EP3814827A1

EP3814827A1 - Method, apparatus and system for generating a highly dynamic power density distribution of a laser beam

Info

Publication number: EP3814827A1
Application number: EP19741972.4A
Authority: EP
Inventors: Thomas WESTPHALEN
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2018-06-29
Filing date: 2019-06-27
Publication date: 2021-05-05
Also published as: WO2020002492A1; DE102018210698A1

Abstract

The invention relates to a method, an apparatus and a system for producing a highly dynamic power density distribution of a laser beam, for example of a laser beam in a laser processing apparatus. In the method according to the invention for adapting the power density distribution of laser radiation, the laser radiation is guided along a fibre and emerges at a fibre end (F1). In the object space, i.e., the space where the fibre end is situated, the laser beam is deflected by means of a micro-optical system component (F2, F3, F4, F5) in a plane. A micro-scanner according to the invention for adapting the power density distribution of laser radiation guided along a fibre and emerging at a fibre end (F1) is characterized in that the micro-scanner comprises a micro-optical system component (F2, F3, F4, F5) for deflecting the laser beam in the object space in a plane. A system according to the invention for adapting the power density distribution of laser radiation guided along a fibre and emerging at a fibre end (F1) is characterized in that the system comprises an above-described apparatus and a macro scanner.

Description

Method, device and system for generating a highly dynamic power density distribution of a laser beam

Description:

The invention relates to a method, a device and a system for generating a highly dynamic power density distribution of a laser beam, for example a laser beam in a laser processing device.

Lasers have been used for some time to machine workpieces made of metal, for example. Such machining processes include, for example, drilling, milling, polishing, engraving, etc. Such

In most cases, laser machining processes require a very specific power density distribution of the laser beam on the workpiece to be machined. For example, laser polishing requires a notched, homogeneous one

Rectangular distribution, while laser drilling a circular, homogeneous

Distribution requires. The required distributions are typically determined by a static, i.e. Beam shaping of the raw laser beam realized. The static

However, the beam profile leads to limitations in the laser process. For example, the laser polishing process with the static, asymmetrical beam profile can only be done in one direction, e.g. from left to right. The diameter of a homogeneous beam profile is usually limited to a fixed dimension. This means that for different industrial laser processes either changeover times of a system have to be accepted or even different systems have to be used. There is also a greater degree of flexibility in R&D

Laser parameters such as a variable spot radius are an advantage.

The usual approach in laser material processing is to magnify the fiber end of an optical fiber onto the workpiece. The radiation is first collimated by a collimation lens and then focused on the workpiece via a deflection unit and an F-theta lens. The F-Theta lens is a lens system that is inserted into the beam path after the scan head. The lens focuses the laser beam on the focal point and, when scanning, ensures that this focal point is always in the working plane perpendicular to the optical axis of the lens.

The size of the image of the fiber core is determined by the fiber core diameter and the magnification, which result from the focal lengths of the collimator and the F-theta lens. The focal lengths are usually fixed, which means that the diameter of the beam profile is also fixed. If the beam diameter is to be changed, the imaging system must be modified. It is common to change the F-theta lens.

The shape and flomogeneity of the beam profile are defined by the spatial distribution of the power density at the exit of the fiber core. With basic mode fibers, the beam profile is Gaussian, with multimode fibers almost rectangular (top hat profile). In most cases there is either a Gaussian or a homogeneous, circular distribution. Various possibilities are known in the prior art for converting a Gaussian profile into a round, homogeneous top hat profile. For example, the Gaussian beam can be coupled into a homogenizing element such as a multimode fiber. At the exit of the

Multimode fiber then has an almost homogeneous power density distribution. However, this method does not allow a dynamic change between different beam profiles or diameters in the running process. The

Homogeneity of the beam profile can also be modeled by inserting an aperture with gradual transmission. However, this leads to performance losses and the change process cannot take place in the running process.

