity cross-section) and FIG. 24 (2D-top view). One
recognizes in the intensity cross-section of FIG. 23 that--in
comparison to FIG. 15--the local maximum is broadened in the radial
direction and modulated. The result is a respectively radially
extended modulated ring structure 549.
[0179] FIG. 25 shows the focusing of such an output intensity
distribution 551. The result is a longitudinal quasi-homogenized
intensity distribution (flat top) 561 over a range from about 700
.mu.m in Z direction.
[0180] In analogy to FIG. 6, FIG. 26 shows modification zones 565
(modifications) in a transparent material 9. The upper four images
illustrate again the threshold behavior for pulse group energies Eg
from about 20 .mu.J to 40 .mu.J, while the lower four images show
increasing pulse group energies Eg from about 30 .mu.J to 200
.mu.J. One recognizes that, when the threshold is passed, the
modification zones form essentially always over the same range of
extent in Z direction within workpiece 9. This is based on the
essentially constant intensity having only a short increase and
drop off. With increasing energy, however, not only the strength
but also the lateral extent of the modification zones
increases.
[0181] Another embodiment is shown in FIG. 27, which allows
reaching a sequence of intensity enhancement in propagation
direction. In general, supplemental phase imposing can be done in
the area of the image side focal plane of near field optics 33B
such as lateral and/or longitudinal multi-spot phase imposing.
Specifically, one recognizes in FIG. 27 a sequence of three
intensity maxima 661A, 661B, and 661C, which each have an intensity
distribution in accordance with FIG. 4.
[0182] This sequence can be generated by a longitudinal multi-spot
phase imposing or the use of a multi-focal lens as near field
optics 33B. So, for example, an additional diffractive optical
element may be provided in the area of the Fourier plane (focal
plane of near field optics 33B) or close to near field optics 33B,
which provides an additional phase-modulation for the three foci.
Such phase adaptations are known, for example, from EP 1 212 166
B1, incorporated herein by reference in its entirety.
[0183] In connection with FIGS. 28 to 31, a further potential
formation of an elongated focus zone is illustrated for the case of
an accelerated Airy beam shape.
[0184] FIG. 28 shows a phase distribution 743 as it can be imposed
within beam shaping element 31 onto the input intensity profile.
Here, face distribution 743 includes the phase distribution, which
is required for a generation of the accelerated beam, and the phase
distribution of a concave lens, which compensates a raw beam
convergence. In general, a phase mask of an accelerated beam
creates a well collimated beam which does not change significantly
over the propagation distance and which is then focused with the
near field component in a so-called accelerated beam shape.
[0185] FIGS. 29 and 30 illustrate the associated output intensity
profile 751 in the cut view (FIG. 29) and in the top view (FIG.
30). One recognizes that the intensity maximum is displaced
slightly from the center (i.e. beside the beam axis 45) in Y
direction. Thus, the transverse output intensity profile 751 is
modified with respect to the input intensity profile with a local
maximum 749, which is located outside of beam axis 45.
[0186] The focusing of such an output intensity profile 751 results
in elongated and curved focus zone 707 that is illustrated in FIG.
31. Thereby it is allowed that such an accelerated beam pro-file
can be used also in combination with non-transparent media, if the
focus zone is guided, for example, in Y direction to the border of
such a material. The resulting interaction would, for example,
provide a rounding of the side of the material. In other
embodiments, such a beam profile can be used with transparent
materials for cutting with curved cutting faces.
[0187] In some embodiments, an optical system is configured, for
example, such that a real intensity enhancement in accordance with
FIG. 7 as well as a virtual intensity enhancement in accordance
with FIG. 3 is created. Thereby, the longitudinal extent of
modification zones can be widened.
[0188] FIG. 32 shows schematically an exemplary optical system with
a binary DOE 31C. If a laser beam 3 falls onto binary DOE 31C, on
the one hand, a real intensity enhancement 871 is formed, for
example, a quasi-Bessel beam downstream of DOE 31C. On the other
hand, a beam portion is formed, which is associated with a virtual
image 853-- downstream of DOE 31C-- of an elongated focus zone
807A, for example, in the shape of an inverse quasi-Bessel
beam.
[0189] The optical system includes further a telescope system 833
with a far field optics 833A and a near field optics 833B.
Telescope system 833 images virtual image 853 as well as real
intensity enhancement 871 into the material 9 to be processed. For
that purpose, binary DOE 31C is positioned in or close to the focal
plane of far field optics 833A.
