Beam shaping systems in laser welding processes

Abstract

The beam shaper for converting the MM beam with a flat-top intensity distribution profile includes an end block fused to the downstream end of the fiber that outputs the MM beam along a path within the laser head. The beam shaper further includes a collimator mounted on the laser head downstream from the end block. The collimated MM beam is then focused into a processing zone with a beam waist characterized by a Gaussian intensity profile. The Gaussian region can be provided near the beam waist by positioning the collimator so that the Gaussian region of the MM flat-top beam is located at the focal plane of the collimator within the end block. Alternatively, the Gaussian region can be provided within the waist using a diffractive optical element that converts the flat-top distribution profile into a donut-shaped profile.

Description

[Technical Field]

[0001] The present disclosure relates to laser material processing applications. In particular, the present disclosure relates to beam shaping systems integrated into industrial lasers. [Background technology]

[0002] Beam shaping is the process of redistributing the irradiance and phase of beam optical radiation. The shape of the beam is the primary factor in determining the propagation characteristics of the beam profile. Applications of beam shaping include, among others, metal processing applications that were previously performed using traditional high-flux heat sources such as reactive gas jets, electrical discharges, and plasma arcs. In laser welding, two adjacent or stacked metal pieces are fused together by melting the parts at the weld seam.

[0003] There are three basic welding modes, corresponding to the level of peak power density contained within the focal spot size: conduction mode, transition keyhole mode, and penetration or keyhole mode. Each mode has its advantages and disadvantages. For example, in keyhole mode, it is highly desirable for the keyhole to have a consistent width and depth along the weld area. However, in practice, a uniform keyhole is virtually impossible to create due to the so-called keyhole collapse phenomenon, which is well known to those skilled in the art. Another detrimental characteristic of the keyhole process is the formation of pits and cracks. Overall, the keyhole process is unstable. Conduction mode is known for its stability due to minimal evaporation. However, due to the relatively low level of power, the weld penetration is significantly smaller than that of the keyhole process. To achieve the desired results, it is necessary to create a very large heat-affected zone, which results in a large heat input and, ultimately, deformation of the workpiece. In each mode, the melt pool characteristics depend on laser parameters, including energy, fluence, and spot size, among others.

[0004] The beam shape is determined by the irradiance distribution of the shaped beam, as is well known to those skilled in the art. The radiation (also referred to as intensity or power density) of a single-mode (SM) beam is mathematically described by a Gaussian function, and thus has a bell-shaped shape. Many applications can only benefit from a Gaussian beam, but as is well known, the power output of an individual SM laser can be insufficiently small to process / weld certain materials.

[0005] To overcome this problem, the multiple SM outputs from each laser are combined into a single beam with more than a single mode, and hence further referred to as a multimode (MM) beam. M, a measure of the number of modes, typically ranges from 2 to 10, or even 20. 2 An MM output beam with a factor may be referred to as a low-mode (LM) beam. However, since both MM and LM beams each have more than one mode, within the context of this disclosure, a flat-top laser beam is one with an M factor ranging from 2 to 20. 2 factor and are still referred to as MM beams.

[0006] Due to modal pulsation along the optical path including the MM delivery fiber, the resulting intensity profile of the MM beam at the downstream end of the fiber has a flat-top shape. The flat-top intensity profile of an MM beam is advantageous for many material laser processing operations due to the substantially uniform distribution of intensity across the beam at the focal plane.

[0007] As the MM beam propagates further along a path that includes various optical elements, such as a focusing lens, it has multiple beam regions, including a beam waist formed at the focal plane of the focusing lens. The waist is the narrowest beam region and therefore has the highest power density along the beam. While the beam waist is characterized by the same flat-top intensity distribution, beam regions positioned before the waist have their own intensity profiles that may differ from the flat-top shape. One of these pre-waist beam regions, spaced a large distance from the waist, is characterized by a quasi-Gaussian intensity profile. The beam region where the beam attains the quasi-Gaussian intensity is further referred to as the (upper) Gaussian region. The propagating beam is symmetrical about the waist. Therefore, a second Gaussian region is spaced downstream from the waist at the same distance as the distance between the upper region and the waist.