Special applications require power density distributions that cannot be produced by imaging the fiber end. For example, the notched rectangular distribution usually used in the laser polishing of metallic surfaces cannot cover the image of the fiber end

getting produced. This distribution can be realized by mapping an aperture. The aperture is illuminated at the point of an intermediate image with a homogeneous rectangular profile. A wedge-shaped part hides the radiation in a defined manner, so that the desired notched profile is created on the workpiece. By exchanging the panels, almost any beam profile can be realized. However, the aperture can not be changed during operation and the

hidden parts of the radiation are no longer available for the laser process.

The statically transformed homogeneous power density distributions are usually generated by means of multi-mode laser radiation. The advantages of basic mode laser radiation such as high beam quality and the associated lower divergence angle of the beam caustic, i.e. Lower taper of bores and a larger working distance can be achieved with the above-mentioned static

Beam shaping can not be used.

In more recent technical applications, the spatial-temporal variation of the laser spot is used, the laser spot being deflected by rapid movements of the galvanometer scanner mirror, the so-called wobble. The

However, the fiber output remains static. The wobble is used, for example, for uniform heating of soldering or joining points. Due to the large dimensions and masses of the galvo mirrors in relation to the object space, only frequencies from a few 100 Hz to approx. 1 kHz can be achieved. If a wobble function is superimposed on the uniform movement of a galvo mirror, this reduces the maximum performance of the galvo drive.

The object of the invention is to provide a device with which the beam profile of laser radiation in material processing processes, in particular in

comparatively sluggish, e.g. thermal material processing processes, is flexibly adaptable, the disadvantages described above with regard to dynamic change between different beam profiles or diameters in the running process, homogeneity of the beam profile and power losses to be avoided. In addition, it is an object of the invention to provide a corresponding system for workpiece machining using a laser beam.

According to the invention, this object is achieved by a micro scanner according to claim 1. Advantageous embodiments of the microscanner result from the Subclaims 2 to 10. Furthermore, the object is achieved by a system according to claim 11.

In the method according to the invention for adapting the power density distribution of a laser radiation, the laser radiation is guided over a fiber and emerges at one fiber end. In the object space, i.e. In the room in which the fiber end is located, the laser beam is deflected in one plane using a micro-optical system component. Micro-optical system components are understood here and below to mean components of classic optics, such as lenses, mirrors, prisms, etc., the dimensions of which are 10 mm or less.

A microscanner according to the invention for adapting the power density distribution of one which is guided over a fiber and emerges at one fiber end

Laser radiation, wherein the microscanner has a micro-optical system component for deflecting the laser beam in the object space in one plane, is characterized in that the micro-optical system has at least two micro-optical elements, wherein the micro-optical elements have at least one rotatable plate or at least one movable mirror or at least one

sliding microlens include.

In an advantageous embodiment, the laser beam is deflected in a plurality of planes in the object space. If, for example, the laser beam is deflected in two planes in the object space, the beam profile can be set in two dimensions. For example, the notched rectangular profile can be generated for laser polishing.

In a further advantageous embodiment, the fiber end

deflected micromechanically. As a result, the laser beam leaves the fiber at the same angle at which the fiber end is deflected. A microscanner has an im

Object-arranged micromechanical element for deflecting the

End of fiber. Here and below, a microscanner means a device for adapting the power density distribution of a laser radiation guided over a fiber and emerging at one fiber end, the device a micro-optical system component for deflecting the laser beam in the

Has object space in one plane.

For example, the weight of an approximately 5 mm long, free-standing

Singlemode fiber end (with a fiber core diameter of approx. 5 pm) approx. 0.5 g. If the fiber core is magnified on the workpiece, a Gaussian laser spot with a width of 100 pm is created on the workpiece. A

highly dynamic deflection s _{x of} the fiber end by +/- 10 pm leads to

comparatively sluggish, thermal laser processing processes to an almost homogeneous line distribution with an average beam diameter of ^d x = 500 pm. The fiber end can be, for example, by a piezo drive or

electromagnetic actuators similar to the coils in speakers are deflected.

