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

Abstract

A method for generating a laser output beam having a functionally homogenized intensity distribution is disclosed. According to some embodiments, a distribution of several modes in a multimode confinement core is excited by applying a low-mode source beam to the multimode confinement core, resulting in the distribution exhibiting an unstable intensity distribution. The unstable intensity distribution is functionally homogenized by modulating a phase shift in the multimode confinement core and / or by varying the conditions for launching the low-mode source beam into the multimode confinement core.

Description

Related Applications

[0001] This application claims priority to U.S. Provisional Patent Application No. 62 / 865,902, filed June 24, 2019, and U.S. Provisional Patent Application No. 62 / 882,442, filed August 2, 2019. Both related applications are incorporated herein by reference. [Technical Field]

[0002] The field of the disclosure relates generally to fiber lasers and fiber-coupled laser systems that provide light beams used in additive manufacturing, and more particularly to improved annular laser beam profiles for powder bed fusion applications. [Background technology]

[0003] There are several types of additive manufacturing that have the ability to process metal workpieces. Two such categories of additive manufacturing include powder bed fusion and directed energy deposition (DED).

[0004] Powder bed fusion is a type of additive manufacturing that involves fusing powder with thermal energy provided by a light beam or electron beam. Currently, there are two types of powder bed fusion that use a light beam.

[0005] The first type of powder bed fusion is called selective laser sintering (SLS). In SLS, a laser beam sinters powdered materials—such as plastics, nylon, and ceramics. Direct metal laser sintering (DMLS) is a similar technique where the powder is metal.

[0006] The second type of powder bed fusion is called selective laser melting (SLM). In the SLM process, a laser creates a molten pool within the powder bed, which rapidly cools and solidifies to form the part.

[0007] DED includes laser direct deposition modeling (LENS) and electron beam additive manufacturing (EBAM): instead of sintering or melting powder layers, the feedstock is hardened by thermal energy as it is deposited.

[0008] Efforts have been made to evaluate the use of annular intensity distributions in additive manufacturing. For example, in a 2015 paper titled "Simulation of the Effect of Various Laser Beam Intensity Profiles on Heat Distribution in Selective Laser Melting," Wischeropp et al. described a 2D FEM model that qualitatively simulated the heat distribution of melting TiAl6V4 powder on top of solid TiAl6V4. The heat distribution during single-track melting was simulated for three different laser beam intensity profiles at various scanning speeds and laser powers. The results touted in the paper show increased energy efficiency and reduced volatilized material when using a donut-shaped laser beam intensity profile as opposed to a Gaussian-shaped laser beam intensity profile. The authors suggest that a donut-shaped laser beam intensity profile increases the production rate of powder bed fusion.

[0009] Doughnut-shaped laser beam intensity profiles—more commonly referred to as annular intensity distributions (including saddle-shaped profiles)—have been attempted by exciting a population of many modes. In other words, a multimode input is used to essentially fill a fiber optic segment with an annular core, thereby exciting many modes that are delivered to the output of the segment. Summary of the Invention [Problem to be solved by the invention]

[0010] This disclosure describes low-mode source excitation of a few modes within a multimode annular confinement core (i.e., a core with a ring-shaped cross-sectional profile). It is the inventors' belief that the few modes, as opposed to a high-mode population fed to the output, provide a higher quality beam in terms of BPP and Rayleigh range, which dramatically improves performance in terms of increased throughput, reduced volatilized materials (less smoke and soot production), and reduced manufacturable feature size. [Means for solving the problem]

[0011] In some embodiments, the minority modes produce a non-uniform intensity distribution delivered at the output. Accordingly, this disclosure also describes embodiments that include externally applied perturbations to produce functionally homogenized annular intensity distributions with relatively wide Rayleigh ranges for additive manufacturing. The disclosed embodiments rely on various optical properties and mechanisms to homogenize the annular intensity distribution. Accordingly, the embodiments (and their underlying mechanisms) are generally referred to as phase-change and variable-mode excitation embodiments.

