System and method for high throughput and high-resolution volumetric additive manufacturing using time multiplexed computer-generated holograms

Abstract

A method for volumetric manufacturing of a three-dimensional object, comprising the steps of spatially modulating a first optical beam (300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300) with a first spatial light modulator (310, 410, 510, 610, 710, 810, 910, 1010, 1110, 1210, 1310), to generate a first spatially modulated optical beam (320, 420, 520, 620, 720, 820, 920, 1020, 1120, 1220, 1320), and introducing said first modulated beam (320, 420, 520, 620, 720, 820, 920, 1020, 1120, 1220, 1320) at a first projection direction into a photosensitive resin (350, 450, 550, 650, 750, 850, 950, 1050, 1150, 1250, 1350) that is provided in a photosensitive resin container (340, 440, 540, 640, 740, 840, 940, 1040, 1140, 1240, 1340), so as to generate a three-dimensional light intensity distribution (360, 460c, 660, 760, 860, 1060, 1260) in said photosensitive resin (350, 450, 550, 650, 750, 850, 950, 1050, 1150, 1250, 1350), wherein the first spatially modulated optical beam provides time multiplexed pattern projections. The present invention is furthermore related to a device for performing said method.

Description

[0001] SYSTEM AND METHOD FOR HIGH THROUGHPUT AND HIGH-RESOLUTION VOLUMETRIC ADDITIVE MANUFACTURING USING TIME MULTIPLEXED COMPUTERGENERATED HOLOGRAMS

[0002] Field of the invention

[0003] Background

[0004] The conventional approach in three-dimensional printing (3D printing) or additive manufacturing (AM) relies on adding material in a layer-by-layer fashion. This fabrication process is based on a sequential operation which consists of constructing the 3D object by piling two dimensional layers on top of each other. An example is stereolithography (SLA) (see for example US-5,344,298) , where the object is formed one layer at a time by the solidification of a photocurable resist under light irradiation before application of a subsequent layer.

[0005] The successive layers of the object can be defined for example by scanning a laser beam point-by-point , as suggested in US- 5,344,29, or by digital light processing (DLP) technology, as described in US-6,500,378. This sets some limitations on the applications and geometries for which this technique is suitable, such as printing onto a substrate or around a pre-existing structure .

[0006] In contrast, a true 3D printing approach was disclosed ("Volumetric additive manufacturing via tomographic reconstruction", Science 363, 6431 (2019) ; and "High-resolution tomographic volumetric additive manufacturing (TVAM) ", Nature communications 11, 1 (2020) ) . The idea, as described in WO 2019 / 043529 Al and US- 10, 647, 061, consists of irradiating a volume of transparent and photo-responsive material with computed two-dimensional light patterns from multiple angles . Typically, the number of angles are between 200 to 600 per revolution . The light exposure results in a volumetric energy dose which is suf ficient to solidi fy the material in the desired geometry . The main advantages of this method compared to existing methods are its very rapid manufacturing time ( down to a few tens of seconds ) , and its ability to print complex hollow structures without the need for support structures as required in layer-by-layer manufacturing systems .

[0007] To achieve a correct three-dimensional light dose deposition in the build volume , the light patterns proj ected from multiple angles must illuminate the entire build volume . This restricts the printing technique to mainly transparent materials ( i . e . small amount of scattering of light can be corrected) and an absorbance value which allows signi ficant light propagation to the centre . This sets , in particular, an upper limit on the photoinitiator concentration in the material to be exposed to the light patterns .

[0008] The tomographic-based methods are called volumetric because they depart from the layered fabrication to construct the obj ect in a true three-dimensional fashion . The build volume is determined by the cross section of the proj ected patterns (x, y) and the diameter of the resin container ( z ) in which the material to be exposed is provided . Obj ects with scale of the order of 2 cm x 2 cm x 2 cm with resolution of 80 micrometers have been manufactured ("Volumetric Bioprinting of Complex Living-Tissue Constructs within Seconds" , Adv Mater 31 , 1970302 ( 2019 ) ) . Resolution down to 30 micrometers have been recently achieved in a smaller volume ("Volumetric additive manufacturing of silica glass with microscale computed axial lithography", Science, 376(6590) 2022) . As one could see in this work, the final resolution of the printing structure is determined by the effective pixel size of the projected images at the center of the build volume. A number of effects, such as the diffusion of chemical species or the optical aberrations inherent to the setup, the wobble of the vial container, the non-constant thickness of the vial container, can drastically decrease the resolution of volumetric printing methods.

[0009] It was demonstrated by Schusteff et al. ("One-step volumetric additive manufacturing of complex polymer structures", Science Advances, Sci Adv, 3 (212) , 2017) that 3D aperiodic structures can be realized using the orthogonal interferometric intersection of three spatially modulated beams in a resin container. The type of 3D structures that can be realized by the interferometric intersection of the three spatially modulated beams is limited to the number of 2D pixels in a spatial light modulator (SLM) , which is typically of the order of 1 million to 10 million pixels for the larger spatial light modulators on the market. These pixels are then distributed in the 3D resin container by holographic projection (near the Fourier plane of the projection lens) . A 3D object of volume Ixlxl cm having 10 pm resolution requires 1000 mega voxels, which is 2 orders of magnitude more than the number of pixels in the largest commercial SLM.

[0010] The orthogonal interferometric intersection of the three beams provides an optimal spatial resolution which is only limited by the numerical aperture NA of the Fourier lens. This is so, because the lateral resolution is approximately equal to X / 2NA and the axial resolution is approximately equal to 2X / NA2, wherein X is the wavelength of the light beams. Thus, the limitation of the one-step technique is not the resolution but rather the lack of a suitably high number of pixels on the SLM.

[0011] The TVAM approach above provides the missing number of voxels by providing, cumulatively, a high (200-600) number of projections. Such a high number of frames obtained by rotation brings challenges such as, generating a rotating vial, obtaining a low wobble rotation, and optical aberrations caused by vial. It has been shown that all this can be corrected, but the correction adds complexity and cost to the system.

[0012] A related approach using multiple beams (more than 3) is proposed by Chi Chung Li ("Multi-beam phase mask optimization for holographic volumetric additive manufacturing" in https: / / doi.org / 10.48550 / arXiv.2401.15590) . Here multiple beams are configured from discrete angles to form a 3D object.

[0013] There is a need for a method of printing arbitrary 3D objects in a volumetric manner which overcomes the above discussed problems from the prior art.

[0014] Summary of the invention

[0015] The present invention is related to new strategies and detailed protocols to print arbitrary 3D objects in a volumetric manner.

[0016] In detail, the present invention is related to a method for volumetric manufacturing of a three-dimensional object, comprising the steps: spatially modulating a first optical beam with a first spatial light modulator, to generate a first spatially modulated optical beam, introducing said first modulated beam at a first proj ection direction into a photosensitive resin that is provided in a photosensitive resin container, so as to generate a three-dimensional light intensity distribution in said photosensitive resin, characteri zed in that the first spatially modulated optical beam provides time multiplexed pattern proj ections .

[0017] In general terms , the invention proposes to use a single proj ection angle or at most two proj ections angles and using time multiplexed pattern proj ections to create a time variant 3D dose distribution, which upon incoherent addition, approximate the 3D shape of the desired 3D obj ect . Because the photosensitive resins have a threshold dose for solidi fication, the final three- dimensional obj ect is constructed with high fidelity . Furthermore , by combining lateral motion, discrete or continuous , of the proj ection system orthogonal to the axial direction of the pattern proj ection, larger three-dimensional obj ects can be constructed as well with increased axial resolution .

