This PDF file contains the front matter associated with SPIE Proceedings Volume 11698, including the Title Page, Copyright Information, and Table of Contents.

Welcome and Introduction to SPIE Photonics West OPTO conference 11698: Emerging Digital Micromirror Device Based Systems and Applications XIII

The effect of light scattering is non-negligible in micro-scale DMD-based 3D printing. Light scattering results in unwanted exposure and polymerization, thus deteriorating the fabrication resolution and fidelity. We report using machine learning (ML) approach to mitigate the effect of light scattering. A neural network (NN) was designed to study the highly-sophisticated relationship between the input digital masks and their corresponding output 3D printed structures. After trained with 300 pairs of digital masks and printed structures, the neural network was able to optimize the 3D printing process by suggesting the optimal grayscale digital masks which are not necessarily identical to the desired structures. Verification results showed that using NN-generated digital masks yielded significant improvements in printing resolution and fidelity compared to using masks that are identical to the desired structures.

Volumetric additive manufacturing is a novel 3D printing method to form parts in a single exposure, in contrast to traditional stereolithography 3D printing which builds parts layer-by-layer. Photopolymerizable resin is exposed with grayscale images from different angles over 360 degrees to deliver a 3D distribution of light energy which cures the resin to form the desired part. The exposure images are calculated using computed tomography (CT), the same principles which are used to convert X-ray images obtained during a computerized axial tomography (CAT) scan to reconstruct a 3D volume of the patient. A digital micromirror device is used to project the sequence of calculated exposure images into a cylindrical vial of resin as it is rotated thus exposing over 360 degrees. The polymerization kinetics of the resin, the alignment of the resin vial to the projected images, and the uniformity of the illumination field are all tightly coupled together and must all be understood and controlled to generate quality parts. Projecting a good image is challenging as the vial and resin are essentially a strong cylindrical lens located at the image plane, and the desired build space is not a plane but a volume. We perform an analysis of the cylindrical lensing on the image quality (and thus the print quality) and investigate methods to correct for the lensing. We demonstrate different methods to improve print quality and discuss the benefits and limitations of each technique.

I will present our recent work on parallelization of the two-photon polymerization (TPP) process based on temporal focusing via a digital micromirror device, where programmable femtosecond light sheets are formed and used to substantially improve the fabrication rate without sacrificing resolution. In the experiments, we demonstrated the fabrication of arbitrarily complex structures at a record-breaking resolution and speed, i.e., lateral/axial resolution: 140 nm/175 nm at 10s mm3/min, which is approximately 3 orders of magnitude higher than any existing fabrication methods. Importantly, the throughput enhancement also effectively reduces the cost by 90% (~$1.5/mm3), which makes TPP an economic solution for nanofabrication.

Computed axial lithography (CAL) is a volumetric additive manufacturing method in which a three-dimensional light dose distribution is constructed in a photopolymer from the superposition of illumination patterns from many different angles. The technique’s advantages over layer-by-layer light printing methods stem from the fact that in CAL hydrodynamic stresses are effectively eliminated from the resin precursor material during printing. This key difference allows a wider range of materials to be processed, including high-viscosity or thermally gelled precursors, and allows polymeric objects to be printed around pre-existing solid objects (‘overprinting’). In this talk we describe some of the current limitations on spatial resolution, printing speed, and mechanical properties in CAL. We also describe a computationally efficient approach to modeling the occlusion of light by objects suspended in the printing volume, which supports the optimization of overprinting processes.

Collagen is the most abundant protein and the main load-bearing material in the human body. Collagen architecture and behavior under load are important for understanding their functions in healthy tissues and the pathophysiology of diseased tissues. Structured polarized light imaging (SPLI) has proved to be an effective method to quantify collagen properties of thick collagenous tissues. Compared to without using structured illumination, SPLI provides improved accuracy in quantification and enhanced contrast for visualization of collagen structures by suppressing highly scattered light from the deeper tissue. However, current SPLI requires multiples images, typically 12, which is a challenge for imaging tissue dynamics where acquisition time is highly constrained. In this study, we developed real-time SPLI to address this challenge. The real-time imaging capability was achieved with a polarization-sensitive camera, capable of acquiring images at four polarization states simultaneously at 75 frames/second. Using a rolling image processing technique, we achieved the same frame rate for visualizing tissue dynamics. The performance of real-time SPLI was validated by imaging a rotating chicken tendon. Additionally, we integrated real-time SPLI with a tissue tester and demonstrated that pig scleral tissue, under uniaxial stretching, exhibited nonlinear and region-dependent deformation.