[0008] Those skilled in the art of laser-based material processing are well aware that Gaussian beams are associated with high-quality welds. Due to their bell-shaped intensity distribution profile, the intensity is not uniformly distributed across the beam spot with the highest intensity at the central apex region gradually shrinking toward the base boundary. This profile creates a smooth temperature gradient across the surface being laser-treated, first by the leading wings, then by the intensity peak, and finally by the trailing wings, gradually cooling the irradiated area. Such thermodynamics are attractive for a significant number of material processing methods. Regardless of shape, the laser beam is delivered to the weld area through a laser head, which is the most downstream component of an industrial laser system.

[0009] FIG. 1 shows an exemplary laser head 25 typically mounted on a robotic arm. The laser head 25 encloses a beam guidance schematic that steers the MM flat-top laser beam 10 after the delivery fiber 22 outputs it to the laser head. The optical schematic includes an endblock 15 fused to the downstream end of the delivery fiber 22, which receives the combined beam 10 from a combiner that combines the output from each SM laser source. As known to those skilled in the laser art, the endblock 15 is typically made of quartz and is configured to protect the fiber end 22 from burning, which is unavoidable at industrial laser power levels ranging from several hundred watts to several hundred megawatts. The beam 10 diverges while propagating through the endblock 15 before impinging on a collimating lens or collimator 1. The collimator 1 is an optical element that transforms the diverging beam 10 emanating from the downstream fiber end 22 into a beam of parallel rays. The downstream fiber end 22 is therefore positioned in focus, ie spaced from the collimator 1 at a distance equal to the focal length F1 of the collimator.

[0010] A focusing lens 6 with focal length F2 focuses the collimated beam 10 onto a surface 12, thus forming a beam waist with a flap top intensity profile. A Gaussian region 14 of the focused beam is spaced from the beam waist.

[0011] One of the important factors related to beam divergence is the depth of field (DOF), which is closely related to the so-called process window. In the context of material processing, DOF is the distance that a laser-processed workpiece can be moved away from the center of the beam waist while still maintaining the focused beam size. More specifically, DOF can be defined as the Rayleigh range, which is well known to those skilled in the art of optics. In the scheme disclosed above, the maximum Rayleigh range is at the beam waist. The Rayleigh range in the Gaussian region 14 is much smaller than the Rayleigh range at the waist. A small DOF is disadvantageous in laser-based material processing applications for reasons that will be explained later.

[0012] To operate in the Gaussian region 14, the beam 10 must be defocused. This can be achieved by displacing the focusing lens 6 and the surface 12 relative to each other. Nevertheless, the defocusing results may not be acceptable because the light spot formed on the surface by each region 14 is large and may have insufficient energy. For example, if the light spot is changed by a defocused beam 10 by more than 10%, the power density decreases rapidly because density and spot size are quadratically related to each other. Even if the power density is sufficient, the DOF in the Gaussian region 14 is small. This means that both part tolerances (workpieces being welded are often not ideally uniform) and / or errors caused by robot motion can significantly affect the quality of the weld. Therefore, robot motion during welding using the Gaussian region 14 of the beam 10 is very difficult to control, which results in sophisticated software that is understood to result in high manufacturing costs.

[0013] It would be highly advantageous to configure a laser welding apparatus with a beam shaping system that can transform the beam 10 so that the Gaussian region 14 is located within the beam waist. Such a beam shaping system provides an extended DOF, which minimizes the detrimental effects of robotic motion errors and increased energy. The extended DOF also helps minimize damage to the workpiece, which is expensive but not always uniform. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] U.S. Patent Application Publication No. 20160368089 [Patent Document 2] U.S. Patent Application Publication No. 20180369964 Summary of the Invention [Problem to be solved by the invention]

[0015] It is therefore desirable to provide a beam shaping system for industrial laser-based robotic welding equipment that is configured to form a waist of a flat-top MM beam characterized by a Gaussian intensity distribution at the surface of a workpiece being laser treated.