[0018] According to the present invention, time multiplexing refers to sending di f ferent pattern proj ections sequentially in time . This can be achieved, for example , by generating pattern proj ections such as two-dimensional computer-generated holograms ( CGH) on a spatial light modulator ( SLM) , and sending those pattern proj ections sequentially into a volume of a photosensitive resin where they are incoherently summed ( i . e . accumulated) . The generation of pattern proj ections such as two-dimensional computer-generated holograms ( CGH) on a spatial light modulator ( SLM) is known in the art .

[0019] According to the present invention, the pattern proj ections are generated by directing an optical beam from a light source such as a laser onto a spatial light modulator ( SLM) , which generates a spatially modulated optical beam that exhibits the pattern proj ection displayed on the spatial light modulator .

[0020] According to preferred embodiments of the present invention, a second optical beam is provided and spatially modulated with a second spatial light modulator to generate a second spatially modulated optical beam that provides time multiplexed pattern proj ections and is introduced at a second proj ection direction through the same surface of the photosensitive resin container as the first spatially modulated optical beam into the photosensitive resin .

[0021] Preferably, the first and second proj ection direction form an angle of intersection, preferably 90 ° . However, it is also possible to arrange these proj ection directions parallel to each other, in order to illuminate larger portions of the photosensitive resin container in parallel , thus improving manufacturing speed .

[0022] According to preferred embodiments of the present invention, the time multiplexed pattern proj ections are optimi zed by comparing initially guessed pattern proj ections with the three-dimensional obj ect to be manufactured over a loss function and minimi zing the loss function i f the result of the comparison of the initially guessed pattern proj ections with the three-dimensional obj ect is not suf ficient . This will be explained hereinafter in detail .

[0023] According to preferred embodiments of the present invention, the spatially modulated light beams are introduced into the photosensitive resin via a first and optionally a second proj ection lens that is preferably at least partly immersed into the photosensitive resin . This avoids any aberrations that may be generated at the interface between di f ferent media, such as air and ( liquid) photosensitive resin .

[0024] The present invention is also related to a device for performing the method described herein, comprising a first proj ection system comprising a light source for generating a first optical beam, a first spatial light modulator configured to generate a first spatially modulated optical beam from said first optical beam that can provide time multiplexed pattern proj ections , and optionally at least a first proj ection lens ; a photosensitive resin container for a photosensitive resin, wherein said photosensitive resin container has at least one surface that is transparent for the spatially modulated optical beam .

[0025] According to preferred embodiments of the present invention, the device furthermore comprises a second proj ection system comprising a second spatial light modulator configured to generate a second spatially modulated optical beam that can provide time multiplexed pattern proj ections , and optionally a second proj ection lens .

[0026] The main component of the first and optionally second proj ection system is a spatial light modulator ( SLM) that may be preferably selected from the group consisting of a binary amplitude digital micromirror device , a piston light mirror, and a liquid crystal display . Such SLMs are known in the art . The first and optionally second proj ection system contain a light source from which light can be directed onto the SLMs . According to the present invention, a spatially coherent light source is required that provides means for interference such that the computer-generated holograms ( CGH) generate a 3D pattern of intensity . As an example , lasers may be mentioned .

[0027] As will be discussed below, the optical beam emitted by said light source is capable of modi fying the phase of the photosensitive resin, for example from liquid to solid, thereby hardening the photosensitive resin and forming the desired 3D obj ect .

[0028] Preferably, the spatially modi fied optical beam that is emitted from the SLM is directed into the photosensitive resin container through a proj ection lens . Any proj ection lens , such as those typically used in volumetric manufacturing, but not limited thereto , can be used in the present invention .

[0029] In embodiments where several , for example two spatially modi fied optical beams are directed into the photosensitive resin container, these spatially modi fied optical beams can be provided through separate arms each comprising separate proj ection systems , or by dividing an optical beam in a beam-splitter . Both variants are shown below with respect to the embodiments in the drawings .

[0030] According to preferred embodiments of the present invention, the device furthermore comprises a lateral motion system for lateral movement of the first and optionally the second proj ection system, with respect to the photosensitive resin container . Thus , the first and optionally the second proj ection direction are modi fied by a lateral movement of the optical axis of the first and optionally second spatially modulated optical beam . This allows illuminating larger parts of the volume of the photosensitive resin during the method of the present invention .

[0031] According to preferred embodiments of the present invention, the device furthermore comprises a vertical motion system for vertical movement of the photosensitive resin container towards the first and optionally the second proj ection system, and / or for vertical movement of the first and optionally the second proj ection system towards the photosensitive resin container . This allows modi fying the depth with which the spatially modi fied optical beam enters the photosensitive resin . Moreover, in embodiments where the spatially modi fied optical beam is directed into the photosensitive resin through a proj ection lens , by said vertical movement it can be achieved that the proj ection lens is at least partially immersed in the photosensitive resin . Especially preferred, only a portion of the proj ection lens that has a small diameter ( in particular a smaller diameter that at least another portion of the lens ) is immersed in the photosensitive resin .

[0032] According to the present invention, the photosensitive resin container does not have to be cylindrical , as in the prior art method shown in Fig . 2 . The photosensitive resin container can have any form . According to one embodiment of the present invention, the container can be flat (wafer-like ) with an upper surface being transparent for the spatially modi fied optical beam, i . e . the spatially modi fied optical beam has to be capable of passing into the photosensitive resin container . According to another preferred embodiment of the present invention, the container is a cuboid or a rectangular cuboid ( e . g . cuvette-like ) . At least one surface , preferably one or two surfaces that are orthogonal to each other, of the photosensitive resin container has to be transparent for the spatially modified optical beam, i.e. the spatially modified optical beam has to be capable of passing into the photosensitive resin container.

[0033] According to preferred embodiments of the present invention, the photosensitive resin container may comprise compartments, such as in multi-well plates that are well-known in the art of biology of high-throughput screening. This will be discussed in detail below.

[0034] According to preferred embodiments of the present invention, the device furthermore comprises an optical measurement system, for providing a feedback control.

[0035] The present invention is illustrated with respect to several embodiments .

[0036] According to a first embodiment, a single projection direction from one side of a photosensitive resin container through its transparent window is presented. The projection consists of a two-dimensional computer-generated hologram (CGH) displayed on a spatial light modulator (SLM) , which can be, but is not limited to, a binary amplitude digital micromirror device, a piston light mirror (PLM) , or a liquid crystal display, phase or amplitude. The CGH is designed to provide a three-dimensional light intensity distribution in the photosensitive resin. By multiplexing i.e. sending different CGHs sequentially in time, the photosensitive resin accumulate (incoherently sum) , the 3D intensity distribution generated by each individual CGH. An optimization algorithm is devised to optimize individual time multiplexed CGHs so as to form the desired three-dimensional object upon thresholding the 3D dose. The latter corresponds to the hardening of the resin from liquid to solid. According to a second embodiment, two independent projection systems of the type as described in the first embodiment provide an intersection of the two projected beams in the photosensitive resin container. The preferred angle of intersection is approximately, but not restricted to, 90 degrees so as to obtain an effective spatial resolution of the voxel in an xyz coordinate system of the order of X / 2NA. In this embodiment, the two beams go through the same side of the container.

[0037] According to a third embodiment, the single projection system of embodiment 1, wherein the projection system is composed of either a combination or single lens and / or microscope objective and / or gradient index lens, is immersed in the photosensitive resin container to avoid the presence of an interface with air or the transparent photosensitive resin container to decrease optical aberrations.

