Patent Publication Number: US-2022212406-A1

Title: Three-dimensional printer resin curing system using Risley prisms

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Application Ser. No. 63/134,749 entitled “Three-dimensional printer resin curing system using Risley prisms”, filed 7 Jan. 2021, the contents of which are incorporated herein by reference in their entirety. 
    
    
     ORIGIN OF THE INVENTION 
     The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefore. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure generally relates to additive manufacturing systems, and more particularly to stereolithography (SLA) three-dimensional (3D) printer. 
     2. Description of the Related Art 
     Stereolithography (SLA) 3D printers currently achieve ˜50 μm resolution in lateral dimensions with a comparable layer thickness. Several models use a spatial light modulator (SLM) such as a liquid crystal on silicon (LCoS) or a digital micro-mirror devices (DMD) to generate a pattern in the liquid resin, hardening it with the ultra-violet (UV) light that illuminates the SLM. For a 2 k format printer, such an implementation yields a fixed 2.7″×4.7″ lateral build size. Higher resolution and/or the ability to adapt to larger build sizes is of interest for a variety of applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description of the illustrative embodiments can be read in conjunction with the accompanying figures. It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements. Embodiments incorporating teachings of the present disclosure are shown and described with respect to the figures presented herein, in which: 
         FIG. 1  illustrates a side view of an example three-dimensional printer resin curing system using a single pair of Risley prisms, according to one or more embodiments; 
         FIG. 2  illustrates a side view of an example three-dimensional printer resin curing system using first and second pairs of Risley prisms, according to one or more embodiments; 
         FIG. 3  illustrates a side view of an example three-dimensional printer resin curing system using a pair of Risley prisms and an annular Risley prism, according to one or more embodiments; 
         FIG. 4A  illustrates a side view of a pair of Risley prisms in first rotational positions to steering with no angle, according to one or more embodiments; 
         FIG. 4B  illustrates a side view of the pair of Risley prisms of  FIG. 4A  in second rotational steering positions to steering with an induced angle, according to one or more embodiments; 
         FIG. 5  illustrates a block diagram of a control system of three-dimensional printer resin curing system, according to one or more embodiments; and 
         FIG. 6  presents a flow diagram of a method for controlling at least one pair of Risley elements to control a pattern size of a three-dimensional printer resin curing system, according to one or more embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The present innovation provides an opto-mechanical solution for curing patterns in resin in a stereolithography (SLA) 3D printer. The invention primarily consists of a laser beam steering mechanism and laser beam conditioning for a laser and does not directly include mechanical housing. Build platform motion control (x, y, z) and light sources are discussed in alternate embodiments. 
     The function of Risley Prisms can be accomplished via any mechanism that results in a fixed angular offset of light. In the case of Risley Gratings [1], diffraction is used to generate the angular offset by sending all of the light into a particular nonzero diffracted order, typically in a polymerized grating form. This technique allows a very lightweight solution to Risley functions, reducing the demand on rotation stages and associated motor control as well as eliminating the chromatic aberration associated with the prism. The disadvantage of the Risley Grating is that 50% of the original source light is likely to be lost through a polarization process needed to reduce the grating to a single order output. In some cases, a circularly polarized source can be used in which case the loss would not occur. Achromatic Risley Gratings are available for custom purchase. 
     An additional method for the angular deviation of light is a particularly designed photonic crystal structure. In order for the photonic crystals to have the beam diverting properties necessary, they must be spatially variant photonic crystals [2-5]. These have recently been reported to be self-collimating while re-directing energy at an angle of 90 degrees [2-3]. Other, lower angular deviations are also possible using the same techniques. Designs could theoretically be extended to achromatic function. 
     Fabrication methods are generally specific to the photonic crystal design selected. As such, fabrication methods for this invention cannot be prescribed in general, but several methods are available in literature [4-9] and several methods have been patented [11-16]. 
     Several variants of the present innovation are possible, each intended to accomplish the same basic process of steering UV light around the print bed. The most fundamental but not necessarily optimal form will be described here, and other variations will be included in the Alternatives section. 