In an advantageous embodiment, the micro-optical element has the

Diamond material. This means that even smaller dimensions of the optics can be realized. This is made possible above all by the high thermal conductivity of diamond. As a result, heat losses in the anti-reflective coating on the entry and exit facets are dissipated much better, which significantly increases the destruction threshold for the permissible power density. This is especially for

High-performance applications such as material processing are an advantage.

In a further, alternative or additional advantageous embodiment, the laser beam is deflected by means of a micro-optical system arranged in the object space. The micro-optical system contains at least two micro-optical elements, for example rotatable plates or movable mirrors. Instead of directly manipulating the fiber end, the optical axis can also be shifted in the image space, for example by rotating two plane-parallel plates directly behind the fiber or behind an intermediate image. The rotation can take place, for example, by means of galvometer drives.

In certain geometries, the fiber end is not directly accessible through other components such as a spliced quartz block. In a further advantageous embodiment, the fiber end can by means of of an imaging system are mapped as a first intermediate image between the rotatable plates. The micro-optical system thus has two plane-parallel plates that can rotate about themselves. The assignment between the rotation of the plates and the displacement of the optical axis is analogous to the arrangement mentioned above directly after the fiber end. However, since the intermediate image is very accessible, significantly smaller plane-parallel plates can be installed here, which increases the dynamics.

In a further exemplary embodiment, the microscanner has two microlenses which can be displaced perpendicular to the beam direction. The two microlenses displaceable perpendicular to the beam direction can be arranged behind the fiber end or an intermediate image. In addition, the microscanner can do two in the beam direction

have displaceable microlenses. A lens collimates the beam and creates an intermediate image, which passes through the collimator and an F-theta lens

Image space or on the workpiece. This can be done on several levels. If the fiber end is not accessible close enough, it can be imaged on an intermediate image using an imaging system. A shift of the lenses also leads to a shift of the intermediate image. Here, too, the intermediate image is much more accessible. Therefore, significantly smaller microlenses can be installed in this arrangement.

In a further advantageous embodiment, the microscanner has

Imaging system for imaging the fiber end as the first intermediate image.

The various exemplary embodiments can be combined with one another. For example, the fiber end can be deflected directly in one direction with a piezo drive. The beam deflection in another direction can

for example by rotating a plane-parallel plate or shifting a microlens.

A system according to the invention for adapting the power density distribution of a laser radiation guided over a fiber and emerging at one fiber end is characterized in that the system has a device as described above and a macro scanner. Macroscanners become conventional Understand beam deflection system such as a galvanometer or polygon scanner. The origin coordinates of the microscanner can be via the

Macro scanner can be approached. From there, the laser spot can experience a temporally highly dynamic meandering deflection in different planes through the device.

The beam deflection of the microscanner can be performed without loss of performance

Macro scanners can be switched into the beam path and changed.

Further advantages, special features and expedient developments of the invention result from the subclaims and the following illustration of preferred exemplary embodiments with the aid of the figures.

The illustrations show:

Fig. 1 Schematic representation of the power distribution homogenized by the microscanner

2 shows a schematic representation of a) the notched rectangular distribution and b) the distribution scanned by means of a microscanner

Fig. 3 Schematic representation of the direct deflection of the fiber end

Fig. 4 Schematic representation of the direct deflection of the fiber end in the x direction by a piezo drive

Fig. 5 Schematic representation of the direct deflection of the fiber end in the z direction by a piezo drive

Fig. 6 Schematic representation of the effect of the direct deflection of the

Fiber end in the z direction through a piezo drive

Fig. 7 Schematic representation of a 2D microscanner using rotatable plane-parallel plates

Fig. 8 Schematic representation of the manipulation of the optical axis in

Object space through two rotatable plane-parallel plates in front of the fiber end Fig. 9 Schematic representation of the manipulation of the optical axis in

Object space through two rotatable plane-parallel plates arranged around the intermediate focus

10 Schematic representation of a 2D microscanner using displaceable microlenses in two embodiments

Fig. 11 Schematic representation of a 2-D microscanner with two, before

Microlenses arranged at the fiber end

Fig. 12 is a schematic detailed representation of a 2D microscanner, in which two

Microlenses L2 and L3 are arranged in front of the intermediate image L7.