[0038] According to a fourth embodiment, a lateral motion orthogonal to the direction of the light in the projection system of embodiment 1 or 2 is added so as to increase the lateral size of the three-dimensional object. The lateral motion is preferably continuous; however it is possible to conceive the lateral motion in discrete steps as well. Furthermore, due to the numerical aperture (NA) of the projection system which can be from approximately 0.01 to 1.5, and the lateral motion of the projected patterns, every voxels in the resin container is sequentially illuminated, via the lateral motion, by the different angles within the said NA of the projection system. This multiple exposure, both time multiplexed and via lateral motion enhances the axial resolution and increase the size of the 3d printed object beyond the fi eld of view of the projection system. According to a fi fth embodiment , the photosensitive resin container in embodiment 1-3 can be composed of many individual compartments , such as microplates also called well plates , known in biology containing a number of individual , compartmentali zed wells , typically, but not restricted to 24 , 96 , 384 , 1536 wells . According to a sixth embodiment , the two proj ections systems of embodiment 5 are incident on two di f ferent sides of the photosensitive resin container . For example , but not limited to , the photosensitive resin container is a rectangular cuvette having 4 polished sides making an angle of 90 degrees . A first proj ection beam is incident on one side and the second proj ection beam is incident on an orthogonal side .

[0039] According to a seventh embodiment , each of the system presented in embodiment 1 to 6 is presented with an optical measurement system capable of measuring the change in index of refraction, in situ, during the fabrication of the 3d obj ect . The information of the 3D measurement can be used to stop the proj ection and / or used as a feedback signal to change the proj ection patterns .

[0040] BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The invention is described hereinafter in detail with non-limiting preferred embodiments and non-limiting drawings , wherein :

[0042] Fig . 1 is a scheme of principle of a prior art method using three beam interference to produce a desired 3d obj ect as described in the publication "One-step volumetric additive manufacturing of complex polymer structures" , Science Advances 2017 .

[0043] Fig . 2 is a schematic illustration describing the prior art method of tomographic volumetric additive manufacturing to produce a 3D object m a vial according to the description in publications "Volumetric additive manufacturing via tomographic reconstruction" Science, 2019 and "High-resolution tomographic volumetric additive manufacturing", Nature Communications, 2020.

[0044] Fig. 3a is a schematic illustration representing a single optical beam, time multiplexed, directed into a photosensitive resin, according to the first embodiment of the present invention that can be used to print a 3D object.

[0045] Fig. 3b are pictures of a physical simulation of a 3D printed object realized with the embodiment of Fig. 3a.

[0046] Fig. 4a is a schematic illustration representing two optical beams, time multiplexed, intersecting non interf eromet- rically in a photosensitive resin, according to the first embodiment of the present invention that can be used to print a 3D object.

[0047] Fig. 4b illustrate a 90 degree geometry crossing of the two modulated beams method of Fig. 4a.

[0048] Fig. 4c is a physical simulation of a 3D printed object realized with the invention of Fig. 4a.

[0049] Fig. 5a presents an embodiment of Fig. 3a wherein the focusing element is dipped into the photosensitive resin contained in wafer-like container to produce a 3D object.

[0050] Fig. 5b presents another embodiment of Fig. 5a wherein the focusing element is dipped into the photosensitive resin contained in wafer-like container and a relative continuous motion of the focusing element provides a larger 3d object than the field of view of the focusing element.

[0051] Fig. 6 presents an embodiment wherein two or more single beam system as described in Fig. 3a are used on a single wafer filled with photosensitive resin to increase production speed. Fig . 7 presents an embodiment of Fig . 3a wherein the focusing element is dipped into the photosensitive resin contained in a multi-well container to produce one or more 3D obj ects in each well .

[0052] Fig . 8 presents an embodiment of Fig . 7 wherein two or more single beam systems as described in Fig . 3a are used in a photosensitive resin contained in a multi-well container to produce one or more 3D obj ects in each well .

[0053] Fig . 9 presents an embodiment wherein the focusing system described in Fig . 3a contains a thin focusing element , such as a gradient index lens ( GRIN) that is dipped in the wafer filled with photosensitive resin . The thin optical element produces less drag when moved into the resin .

[0054] Fig . 10a presents an embodiment that uses two optical beams as described in figure 4a, using only one spatial light modulator, wherein the focusing of the beam into the photosensitive resin occurs through an air / resin interface .

[0055] Fig . 10b presents another embodiment of Fig . 10a that uses two optical beams , using only one spatial light modulator, wherein the focusing system is dipped into the photosensitive resin .

[0056] Fig . 10c presents another embodiment of Fig . 10a that uses two optical beams , using only one spatial light modulator, wherein the focusing of the beam into the photosensitive resin occurs through a glass corrected cover-slip ( glass / resin interface ) .

[0057] Fig . 11 presents an embodiment that uses two optical beams as described in Fig . 4a, using only one spatial light modulator, wherein the focusing of the two beams is made from di f ferent sides of a corrected glass cuvette . Fig . 12 present another embodiment of Fig . 10a wherein the photosensitive resin container is a multi-well container such as used in biological sciences for high throughput screening .

[0058] Fig . 13 presents yet another embodiment of Fig . 12 , wherein the focusing optics contains a thin focusing element , such as a gradient index lens ( GRIN) that is dipped in the wafer filled with photosensitive resin . The thin optical element produces less drag when moved into the resin .

[0059] Fig . 14 presents an in-situ optical measuring system that monitors the fabrication of the 3D obj ect fabricated using any of the embodiments described in any of the above-described figures .

[0060] In the drawings , reference number that share the last two numbers designate the same component . For example , the reference numbers 330 , 430 , 530 etc . stand for a proj ection lens .

[0061] DETAILED DESCRIPTION

[0062] In tomographic volumetric additive manufacturing (herein also designated as volumetric manufacturing) , a resin container comprising photo-responsive material is illuminated with patterns of light from multiple angles . The resin (photo-responsive material ) has the property to be photo-sensitive , which means here that a photo-initiator under light exposure can trigger the solidi fication of the material . The main advantage of this technique is the capability to rapidly print a full centimeter-scale obj ect at once with high resolution .

[0063] A first approach involving a single shot fabrication is taught in Schustef f et al . ("One-step volumetric additive manufacturing of complex polymer structures", Science Advances, Sci Adv,

[0064] 3 (212) , 2017) .

[0065] Such an apparatus from the prior art is shown in Fig 1. A light beam 100 is incident on a liquid crystal spatial light modulator 110. A lens 130 is placed at a distance equal to its focal length from the modulator 110. A container 140 having inside a photosensitive resin is placed at a distance equal to the focal length of the lens 130. Three different areas of the modulator 110 provide a specific spatial modulation on the beam 100. Specifically, a first portion of the modulator 110 provides a modulated beam 120 which is incident on the front surface of resin container 140. A second portion of the modulator 110, spatially separated from the said first portion, produces a spatially modulated beam 121. A third portion of the modulator 110, spatially separated from the said first and second portion, produces a spatially modulated beam 122. Beam 121 is reflected at ninety degree by a mirror 101 and enters the resin container 140 through a second surface of said container 140. Beam 122 is reflected at ninety degree by a mirror 102 and enters the resin container 140 through a third surface of said container 140. The three modulated beams 120,121 and 122 intersect at 90 degrees with respect to each other inside the resin container 140. The intersection provides an interference between the three beams which forms the desired 3D object.