     A single pair of Risley elements directs the incoming light to an angle away from the optical axis. By adding a second pair of Risleys some distance away from the first pair, but centered on the optical axis, the light from the first pair of Risleys can be used as compensators and direct the light to the print bed normal to the print layer. 
     In one or more embodiments, the present disclosure is directed to an opto-mechanical solution for curing patterns in resin in a SLA 3D printer. The invention primarily consists of a laser beam steering mechanism and laser beam conditioning for a laser and does not directly include mechanical housing. Build platform motion control (x, y, z) and light sources are discussed in alternate embodiments. The present disclosure has the capability of providing ˜6 μm lateral resolution over a substantial region that depends on the desired print rates, cost and build platform motion options. Vertical resolution (layer thickness) would be accomplished with standard processes to reach layer thicknesses ˜25 μm, or use a much higher cost commercial off-the-shelf (COTS) vertical lift mechanism to drive the layer thickness down to something comparable to the expected 6 μm lateral resolution. 
     The primary innovation in this invention is the use of Risley prisms or their equivalents to steer a laser beam around a print area. The laser (or other light source) spot is generated using a cycloidal diffractive waveplate to achieve the small spot size without the restriction of lithography lens being in close proximity to the resin layer being hardened. This long standoff distance is critical to use with the proposed Risley steering system in the printer. 
       FIG. 1  illustrates an example three-dimensional printer resin curing system using a single pair of Risley prisms.  FIG. 2  illustrates an example three-dimensional printer resin curing system using first and second pairs of Risley prisms.  FIG. 3  illustrates an example three-dimensional printer resin curing system using a pair of Risley prisms and an annular Risley prism. 
     Risley prisms are basic wedge prisms that have been used for decades to provide optical pointing and steering functions.  FIG. 4A  illustrates a pair of Risley prisms in first rotational positions to steering with no angle.  FIG. 4B  illustrates the pair of Risley prisms of  FIG. 4A  in second rotational steering positions to steering with an induced angle. Typically, the Risley prisms exist in pairs and are rotated independently of each other, allowing energy to be steered over a hemisphere of space, with the limitation in angular precision being established by the precision of rotation of the prisms. The steering function is a result of the refraction-induced angular change of light passing through the prism. The magnitude of angular change is fixed for a given prism but the direction of the output light can be adjusted by rotating the prism. 
       FIG. 5  illustrates a block diagram of a three-dimensional printer resin curing system  500  that enables a larger production size of a conventional additive manufacturing system  502 . using a controller  504  controls an optical steering system  506  that can expand a size a pattern produced by a conventional additive manufacturing system  502  using one or more pairs of Risley elements  506   a - 506   b  and  508   a - 508   b . In particular, the controller  504  rotates each Risley element  506   a - 506   b  and  508   a - 508   b . In one or more embodiments, Risley elements  508   a - 508   b  are annular Risley elements. A laser  509  of the conventional additive manufacturing system  506  projects a laser beam  510  that is reduced in size by a polarization grating point image  512  to a reduced size laser beam  514 . A shutter  516  selectively allows the reduced size laser beam  514  to pass through to the first pair of Risley element  506   a - 506   b  to increase an angular offset from a longitudinal axis of the optical steering system  506  to a current print layer  518 . In one or more embodiments, the second pair of Risley elements  508   a - 508   b  can further expand or reduce the angular offset. Risley elements  506   a - 506   b  and  508   a - 508   b . are respectively held in holders  520   a - 520   d . The controller  502  drives rotation actuators  522   a - 522   d  to respectively and independently rotate the holders  520   a - 520   d . In particular, the controller  502  includes a processor  524  that accesses an optical steering application  526  stored in device memory  528  to execute the rotations via a device interface  530  according to a lookup table  532  that defines rotational positions required for Risley elements  506   a - 506   b  and  508   a - 508   b.    