[0012] More specifically, in a first embodiment, rapid vibration is applied to the free end of the optical fiber to introduce mechanical vibrations (e.g., about 70 Hz) that rapidly change the interference pattern of the few modes. The change in the interference pattern rapidly moves high intensity regions within the annular intensity distribution to functionally homogenize the annular intensity distribution as seen by the powder material. In other words, so-called hot spots are rapidly distributed to avoid excessive smoke and soot while providing a relatively low BPP and a high Rayleigh range.

[0013] In a second embodiment, an external perturbation is applied that modulates the launch conditions under which a minority of modes are excited. The source beam thus rapidly varies the population of excited modes. When the launch conditions are varied rapidly enough, this rapid modulation has the effect of homogenizing the annular intensity distribution (as seen by the powder material) provided at the output.

[0014] Additional aspects and advantages will be apparent from the following detailed description of the embodiments, which proceeds with reference to the accompanying drawings. [Brief explanation of the drawings]

[0015] [Figure 1] 10 is an annotated image of the annular intensity distribution delivered to the output of an optical fiber. [Figure 2] 1 is a cross-sectional view of a typical fiber structure that provides a beam with variable beam characteristics. [Figure 3] 1 is a cross-sectional view of a typical fiber structure that provides a beam with variable beam characteristics. [Figure 4] 1 is a cross-sectional view of a typical fiber structure that provides a beam with variable beam characteristics. [Figure 5] 1 is an image of the functionally homogenized annular intensity distribution delivered at the output of an optical fiber. [Figure 6] FIG. 1 is a perspective view of an external perturbation device. [Figure 7] 1 is an isometric view of an external perturbation device, with the internal components of the external perturbation device shown in dash-dot lines. DETAILED DESCRIPTION OF THE INVENTION

[0016] FIG. 1 shows experimental results of an annular intensity distribution provided at the output of a ring confinement core of an optical fiber segment (see FIGS. 2-4 for segment illustrations below). The annular intensity distribution 100 is generated in response to a single-mode source (SM) input into the fiber segment, thereby exciting a minority population of minority modes within the ring confinement core. The inventors have recognized that generating an annular intensity distribution 100 that includes a minority of modes can provide a desirable laser beam for improving additive manufacturing performance despite a non-uniform energy distribution 110.

[0017] However, the otherwise non-fully populated modes maintained within a multimode waveguide tend to produce a lumpy, non-uniform intensity distribution (also called lobed structure) at the output. The non-uniform energy distribution 110 is characterized by regions of relatively low and high intensity, or so-called hot spots. When there are a few modes excited in the disclosed system, the distribution of the few modes under static perturbation and mode excitation conditions will appear somewhat lumpy at the output. This lumpy state may be tolerable (i.e., statically fed) in embodiments with materials and scan speeds that are not susceptible to hot spots. This non-uniformity may be time-dependent. This time-dependence can cause the beam to become unstable.

[0018] In other applications, however, the inhomogeneous energy distribution 110 is functionally homogenized by an externally applied perturbation that dynamically modulates the phase change and / or rapidly changes the launch conditions to alter the population of the minority excited modes. Although the resulting beam may still be inhomogeneous at a particular moment in time, the perturbation is fast enough to allow the beam to behave as it interacts with the workpiece (such as a powder bed) as if it were homogeneous and stable. Thus, the present disclosure describes methods for generating inhomogeneous, non-uniform, or asymmetric intensity distributions that retain relatively high quality (e.g., in terms of object depth and Rayleigh range) for use in industrial laser processing applications.

[0019] There are multiple fiber optic devices capable of producing annular intensity distribution 100. Three such embodiments are described below, although one skilled in the art will recognize that other embodiments are possible in light of this disclosure. While the following examples are described in the context of annular intensity distributions, the disclosed methods have broadened the applicability of various types of multimode waveguide structures (e.g., rectangular, hexagonal, etc.) with modes associated with inhomogeneous, non-uniform, or asymmetric beams of various shapes (e.g., top-hat beams with hot spots), without being fully accounted for.