[0066] Another apparatus and a method for tomographic additive manufacturing is described in detail in e.g. WO 2019 / 043529 Al or US 2018 / 0326666 Al.

[0067] Such an apparatus from the prior art is shown in Fig. 2. A light beam 200, which can be generated from a light source such as a laser, is directed to a spatial light modulator (SLM) 210 such as a DLP modulator, which produces computed light patterns 211 . The patterns represent proj ections of the obj ect to fabricate as seen from di f ferent rotational angles , and they are computed by a Radon trans form similarly to X-ray computed tomography . The light patterns 211 are guided by lenses 230 and 245 into a photosensitive resin container 240 , in order to form the desired 3D obj ect in the photosensitive resin 250 provided in said container 240 . The photosensitive resin container 240 is cylindrical and set into rotation while it is being irradiated from the side with computed light patterns 211 . The light patterns 211 are displayed in synchroni zation with the rotational movement of the photosensitive resin container 240 . At any given time , the intensity of a single light pattern 211 is insufficient to solidi fy the resin completely . However, after the photosensitive resin container 240 has been illuminated from every angle by all the light patterns 211 , a three-dimensional distribution of accumulated light dose is created . This three-dimensional dose distribution causes the resin to locally reach its gelation threshold, thus resulting in the solidification of the desired ob j ect .

[0068] There are a number of issues which makes the technique di f ficult to reach high resolution .

[0069] In particular, the rotation of the photosensitive resin container 240 has to be well centered, and the geometry of the photosensitive resin container 240 such as its wall thickness and non-perfect circular shape af fect the propagation of the patterns and thus the cumulative dose to create the 3D obj ect . The cylindrical shape of the photosensitive resin container 240 also produces aberrations of the optical beam . Although these ef fects could be taken into account by knowing the parameters of each photosensitive resin container 240 and correcting for it electronically, such an approach is not practical for high volume manufacturing as it would require to calibrate each photosensitive resin container 240 .

[0070] In the apparatus of Fig . 2 , these ef fects have to be compensated to the extent possible by an index-matching liquid bath 241 . Moreover, an observation system composed of a second light beam 275 generated from a light source such as a laser and which is directed orthogonally to the light patterns 211 into the photosensitive resin container 240 , lenses 291 and 292 , and a camera 293 is provided in order to allow an improved fabrication of the 3D obj ect by a feedback control . These additional elements increase the complexity of the system .

[0071] Here , a method is proposed to 3D print obj ects in a volumetric manner which does not require a rotation of a photosensitive resin container such as a vial , nor a cylindrical vial at all . The method is also yielding increased resolution due to a simpler set-up .

[0072] According to a first embodiment of the present invention, a concept is disclosed to signi ficantly improve the printing resolution of volumetric additive manufacturing .

[0073] The first embodiment of the present invention, described in a schematic illustration in Fig . 3a, presents a single proj ection system providing a single proj ection direction . An optical beam 300 of a wavelength suitable for photopolymeri zation of the photosensitive resin 350 provided in a photosensitive resin container 340 , is spatially modulated by a spatial light modulator 310 . According to the present invention, a spatial light modulator (SLM) is a component that is selected from the group consisting of a binary amplitude digital micromirror device (DMD) , a phase light modulator (called PLM and composed of an array of vertically actuated reflecting pistons) , and a phase liquid crystal display (LCD) . Such SLMs are known in the art.

[0074] A modulated beam 320 is shaped according to a computed-generated hologram (CGH) which is displayed on the SLM 310. The CGH is designed to provide a three-dimensional light intensity distribution 360, of which in Fig. 3a a few intensity voxel are illustrated, in the photosensitive resin container 340 located near the Fourier plane of a projection lens 330, by means of said projection lens 330.

[0075] The lens 330 can also be a compound system of lenses to provide an appropriate aberration correction. By time multiplexing, i.e. sending different CGHs sequentially in time, the photosensitive resin 350 accumulates (incoherently sums up) the 3D intensity distribution generated by each individual CGH.

[0076] An optimization algorithm is devised to optimize individual time multiplexed CGHs so as to form the desired three-dimensional object upon thresholding the 3D dose. The latter corresponds to the hardening of the resin from liquid to solid.

[0077] A simulation using wave optical propagation is performed and shown in Fig. 3b. Figure 3b shows the general working principle of the one-beam phase optimization according to the present invention . Picture I in Fig. 3b shows example patterns in Fourier space. Shown is an example of an optimized phase pattern, among multiple time multiplexed phase patterns, loaded on a spatial light modulator 310 that is configured as a phase modulator.

[0078] With a Fourier lens, the patterns are projected into the volume of a photosensitive resin. The field amplitude near the Fourier plane of a lens 330, is represented in picture II of Fig. 3b.

[0079] The result of incoherently summing the multiplexed optimized field amplitudes in a deposited energy dose in the photosensitive resin 350 is displayed as one 2D slice of the desired 3D object in picture III of Fig. 3b.

[0080] Picture IV of Fig. 3b is a schematic example of the polymer crosslinking conversion as a function of light dose. A polymerization threshold is indicated from a specific light dose. Below said dose, no conversion of the photosensitive resin 340 from liquid to solid occurs, whereas above said dose, the desired conversion of the photosensitive resin 340 from liquid to solid occurs .

[0081] Applying this threshold for polymerization (i.e. material hardening from liquid to solid) to the 2D slice in picture III of Fig. 3b yields the binary object in picture V of Fig. 3b. One can see that, the form of a section of the well-known Benchy boat (the white part of the image illustrated in picture VII of Fig. 3b) can be clearly seen although elongated. This is because the numerical aperture in the simulation was NA=0.5, which yields a rather large depth of focus compared to the lateral resolution . A NA2NA 1

[0082] A factor - - = — = - means that for NA=0 . 5 , the depth of fo- cus in the axial direction is 8 times larger than in the transversal direction . I f a large numerical aperture is used and / or a scavenging species to inhibit polymeri zation, such as the well-known oxygen or TEMPO molecules , the aspect ratio between the axial and lateral dimensions can be tuned .

[0083] Picture VI of Fig . 3b ) shows the light dose histogram ( intensity histogram) of the voxels . It is computed by wave optics simulation and the optimi zation algorithm yields phase patterns that , when summed, sets a light dose over the polymer hardening threshold for the voxels belonging to the 3D obj ect and below the said threshold for voxels that do not belong to the obj ect . The histogram shows that at the dose threshold, approximately 99 percent of the voxels belong to the correct category .

[0084] The intensity histogram tells how many voxels receive how much energy . There is a small amount of overlap, but mostly the obj ect voxels receive more light than the threshold and void pixels stay below the threshold .

[0085] For all embodiments according to the present invention, suitable a wavelength for polymeri zation is , but not restricted to , the ultra-violet (UV) , blue ( 300-410 nm) the visible range ( 410 nm to 700 nm) or the infrared range (more than 700 nm) .

[0086] Examples of photosensitive resins , preferably resins that are transparent to the illumination wavelength, that can be used according to the present invention are :

[0087] - Acrylate resins that include monomers , oligomers , and polymers with acrylate functional groups . They are known for rapid polymeri zation and are commonly used in UV-curing applications . - Epoxy Acrylates, which are polymers that comprise a combination of epoxy and acrylate groups. These resins offer the advantageous properties of both components, such as high chemical resistance from epoxy resins and rapid curing from acrylates .

[0088] - Polyurethane acrylates, which combine the toughness and flexibility of polyurethanes with the fast curing properties of acrylates, making them suitable for applications requiring durable, yet flexible materials.

