Functionally homogenized intensity distribution for additive manufacturing or other industrial laser processing applications

Abstract

The present invention discloses a technique for generating a laser output beam having a functionally homogenized intensity distribution. According to some embodiments, a population of a small number of modes in a multi-mode confined core is excited by applying a low-mode source beam to the multi-mode confined core, so that the population exhibits an unstable intensity distribution. The unstable intensity distribution is functionally homogenized by providing one or both of modulation of a phase shift in the multi-mode confined core and a change in the emission conditions of the low-mode source beam entering the multi-mode confined core.

Description

[0001] Related Applications

[0002] This application claims the benefit of priority to U.S. Provisional Patent Application No. 62 / 865,902, filed on June 24, 2019; and U.S. Provisional Patent Application No. 62 / 882,442, filed on August 2, 2019. The entirety of these two related applications is hereby incorporated by reference. Technical Field

[0003] The field of the present disclosure relates generally to fiber lasers and fiber-coupled laser systems for delivering beams for use in additive manufacturing, and more particularly to improved annular laser beam profiles for powder bed fusion applications. Background Art

[0004] There are several types of additive manufacturing that are capable of processing metal workpieces. Two such categories of additive manufacturing include powder bed fusion and direct energy deposition (DED).

[0005] Powder bed fusion is a type of additive manufacturing that requires melting powder using thermal energy provided by a light beam or electron beam. Currently, there are two main types of powder bed fusion that employ light beams.

[0006] The first type of powder bed fusion is called selective laser sintering (SLS). In SLS, a laser beam sinters powdered materials such as plastics, nylons, and ceramics. Direct metal laser sintering (DMLS) is a similar technology, where the powder is metal.

[0007] The second type of powder bed fusion is called selective laser melting (SLM). In the SLM process, a laser forms a molten pool in a powder bed. The molten pool quickly cools and solidifies to form the part.

[0008] DED includes Laser Engineered Net Shaping (LENS) and Electron Beam Additive Manufacturing (EBAM). Instead of sintering or melting powder layers, the raw materials are simultaneously deposited and cured with thermal energy.

[0009] Some attempts have been made to characterize the use of annular intensity profiles in additive manufacturing. For example, in a 2015 paper by Wischeropp et al. entitled “Simulation of the effect of different laser beam intensity profiles on heat distribution in selective laser melting”, a 2D-FEM model for qualitatively simulating the heat distribution of TiAl6V4 powder melting on top of a solid TiAl6V4 block is described. The heat distribution during single-pass melting was simulated for three different laser beam intensity profiles at different scan speeds and laser powers. The results claimed in the paper indicate that energy efficiency is improved and the amount of vaporized material is reduced when an annular laser beam intensity profile is employed compared to a Gaussian-shaped laser beam intensity profile. The authors believe that the annular laser beam intensity profile provides a higher build rate for powder bed fusion applications.

[0010] Ring-shaped laser beam intensity profiles, more generally referred to as annular intensity distributions (which include a saddle shape), have previously been attempted by exciting a population formed by many modes. In other words, a multimode input is used to essentially flood a fiber segment having a ring-shaped core in order to excite many modes that are delivered at the output of the segment. Summary of the invention

[0011] The present disclosure describes low mode source excitation of a small number of modes in a multi-mode annular confined core (i.e., a core having a cross-sectional profile in the shape of an annulus). The inventors presently believe that a small number of modes provides higher beam quality in terms of BPP and Rayleigh range compared to a large number of modes delivered at the output, which greatly improves performance in terms of increasing processing speed, reducing vaporized material (reducing smoke and soot generation), and reducing the size of manufacturable features.

[0012] In some embodiments, a small number of modes produce inhomogeneities in the intensity distribution delivered at the output. Therefore, the present disclosure also describes embodiments including externally applied perturbations to establish a functionally homogenized annular intensity distribution with a relatively high Rayleigh range for use in additive manufacturing. The disclosed embodiments rely on different optical properties and mechanisms by which the annular intensity distribution is homogenized. Therefore, the embodiments (and inherent mechanisms) are generally referred to as phase shift embodiments and variable mode excitation embodiments.