[0190] The imaging results in an enlarged interaction region that
includes elongated focus zone 807A and focus zone 807B that
originates from the real intensity enhancement 871. In the
resulting sequence of successive focus zones 807A and 807B, the
intensity for (inverse) quasi-Bessel beams is at first in
accordance with the intensity distribution shown in FIG. 4 and
there-after in accordance with the intensity distribution shown in
FIG. 8. The result is an intensity distribution with a low
intensity intermediate space that is formed by the strong intensity
drop 61B and the strong intensity raise 81A. That low intensity
intermediate space can, for example, be provided in the region of a
contact zone when processing a pair of on each other lying
workpieces. In addition, this approach allows that one can achieve
twice the length for the interaction for identical input beam
diameter and identical angular range, which is covered by the
optical system.
[0191] In some embodiments, the non-phase-modulated portion can be
focused in the area between the successive focus zones 807A and
807B. A respective Gaussian focus 807C is additionally shown
schematically in FIG. 32. In such an embodiment, an adaptation of
the efficiency of the diffraction may become possible, because the
non-phase-modulated beam may be used for filling intensity
voids.
[0192] Herein, some aspects were described exemplarily based on
selected virtual beam profiles. In general, those aspects can be
transferred onto the herein as (inverse) virtual beam shapes
described types of beams such as inverse quasi-Bessel/Airy beam
shapes, e.g. inverse quasi-Bessel beam profiles or inverse
modulated or homogenized quasi-Bessel beam profiles.
[0193] In connection with FIGS. 33A to 33D and 34, the propagation
from the beam shaping element to the near field optics is explained
by beam profiles and amplitude courses for an in-verse quasi-Bessel
beam. Lighter grayscale values correspond to larger amplitudes. A
respective inverted quasi-Bessel beam can be generated with the
herein disclosed refractive, reflective, and diffractive optical
systems, for example, with the hollow cone axicon systems and the
DOE systems. A DOE system can be based, for example, on the phase
distribution of a phase mask shown in FIG. 14, in which a focusing
phase contribution is provided in addition to the phase required
for the inverse quasi-Bessel beam.
[0194] It is assumed that a laser beam having a rotationally
symmetric Gaussian beam profile is irradiated onto the beam shaping
element. A Gaussian beam profile includes a transverse amplitude
course that runs through the beam center in a Gaussian manner. The
FIGS. 33A, 33B, 33C, and 33D show respectively the development of
the beam profiles 900A, 900B, 900C, and 900D and the respective
schematic amplitude courses 902A, 902B, 902C, and 902D, the latter
directly after the beam shaping element at z=0 mm and at a distance
downstream at z=10 mm, z=50 mm as well as in the focal plane of the
successive near field component at z=200 mm. A transition of 100%
is assumed, i.e., one does not generate a stray radiation portion
e.g. in terms of non-phase-modulated or scattered light.
[0195] FIG. 34 shows the amplitude distribution for a step along
the beam axis Z beginning at the exit of the beam shaping element
at z=0 up to the near field lens at z=250 mm. The positions of the
beam profiles 900A, 900B, 900C, and 900D are indicated in FIG. 34
with arrows.
[0196] One recognizes that, due to the pure phase mask, a Gaussian
beam profile 900A and a Gaussian amplitude course 902A are still
present directly after the beam shaping element similar to the
Gaussian beam. A sharply limited hole is then immediately formed,
however, caused by the imposed phase, which yields the additional
divergence. Already at z=10 mm, one recognizes a clear dark spot
904 in the center of the beam profile 900B. The same is
continuously growing. At the same time, a ring area 906 with higher
amplitude is formed.
[0197] Ring area 906 is sharply limited towards the inside, which
can be seen at a step shape in the radial amplitude/intensity
distribution. A flank 907 of the circumferential step faces towards
that beam axis/towards the beam center. With increasing z values,
the opposing sections of flank 907 get separated, i.e. the central
sharply limited hole grows fast in diameter (D1<D2).
[0198] In the radial amplitude/intensity distribution, ring area
906 drops towards the outside with increasing z values faster and
faster. This development is schematically shown in the falling
flanks 908A to 908C of the amplitude courses 902A to 902C. In the
far field, i.e., for example in the overlapping focal planes of the
imposed focusing (far field) action and the near field optics, a
sharp ring 908D is formed within beam profile 900D, that thereafter
diverges (see FIG. 34). Thereby, now a sharp edge is performed at
the outer side, i.e., the step of the inner flank now faces towards
the outside.
[0199] In FIG. 34, one recognizes the sharp edge in the transition
between dark area 910A, which broadens in Z direction, and border
area 910B, which narrows in Z direction and is more bright, whereby
the grayscale values in brighter border area 910B at first have
higher values radially inside and then, beginning at the focal
plane, have higher values radially outside.