[0089] - Vinyl ethers, which present fast curing rates and low shrinkage, and can be used in applications where dimensional stability is crucial.

[0090] - Thiolene resins that involve the reaction between thiol and alkene (ene) functional groups. Thiolene resins offer low shrinkage and high durability.

[0091] - Methacrylates, which are similar to acrylates but generally exhibit slower curing. Methacrylates provide a good balance between hardness and flexibility.

[0092] - Silicone acrylates, which combine the properties of silicones and acrylates to yield flexibility, thermal stability and resistance.

[0093] - Cationic epoxy resins, which exhibit a cationic polymerization mechanism which bring advantages such as high adhesion, chemical resistance, and low shrinkage.

[0094] - Unsaturated polyester resins, which are used for their mechanical properties and resistance; and

[0095] - hybrid systems which involve combinations of different of the above resin systems to tailor material properties to specific applications, offering a balance of properties like toughness, flexibility, and curing speed.

[0096] Examples of photoinitiators that can be used according to the present invention include: - Photoinitiators sold under the general trade name Irgacure , such as Irgacure 651 , Irgacure 819 , and Irgacure 1173 , which are widely used due to their strong absorption in the UV range and compatibility with a variety of resins .

[0097] - Camphorquinone ( CQ) , which is commonly used in dental materials and visible light-cure systems . CQ is ef fective with amines as co-initiators , absorbing in the blue light spectrum .

[0098] - Phenylbis ( 2 , 4 , 6-trimethylbenzoyl ) phosphine Oxide (BAPO)

[0099] - 2 , 2-Dimethoxy-2-phenylacetophenone ( DMPA)

[0100] - 2 , 4 , 6-Trimethylbenzoyl-diphenylphosphine Oxide ( TPO)

[0101] - 2-Hydroxy-2-methylpropiophenone (HMPP ) , which is used in applications requiring fast curing and high resolution .

[0102] - Benzoin Methyl Ether (BME )

[0103] - 2-Methyl- l- [ 4- (methylthio ) phenyl ] -2-morpholinopropan- l-one (MTMP )

[0104] - Michler' s Ketone (MK) , which is used in combination with other photoinitiators to enhance the sensitivity of the photopolymer to visible light , particularly in the blue spectrum .

[0105] - Anthraquinone derivatives , which are used for their absorption in the UV and visible spectra, and are suitable for systems requiring speci fic wavelengths for curing .

[0106] A second embodiment of the present invention is described by a schematic illustration in Fig . 4a . In said second embodiment , two independent proj ection systems of the type as described in the first embodiment provide an intersection of two proj ected beams in the resin container 440 . A first proj ection system is composed of a first optical beam 400 which is spatially modulated by a CGH loaded on a first spatial light modulator 410 to provide a first spatially modulated beam 420 incident onto a first proj ection lens 430 that produces a first beam 460a for generating a 3D light intensity distribution 460c in the resin container 440 .

[0107] A second proj ection system is composed of a second optical beam 405 spatially modulated by a CGH loaded on a second spatial light modulator 415 to provide a second spatially modulated beam 425 incident onto a second proj ection lens 435 that produces a second beam 460b for generating a 3D light intensity distribution 460c under a speci fic angle of intersection with the first modulated light beam 460a from the said first proj ection system . Both beams 460a and 460b are directed to the same face of the resin container 440 . Said container 440 contains a photosensitive resin 450 .

[0108] The preferred angle of intersection of the beams is approximately or precisely, but not restricted to , 90 degrees so as to obtain an ef fective spatial resolution of the voxel of the order of X / 2NA.

[0109] In this embodiment , the two beams 460a, 460b go through the same face of the container . Similarly to the first embodiment , the CGHs are computed so as to provide a 3D light intensity distribution 460c within the resin container 440 . The two beams 460a, 460b need not be simultaneously intersecting . Contrary to the prior art described in Fig . 1 , the two beams 460a, 460b do need to be coherent with each other . The intensity of the two intersecting 3D distributions are summed (not the amplitude as in Fig . 1 ) . The advantage of the two intersecting beams 460a, 460b is that the spatial resolution is given by the lateral resolution X / 2NA as can be seen in the inset of Fig . 4a, by the intersection of the beams 460a and 460b . When the intensity is summed and thresholded at the focus 490 , a smaller spot si ze equivalent to the lateral resolution X / 2NA is reached . There is thus and advantage in having two orthogonally intersecting beams instead of just one beam when the numerical aperture is smaller than 1. The exact orientation of the two beams need to be precisely calibrated in order to take advantage of this resolution increase.

[0110] Similar to the first embodiment, the CGHs are time multiplexed. Depending on the resolution required, the number of time multiplexed can be adjusted. It can vary approximately from a few tens to 1000 in practice.

[0111] Figure 4b illustrates the same embodiment with exactly a 90 degree configuration. The optical axis of the two beams 420 and 425 form a 90 degree angle. The figure shows the intersection of two Gaussian beams for the sake of illustrating that the voxel at the intersection of the beams as the same dimension in xyz which is given by the lateral resolution of each beam, i.e. X / 2NA.

[0112] A simulation using wave optical propagation is performed and shown in Fig. 4c. Figure 4c shows the general working principle of the two-beam phase optimization according to the present invention .

[0113] Picture I of Fig. 4c shows example patterns in Fourier space. Shown is an example of one optimized phase pattern, among multiple time multiplexed phase patterns, loaded on one of the two spatial light modulators 410, 415 of the device shown in Fig. 4a, 4b, that are configured as phase modulators.

[0114] With Fourier lenses, the patterns are projected from two orthogonal angles into the volume of a photosensitive resin. The field amplitude near the Fourier plane of each lens (430,435 in Fig. 4a, 4b) , is represented in picture II of Fig. 4c. The result of incoherently summing the multiplexed optimized field amplitudes from both beams in the photosensitive resin 450 is displayed as one 2D slice of the desired 3D object in picture III of Fig. 4c. The incoherent sum results in a deposit energy dose .

[0115] Picture IV of Fig. 4c is a schematic example of the polymer crosslinking conversion as a function of light dose. A polymerization threshold is indicated from a specific light dose. Below said dose, no conversion of the photosensitive resin 440 from liquid to solid occurs, whereas above said dose, the desired conversion of the photosensitive resin 440 from liquid to solid occurs .

[0116] Applying this threshold for polymerization (i.e. material hardening from liquid to solid) to the 2D slice in picture III of Fig. 4c yields the binary object in picture V of Fig. 4c. One can measure that, the form of a section of the well-known gyroid shape (the white part of the image illustrated in picture VII of Fig. 4c) is exactly reproduced. This is because the voxel resolution in this implementation with two beams is the same in xyz .

[0117] Picture VI of Fig. 4c) shows the light dose histogram (intensity histogram) of the voxels. It is computed by wave optics simulation and the optimization algorithm yields phase patterns that, when summed, sets a light dose over the polymer hardening threshold for the voxels belonging to the 3D object and below the said threshold for voxels that do not belong to the object. The histogram shows that at the dose threshold, 100 percent of the voxels belong to the correct category. The intensity histogram tells how many voxels receive how much energy. There is a small amount of overlap, but mostly the object voxels receive more light than the threshold and void pixels stay below the threshold .