[0013] More specifically, in a first embodiment, rapid vibrations are applied toward the free end of the fiber to introduce mechanical oscillations (e.g., about 70 Hz) that rapidly change the interference pattern of a small number of modes. The change in the interference pattern rapidly moves any high intensity regions within the annular intensity distribution to functionally homogenize it from the perspective of the powder material. In other words, any so-called hot spots are quickly distributed to avoid excessive smoke and fumes while still delivering a relatively low BPP and a relatively high Rayleigh range.

[0014] In a second embodiment, an external perturbation is applied to modulate the emission conditions by which a small number of modes are excited. Thus, the source beam rapidly changes the excited mode population. When the emission conditions vary rapidly enough, the rapid modulation has the effect of homogenizing the annular intensity distribution delivered at the output (from the perspective of the powder material).

[0015] Additional aspects and advantages will become apparent from the following detailed description of the embodiments, which proceeds with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is an annotated pictorial view of the annular intensity distribution delivered at the output of the optical fiber.

[0017] Figures 2 to 4 is a cross-sectional view of an exemplary optical fiber structure for delivering a light beam having variable beam characteristics.

[0018] Figure 5 is a pictorial view of the functionally homogenized annular intensity distribution delivered at the output of the optical fiber.

[0019] Figure 6 and Figure 7 are perspective and isometric views of the external perturbation device, respectively. Figure 7 The internal components of the device are also shown in dotted lines. DETAILED DESCRIPTION

[0020] Figure 1 1 shows experimental results of an annular intensity distribution 100 provided at the output of an annular confining core of an optical fiber segment (see for example for illustrating a segment described later) Figures 2 to 4 ). An annular intensity distribution 100 is generated in response to a single mode (SM) source input to the fiber segment, thereby exciting a smaller population of a small number of modes in the annular confined core. The inventors have recognized that generating an annular intensity distribution 100 consisting of a small number of modes can provide a desired laser beam to enhance the performance of additive manufacturing without suffering from any non-uniform energy distribution 110.

[0021] However, the not fully populating modes that are originally supported in the multimode waveguide tend to result in an uneven, non-uniform intensity distribution (also known as convex corner structure) at the output. The uneven energy distribution 110 is characterized as areas of relatively low intensity and areas of relatively high intensity or so-called hot spots. Since there are a small number of modes excited in the disclosed system, their distribution under static perturbations and mode excitation conditions appears somewhat uneven at the output. In some applications where the material and scanning speed are not sensitive to hot spots, this unevenness is tolerable (i.e., static delivery). This unevenness may have a time dependency, which will also make the beam unstable.

[0022] However, for some other applications, the non-uniform energy distribution 110 is functionally homogenized by an externally applied perturbation that dynamically provides one or both of modulation of the phase shift and rapid changes in the emission conditions to vary the population of the small number of modes that are excited. Even though the resulting beam may still be instantaneously non-uniform at any particular time, the perturbation is fast enough to allow the beam to behave as if it were uniform and stable in its interaction with a workpiece (e.g., a powder bed, etc.). Thus, the present disclosure describes techniques for homogenizing a non-uniform, non-uniform, or asymmetric intensity distribution so that it maintains relatively high quality (e.g., in terms of depth of field and Rayleigh range) for use in industrial laser processing applications.

[0023] There are a variety of fiber optics devices that can produce annular intensity distribution 100. Three such embodiments are described below, but those skilled in the art will appreciate in light of this disclosure that other embodiments are possible. Although the following examples are described in the context of annular intensity distributions, the disclosed techniques have broad applicability to different types of multimode waveguide structures (e.g., rectangular, hexagonal, etc.) having modes that are not completely filled and are therefore associated with uneven, non-uniform, or asymmetric beams of various shapes (e.g., a top-hat beam with a hot spot).

