A more elegant and scalable extension of multiple laser beams could be achieved by spatially shaping a single, wide-area high power beam as was recently demonstrated at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory [ref Heebner]. Beam shaping in NIF is achieved using an Optically-Addressable Light Valve (OALV), and is performed for the purpose of reducing the intensity of portions of the beam which would otherwise cause or exacerbate laser damage on certain optics. The ability to temporarily shadow, or “block” these few isolated sites from high fluence laser pulses has enhanced operational flexibility by enabling uninterrupted use of NIF at near peak laser performance until such time as the optic can be removed, repaired offline either through recycling or refinishing, and replaced [10]. However, the application and requirements of an OALV for laser materials processing, e.g. additive manufacturing, has not been explored. Here we propose an alternative method for creating additively manufactured parts using a wide-area, optically-addressable photomask supplied with a hybrid light source consisting of a low cost diode laser array and a Q-switched pulsed laser. Since the majority of the heat input is supplied by the diode laser component, our system can be shown to scale more effectively as opposed to the more costly fiber laser-based counterparts even with the addition of the pulsed laser source. The novel use of an OALV [11, 12], not only allows addressing on and off regions of the build but also offers the potential to ‘grayscale’ an image providing selective thermal gradients to be imposed to control residual stress and microstructure. 2
Figure 1 displays the optical systems used for layer-by-layer printing using our hybrid two laser, OALV-based approach. Figure 1(a) shows the Diode-based Additive Manufacturing (DiAM) laser system which is comprised of a set of optical subsystems. These subsystems are a set of diode lasers, an incoherent beam combining optical system, a short pulse laser and delivery optics, the OALV, image projector, and image delivery optics to the print plane. Figure 1(b) shows the layered structure of the OALV and illustrates its basic functionality. From left to right, the OALV consists of a BK7 glass substrate, a layer of conductive indium tin oxide (ITO), a short gap defined by spacer spheres, a twisted nematic liquid crystal layer, an additional spacer gap, a photoconductive Bismuth Silicon Oxide (BSO) layer, and a final conductive ITO layer. When 470 nm light from an incoherent projector is incident on the photoconductive layer, there is a local short to the BSO layer causing the LC to align with the applied field, thus disabling polarization rotation. Additional details regarding the OALV device performance are discussed below. The laser diode sources are made up of a set of four diode arrays (60 individual bars each; Trumpf, Germany) and can produce 1.25 kW each in continuous wave (CW) operation, 4.8 kW total at 150 A. The laser output can be pulsed with a width as short as 80 μs at full peak intensity with a center wavelength of 1007 nm and a +/−3 nm FWHM bandwidth. A set of turning mirrors and lenses spatially multiplexes the output of the four diode arrays onto a homogenizer. The homogenizer is a hollow rectangular cuboid with gold plating along the inner surface. The purpose of the homogenizer is to take a non-uniform tiled beam input and convert it to a uniform square output with a large number of modes (low speckle size) in a rectangular 6x6 mm2 shape. Following the homogenizer is a polarizer and a set of relay optics that image the homogenizer output onto the OALV with a de-magnification ratio of 2.9. This results in a uniform intensity 18x18 mm2 square spot at the OALV. In addition to the diodes, a 7 ns short pulse beam from a Q-switched Nd:YAG laser (Continuum, USA) is used to finally initiate the process and melt the powder. A custom powder spreader was used for DiAM builds. 1” pistons are adjusted using motorize lab jacks that have better than 1 μm repeatability. The recoater is mounted on a translation stage with a maximum velocity of 200 mm/s. The build piston is moved down 50 μm to set the layer thickness before recoating and at the end of printing is lifted up to remove the substrate with the printed part attached to it. The entire spreading mechanism is placed in a standard glove box that is purged with ultrahigh purity (99.999%) argon. Sn powders were acquired from Goodfellow, USA. In what is presented here, the OALV was used in a binary-on/off configuration. However, as shown below and described previously [13], OALVs can be easily operated in grayscalemode by appropriately scaling the intensity in each pixel of the 470 nm projector image. The OALV transmission versus incident 470 nm intensity depends on the voltage and frequency at which the OALV is driven. When operating in the binary configuration it is desirable to use settings that produce a sharp onset of IR transmission as opposed to a gradual rise with input intensity. One major advantage of using an OALV as a beam shaper for additive manufacturing is that the input beam does not need to be single mode or low divergence. Unlike alternate methods which rely on scanning a beam with a small spot size along the print