[0016] Still other needs exist for laser-based material processing that incorporates improved beam shaping systems. [Means for solving the problem]

[0017] The disclosed apparatus is configured to address at least some of the concerns discussed above. The apparatus generally includes a laser source, preferably a fiber laser or a YAG source, having a M range of 2 to 20. 2The MM laser beam may include multiple SM continuous wave (CW), quasi-CW, or pulsed lasers that output an MM laser beam, with a factor of 100 and a maximum power output of 20 kW, although greater power outputs are clearly possible. The MM flat-top laser beam is mounted to the laser head and guided along a delivery fiber fused to a quartz block configured to prevent burning of the end of the fiber. Upon expanding in the quartz block, the flat-top laser beam is guided along a path through the laser head by guidance optics, which may include, among other things, a collimator and focusing lenses. Preferably, but not necessarily, a scanner comprising several movable mirrors is also mounted to the laser head, as disclosed in detail in U.S. Patent Nos. 5,929,999 and 5,929,999, both of which are incorporated herein by reference in their entireties.

[0018] The laser head is provided with a beam shaping system of the present invention configured to convert a laser beam with a flat-top intensity distribution into a Gaussian intensity distribution profile. In contrast to the known prior art shown in Figure 1, where the Gaussian region is located outside and away from the beam waist, the present invention provides for locating the Gaussian region either in the immediate vicinity of the waist or exactly within the waist.

[0019] According to one disclosed general aspect, this is accomplished by providing the beam shaping system with additional diffractive elements, such as axicons, homogenizers, and others. Unlike converging lenses, which are designed to focus a light source to a single point on the optical axis, axicon lenses use interference to create a focal line along the optical axis. Within the beam overlap region, called the DOF, axicons replicate the properties of Bessel beams, which are beams composed of rings of equal power.

[0020] Bessel beams can be mathematically represented by Bessel functions, with planar cross-section intensity profiles that, not surprisingly, comprise a set of concentric rings in the focal plane. For example, for a zero-order beam, the Bessel beam intensity profile has a donut-shaped planar cross-section characterized by relatively low energy, while for first orders, the planar cross-section has a light spot exactly at the center.

[0021] The use of such a diffractive element makes it possible to recreate a region of the MM beam with Gaussian intensity very close to the beam waist without displacing the focusing lens. In other words, this scheme effectively creates a Gaussian region adjacent to and even within the waist. Recreating a Gaussian region next to and effectively within the waist increases the DOF and increases the energy compared to the prior art scheme of FIG. 1.

[0022] In the previously disclosed scheme, the downstream end of the delivery fiber and the collimator are spaced apart from each other by the focal length of the collimator. Without an additional diffractive optical element, the Gaussian region would be located far away from the waist, as discussed with reference to the prior art in FIG.

[0023] Yet another embodiment of the present invention does not involve an additional diffractive element. In contrast to the optical schematic disclosed previously, the collimator is spaced at a focal distance from the Gaussian region of the top-hat beam, rather than from the downstream end of the MM delivery fiber. Thus, the beam waist now contains a light spot with a Gaussian profile at the target plane exactly within the beam waist, instead of a flat-top intensity profile.

[0024] Both of the discussed embodiments are applicable to step-index MM fibers. However, the second embodiment of the invention outlines graded fibers. Graded fibers do not use total internal reflection to guide light. Instead, they use refraction. The refractive index of the fiber gradually decreases with distance from its center, eventually decreasing to the same value as the cladding at the edge of the graded-index core. It is possible to establish relative positions between the fiber end, collimator, and focusing lens such that the beam waist formed at the treated surface is characterized by an approximately Gaussian intensity distribution profile associated with increased energy.