[0024] Figure 2 A first embodiment capable of producing an annular intensity profile 100 is shown comprising a variable beam characteristic (VBC) fiber 200 similar to those described in U.S. Patent No. 10,295,845 to Kliner et al. The '845 patent Figure 7 to Figure 1 FIG. 0 shows experimental results of VBC fiber 200 and illustrates the response of the light beam to the perturbation of VBC fiber 200 when perturbation assembly 210 acts on VBC fiber 200 to bend the fiber. Figures 4 to 6 is the simulation result, and Figure 7 to Figure 10 is an experimental result, in which a beam from an SM 1050nm source is launched into an input fiber (not shown) having a core diameter of 40 microns. The input fiber is spliced ​​to a first section of fiber 204 having a first refractive index profile (RIP) 212. The first section of fiber 204 is spliced ​​to a second section of fiber 208 having a second RIP 214 different from the first RIP at an optically inert junction 206 (splice joint, refractive index matching glue, etc.). Therefore, the first section of fiber 204 carries a beam that excites a mode population in the second section of fiber 208, which includes confinement regions arranged coaxially to form an outer ring, an optional inner ring, and an optional central core. By introducing a lateral displacement, such as the '845 patent Figure 6 As shown, the modes in the outer ring (or inner ring) are excited to produce a ring-shaped intensity distribution at the output of the second section of optical fiber 208. Additional details on producing a ring-shaped intensity distribution are described in US Patent No. 10,663,768 to Martinsen et al.

[0025] To further enhance the generated beam for use in additive manufacturing, the inventors tested SM input 280 that excites a small number of modes 286 in the second section of optical fiber 280, which typically will support many modes. In other words, the low-mode input 280 delivered by the first section of optical fiber 204 at the junction 206 excites a relatively small population of modes 286 in the second section of optical fiber 208 (which serves as a waveguide for guiding the modes 286). In a representative experiment, a single-mode beam was launched into an annular guiding region with an inner diameter of about 40 μm and an outer diameter of about 60 μm. Filling all the modes in the annular region will result in an M of about 30 2 The actual measured M value of the annular beam is 2 is about 8 (due to its low mode excitation). This 3.8× improvement in beam quality results in a 3.8× increase in the depth of field (Rayleigh range) of the focused beam, providing significant processing advantages (larger processing window, less sensitivity to optical alignment).

[0026] The exact number of modes in the small group may vary based on empirical results. The inventor currently believes that the excitation of about half (i.e., 50%) or less of the supported modes provides the desired benefits associated with powder bed melting. In other embodiments, the number of excited modes may include a range from two to ten modes, which may be about 10% or less of the possible modes that can actually be guided by the waveguide. Other percentages and ranges of excited (compared to supported) modes are also considered to fall within the scope of the present disclosure. Similarly, the low number of modes of the source can be expressed in proportion to the small number of modes excited at the output. For example, an SM source is suitable for exciting ten or less modes, and more generally, a low mode source (e.g., four modes) is suitable for exciting 10% or less of the supported modes. The actual percentage may vary depending on the number of modes supported in a multimode optical fiber, covering a wider range for different optical fiber designs. Some designs support 10 to 20 modes, in which case a low-mode input may excite about 80% of those modes, while other designs support over 1000 modes with a low-mode input exciting a much smaller percentage.

[0027] Figure 3 A second embodiment capable of producing an annular intensity distribution 100 is shown. The offset spliced ​​optical fiber 300 includes a first section of optical fiber 304, an offset splice node 306, and a second section of optical fiber 308. The first section of optical fiber 304 includes a first RIP 312. The second section of optical fiber 308 includes a second RIP 314 defined by one or more annular cores. Specifically, the annular core 320 is laterally offset from the central SM constraining core 322 of the first section of optical fiber 304. Thus, the annular constraining core 320 faces the central SM constraining core 322. Thus, due to the offset splice node 306, the light beam 332 propagating through the central SM constraining core 322 is directly emitted into at least a portion of the annular constraining core 320.

[0028] Figure 4 A third embodiment is shown that is capable of producing an annular intensity distribution 100. In this example, two optical fibers are separated by a free space optic 410 in between. The optic 410 is used to launch a light beam into an annular confining core.