Quantitative phase microscopy (QPM) has been applied to a wide range of applications, especially in biomedical imaging. The label-free nature of QPM measurement enables sample characterization without complicated and invasive chemical staining procedures on samples. Higher imaging resolution is always preferred for imaging microscopic structures. Synthetic aperture method can be used to enhance the resolution in QPM, namely forming a synthetic aperture phase microscope (SAPM). In SAPM, multiple images, captured under different sample illumination angles, are synthesized to enlarge the object spectrum to reconstruct a high-resolution phase image. However, multiple acquisitions decrease the imaging speed in SAPM. To solve this problem, we propose a single-shot synthetic aperture phase microscope method through reference beam modulation, enabled by digital micro-mirror devices (DMDs). The proposed system provides a resolution enhancement of around 1.4 folds without sacrificing the camera frame rate and field of view.

We propose and demonstrate a digital micro-mirror device (DMD) based laser-illumination Fourier ptychographic microscopy (FPM) method for high-speed and high-resolution label-free imaging applications. Using this method, we can simultaneously retrieve phase and intensity images of samples, such as stained and unstained cancer tissue slices and cells. The system illumination is provided by a 532 nm laser, while two DMDs are utilized for achieving active illumination angle scanning and dynamic filtering. Our current system has achieved real-time imaging at 42 fps with around 1 𝜇𝑚 resolution.

In this paper, we report a dispersion-eliminated coded-aperture light field (DECALF) imaging system based on digital micromirror devices. Using a dual-DMD design to compensate for dispersion in the entire visible spectrum, the DECALF imaging system captures 1280×1024×5×5 (𝑥, 𝑦, θ,φ) color light field images at 20 Hz. Using three-dimensional (3D) color scenes, we experimentally demonstrate multi-perspective viewing and digital refocusing by DECALF imaging system.

Spin Glasses (SG) are paradigmatic models for physical, computer science, biological and social systems. The problem of studying the dynamics for SG models is NP-hard, i.e., no algorithm solves it in polynomial time. Here we implement the optical simulation of an SG, exploiting the N segments of a Digital Micromirror Device to play the role of the spin variables, combining the interference at downstream of a scattering material to implement the random couplings and measuring the transmitted light intensity to retrieve the system energy. We demonstrate that This optical platform beats digital computation for large-scale simulation (N>12000).

A holographic lidar concept established on the Digital Micromirror Mirror (DMD)-based hybrid light modulation is reported, which multiplexes coarse-steering by sawtooth phase modulation and fine-steering by binary amplitude modulation. The hybrid steering is achieved by overlaying displayed Computer-Generated Holograms (CGHs) with a sawtooth blazed grating phase mask, which the blaze angle programmed by synchronized short-pulse illumination of transitioning micromirrors creating the CGHs. The steering principle is demonstrated as a 2D beam steering scheme with a 532 nm visible pulse laser, and implemented into a 905 nm lidar system with a 44° field-of-view, 0.9°×0.4° angular resolution, 7.8 FPS video frame rate, and 1 m detection distance.

A frequency modulation technique is described to distinguish between particle signals and stray light in a prototype of a photolithographic mask inspection system. Strict cleanliness requirements apply to the reticles used in photolithography, and to guarantee correct alignment and pattern reproduction both sides of a reticle (pellicle on front side and reticle back side) need to be inspected for contamination. Reticles constitute difficult inspection targets for imaging systems, because their diffractive patterns can redirect light in arbitrary directions which may potentially result in false positive detections. In this paper we introduce a frequency encoded structural illumination system which enables automatic delineation between stray light and particle signal. In our approach, a sinusoidal pattern whose temporal frequency changes with spatial coordinates is used to obliquely illuminate the surface under test. Incoherent illumination is used, and thus the signal recorded by the imaging optics positioned normal to the inspected surface is a superposition of light scattered by particles and stray light redirected by the photolithographic pattern. Because the inspected surface and reticle pattern are from the perspective of an observation system axially separated, the frequency response of each individual detector point is z position dependent. Therefore, Fourier analysis can be used to analyze the frequency content of each pixel individually and by comparing measured with expected frequencies, one can distinguish between light scattered by particles and stray light. The frequency encoding technique can be used in both imaging and scatterometry based systems. Example experimental results are presented.