[0025] These and other features will become more apparent with reference to the accompanying drawings, which are not drawn to scale. The drawings provide an illustration and further understanding of various aspects and features and constitute a part of this specification, but do not represent any particular outline or limitation of the aspects. In the drawings, each identical or nearly identical component appearing in various figures is designated by a like reference numeral. For purposes of clarity, not all components may be labeled in every figure. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is an optical schematic diagram of a typical known laser head configured to steer a flat-top MM beam to a target. [Figure 2] FIG. 1 is a diagram of a laser head of the present invention configured to process a laser treated workpiece with one of the Gaussian regions of an MM beam according to the concepts of the present invention. [Figure 3] FIG. 3 is an optical schematic diagram of the laser head of the present invention of FIG. 2. [Figure 4] 4 is a diagram of the beam downstream from the focusing lens of the optical schematic of FIG. 3 and the cross-sectional intensity distribution profile of the beam. [Figure 5]4 is a diagram of the beams formed by the optical scheme of FIG. 3 and the depth penetration of each plane of the beams. [Figure 6] 1 is an optical schematic diagram of a modified beam shaping constructed in accordance with the concepts of the present invention; DETAILED DESCRIPTION OF THE INVENTION

[0027] The inventive concept provides a larger process window in laser-based material processing that typically requires the use of high power and high quality MM beams, and is realized by the inventive optical scheme that converts a non-Gaussian intensity profile to a Gaussian intensity profile in the vicinity of the shaped beam waist.

[0028] FIG. 2 shows an exemplary laser head 50 constructed in accordance with the concepts of the present invention and equipped with the optical schematic of the present invention. The laser head 50 is a critical part of an industrial laser system positioned upstream from the workpiece to be laser processed. The laser head of the present invention typically includes, among other optical and possibly electronic components, a beam-shaping optical schematic including a collimating lens 1 focused at the downstream end of the laser beam delivery fiber 22 and thus spaced at a focal distance from the laser beam delivery fiber 22. In simplest terms, collimation ensures that light rays incident on the input of the collimator 1 travel parallel to each other downstream from its output. The laser head 50 may optionally include two rotating mirrors 3 and 5 and a stationary mirror 4. Finally, the collimated beam impinges on a focusing lens 6, which focuses the beam on the surface of the workpiece to be laser processed. Up to this point, the illustrated schematic is identical to the schematic of FIG. 1, which shows a workpiece being laser processed with a flat-top shaped MM beam. The objectives of the disclosed beam shaping scheme are: 1. to irradiate the workpiece with a beam having a Gaussian profile, and 2. to place the desired Gaussian region of the beam virtually in the vicinity of the waist, i.e., immediately adjacent to or within the waist region. Stated differently, the illustrated scheme includes a combination of optical elements arranged to convert a flat-top or other shaped MM beam into a Gaussian beam with both increased energy and DOF compared to known prior art.

[0029] 2 and 3, the concept of the present invention is realized by introducing a diffractive optical element 2 mounted somewhere between the collimator 1 and the focusing lens 6, or at a very short distance downstream from the focusing lens 6, depending on the focal length of this lens. For example, for a focal length of 200 mm, this distance does not exceed 10 mm. The combination of the diffractive element 2 and the focusing lens 6 creates a region 20 where the light has a Gaussian intensity distribution. In other words, the lens-diffractive element doublet generates a Bessel-Gaussian beam. In the illustrated schematic, the diffractive element 2 is mounted at a distance F from the focusing lens 6, which is slightly shorter than the focal length F2 of the lens 6 in FIG. 1. 21 1, the Gaussian beam region 20 is provided so that it is virtually adjacent to or within the beam waist. In fact, the Gaussian region 14 is brought close to the waist so that it is considered here to be a Gaussian region within the waist that includes the illuminated surface 12.

[0030] Diffractive element 2 may include, among others, a homogenizer, a hologram, and an axicon. In the illustrated configuration, element 2 is an axicon lens, which is well known to those skilled in the art of optics. Within the context of this disclosure, axicon 2 transforms the flat-top intensity profile of beam 10 into a beam shape that can be mathematically represented by a Bessel function and may have a donut-shaped intensity profile within the waist of the transformed beam. The region of the transformed MM Bessel beam 10 with a Gaussian distribution is not symmetric; only the upper region 14 has the desired energy, as will be discussed later. The operating principles of an axicon are common to any suitable diffractive optical component.