[0029] Figure 5 An example of a functionally homogenized annular intensity distribution 500 is shown, showing Figure 1 How can the hot spots be quickly moved so that the average power is smoothed over the annulus 510 (or other shaped confined area). At least two embodiments are proposed in the present disclosure for achieving such uniformity.

[0030] In a first embodiment, laboratory experiments conducted by the inventors have demonstrated that Figure 1The power distribution is shown to be sensitive to the motion of the fiber with the annular confined core. Thus, the inventors recognized that the output of the fiber with the annular confined core can be rapidly perturbed in order to produce an apparently uniform power distribution. At any instant, the power remains non-uniformly distributed, but the rapid perturbation provides the appearance of a homogenized beam relative to the material to which the homogenized beam is applied. In other words, from the perspective of the material (i.e., the thermal mass properties of the material), a functionally homogenized beam with a small number of modes behaves essentially as if the beam were actually scrambled in azimuth.

[0031] In terms of the underlying mechanism that produces the functional result, it is important to note that the externally applied perturbation changes the phase relationship between modes at the output by a small amount, and does not necessarily change the number of modes excited. Thus, the change in phase results in rapidly changing minima and maxima, as well as positive and negative interference, between modes in the second fiber section, which in turn rapidly changes the azimuthal position of the hotspot. Thus, when the phase changes are fast enough, the average intensity appears to be homogenized.

[0032] Figure 6 and Figure 7 An example of how an external perturbation device 600 can apply perturbations externally and directly to a jacketed optical fiber 610 within a laser system housing (not shown) or near a processing head of the type shown in Victor et al. Publication No. US 2018 / 0180803 A1 (see, e.g., FIG. 31A ) or another type of additive manufacturing system is shown. The device 600 uses a second fiber segment 208 fixed to an output fiber (i.e., a VBC fiber, Figure 2 ) is aligned with the jacketed optical fiber 610 and is powered by a small power source (not shown). In other embodiments, instead of or in addition to vibrating the optical fiber through a protective jacket or cable, the second optical fiber 208 is directly vibrated.

[0033] Figure 7 The device 600 is shown to include a commercially available 5V DC electric vibration motor 710, which is contained in a 3D printed housing 720 that is clamped onto the optical fiber conduit. It can also be secured with a cable tie or other attachment (such as a clamp). The motor 710 rotates the counterweight 740 at about 70 Hz (4,200 RPM), which produces vibrations that change the phase relationship inside the optical fiber 610 (as previously described). Higher or lower frequencies outside the audible range can also be used.

[0034] Various other types of perturbation devices are also possible. For example, many other devices may be used inside or outside the laser box: piezoelectric devices, voice coils, solenoid actuators, alternating electromagnetic fields, fans / air or other vibration devices and sources for vibrating the fiber. FIG. 24 of the '854 patent shows examples of different types of perturbation devices for varying the mode population being excited, and these types of devices are also suitable for varying the phase relationship within the fiber 610. Other mechanical actuators include linear or rotary motors that drive vibrations directly or via a frequency-changing linkage (e.g., an eccentric rotating mass), pneumatic actuators, and electro- or magnetostrictive devices. Perturbations may also be imparted by pushing or compressing a ring-shaped fiber, introducing small microbends into a ring-shaped fiber, and including some geometric shapes in the fiber cladding.

[0035] In a second embodiment, the inventors have recognized that rapid changes in launch conditions can also be used to produce functionally homogenized results. For example, U.S. Patent No. 10,677,984 to Brown et al. describes a technique for producing temporally distinct intensity distributions by exciting modes in different cores through rapid, externally applied perturbations at a VBC fiber. The technique can also be applied to dithering between different smaller groups of a small number of modes excited in the same core, thereby rapidly changing hot spots to deliver high-quality beams, thereby enhancing powder bed melting. Dithering can cause the launch conditions to vary between two coaxial cores or within a single annular core (e.g., by moving a portion of the beam between the cladding and the waveguide portion or by rapidly lateral displacement of the beam emitted in a single annular core). In some embodiments, a static perturbation is applied to impart a lateral displacement, and a high-frequency dynamic complementary perturbation is applied to rapidly vary the launch conditions in the single annular core.