Diffractive beam steering by Digital Micromirror Device enables an efficient way to simultaneously manipulate light both in the spatial and angular domain by spatial and time multiplexing while keeping large area and angular throw product. Long-term stability and susceptibility of beam and image steering rely on how synchronization of ns laser pulse and transition of micromirrors is maintained over time and through variation of device temperature. The long-term performance of the beam steering is evaluated by monitoring diffraction efficiency over 350 hrs. with a 360 Hz repetition rate. Also, diffraction efficiency was monitored while increasing the temperature of the mirror array from 45 to 75 degrees C. Over the period, stable beam steering was observed. A decrease in diffraction efficiency under high temperature was observed. We confirmed readjusting synchronization timing recovered the diffraction efficiency to the level of room temperature. The experimental results show a stable operation of diffractive beam steering by DMD is feasible for the long term, and even under variation of temperature by adaptively adjusting synchronization timing of laser pulse to starting timing of DMD mirror transition.

This Conference Presentation, “Boosting single-shot ultrafast imaging to 70 trillion frames per second and 1000 frames,” was recorded for the Photonics West 2021 Digital Forum.

High-speed three-dimensional (3D) fringe projection profilometry (FPP) is widely used in many fields. Recently, researchers have successfully tested the feasibility of performing fringe analysis using deep convolutional networks (CNN). However, the existing methods require tremendous real-world scanning trials for model training, which is not trivial. In this work, we propose a framework to establish the digital twin of a real-world system in a virtual environment and a process to automatically generate 3D training data. Experiments are conducted to demonstrated that a physical system can adopt the CNN trained in the virtual environment to perform accurate real-world 3D shape measurements.

We report single-pixel three-dimensional (3D) dual photography. Inspired by a tri-linkage between single-pixel imaging, fringe projection profilometry (FPP), and dual photography, we propose a method for 3D imaging that allows for the synthesis of dual and relit images in a camera-free context. By using FPP calibration methods and implementing a direct coordinate mapping with phase shifting, we obtain highly efficient measurements of the direct component of the scene light transport matrix, while simultaneously sensing the surface profiles of 3D objects. Through the addition of calibration information and by exploiting Helmholtz reciprocity, dual photography and scene relighting can thus be performed on 3D images. To verify the proposed imaging method, we have developed a light-path symmetric single-pixel imaging system based on two digital micromirror devices (DMDs). As a benefit of this choice of imaging platform, the traditionally distinct roles of projector and camera hardware can be freely interchanged, depending only on the DMD hardware chosen for the deployment of modulation patterns. For the acquisition of images, binary cyclic S-matrix patterns and binary sinusoidal fringe patterns are loaded onto each DMD for scene encoding and virtual fringe projection, respectively. Using this system, we further demonstrate the broadening of traditional dual photography image synthesis to viewing and relighting of 3D images at user-selectable perspectives. Our work extends the conceptual scope and the imaging capability of dual photography.

We report on a novel kind of accelerating beams that follow parabolic paths in free space. This accelerating peculiar polygon beam (APPB) not only exhibits autofocusing property, but also possesses two types of accelerating intensity maxima, i.e., the cusp and spot structure. We also provide a detailed insight into the theoretical origin and characteristics of this spatially accelerating beam through catastrophe theory. Moreover, experimental results confirm the peculiar features presented in the theoretical findings, and the APPB is further verified to exhibit self-healing property during propagation with either obstructed cusp or spot-like main lobe reconstructing after a certain distance. This accelerating beam with multiple types of main lobes has never been achieved previously, and we anticipate that APPB will facilitate the applications in the areas of biomedical manipulation and optofludics.