[0031] Figure 4 shows a plan view of the region of a Bessel beam, along the optical path between diffractive element 2 and the plane containing surface 12 downstream from the beam waist, with the respective intensity profiles obtained according to the schematics of Figures 2 and 3. As shown, the uppermost beam region, or plane 1, and the lowermost beam region, or plane 9, are 40 mm apart and positioned symmetrically with respect to the waist extending between planes 4 and 5. Planes 1, 2, and 6-9 all show different profiles of the Bessel beam that differ from a Gaussian profile. In contrast, the profiles at plane 3 and, especially, plane 4, are very close to a Gaussian distribution. Plane 4 is the most appealing cross section because it is effectively located within the waist, indicating that the energy is close to its maximum value. As for plane 5, the profile shown differs slightly from a Gaussian one but is still appropriate for the intended purpose.

[0032] FIG. 5 shows that the DOF, the distance between adjacent planes 4 and 5 that define the waist between them, is equal to 5 mm. Interestingly, the same schematic without an axicon, such as that shown in FIG. 1, only provides a DOF of 1 mm in the Gaussian regime. Naturally, the DOF depends on the laser power and the respective parameters of all optical components of the laser head 50, including the focal length of the focusing lens 6, which in this experiment is 150 mm, and the collimator, which is equal to 100 mm. The smallest spot size, i.e., the highest density, is at plane 5 and is equal to 350 μm. Plane 4 has a light spot size approximately equal to that of plane 5. In contrast, plane 8 is characterized by a spot size of 2500 μm, the largest of the planes shown.

[0033] FIG. 6 shows another optical schematic of a beam shaper. This beam shaper shares the same optical elements as the prior art schematic of FIG. 1, including, among other things, endblock 15, collimating, and focusing lenses 1 and 6, but does not have the diffractive element 2 that was important to the schematics of FIGS. 2 and 3. Instead, this beam shaper utilizes a MM flat-top beam 10 that has a region with a Gaussian intensity distribution by displacing collimator 1 downstream from fiber end 22. The collimator is displaced so that the Gaussian region is not at the fiber end 22, but within endblock 15, which is spaced from the collimator by a distance corresponding to the focal length of collimator 1. As a result, the waist of beam 10 at surface 12, spaced from lens 6 at the original focal length F2, is characterized by a Gaussian intensity distribution region.

[0034] The delivery fiber 22 used in all previously disclosed schemes comprises a refractive step-index fiber. However, the scheme shown in Figure 6 can be used in combination with a graded-index fiber, which is by definition not an SM fiber. The operation of the scheme of Figure 6 utilizing a graded-index fiber is the same as for a step-index fiber.

[0035] The schematics disclosed herein according to the present invention are not limited in their application to the details of construction and arrangement of components set forth in the following description or illustrated in the accompanying drawings. These aspects may take on other embodiments and be practiced or carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, acts, components, elements, and features discussed in connection with any one or more embodiments are not intended to be excluded from a similar role in any other embodiment.

[0036] Additionally, the phraseology and terminology used herein are for descriptive purposes and should not be construed as limiting. Any reference to system and method examples, embodiments, components, elements, or acts referred to herein in the singular can encompass embodiments that include the plural, and any reference to any embodiment, component, element, or act herein in the plural can encompass embodiments that include only the singular. References in the singular or plural are not intended to limit the systems or methods disclosed herein, their components, acts, or elements. The use herein of "including," "comprising," "having," "containing," "with," and variations thereof are meant to cover the items listed thereafter, and equivalents thereof, as well as additional items. References to "or" may be construed as inclusive, such that terms described using "or" can refer to any of one, more than one, and all of the listed terms.