13 shows a schematic detailed illustration of a 2D microscanner, in which two

Microlenses L2 and L3 are arranged in front of the intermediate image L7.

14 shows a schematic illustration of a laser drilling process

1 shows a schematic representation of the power distribution homogenized by the microscanner (1). A Gauss-to-Tophat transformation is shown as an example. In Fig. 1 a) it is shown that the center of a Gaussian laser spot of a fiber laser by a conventional

Macro scanners, such as a 2D galvo scanner, on the

Workpiece coordinates (xo; yo) is mapped. The beam diameter is d _x . 1 b) shows how the microscanner (1) additionally deflects the center point in the x direction by the amount s' _x (t). The beam deflection takes place so quickly that for comparatively slow ones, for example thermal ones

Material processing processes only the time-averaged beam diameter <is effective (see Fig. 1 c). In other words, the

Laser spot in the x-direction widened or smeared into an approximately top-hat-shaped profile with the width. The homogeneity of the beam profile averaged over time can be controlled by the function s' _x (t).

It is remarkable that the beam always has its (basic mode)

Maintains beam quality.

2 shows a schematic representation of a) the notched rectangular distribution and b) the power distribution scanned by means of microscanner (1). The Origin coordinates of the microscanner (1) (xo; yo) are approached via the macroscanner. From there, the laser spot experiences a temporally highly dynamic meandering deflection in the x and y directions. The notched

Rectangle distribution is "painted". The deflection in the y direction s' _y (t) takes place analogously to that in the x direction, so that the beam profile can be set in two dimensions. For example, the notched rectangle profile for the

Laser polishing processing can be generated.

Fig. 3 shows a schematic representation of the direct deflection of the fiber end F1. The fiber end F1 is here, for example, by a piezo drive F2 (FIG. 3 a)) or a first electromagnetic actuator F4 and a second

electromagnetic actuator F5 (Fig. 3b)) similar to the coils in speakers

FIG. 4 is a schematic representation of the direct deflection of the fiber end F1 in the x direction by a piezo drive F2. 4 a) shows a

Embodiment in which the fiber end F1 is attached to a piezo element F2. In the situation shown in FIG. 4 b) it is shown how the piezo element is deflected by the amount a _x . This raises the optical axis by the distance s _x = a _x . The microscopic deflection s _x (t) into a macroscopic deflection s' _x (t) of the beam in the image space, for example on the workpiece, is effected via the magnification defined, for example, by the focal lengths of the collimator and the F-theta objective.

5 is a schematic representation of the direct deflection of the fiber end F1 in the z direction by a piezo drive F2. 5 a) shows one

Embodiment in which the fiber end F1 is attached to a piezo element F2. In the situation shown in FIG. 5 b) it is shown how the piezo element is deflected by the amount a _z . This raises the optical axis by the distance s _z = a _z . As before, the magnification defined by the focal lengths of the collimator and the F-theta lens causes the microscopic deflection s _z (t) into a macroscopic deflection s' _z (t) of the beam in the image space, for example on the workpiece. 6 is a schematic representation of the effect of the direct deflection of the fiber end F1 in the z direction by a piezo drive F2. 6 a) shows how the radiation without a piezo element F2 is imaged onto the focus F 7 by the focusing lens F6, for example an F-theta lens. FIG. 6 b) shows how the radiation with the piezo element F2 is imaged by the focusing lens F6 onto the focus F 7. Finally, FIG. 6 c) shows how the image plane shifts by the amount s' _z when the fiber end F1 is deflected by the distance s _z by the piezo drive F2.