Besides low cost, automobile design will be another major factor for the mass adoption of the LiDAR in the autonomous vehicles. Extra aperture in the automobile chassis for LiDAR operation will be undesirable for automobile designers. As a result, it will be advantageous to integrate the LiDAR together with the headlight such that the chassis design of the vehicle does not have to deviate from standard practices. This paper presents optical design of integrated LiDAR and smart headlight using a single DMD such that it replaces the current headlight without impact on the overall chassis design of the vehicle. For low cost considerations, the two functions share the same DMD. Preliminary designs of such system and results will be presented.

Over the last few years, DLP® Products at Texas Instruments, Inc. has been developing a MEMS-based phase-only spatial light modulator that we call the Phase Light Modulator or PLM. The device which shares many of the process steps with the DLP micromirror device consists of a vertically actuated mirror on top of a CMOS array. The current reflective device provides enough vertical range to be suitable for visible wavelengths. This paper discusses a number of metrics important to the performance of a phase-only spatial light modulator and the corresponding characteristics of the PLM. The metrics include diffraction efficiency, number of phase levels, resolution, speed, and die flatness. The paper also discusses in depth recent design improvements such as increasing the number of phase levels to improve efficiency and reduce image noise. Recent lifetime test results collected over more than 1000 hours are also presented. Finally, there is a discussion of our possible approaches to testing the device in a high-volume production environment.

Computer-generated holography (CGH) is a technique to generate holographic Interference patterns for various applications. Typical applications are in the area of information displays, but dynamic illumination application, as an example, an automotive glare free headlamp, would be another possible application. For most applications a real-time computation of the CGH and a high contrast ratio needs to be achieved. One of the major issues related to computer hologram generation is the computational power required to archive high quality images. With emerging MEMS based phase modulators the cross talk between pixels will be removed and the modulation speed will be much higher than in existing LC based micro displays. A novel not iterative algorithm will be used to construct time multiplex phase only sub holograms (TPOSH). Based on the TPOSH we will generate various high dynamic range light distribution for typical automotive headlamps scenarios. To verify the method, simulations and experiments are performed. Numerical simulations and optical experiments are showing good consistence with each other. Finally we will compare the TPOSH results with Gerchberg Saxton (GS) based computer generated holograms.

Computer-generated holography, a disruptive technology for projection displays, enables variable projection distance by combining a variable lens hologram with an image hologram and encoding it onto a phase-modulating spatial light modulator. The basic structure of a holographic projector includes a spatial light modulator (SLM) to modulate the phase of an incident reconstruction light beam, i.e., a coherent light source (i.e., LASER). Speckle appears in the projected content due to the quasi-random interference of a coherent light source on an optically rough surface that serves as a projection screen. A partially coherent light source, such as a light emitting diode can be used in holographic projection, but image sharpness is sacrificed. Furthermore, due to the limitation of the phase retrieval algorithm, which calculates the hologram to be encoded on the spatial light modulator from a given graphic, the resulting projected content can look speckly due to phase retrieval algorithm imperfections. In this work, we demonstrate reduction in speckle arising from phase retrieval algorithm imperfections through the use of a piston-mode spatial light modulator. The SLM is a two-dimensional micromirror array with extraordinarily fast switching characteristics that enables the implementation of a temporal averaging method for speckle reduction without additional optical components and keeps the benefits of holographic projection.

Based the fringe projection profilometry, a compact and flexible positionable measuring head can be combined with optical fiber bundles to perform in-situ inspection tasks in industrial applications. Surfaces of complex geometries can be reconstructed and quantified in metric coordinates by means of a fast, non-contact and high-resolution measurement. Defect segmentation, on the other hand, is rather complex with three-dimensional point clouds, since reference data is required or a deviation determination is ambiguous and susceptible to errors. Due to each reconstructed object point corresponding to a camera pixel, it is possible to apply image processing algorithms or neural networks for defect segmentation. Since image based segmentation is more susceptible to poor illumination and deviating surface curvature or texture, a circular array of miniature LEDs has been coaxially arranged around the imaging optics of the camera’s fiber to provide different illumination directions. By utilizing a directional variable illumination sequence, the advantages of image-based segmentation can be combined with the unambiguousness and metric quantifiability of point cloud data.