[0200] This general behavior of the beam profile and the amplitude
courses enable a test of an optical system with a Gaussian input
beam, for which at first a hole forms with a steep flank facing the
inside and thereby results in a local maximum outside of the beam
axis in the far field. An imaging of the beam profile from the
inner area as well as in the area of the focus zone can identify
the respective beam profile. The use of the optical system is
thereby not necessarily limited to Gaussian beams. In addition, it
is to note that the figures are a result of calculations for the
ideal case. For example, if a non-ideal DOE is used, the addressed
non-phase-modulated portion for higher orders or a portion of a
real quasi-Bessel beam (such as for a binary mask) can be on the
beam axis and can fill the "hole" with intensity.
[0201] An inverse quasi-Bessel beam can therefore comprise a step
with a steep flank in the amplitude course and accordingly in the
intensity distribution. The same can in particular face to the
in-side in the area close to the beam shaping element, for example,
in the area up to half of the far field, and in particular in the
area of a focus length of the far field optics downstream of the
beam shaping element. For a "simple" inverse quasi-Bessel beam
without base at the beam axis, the amplitude/intensity increases in
the range of the step from almost zero to the maximum of the
phase-modulated beam portion. Thereby, the formation of the step
(within the phase-modulated beam portion) is also given for an
exemplary incident beam having essentially a constant radial
intensity (radial flat top) across the beam shaping element,
because the step concerns essentially the beam center.
[0202] The beam characteristic described before upstream of the far
field focal plane is thereafter radially inverted up to the focus
zone. After that focus zone, it inverts radially another time such
that again a step shape can be formed at that position--without
interaction with a material to be processed. The beam profile can,
for example, be analyzed by taking the beam at a respective
position, be it within the optical system after the beam shaping
element or before or after the focus zone. In particular for
setups, which allow a blocking of a central disturbing beam, one
can analyze the intensity distribution of the phase-modulated beam
portion before or after the focus area.
[0203] In this context, it is further referred to the German patent
application filed by the same applicant at the same day that in
particular discusses possibilities of using DOEs when generating
inverse quasi-Bessel beam-like or inverse quasi-Airy beam shapes.
The content of that application is herein incorporated in its
completeness. As is explained therein generally, for example,
several steps can be formed when generating several (inverse)
quasi-Bessel beams, which in the case of the relation to a virtual
image can comprise, strongly pronounced flanks that show in the
respective longitudinal sections to the inside (before far field
focal plane and after focus zone) or to the outside (between far
field focal plane and focus zone).
[0204] Further embodiments and/or further developments of the
herein disclosed aspects are summarized in the following:
[0205] The transverse output profile can correspond to a far field
intensity profile of the virtual optical image and/or of a far
field intensity profile of the focus zone with respect to the near
field optics.
[0206] A given input beam shape of the laser beam can comprise the
transverse input intensity profile, a beam diameter, a transverse
input phase profile, and input divergence, and/or a polarization,
and the optical system can be configured such that the given input
beam shape is transformed into a convergent output beam shape at
the exit of the near field optics, whereby the near field of the
output beam shape forms the elongated focus zone.
[0207] The optical system can comprise a supplementing phase
imposing unit in the area of the image side focal plane of the near
field optics, in particular for the lateral and/or longitudinal
multi-spot phase imposing.
[0208] In general, the herein disclosed focusing elements such as
the far field optics and the near field optics can be configured
as, for example, lens, mirror, DOE, or a combination thereof.
[0209] Moreover, additional optical elements can be inserted into
optical systems. Among others, intermediate images can be inserted
in the imaging system, to realize, for example, a filter function
as well as a scan movement in the area of the image-side focal
plane. Thereby, e.g., the image-side focal plane (e.g. image plane
340 in FIG. 20) can itself be imaged by an additional optical
system. Alternatively or additionally, such optical intermediate
systems can allow, for example, an enlarged working distance and/or
a magnification of the working field for scanner application.
[0210] It is explicitly stated that all features disclosed in the
description and/or the claims are intended to be disclosed
separately and independently from each other for the purpose of
original disclosure as well as for the purpose of restricting the
claimed invention independently of the composition of the features
in the embodiments and/or the claims. It is explicitly stated that
all value ranges or indications of groups of entities disclose
every possible intermediate value or intermediate entity for the
purpose of original disclosure as well as for the purpose of
restricting the claimed invention, in particular as limits of value
ranges.
[0211] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. Accordingly, other embodiments are within
the scope of the following claims.
* * * * *