[0118] According to a third embodiment , illustrated in Fig . 5a, the single proj ection system of the first embodiment is used, wherein the proj ection system is composed of optical beam 500 which is incident on an SLM 510 having a CGH loaded thereon . Preferably, a reflective SLM is used here , but the embodiment can also be reali zed with a transmissive SLM . The spatially modulated beam 520 is then directed to a lens 530 via a beam-splitter 510a . The beam-splitter 510a is used here to reali ze a compact system . Instead of a beam-splitter, a system of prisms can be used for directing the spatially modulated beam 520 , as it is known in the art of DLP proj ection .

[0119] In this embodiment , the lens 530 (which is preferably of the same type as in the first embodiment ) is immersed in a photosensitive resin container 540 and the photosensitive resin 550 provided therein, to avoid the formation of an air interface or glass interface , and so to decrease optical aberrations caused by such interfaces . The extent to which the lens 530 is immersed in the photosensitive resin 550 can be varied, as long as the desired prevention of the formation of an air interface or glass interface is achieved . Preferably, i f both the lens 530 and the photosensitive resin container 540 are fixed, only the front portion, and especially preferred only the front surface of the lens ( from which the beam is emitted) is immersed into the photosensitive resin 550 .

[0120] In this embodiment a 3D obj ect is fabricated within the field of view of the lens 530 , as exempli fied by the light distribution (voxels ) 560 . The photosensitive resin container 540 contains the liquid photosensitive resin 550 . The height of the 3D obj ect formed in the photosensitive resin container 540 can be increased by having a vertical motion system 570 of the container 540 or the lens 530 . Such vertical motion systems are known . For example , the photosensitive resin container 540 can be provided on a platform that can be moved vertically by means of a motor . Alternatively, the lens 530 can be attached to a support that can be moved vertically by means of a motor . In said case , the lens 530 can be immersed in the photosensitive resin 550 to a larger extent , before the method of forming the 3D article is started . During the execution of the method, the lens 530 can be gradually moved out of the photosensitive resin 550 , by either moving the lens 530 upwardly in z direction out of the photosensitive resin 550 , or by moving the photosensitive resin container 540 downwardly in z direction, away from the lens 530 .

[0121] In a fourth embodiment , illustrated in Fig . 5b, a lateral XY motion of a proj ection system of Fig . 5a composed of an optical beam 500 , a SLM 510 , a beam-splitter 510a, a spatially modulated beam 520 , and a proj ection lens 530 is performed during the 3D fabrication method .

[0122] The motion is preferably approximately or precisely orthogonal to the axis of the proj ection of the spatially modulated beam 520 . The lateral motion is preferably continuous , but can also be implemented as a sequence of discrete steps , by virtue of a lateral motion system 580 . Such lateral motion systems are known . For example , the proj ection system of Fig . 5a can be attached to a support that can be moved laterally by means of a motor, such as a servo motor . The lateral motion is added to increase the lateral size of the three-dimensional object beyond the field of view of the projection lens 530.

[0123] Furthermore, due to the numerical aperture (NA) of the projection system which can be from approximately 0.01 to 1.5, and the lateral motion of the projected patterns, every voxel in the resin container 540 is sequentially illuminated, via the lateral motion .

[0124] As an example, a point 541 located in the resin container 550 is illuminated continuously by projection patterns both in time and space. To illustrate the advantage, two spatially separated projections are shown in Fig. 5b. Point 541 is illuminated by a ray from projection 543 and another ray at a different angle from projection 542. Thus the 3D object is designed by taking lateral motion (and time multiplexing as well) into consideration. This multiple exposure, both time multiplexed and via lateral motion enhances the axial resolution and increase the size of the 3D printed object beyond the field of view of the projection system.

[0125] In a further variant of the fourth embodiment (not shown) , the container 540 can be a roll on which a layer of the photosensitive resin 550 is deposited, as in the art of roll-to-roll manufacturing. In this case, the photosensitive resin 550 is in contact with the roll and in contact with air on the other side. A protective layer can also be included, as if often the case in roll-to-roll manufacturing. The object is fabricated while the roll is moved.

[0126] Figure 6 illustrates yet another variant of the fourth embodiment, wherein two projection systems composed of an optical beam 600, a SLM 610, a beam-splitter 610a, a spatially modulated beam 620, and a projection lens 630, according to embodiments 1,3 and 4, are used to increase fabrication throughput.

[0127] The projection systems are arranged parallel to each other, so that the beams 620 do not enter the photosensitive resin 640 at an angle of intersection, but parallel to each other. Voxels 660 and 661 are generated at different locations in the photosensitive resin 640.

[0128] In the variant shown in Fig. 6, a vertical motion system 670 and / or one or two lateral motion systems 680 may be provided, as described above with respect to Figs. 5a and 5b.

[0129] Figure 7 presents a fifth embodiment according to the present invention. As compared to the first to fourth embodiment, the photosensitive resin container 750 here is composed of several individual compartments 755, such as microplates (also called well plates) . Such well plates are known in the art, for example in biology and high-throughput screening assays.

[0130] The number of compartments 755, such as wells, in the photosensitive resin container 750 are typically, but not restricted to 24, 96, 384, or 1536 compartments.

[0131] In the embodiment shown in Fig. 7, a vertical motion system 770 and / or a lateral motion system 780 may be provided, as described above with respect to Figs. 5a and 5b. By virtue of said lateral motion system 780, the projection system may be moved sequentially to each of the compartments 755, so as to carry out the 3D fabrication method sequentially in each of the compartments 755. Immersing of the lens 730 into the photosensitive resin 760 can be achieved by virtue of the vertical motion system 770. Figure 8 presents yet another variant of the fifth embodiment to achieve high throughput 3D fabrication, wherein multiple projections systems (800, 810, 815, 830) are provided, in accordance with embodiment 1,3,4, and 5. The above discussion on multiple projection systems equally applies. In this variant, the number of projection systems may be at least 2 up to the number of compartments 855 in the photosensitive resin container 840. Preferably, the number of projection systems may be in the range from 2 to 20, more preferably 3 to 10.

[0132] Fig. 9 presents yet another variant with one projection system (900, 910, 910a, 920, 930) according to embodiment 1,3,4, and 5. In this variant, lens 930 is composed of a large diameter lens 930a and a thin diameter projection lens 930b arranged at the exit of the large diameter lens 930a, to facilitate insertion into and possibly motion in the resin container 940. The thin lens 930b also facilitates cleaning of the lens 930 after fabrication. The thin lens is preferably, but not limited to, a cylindrical gradient index lens.

[0133] Fig. 10a presents another embodiment according to the present invention using a single SLM 1010 to produce two independent projections. A beam-splitter 1010a is disposed after the beam 1000 has been spatially modulated by the CGH loaded onto the SLM 1010 to produce two optical beams 1020 and 1020a, one per arm.

[0134] In each arm, an optical switch 1021a, 1021b, such as a filter, is disposed so as to control the blocking or passage of the modulated beam.

[0135] In one variant of this embodiment, the full area of the SLM 1010 encodes one CGH, and only the beam 1020, 1020a in one arm is allowed to pass by activating the optical switch 1021a or 1021b. A sequence of time multiplexed patterns can thus be implemented for each arm .

[0136] In another variant of this embodiment (not shown in Fig . 10a ) , the area of the SLM 1010 is divided into two areas of equal si ze . One area of the SLM 1010 is directed to one arm and the other area is directed to another arm . In this embodiment the optical switches 1021a, 1021b may not be needed .

[0137] Both of the above variants of this embodiment implement the method of the second embodiment of two beams intersecting in the photosensitive resin 1050 , under an angle of intersection 1045 . For this , the lenses 1030 , 1035 may be arranged in an inclined manner, and the spatially modi fied beams 1020 , 1020a, are directed into the lenses 1030 , 1035 via reflecting elements 1022a, 1022b such as mirrors .