       FIG. 6  presents a flow diagram of a method for controlling at least one pair of Risley elements to control a pattern size of a three-dimensional printer resin curing system. The method  600  includes emitting a laser beam by an additive manufacturing system (block  602 ). The method  600  includes spatially reducing the laser beam to a reduced laser beam using a polarization grating (e.g., cycloidal diffractive waveplate (CDW)) (block  604 ). The method  600  includes determining a change in an angular offset of the laser beam that corresponds to an specified increase in spatial size of a pattern of a current layer produced by the additive manufacturing system (block  606 ). The method  600  includes determining independent rotations of the at least one pair of Risley elements that produce the change in angular offset for the laser beam (block  608 ). The method  600  includes independently rotating a first pair of longitudinally aligned Risley elements to direct the star simulation laser beam onto a pre-defined location on a current print layer (block  610 ). The method  600  includes opening a shutter to enable the reduced laser beam to cure resin at the pre-defined location (block  612 ). Then method  600  returns to block  602 . 
     The following references cited above are hereby incorporated by reference in their entirety:
     (1) Chulwoo Oh, Chulwoo Oh, Jihwan Kim, Jihwan Kim, John F. Muth, John F. Muth, Michael J. Escuti, Michael J. Escuti, “A new beam steering concept: Risley gratings”, Proc. SPIE 7466, Advanced Wavefront Control: Methods, Devices, and Applications VII, 74660J {11 Aug. 2009); doi: 10.1117/12.828005; https://doi.org/10.1117/12.828005.   (2) Rumpf, R. C., Pazos, J. J., Digaum, J. L., &amp; Kuebler, S. M. (2015). Spatially variant periodic structures in electromagnetics. Philosophical Transactions of the Royal Society A: Mathematica,l Physical and Engineering Sciences, 373(2049).   (3) Jennefir L. Digaum, Rashi Sharma, Daniel Batista, Javier J. Pazos, Raymond C. Rumpf, Stephen M. Kuebler, “Beam-bending in spatially variant photonic crystals at telecommunications wavelengths”, Proc. SPIE 9759, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics IX, 975911 (14 Mar. 2016).   (4) Pazos, j. (2010). Digitally manufactured spatially variant photonic crystals. Phd. University of Texas at El Paso.   (5) Liu, Longju &amp; Hurayth, Abu &amp; Li, Jingjing &amp; Hillier, Andrew &amp; Lu, Meng. (2016). A strain-tunable nanoimprint lithography for linear variable photonic crystal filters. Nanotechnology. 27.295301.   

     While the disclosure has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular system, device or component thereof to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiments disclosed for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. 
     In the preceding detailed description of exemplary embodiments of the disclosure, specific exemplary embodiments in which the disclosure may be practiced are described in sufficient detail to enable those skilled in the art to practice the disclosed embodiments. For example, specific details such as specific method orders, structures, elements, and connections have been presented herein. However, it is to be understood that the specific details presented need not be utilized to practice embodiments of the present disclosure. It is also to be understood that other embodiments may be utilized and that logical, architectural, programmatic, mechanical, electrical and other changes may be made without departing from general scope of the disclosure. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present disclosure is defined by the appended claims and equivalents thereof. 
     References within the specification to “one embodiment,” “an embodiment,” “embodiments”, or “one or more embodiments” are intended to indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present disclosure. The appearance of such phrases in various places within the specification are not necessarily all referring to the same embodiment, nor are separate or alternative embodiments mutually exclusive of other embodiments. Further, various features are described which may be exhibited by some embodiments and not by others. Similarly, various requirements are described which may be requirements for some embodiments but not other embodiments. 
     It is understood that the use of specific component, device and/or parameter names and/or corresponding acronyms thereof, such as those of the executing utility, logic, and/or firmware described herein, are for example only and not meant to imply any limitations on the described embodiments. The embodiments may thus be described with different nomenclature and/or terminology utilized to describe the components, devices, parameters, methods and/or functions herein, without limitation. References to any specific protocol or proprietary name in describing one or more elements, features or concepts of the embodiments are provided solely as examples of one implementation, and such references do not limit the extension of the claimed embodiments to embodiments in which different element, feature, protocol, or concept names are utilized. Thus, each term utilized herein is to be given its broadest interpretation given the context in which that terms is utilized. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope of the disclosure. The described embodiments were chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.