[0037] Having thus outlined some of the concepts of the present invention, it is to be understood that various alternatives, modifications, and improvements will readily occur to those skilled in the art. For example, the examples disclosed herein may be used in other contexts. Such alternatives, modifications, and improvements are intended to be part of this disclosure and are intended to be within the scope of the examples discussed herein. Accordingly, the foregoing description and drawings are by way of example only. [Explanation of symbols]

[0038] 1 Collimating lens, collimator 2. Diffractive optical element, axicon 3.5 Rotating mirror 4 Unchanging Mirror 6. Focusing Lens 10 MM Bessel beam, MM flat top beam, MM flat top laser beam 12 Surface 14 Gaussian region, upper region 15 End Block 20 Gaussian beam region 22 laser beam delivery fiber, downstream fiber end 22 25 laser head 50 laser head F1 Focal length of collimator 1 F2 focal length of focal lens 6

Claims

1. 1. A beam shaper for transforming a MM beam with a non-Gaussian intensity distribution profile, comprising: an end block fused to a downstream end of a fiber that guides the MM beam along a path; a collimator downstream from the endblock for receiving and collimating the MM beam; a focusing lens positioned at a fixed position, the focusing lens forming a beam waist at a focal plane of the focusing lens at a workpiece being laser machined, the beam waist having a Gaussian intensity distribution profile; Equipped with 10. The beam shaper according to claim 9, wherein the beam shaper is configured such that a Bessel beam is generated at the beam waist.

2. 2. The beam shaper of claim 1, wherein the collimator is spaced from an interface between the downstream end of the fiber and the end block by a distance equal to a focal length of the collimator, and the MM beam has a flat-top intensity distribution profile.

3. 3. The beam shaper of claim 2, further comprising a diffractive optical element spaced downstream from the end block and configured to convert the MM beam into the Bessel beam, the beam waist of the Bessel beam being located at the focal plane of the focusing lens at the workpiece and having the Gaussian intensity distribution profile.

4. The beam shaper of claim 3 , wherein the diffractive optical element is positioned between the collimator and the focusing lens.

5. The beam shaper of claim 3 , wherein the diffractive optical element is positioned downstream from the collimator.

6. The beam shaper of claim 3 , wherein the diffractive optical element is an axicon, a hologram, or a homogenizer.

7. The beam shaper of claim 3 , wherein the end block, the collimator, the diffractive optical element, and the focusing lens are mounted to a housing of a laser head of a high-power fiber laser welding system.

8. 2. The beam shaper of claim 1, wherein the collimator is positioned within the end block and spaced downstream from the end block such that a focal plane of the collimator coincides with a Gaussian beam region of the MM beam having a Gaussian density distribution, and the Gaussian beam region is focused onto the focal plane of the focusing lens at the workpiece being laser processed.

9. 2. The beam shaper of claim 1, wherein the end block fused to the downstream end of the fiber, the collimator, and the focusing lens are mounted to a housing of a laser head of a high-power fiber laser welding system.

10. 10. The beam shaper of claim 9, wherein the fiber is a step-index fiber or a graded-index fiber.

11. The beam shaper of claim 1 , further comprising a plurality of movable mirrors positioned upstream from the focusing lens and mounted to a laser head along with the collimator and the focusing lens.

12. A beam shaper according to claim 11, a robot arm supporting the laser head; The fiber delivers the MM beam from a fiber laser or YAG laser source operating in the CW, QCW, or pulsed regime.

13. 1. A method of transforming a MM beam with a non-Gaussian intensity distribution profile, comprising: guiding the MM beam in a delivery fiber; coupling the MM beam into an endblock of a laser head, the endblock being joined to a downstream end of the delivery fiber; collimating the MM beam in the laser head with a collimator; focusing the collimated MM beam at a surface of a workpiece to be laser machined by a collimating lens in the laser head, thereby forming a waist of the MM beam at the workpiece to be laser processed, wherein a region of the MM beam characterized by a Gaussian intensity distribution is formed near the waist of the MM beam without displacing the collimating lens; forming a waist in the MM beam comprises: focusing the collimator on the downstream end of the delivery fiber; converting the collimated MM beam into a Bessel beam; and focusing the Bessel beam so that a Gaussian region of the MM beam is located within the waist.

14. 14. The method of claim 13, wherein forming the waist of the MM beam comprises focusing the collimator on the Gaussian region of the MM beam within the end-block.

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