[0036] Finally, the skilled person will appreciate that many changes may be made to the details of the above-described embodiments without departing from the basic principles of the invention. For example, features of the first and second embodiments for introducing externally applied perturbations may be combined into a third embodiment having both phase relationships and modal excitation homogenization. Furthermore, the skilled person will appreciate that the modulation frequency and rate of change of the firing conditions are a function of the desired average intensity distribution, the type of laser processing, and the thermal material properties of the workpiece (such as thermal conductivity, thermal diffusivity, specific heat, melting point, or other properties). Accordingly, the scope of the invention should be determined solely by the appended claims.

Claims

1. A method of producing a laser output beam having a functionally homogenized intensity distribution, the method comprising: exciting a population of a small number of modes in a multimode confining core by applying a low-mode source beam to the multimode confining core such that the population exhibits a non-uniform intensity distribution characterized as an azimuthally asymmetric hot spot, wherein the low-mode source beam excites 50% or less of the modes supported by the multimode confining core; and One or both of modulation of a phase shift in the multi-mode confining core and a change in launch conditions of the low-mode source beam entering the multi-mode confining core are provided, thereby functionally homogenizing the non-uniform intensity distribution to produce the laser output beam.

2. The method of claim 1, wherein: The low-mode source beam has four or fewer modes.

3. The method of claim 1, wherein: The low-mode source beam has a single mode.

4. The method of claim 1, wherein: The low-mode source beam excites 10% or less of the modes supported by the multi-mode confining core.

5. The method of claim 1, wherein: The population of the small number of modes in the multi-mode confining core includes ten or fewer modes.

6. The method of claim 1, wherein: The method further includes providing modulation of the phase shift by coupling a perturbation device to an optical fiber including the multimode confined core.

7. The method of claim 6, wherein: The perturbation device includes a voice coil located in a housing that conforms to the jacket of the optical fiber.

8. The method of claim 6, wherein: The agitation device comprises a rotary electric motor located in a housing conforming to the jacket of the optical fiber.

9. The method of claim 1, wherein: The method further includes providing a change in the launch condition of the low-mode source beam by coupling a perturbation device to a junction of a variable beam characteristic (VBC) fiber.

10. The method of claim 1, wherein: The multi-mode constraining core is an annular constraining core.

11. The method of claim 1, wherein: The method further includes applying the laser output beam to an additively manufactured workpiece.

12. An apparatus for producing a laser output beam having a functionally homogenized intensity distribution, the apparatus comprising: a first section of optical fiber, the first section of optical fiber being used to guide a low-mode source beam; a second length of optical fiber having a multimode confining core configured to receive the low-mode source beam and thereby excite a population of a small number of modes in the multimode confining core such that the population exhibits a non-uniform intensity distribution characterized as an azimuthally asymmetric hot spot, wherein the low-mode source beam excites 50% or less of the modes supported by the multimode confining core; as well as A perturbation device for providing one or both of a modulation of a phase shift in the multi-mode confined core and a change in an emission condition of the low-mode source beam entering the multi-mode confined core, thereby functionally homogenizing the non-uniform intensity distribution to produce the laser output beam.

13. The device of claim 12, wherein: The first length of optical fiber and the second length of optical fiber include variable beam characteristic (VBC) optical fibers.

14. The apparatus of claim 12, wherein: The first length of optical fiber and the second length of optical fiber include offset-spliced ​​optical fibers.

15. The apparatus of claim 12, wherein: The first segment and the second segment include a first optical fiber and a second optical fiber, respectively, separated by free space optics between a free end of the first optical fiber and a free end of the second optical fiber.

16. The apparatus of claim 12, wherein: The perturbation device includes a voice coil coupled to the second length of optical fiber.

17. The apparatus of claim 12, wherein: The perturbation device includes the internal geometry of the second length of optical fiber.

18. The apparatus of claim 12, wherein: The first section of optical fiber is a single-mode optical fiber.

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