[0020] FIG. 2 shows a first embodiment capable of generating an annular intensity distribution 100, including a variable beam characteristic (VBC) fiber 200 similar to that described in U.S. Pat. No. 10,295,845 to Kleiner et al. Figures 7-10 of U.S. Pat. No. 10,295,845 show experimental results for the VBC fiber 200 and the beam response to perturbations of the VBC fiber 200 when a perturbation assembly 210 acts to bend the VBC fiber 200. Figures 4-6 of U.S. Pat. No. 10,295,845 are simulated experiments, while Figures 7-10 show experimental results in which a beam from a SM 1050 nm source was launched into a 40 μm core diameter input fiber (not shown). The input fiber was spliced ​​to a first length of fiber 204 having a first refractive index profile (RIP) 212. The first fiber -2 04 is a second length of fiber 20 having a second RIP 214 different from the first RIP. 8 to the joint 206 (joint The first length of fiber 204 is bonded with a diaphragm, a refractive index-matching adhesive, etc., so that the first length of fiber 204 carries a beam that excites a population of modes in the second length of fiber 208. The second length of fiber 208 has coaxially arranged confinement regions comprising an outer ring, an optional inner ring, and an optional central ring. By introducing a lateral displacement as shown in FIG. 6 of U.S. Pat. No. 10,295,845, the modes in the outer (or inner) ring are excited to produce an annular intensity distribution at the output of the second length of fiber 208. Further details on the generation of an annular intensity distribution are described in U.S. Pat. No. 10,663,768 to Martinsen et al.

[0021] To further improve the resulting beam for use in additive manufacturing, the inventors tested an SM input 280 that excites a small population mode 286 within the second length of fiber 208. In other words, the low-mode input 280 provided by the first length of fiber 204 at the junction 206 excites a small population mode 286 within the second length of fiber 208, which acts as a waveguide to guide the small population mode 286. In a representative experiment, a single-mode beam was launched into an annular waveguiding region with an inner diameter of approximately 40 μm and an outer diameter of approximately 60 μm. While dominating all modes in the annular region would result in an M value of approximately 30, the measured M value for the actual annular beam (due to low-mode excitation) is approximately 8. This 3.8-fold improvement in beam quality results in a 3.8-fold increase in the irradiated depth (Rayleigh range) of the focused beam, providing substantial processing benefits (wider process window, less sensitivity to optical alignment).

[0022] The exact number of modes in the small population may vary based on past results. It is the inventors' belief that exciting no more than about half (i.e., 50%) of the sustained modes provides desirable benefits for powder bed fusion bonding. In other embodiments, the number of excited modes may range from 2 to 10 modes. The number of excited modes may be no more than about 10% of the possible modes actually guided by the waveguide. Other percentages and ranges of excited (relatively sustained) modes are also considered within the scope of this disclosure. Similarly, the low number of modes in a source may be expressed in terms of the proportion of the few modes excited at the output. For example, SM sources are suitable for exciting no more than 10 modes. More commonly, low-mode sources (e.g., 4 modes) are suitable for exciting no more than 10% of the sustained modes. The actual proportion may vary depending on the number of modes sustained in multimode fibers, which ranges widely for various designs. Some designs support 10 to 20 modes, with the low-mode input exciting approximately 80% of these modes. On the other hand, some support over 1000 modes, with the low mode input exciting a much smaller fraction.

[0023] 3 illustrates a second embodiment capable of producing an annular intensity distribution 100. The offset spliced ​​fiber 300 includes a first length of fiber 304, an offset splice 306, and a second length of fiber 308. The first length of fiber 304 includes a first RIP 312. The second length of fiber 308 includes a second RIP 314 defined by one or more annular cores. Specifically, the annular core 320 is laterally offset from a central SM confinement core 322 of the first length of fiber 304, such that the annular confinement core 320 opposes the central SM confinement core 322. A beam 332 propagating through the central SM confinement core 322 is launched directly into at least a portion of the annular confinement core 320 by the offset splice 306.