The rapid developments in the micro camera industry, driven by the smartphone sector, offer a wide range of innovations with respect to the performance and miniaturization of endoscopic instruments. For the fast 3D inspection of inaccessible components such as turbine blades in partially disassembled aircraft engines, a borescopic fringe projection system was developed. This study provides a methodology for the comparison of different camera and projection configurations within borescopic fringe projection. Furthermore, the current limits for the use of multimedia sensors in metrological applications are shown and quantified. The projection unit of the developed measuring system is based on a digital micromirror device, which generates structured light that is imaged into the measurement scene by means of an objective lens and a borescope. Threedimensional, high-resolution reconstructions are carried out via chip-on-the-tip miniature cameras based on the MIPI interface by forming a triangulation base with the projection unit. To enable inspections in confined spaces, the cameras are connected to external framegramer boards. In this study, 1/6” and 1/4” sensors with fixedfocus lenses are evaluated to assess the trade-off between the physical sensor size and the possible reconstruction accuracy. Typical camera parameters such as the sensitivity with respect to the signal-to-noise ratio are determined by means of a standardized test setup. In addition, the application in the triangulation system is evaluated through the modulation signal strength as well as the suitability for typical system calibrations based on the widely used pinhole camera model in combination with a distortion polynomial correction according to the approach of Conrady and Brown. Finally, the sensors are compared regarding 3D reconstructions of calibrated geometric features in accordance with ISO/IEC Guide 98-3:2008 and VDI/VDE 2634-2.

High-speed three-dimensional (3D) surface imaging by structured-light profilometry is currently driven by industrial needs, medical applications, and entertainment. However, the limitation of pattern projection speed has prevented the structured illumination to reach the kilohertz (kHz) level. The limited bandwidth of the data transmission has prevented the camera from streaming data continuously, which thus has brought difficulties in kHz-level image acquisition, processing, and display of 3D information during the occurrence of dynamic events (i.e., in real time). Besides, the tradeoff between the camera’s sensor readout rate and the activated pixel numbers has strained the existing methods from reaching a large field of view (FOV) at kilohertz (kHz)-level acquisition. To overcome these limitations, we have developed highspeed band-limited illumination profilometry (BLIP) in two configurations. The first configuration, employing a single camera with a CoaXPress interface (CI), enables real-time 3D surface information reconstruction at 1 kHz. The second configuration, employing two cameras with a CI, uses temporally interlaced acquisition (TIA) to improves the 3D imaging over 1000 frames per second on a field of view (FOV) of up to 180×130 mm² (corresponding to 1180×860 pixels) in captured images. We have demonstrated the systems’ performance by imaging various static and fast-moving 3D objects. CI-BLIP has been applied to fluid mechanics by imaging dynamics of a flag, which allowed observation of the wave propagation, gravity-induced phase mismatch, and asymmetric flapping motion. Meanwhile, TIA-BLIP has empowered the 3D visualization of glass vibration induced by sound. We expect BLIP systems to find diverse scientific and industrial applications.

Digital micromirror device (DMD) allows very fast binary light modulation. For digital hologram displaying using DMD, binarization should be applied. In this work, 12 error diffusion and 8 dot diffusion binarization algorithms were considered. All methods were used in both iterative and non-iterative forms. Quality of reconstructed images from binarized holograms was compared. Optically registered digital holograms and synthesized holograms of binary and grayscale images were used. Among the error diffusion methods, Stucky’s and Atkinson's standard error diffusion methods and dot diffusion methods provide the highest quality of reconstruction. The effect of the pixel bypass matrix on the quality of reconstructed images was also evaluated. As bypass matrices, standard variants (for example, direct and reverse sequential bypass of pixels on rows or columns, a matrix with a line-by-line alternating direction of bypass of pixels, a spinning and swirling spiral matrices), and more complex types (for example, used in the Knuth dot diffusion method) were used. Non-iterative error diffusion with use of several specialized bypass matrices improves the quality up to 7%. Iterative realization of the error diffusion allows to increase quality to 15%.