plane, this method can generate an entire print layer in one pulse using lasers with high divergence and a large number of modes. Laser diode arrays with linearly polarized output are well-suited for this task. The primary advantage of using low cost diodes to selectively melt material is the ease in scaling up to larger areas and correspondingly higher powers using spatial or wavelength multiplexing of sources. However, when considering the use of this technique in material processing of micron-scale layers of powders, the temporal characteristic of the incident beam and thermal properties of the material must be considered. For a 20 ms pulse and a thermal diffusivity D~0.11 cm2/s, the thermal diffusion length in Sn powder is approximately given by L 2 D τ = ~0.94 mm which would negate the fine feature scale afforded by the optical resolution of the system in the x-y plane and the powder particle size in the z-axis. In order to precisely modify the energy deposition to limit the extent of material phase change, we introduce a second laser source with a pulse length τs << L2/4D. This additional short-pulse beam is generated from the 7 ns Nd:YAG laser source with a maximum energy of 2 J at 1064 nm and is combined with the 1007 nm diode beam using a dichroic filter just prior to the OALV (see Fig. 1). Finally, a projector image at 470 nm is combined with the two laser beams using another dichroic mirror. The projector image and the image of the homogenizer output are coincident on the OALV. The OALV rotates the polarization of the “off” pixels, while the “on” pixels remain in their original orientation. A final polarizer after the OALV rejects the light corresponding to the “off” pixels while allowing the light corresponding to the “on” pixels to pass through to the final image relay optics and the build plane. 3. OALV operation and performance Conventional liquid crystal (LC) light modulators such as those found in LC-based displays or projectors generally consist of a 2D array of discrete LC cells or pixels. The transmission of each pixel is addressed on an individual basis through a matrix of applied voltages. By contrast, OALVs based on liquid crystals consist of a single, large liquid crystal cell that is addressed by a 2D light image. The local intensity of the address image controls the local transmission of a patch of the OALV. This addressing mechanism is achieved with a photoconductive layer inserted in series with the LC layer, both surrounded by electrodes that provide a common voltage across the entire device. Illuminating the photoconductive layer with a light image above the bandgap of the photoconductor locally shorts patches where high intensity is present enabling the voltage to be applied locally across the liquid crystal layer. Where no address light is present, the voltage resides across the high impedance photoconductive layer. Because of this unique property, LC OALVs have been used for niche applications including light-by-light switching, ultrafast pulse shaping, and laser beam shaping [14–17]. Optical addressing circumvents the need to fabricate a pixelated matrix with electrical backplane. Through this approach, the auxiliary addressed image can be patterned with a conventional, pixelated electrically addressed modulator. This neatly divides the challenges into two subcomponents: 1) auxiliary images can be created at low power, low coherence, and can leverage any of a number of commercial LC arrays and 2) the OALV does not require an electrical backplane and hence can be scaled to large apertures and handle high powers [15]. An example light pattern based on an image resolution target is shown in Fig. 2. The OALV used in this system implements BSO as the photoconductor which is addressed at 470 nm and modulates the amplitude of the diode light (through polarization rotation). It can support a 50 μm spatial resolution at the OALV image plane, 10 ms refresh rate, and up to 100:1 extinction ratio over a 22 mm × 34 mm clear aperture. Figure 2 shows the image resolution target at three planes of interest for comparison. The digital image is sent to the projector, as shown in Fig. 2(a). The projector image is relayed to the light valve plane (b) where the image projector light (470 nm), diode beam (1007 nm), and short pulse beam (1064 nm) all intersect. As described above, the liquid crystal region of the OALV rotates the polarization of the
incoming IR beams when there is no incident 470 nm light on a particular pixel. The rotated beam is then rejected by a subsequent polarizer with an efficiency of 95%. However, the OALV responds to the 470 nm “on” regions by preventing rotation, allowing the IR beams to locally pass through the polarizer and propagate the image to the build plane shown in Fig. 2(c). The imaging target shown in Fig. 2(a) is scaled at the projector and undergoes a slight magnification of 0.85 at the relay to the OALV plane, finally being further magnified by 0.32 as compared to the original image size in the relay to the print plane. This target shows there are clearly discernable features in the OALV plane down to less than 40 μm (e.g. group 3, element 5), and features in the print plane down to less than 50 μm (group 1, element 6). For comparison, typical SLM systems operate with raster-scanning beam diameters in the range of 50-150 μm depending on the build chamber height and aperture size of the last optical element (which roughly determines the numerical aperture of the system).
Fig. 2. Comparison of (a) original digital (binary) file, (b) 470 nm wavelength projected light pattern incident on the OALV and (c) ~1 μm wavelength hybrid beam pattern projected onto the sample plane (false color). Images shown in (b) and (c) were collected at the OALV and sample planes respectively using a 12-bit digital camera. (d) Transmission and extinction (1/leakage) performance of the OALV, demonstrating the optical response and potential for grayscale operation at intermediate 470 nm intensities, e.g. 0-10 and ~5-35 mW/cm2 for 500 and 1500 Hz drive frequencies respectively. Quantitative contrast performance measurements are shown in Fig. 2(d) for two system configurations. Both measurements were taken with the OALV driven at 30 V, but the frequency was changed from 500 Hz, producing a sharp transmission rise from 0 to ~96%, to 1500 Hz, producing a more gradual increase from 0 to ~88%. Thus, the OALV can be frequency-tuned to adjust device response such that a more sensitive variation of transmission versus the projector light can be achieved. This grayscale adjustment could be useful for controlling residual stress or microstructure of a build based on tailored laser intensity profiles which can drive thermal gradients [18]. The data was collected by fixing an aperture immediately before the OALV so that the area is known. A variable attenuator was placed directly after the projector, and the total power through the aperture was calibrated to a power |
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