[0024] 4 shows a third embodiment capable of producing an annular intensity distribution 100. In this example, two fibers are separated by a free-space optic 410 that resides between the two fibers. The optic 410 is used to launch the beam into a ring-confined core.

[0025] Figure 5 shows an example of a functionally homogenized annular intensity distribution 500. This illustrates how the hot spots of Figure 1 can move rapidly so that the average power is smoothed across the annulus 510 (or other shape of the confinement region). There are at least two embodiments proposed in this disclosure to achieve such homogenization.

[0026] In a first embodiment, laboratory experiments performed by the inventors have shown that the power distribution shown in FIG. 1 is sensitive to the motion of a fiber with a ring confinement core. Thus, the inventors have recognized that the power of a fiber with a ring confinement core can be rapidly perturbed to produce an apparently homogenous power distribution. In any case, the power is unevenly distributed, but by rapidly perturbing it, a beam that appears homogenous to the material onto which it is irradiated is achieved. In other words, from the perspective of the material—i.e., the heat capacity properties of the material—a functionally homogenized beam with a small number of modes essentially behaves as if the beam were actually azimuthally perturbed.

[0027] Regarding the underlying mechanism that produces the functional result, note that an externally applied perturbation at the output changes the phase between a few modes, but does not change the number of modes excited. Thus, the phase change results in an abrupt change in the maxima and minima and positive and negative interference between modes within the second length of fiber. This abruptly changes the azimuthal location of the hotspot. Thus, the average intensity appears homogenized when the phase change is fast enough.

[0028] 6 and 7 show examples of how an external perturbation device 600 can be applied directly to a coated fiber 610, either inside a laser system case or near an additive manufacturing process head of the type shown in International Patent Application Publication No. US 2018 / 0180803 A1 of Victor et al., or other types. The device 600 is fitted to the coated fiber 610 using a pair of clips affixed to the outer surface of the output fiber (i.e., the second length of fiber 208 of the VBC fiber in FIG. 2) and is powered by a small power source (not shown). In other embodiments, the second length of fiber 208 is vibrated directly, instead of or in addition to vibrating the fiber via a protective jacket or cable.

[0029] Figure 7 shows a device 600 with a commercially available 5V DC electric vibration motor 710 housed in a 3D printed housing 720 that is fastened to a fiber conduit. The device 600 may also be secured with a bundling or other attachment means, such as a clamp. The motor 710 rotates a counterweight at approximately 70 Hz (4200 RPM). The rotation creates vibrations that change the phase relationships within the fiber 610 (as described above). Frequencies outside the audible range may also be used.

[0030] Various other types of perturbation devices are also possible. For example, many other devices may be used inside or outside the laser box, such as piezos, voice coils, solenoid actuators, AC electromagnetic fields, fans / air that vibrate the fiber, or other vibration devices and sources. Figure 24 of U.S. Pat. No. 10,295,845 shows examples of various types of perturbation devices that change the population of excited modes. These types of devices are also suitable for changing the phase relationships within the fiber 610. Other mechanical actuators include linear or rotary motors, pneumatic actuators, and electrostrictive or magnetostrictive devices that drive vibrations directly or via a frequency-changing linkage (e.g., an eccentric rotor). Perturbations may also be imparted by pushing or compressing the ring fiber, introducing small microbends into the ring fiber, and including some geometric structures in the fiber cladding.

[0031] In a second embodiment, the inventors recognized that rapid changes in launch conditions can also be used to produce functionally homogenized results. For example, U.S. Patent No. 1,067,7984 to Brown et al. describes a method for generating temporally distinct intensity distributions by rapidly externally applied perturbations in a VBC fiber to excite different intra-core modes. This method also rapidly changes hot spots to provide a high-quality beam that improves powder bed fusion by dithering between different subpopulations of minority modes excited within the same core. Dithering can change launch conditions between two coaxial cores or within a single ring core (e.g., by moving the beam between the cladding and waveguide section or by applying rapid lateral displacement of the launched beam within the single ring core). In some embodiments, a static perturbation is applied to apply the lateral displacement, and a high-frequency dynamic auxiliary perturbation is applied to change the launch conditions within the single ring core.