7 shows a schematic illustration of a 2D microscanner (1) by means of a first rotatable plane-parallel plate P2 and a second rotatable plane-parallel plate P3. Instead of the direct manipulation of the fiber end F1 as in the previous exemplary embodiments, a shift of the optical axis in the image space can also be achieved by rotating two plane-parallel plates P2 and P3 directly behind the fiber end F1 (FIG. 7 a)) or behind a first intermediate image P6 (Fig. 7 b)) take place. Is the fiber end F1 by other components like

For example, if a spliced-on quartz block is not directly accessible, the fiber end F1 can be imaged as an intermediate image P6 between the rotatable plates P2 and P3 by means of an imaging system P5. The rotation of the two plane-parallel plates P2 and P3 can, for example, be carried out by galvometer drives in a manner similar to that of a conventional macro scanner.

8 a) shows schematically the manipulation of the optical axis in the object space by two rotatable plane-parallel plates P2, P3 in front of the fiber end F1. 8b) shows how the optical axis is shifted in the x direction by the amount s _x . 8 c) shows how the optical axis is shifted in the y direction by the amount s _y when the second plane-parallel plate P3 is rotated by the angle a _y . s _x and s _y are transformed via the collimator P4 and an F-theta lens F6 into the deflections s ' _x and s' _y in the image space on the workpiece.

9 a) shows a schematic representation of the manipulation of the optical axis in the object space by two rotatable plane-parallel plates P2, P3 arranged around the first intermediate focus P6. 9 b) shows how the optical axis is shifted in the x-direction by the amount s _x when the first plane-parallel Plate P2 is rotated by the angle a _x . If the second plane-parallel plate P3 is rotated by the angle a _y , the optical axis is displaced in the y direction by the amount s _y . The assignment between rotation a _x , a _{y of} the first and second plane-parallel plates P2, P3 and the resulting displacement of the optical axis is analogous to the above-mentioned arrangement directly after the fiber end F1. However, since the first intermediate image P6 is very accessible, significantly smaller plane-parallel plates P2, P3 can be installed, which increases the dynamics. 9c) shows how the optical axis is shifted in the y direction by the amount s _y when the plate P3 is deflected by the angle a _y .

Fig. 10 is the schematic representation of a 2D microscanner (1) with

Slidable microlenses L2, L3 in two embodiments. In the embodiment shown in FIG. 10 a), the displaceable microlenses L2, L3 are arranged in front of the fiber end F1. The displaceable microlenses L2, L3 generate an intermediate focus L4, which passes through the collimator P4 into or onto the image space

Workpiece is mapped. The embodiment shown in FIG. 10 b) has an imaging system P5 that generates a first intermediate focus P6. The

Microlenses L2 and L3 map the intermediate focus to the second intermediate focus L4. The beam is then collimated using the collimator P4. This

Embodiment is particularly advantageous when the fiber end F1 is not accessible close enough. A shift of the microlenses L2, L3 also leads to a shift of the second intermediate image L4. Here, too, the first intermediate image P6 is clearly accessible. Therefore, significantly smaller microlenses L2, L3 can be installed in this arrangement.

11 a) is a schematic illustration of a 2-D microscanner (1) with two microlenses L2, L3 arranged in front of the fiber end F1. FIG. 11 b) shows how the second intermediate image L4 is shifted in the x direction by the amount s _x when the first microlens L2 is deflected by the distance l _x . FIG. 11 c) shows how the second intermediate image L4 is shifted in the y direction by the amount s _y when the second microlens L3 is deflected by the distance l _y .

FIG. 12 shows in FIG. 12a) a schematic detailed illustration of a 2D microscanner, in which two microlenses L2 and L3 are arranged in front of the intermediate image L7 are. In FIG. 12 b) it can be seen how the intermediate image L4 is shifted in the x direction by the amount s _x when the lens L2 is deflected by the distance l _x . 12c) shows how the intermediate image L4 is shifted in the y direction by the amount s _y when the lens L3 is deflected by the distance l _y .