[0138] In a further variant (not shown) , the container 1040 can be a roll on which a layer of the photosensitive resin 1050 is deposited as in the art of roll-to-roll manufacturing . In this case , the resin is in contact with the roll (not shown in Fig . 10a ) and in contact with air on the other side . A protective layer can also be included, as is often the case in roll-to-roll manufacturing . The obj ect is fabricated while the roll is moved .

[0139] Figure 10b presents a further variant of the embodiment of Fig . 10a, wherein the proj ection lenses 1030 and 1035 are immersed into the photosensitive resin 1050 , as described above with respect to Fig . 5a .

[0140] Figure 10c presents a further variant of the embodiment of Fig . 10a, wherein the proj ection lenses 1030 and 1035 are placed in front of a transparent cover plate 1061 ( glass or plastic ) that protects the photosensitive resin container 1040 .

[0141] Fig . 11 presents a sixth embodiment according to the present invention, wherein the axes of the proj ection lenses 1130 and 1135 are arranged under an angle of approximately or precisely 90 degrees , and wherein the photosensitive resin container 1140 has surfaces 1141 which are orthogonal to the axes of the respective proj ection lenses 1130 and 1135 .

[0142] Figure 12 presents a further variant of this embodiment , wherein the resin container of Fig . 10a, b, c is replaced by a photosensitive resin container 1240 with compartments 1255 , such as a well plate .

[0143] Fig . 13 presents another variant of this embodiment , wherein the two proj ection lens systems 1230 , 1235 of Fig . 12 , possess a thin diameter proj ection lens 1330b, 1331b, to achieve easier insertion into the photosensitive resin 1350 .

[0144] Fig . 14 present a seventh embodiment according to the present invention, wherein each of the 3D fabrication system presented in embodiments 1 to 6 and Figs . 3 to 13 is presented with an optical measurement system capable of measuring the change in index of refraction, in situ, during the fabrication of the 3D obj ect . The information of the 3D measurement can be used to stop the proj ection and / or used as a feedback signal to change the proj ection patterns .

[0145] An array of light sources 1475 is disposed on one side of the photosensitive resin container 1440 . Each light source 1475 emits a beam of light 1485 , whose wavelength does not polymeri ze the photosensitive resin 1450. Beam 1485 interacts (i.e. refracts, diffracts, absorbed) at an angle with the forming object 1495. An imaging system formed by the projection lens 1491 and lens 1492 produces a focused image of a plane 1496 of the object

[0146] 1495 onto camera 1493. A dichroic filter 1494 lets the curing wavelength from system 1400 go through to the resin container 1440 while it reflects the beam 1485.

[0147] Following the differential phase contrast imaging method known in the art (e.g. "Quantitative differential phase contrast imaging in an LED array microscope", Optics Express Vol. 23, Issue 9, pp . 11394-11403 (2015) ) , the index of refraction of a slice 1465 of the object can be inferred and compared with the desired index of refraction computed. An error signal can be devised to stop the projection system or use the feedback signal to change the projected pattern during projection.

[0148] In all embodiments, additional light beams of different wavelengths can be added co-propagating, but not restricted to, with the optical beams described in the embodiments. The addition can be realized via dichroic mirrors or dispersive elements such as optical diffraction gratings. The light beams with additional wavelengths can polymerize or inhibit polymerization of the photosensitive resin in a way that enables producing multi-material 3D printed objects, as described for example in "Stiffness control in dual color tomographic volumetric 3D printing", Nature Communications 13 (367 (2022) .

[0149] Pattern Optimization

[0150] The pattern optimization for the present invention is detailed here. Patterns are either modulated in Fourier space or real space . By the term "modulated in Fourier space or real space", according to the present invention the following is to be understood: In both cases, the spatial modulation is done using a spatial light modulator (SLM) . In the case of modulation in the Fourier space, the SLM is placed at a distance to a projection lens that is approximately equal to the focal length "F" of said projection lens, and the desired intensity modulation is obtained in the vicinity of the distance "F" from said projection lens. Thus the distance from the SLM to the desired intensity modulation is approximately twice the focal length "F". In the case of modulation in the real space, the SLM is placed at a selected distance to said projection lens, and the desired intensity modulation is obtained in the vicinity of the image plane of said projection lens. For example, in the well-known 4f projection system, for which the two said lenses have the same focal length "F", the distance between the SLM and the desired modulation intensity is approximately four times the focal length "F".

[0151] A wave optical approach as described in "Wave optical model for tomographic volumetric additive manufacturing" Opt. Express 32, 14705-14712 (2024) ) is used. The complex field (amplitude and phase) is propagated through the volume of the photosensitive resin using said wave optical model. The 3D intensity of the field propagating through the volume of the photosensitive resin is summed. It is summed incoherently (hence intensity summing) because the patterns are sequentially propagated as a time multiplexed series.

[0152] This intensity sum results in an energy distribution in the volume of the photosensitive resin. This energy distribution has to be optimized such that object regions in the volume of the photosensitive resin, i.e. the regions where the desired 3D object is to be formed, receive more intensity than an upper threshold TU, and the void regions ( outside the regions where the desired 3D obj ect is to be formed) should stay below a lower threshold TL .

[0153] Mathematically, patterns p(x,y, t) displayed on the SLM are to be found by an algorithm, as described below . Here , the coordinates (x,y) are the transverse coordinates of the SLM and (z) is the propagation direction; the coordinate t is the time . The patterns p(x,y, t') are propagated into the volume of the photosensitive resin with a propagation operator P such that the following loss function is minimi zed :

[0154] Force object polymerization Keep empty space unpolymerized avoid overpolymerization where K can be 1 or 2 for Li or L2 norm for example . is the received intensity (which is then normali zed for the loss function) at every voxel v x,y, z') after propagation of the patterns p(x,y, tm) in the printing volume . The received intensity is the incoherent sum of the intensities ; p(x,y, tm) E IRNxN><Mcorresponding to M di f ferent patterns on a spatial light modulator, each having a si ze of N x N pixels .

[0155] The wave optical operator P propagates the light from a single 2D plane into the printing volume . In the case of real space , the imaging configuration described above , the propagation into the printing volume can be described by a low pass filtering convolution operation where the convolutional kernel depends on the numerical aperture of the imaging system .

[0156] In the case of Fourier space , where the printing volume is near the focal length of the proj ection lens , the propagation is described by a Fourier trans form .

[0157] Once the electric field of the light is computed at the entrance of the printing volume , then the electric field is propagated slice by slice ( changing z) through the printing volume . The calculation from slice to slice is performed, for example but not limited to , by the angular spectrum of plane waves method . The index of refraction of the photosensitive resin n(x,y, z, t) changes in time and space with the accumulated light dose . Hence , the spatio-temporal variation of the refractive index n(x,y, z, t) can be included with the propagation modelled by the thin slice approximation . The above description of the light propagation is called the forward model .

[0158] The patterns p(x, y, tm) can be optimi zed with a gradient descent optimi zer . For this , the gradient of the of the forward model and loss function must be computed .

[0159] Figure 15 shows a flowchart of the algorithmic optimi zation . An initial guess of the patterns ( such as random or constant values ) is fed into the wave optical propagator . This propagator can either take the input field from a conj ugate image plane or the Fourier plane as input . The wave optical propagator propa- gates each pattern into the printing volume and adds the different projected patterns incoherently together. This volume is compared with the target object over the loss function L.