[0032] Finally, those skilled in the art will appreciate that many changes can be made to the details of the above-described embodiments without departing from the underlying principles of the present invention. For example, the features of the first and second embodiments for introducing an externally applied perturbation can be combined into a third embodiment that has both phase relationships and homogenized modal excitation. Furthermore, those skilled in the art will appreciate that the modulation frequency and rate of variation in firing conditions are a function of the desired average intensity distribution, the type of laser processing, and workpiece thermal material properties such as thermal conductivity, thermal diffusivity, specific heat, melting point, or other properties. Therefore, the scope of the present invention should be determined solely by the following claims.

Claims

1. 1. A method for generating an annular laser output beam having a functionally homogenized intensity distribution, comprising: exciting a population of minority modes within the multimode confining core by applying a low-mode source beam to the multimode confining core so that the population exhibits an inhomogeneous intensity distribution characterized by azimuthally asymmetric hot spots; generating the annular laser output beam by dynamically modulating the azimuthally asymmetric hot spot by providing one or both of modulation of phase variations in the multiple modes and variation of launch conditions of the low-mode source beam to vary the population of a minority excited mode, thereby functionally homogenizing the inhomogeneous intensity distribution; A method having the following.

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

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

4. 10. The method of claim 1, wherein the low-mode source beam excites no more than 50% of the modes supported by the multimode confining core.

5. 10. The method of claim 1, wherein the low-mode source beam excites no more than 10% of the modes supported by the multimode confining core.

6. 10. The method of claim 1, wherein the population of minority modes in the multimode confining core includes 10 or fewer modes.

7. 10. The method of claim 1, further comprising modulating the phase change by coupling a perturbation device to an optical fiber including the multimode confining core.

8. 8. The method of claim 7, wherein the perturbation device comprises a voice coil within a housing that fits over the optical fiber jacket.

9. 8. The method of claim 7, wherein the perturbation device comprises a rotary electric motor within a housing that fits over the optical fiber jacket.

10. 10. The method of claim 1, further comprising the step of varying the launch conditions of the low-mode source beam by coupling a perturbation device to a splice of a variable beam characteristic (VBC) fiber.

11. The method of claim 1 , wherein the multimode confinement core is an annular confinement core.

12. 10. The method of claim 1, further comprising directing the annular laser output beam onto an additively manufactured workpiece.

13. 1. An apparatus for producing an annular laser output beam having a functionally homogenized intensity distribution, comprising: a first length of optical fiber guiding the low-mode source beam; a second length of optical fiber configured to receive the lower-mode source beam to excite a population of few modes within the multimode confining core to exhibit a non-homogeneous intensity distribution; a perturbation device that provides one or both of a modulation of phase variations in the multimode and a change in the launch conditions of the lower mode source beam into the multimode confining core to vary the population of a minority excited mode, thereby functionally homogenizing the inhomogeneous intensity distribution to produce the annular laser output beam; An apparatus comprising:

14. 14. The apparatus of claim 13, wherein the first length of optical fiber and the second length of optical fiber comprise variable beam characteristic (VBC) fiber.

15. 14. The apparatus of claim 13, wherein the first length of optical fiber and the second length of optical fiber comprise offset spliced ​​fibers.

16. 14. The apparatus of claim 13, wherein the first length of optical fiber and the second length of optical fiber comprise first and second fibers, respectively, separated by free-space optics between free ends of the first length of optical fiber and the second length of optical fiber.

17. 14. The apparatus of claim 13, wherein the perturbation device comprises a voice coil coupled to the second length of fiber.

18. 14. The apparatus of claim 13, wherein the perturbation device comprises the internal geometry of the second length of optical fiber.

19. 14. The apparatus of claim 13, wherein the first length of optical fiber is a single mode fiber.

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