FIG. 13 shows a schematic detailed illustration of a 2D microscanner in FIG. 13a), in which two microlenses L2 and L3 are arranged in front of the intermediate image L7. 13b) shows how the intermediate image L4 is shifted in the z direction by the amount s _z when the lens L3 is deflected by the distance l _z .

14 shows a schematic illustration of a laser drilling process. The result of the drilling with a homogeneous beam profile of a multi-mode laser beam can be seen in FIG. 14 a). 14b) shows the result of a bore with a beam profile of a basic mode laser beam homogenized by highly dynamic deflection. The divergence angle q ^' _c specifies the taper of the hole. In the case of the homogenized basic mode beam profile, the high beam quality of the laser is retained. Here is the

Divergence angle q ^' _c and thus the conicity of the bore is significantly lower.

The embodiments shown here are only examples of the present invention and should therefore not be understood as restrictive. Alternative embodiments contemplated by those skilled in the art are

equally within the scope of the present invention.

LIST OF REFERENCE NUMBERS

1 microscanner

F1 fiber end

F2 piezo element, piezo drive

F4 first electromagnetic actuator

F5 second electromagnetic actuator

F6 focusing lens, F-theta lens

F 7 focus

L2 first displaceable microlens

L3 second sliding microlens

L4 second intermediate image, second intermediate focus P2 first plane-parallel plate

P3 second plane-parallel plate

P4 collimator

P5 imaging system

P6 first intermediate image, first intermediate focus a _x deflection of the fiber end in the x direction

I _x deflection of the first microlens in the x direction l _y deflection of the second microlens in the y direction s _x microscopic beam deflection in the x direction s ' _x macroscopic beam deflection in the x direction s _y microscopic beam deflection in the y direction s' _y macroscopic beam deflection in y-direction a _x rotation of the first plane-parallel plate a _y rotation of the second plane-parallel plate q ^' _c angle of divergence

Claims

claims:

1. microscanner (1) for adjusting the power density distribution of a laser beam guided over a fiber and emerging at a fiber end (F1), the microscanner (1) being a micro-optical system component (F2, F3, F4, F5, L2, L3, P2, P3 ) for deflecting the laser beam in the object space in one plane, the microscanner (1) having a micro-optical system arranged in the object space for deflecting the laser beam,

characterized,

that the micro-optical system has at least two micro-optical elements, the micro-optical elements at least one rotatable plate or

at least one movable mirror or at least one displaceable

Include microlens.

2. microscanner (1) according to claim 1,

characterized,

that the microscanner (1) has a plurality of micro-optical system components (F2, F3, F4, F5, L2, L3, P2, P3) for deflecting the laser beam in the object space in a plurality of planes.

3. microscanner (1) according to one of the preceding claims,

characterized,

that the microscanner (1) has a micromechanical element arranged in the object space for deflecting the fiber end (F1).

4. microscanner (1) according to claim 3,

characterized,

that the micromechanical element has a piezo drive (F2).

5. microscanner according to one of claims 3 or 4,

characterized,

that the micromechanical element has an electromagnetic actuator (F4, F5).

6. microscanner (1) according to one of the preceding claims,

characterized,

that the micro-optical element is made of diamond.

7. microscanner (1) according to one of the preceding claims,

characterized,

that the micro-optical system has two plane-parallel plates (P2, P3) which can rotate about themselves.

8. microscanner (1) according to one of the preceding claims,

characterized,

that the micro-optical system has two microlenses (L2, L3) that can be displaced perpendicular to the beam direction.

9. microscanner (1) according to claim 8,

characterized,

that the micro-optical system has two additional microlenses (L2, L3) that can be moved in the beam direction.

10. microscanner (1) according to one of the preceding claims,

characterized,

that the microscanner (1) has an imaging system (PS) for imaging the fiber end as the first intermediate image (P6).

11. System for adapting the power density distribution of a laser radiation guided over a fiber and emerging at one fiber end (F1),

characterized,

that the system has a microscanner (1) according to one of the preceding claims and a macroscanner.