[0160] If the loss is not low enough yet according to a preset value, the gradient of the loss with respect to the projection patterns is computed and a gradient descent step is performed. If the loss is below a set value, the optimization is stopped and the final patterns are obtained.

[0161] Figure 16 shows the flowchart with the projection pattern coming from two different angles 0° and 90°. The optimization is similar, but the wave optical propagator has to take into account that the projection happens from two orthogonal directions (or whatever the angle between the two beams are) . During illumination with time multiplexed patterns, some of the regions in the volume polymerize already and thus will alter the refractive index at this location. A wave optical propagator can include scattering of the cured resin with a multi-slice approach and optimize for that.

[0162] Different Implementation for Phase Modulation

[0163] For the phase modulation using spatial light modulators, different hardware devices can be used. A spatial light modulator (SLM) based on liquid crystals allows for continuous phase modulation of the electrical field. Another option is a piston-based MEMS device, such as the phase light modulator (PLM) from Texas Instruments which alters the phase by changing the height of the reflective pistons. Such MEMS modulator have speed above 1 kHz up to 22 kHz. A digital micromirror device (DMD) which operates natively as a binary amplitude modulator can be configured as a phase modulator via the so-called Lee hologram setup or via a super pixel following the method taught in "Superpixel-based spatial ampli- tude and phase modulation using a digital micromirror device", Vol. 22, Issue 15, (2014) . Recent ultra-fast, Giga Hertz phase modulator can also be used such as described in "LNoS : Lithium Niobate on Silicon Spatial Light Modulator", 2024 https: / / doi.org / 10.48550 / arXiv.2402.14608.

Claims

Claims1. A method for volumetric manufacturing of a three-dimensional object, comprising the steps: spatially modulating a first optical beam (300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300) with a first spatial light modulator (310, 410, 510, 610, 710, 810, 910, 1010, 1110, 1210, 1310) , to generate a first spatially modulated optical beam (320, 420, 520, 620, 720, 820, 920, 1020, 1120, 1220, 1320) , introducing said first modulated beam (320, 420, 520, 620, 720, 820, 920, 1020, 1120, 1220, 1320) at a first projection direction into a photosensitive resin (350, 450, 550, 650, 750, 850, 950, 1050, 1150, 1250, 1350) that is provided in a photosensitive resin container (340, 440, 540, 640, 740, 840, 940, 1040, 1140, 1240, 1340) , so as to generate a three-dimensional light intensity distribution (360, 460c, 660, 760, 860, 1060, 1260) in said photosensitive resin (350, 450, 550, 650, 750, 850, 950, 1050, 1150, 1250, 1350) , characterized in that the first spatially modulated optical beam provides time multiplexed pattern projections.

2. The method according to claim 1, characterized in that a second optical beam (405) is provided and spatially modulated with a second spatial light modulator (415) to generate a second spatially modulated optical beam (425) that provides time multiplexed pattern projections and is introduced at a second projection direction through the same surface of the photosensitive resin container (440) as the first spatially modulated optical beam into the photosensitive resin ( 450 ) .

3. The method according to claim 2, characterized in that the first and second projection direction form an angle of intersection, preferably 90°.

4. The method according to any of the preceding claims, characterized in that the time multiplexed pattern projections are varying computer-generated holograms that are sequentially displayed on the spatial light modulator (310, 410, 415, 510, 610, 710, 810, 910, 1010, 1110, 1210, 1310) and incoherently summed in the photosensitive resin (350, 450, 550, 650, 750, 850, 950, 1050, 1150, 1250, 1350) .

5. The method according to claim 1 or 2, characterized in that the time multiplexed pattern projections are optimized by comparing initially guessed pattern projections with the three-dimensional object to be manufactured over a loss function and minimizing the loss function if the result of the comparison of the initially guessed pattern projections with the three-dimensional object is not sufficient.

6. The method according to any of the preceding claims, characterized in that the first and optionally the second spatially modulated optical beam (320, 420, 425, 520, 620, 720, 820, 920, 1020, 1120, 1220, 1320) are introduced into the photosensitive resin (350, 450, 550, 650, 750, 850, 950, 1050, 1150, 1250, 1350) via a first and optionally a second projection lens (330, 430, 435, 530, 630, 730, 830, 930, 1030, 1035, 1130, 1135, 1230, 1235, 1330, 1331, 1335) that is preferably at least partly immersed into the photosensitive resin (350, 450, 550, 650, 750, 850, 950, 1050, 1150, 1250, 1350) .

7. The method according to any of the preceding claims, characterized in that the first and optionally the second projection direction are modified by a lateral movement of the optical axis of the first and optionally second spatially modulated optical beam.

8. A device for performing the method according to any of claims 1 to 7, comprising a first projection system comprising a light source for generating a first optical beam (300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300) , a first spatial light modulator (310, 410, 510, 610, 710, 810, 910, 1010, 1110, 1210, 1310) configured to generate a first spatially modulated optical beam (320, 420, 520, 620, 720, 820, 920, 1020, 1120, 1220, 1320) from said first optical beam (300, 400, 500, 600, 700, 800, 900, 1000, 1100, 1200, 1300) that can provide time multiplexed pattern projections, and optionally at least a first projection lens (330, 430, 435, 530, 630, 730, 830, 930, 1030, 1035, 1130, 1135, 1230, 1235, 1330, 1331, 1335) ; a photosensitive resin container (340, 440, 540, 640, 740, 840, 940, 1040, 1140, 1240, 1340) for a photosensitive resin (350, 450, 550, 650, 750, 850, 950, 1050, 1150, 1250, 1350) , wherein said photosensitive resin container (340, 440, 540, 640, 740, 840, 940, 1040, 1140, 1240, 1340) has at least one surface that is transparent for the spatially modulated optical beam.

9. The device according to claim 8, characterized in that the device furthermore comprises a second projection system comprising a second spatial light modulator (415) configured to generate a second spatially modulated optical beam (425) that can provide time multiplexed pattern projections, and optionally a second projection lens (435) .

10. The device according to claim 8 and 9, characterized in that the first and optionally second spatial light modulator (310, 410, 415, 510, 610, 710, 810, 910, 1010, 1110, 1210, 1310) is selected from the group consisting of a binary amplitude digital micromirror device, a piston light mirror, and a liquid crystal display.

11. The device according to any of claims 8 to 10, characterized in that the device furthermore comprises a vertical motion system (570, 670, 770, 870, 970) for vertical movement of the photosensitive resin container (340, 440, 540, 640, 740, 840, 940, 1040, 1140, 1240, 1340) towards the first and optionally the second projection system, and / or for vertical movement of the first and optionally the second projection system towards the photosensitive resin container (340, 440, 540, 640, 740, 840, 940, 1040, 1140, 1240, 1340) .

12. The device according to any of claims 8 to 11, characterized in that the device furthermore comprises a lateral motion system (580, 680, 780, 880, 980) for lateral movement of the first and optionally the second projection system, with respect to the photosensitive resin container (340, 440, 540, 640, 740, 840, 940, 1040, 1140, 1240, 1340) .

13. The device according to any of claims 8 to 12, characterized in that the first and optionally the second projection lens (930, 1330, 1331) comprises a portion with a small diameter (930b, 1330b, 1331b) .

14. The device according to any of claims 8 to 13, characterized in that the photosensitive resin container (740, 840, 940, 1240, 1340) comprises compartments (755, 855, 1255, 1355) .

15. The device according to any of claims 8 to 14, character- ized in that the device furthermore comprises an optical measurement system.

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