Rapid Continuous 3D Printing via Orthogonal Dual-Color Photoinitiation and Photoinhibition.

Stereolithographic additive manufacturing technology has developed from point-by-point scanning exposure to layer-by-layer masking curing and even volumetric printing. Rapid prototyping is one of the important goals pursued by researchers. A continuous three-dimensional (3D) printing system based on the dual-color photoinitiation and photoinhibition is proposed with the aim of further improving printing speed. The process of continuous 3D printing is realized through the anti-polymerization layer between the cured part and the window generated by the ultraviolet (UV) light sheet (355 nm), and dynamic masking with the blue light (470 nm). The volume of the anti-polymerization layer can be adjusted by the intensity ratio of the incident lights (IUV, 0/Iblue,0) and the size of UV laser spot to enhance the reflow filling rate of the liquid resin. For the orthogonal Gaussian anti-polymerization layer, an intensity ratio of 28.6 allows for an inhibition volume of 97.1% of the desired rectangular anti-polymerization zone with a height of 1 mm. The simulation analysis of continuous 3D printing process by flow-structure interaction reveals that the increase of the thickness of the anti-polymerization layer effectively improves the filling rate of the resin and the cross-sectional area of printing, and reduces the stress of the cured part. The experiments with two different 3D structures printing demonstrate that the filling rate and the stress have virtually no effect on the printing process at a large-scale thickness of the anti-polymerization layer, and the printing speed is capable of reaching 200 µm/s. Certainly, the printing volume and complexity can be further improved with the improvement of the system and the optimization of the resin.

Multi-wavelength confocal displacement sensing using a highly dispersive flat-field concave grating.

A multi-wavelength confocal displacement sensor based on a flat-field concave grating (FFCG) was proposed and designed; the large dispersion and small volume of the FFCG make it an ideal candidate for replacing the complex dispersive lens group. The designed displacement sensor was calibrated by displacement meter, and the characteristics were measured. Consequently, for the proposed displacement sensor, the displacement range of 6.8 mm was measured with the R-square linearity evaluation coefficient of 0.998, and the sensitivity preceded 17.1 nm/mm. The resolution of the displacement sensor was characterized by 70 µm, as well as a full width at half maximum (FWHM) fluctuating around 1.63 nm, indicating high precision and accuracy in displacement measurement. Moreover, the stability and reliability of the sensor were verified within 20 min, with no significant wavelength shifts, and gentle power fluctuations of 557.73 counts at 520 nm and 563.67 counts at 545.05 nm, respectively.

Distance-based generation of a unicursal random path on a non-grid point set for optical polishing.

In modern ultra-precision polishing, sub-aperture technologies are prone to mid-spatial frequency errors due to identical patterns of a path. A random tool path on a regular point set is widely used to suppress mid-spatial frequency errors. In this study, two non-grid uniform point sets, the Fibonacci and the three-directional, were introduced into optical polishing. To solve the time-consuming problem caused by a large amount of distance calculation, a distance-based weighted random (DBWR) algorithm and a linear programming and connecting (LPC) algorithm were presented. The DBWR algorithm reduces the generation time by strengthening the weight of the neighboring points in a specific direction, while the LPC algorithm adjusts the order and distance of points artificially. Then a random stitching method was proposed for the large-scale point set applying to large-sized optical surfaces, which dramatically reduced the generation time. Finally, experiments validated that the algorithms for non-grid sets can be effectively used for optical surface figuring without introducing an apparent mid-spatial frequency.

Target-surface multiplexed quantitative dynamic phase microscopic imaging based on the transport-of-intensity equation.

Microscopic phase digital imaging based on the transport of intensity equation, known as TIE, is widely used in optical measurement and biomedical imaging since it can dispense with the dependence of traditional phase imaging systems on mechanical rotational scanning and interferometry devices. In this work, we provide a single exposure target-surface multiplexed phase reconstruction (SETMPR) structure based on TIE, which is remarkably easy to construct since it directly combines a conventional bright-field inverted microscope with a special image plane transmission structure that is capable of wavefront shaping and amplification. In practice, the SETMPR is able to achieve dynamic, non-interferometric, quantitative refractive index distribution of both static optical samples and dynamic biological samples in only one shot, meaning that the only limitation of measuring frequency is the frame rate. By comparing the measurement results of a microlens array and a grating with a standard instrument, the quantitative measurement capability and accuracy are demonstrated. Subsequently, both in situ static and long-term dynamic quantitative imaging of HT22 cells were performed, while automatic image segmentation was completed by introducing machine learning methods, which verified the application prospect of this work in dynamic observation of cellular in the biomedical field.

Rotation-Assisted Separation Model of Constrained-Surface Stereolithography.

Among a variety of additive manufacturing technologies, constrained-surface image-projection-based stereolithography (SLA) technology has unique advantages in printing precision and commercial maturity. For the constrained-surface SLA process, separating the cured layer from the constrained surface is a crucial step that enables the fabrication of the current layer to accomplish. The separation process limits the accuracy of vertical printing and affects the reliability of fabricating. To reduce the separation force, current existing methods include coating nonsticky film, tilting the tank, sliding the tank, and vibrating the constrained glass. Compared with the above methods, the rotation-assisted separation method presented in this article has the advantages of simple structure and inexpensive equipment. The results of the simulation show that the pulling separation with rotating can reduce the separation force and shorten the separation time effectively. Besides, the timing of rotating is also crucial. A customized rotatable resin tank is used in the commercial liquid crystal display-based three-dimensional printer to reduce the separation force by breaking the vacuum environment between the cured layer and the fluorinated ethylene propylene film in advance. The analyzed results have demonstrated that this method can reduce the maximum separation force and the ultimate separation distance, and the reduction is related to the edge profile of the pattern.

Three-degree-of-freedom autocollimation angle measurement method based on crosshair displacement and rotation.

The classic autocollimation method manages to measure the two-degree-of-freedom (2-DOF) angles, namely pitch and yaw, but fails to measure the roll angle. This paper proposes an autocollimation method that enables the simultaneous measurement of 3-DOF angles in which a carefully designed cooperated reflector (CR) splits the collimated beam into two returning beams parallel to the optical axis. The 3-DOF angles of the CR can be obtained by detecting the displacement and rotation of the crosshair images received by two photodetectors. The measurement principle is dissected, and the experimental results reveal that the constructed system achieves an accuracy of better than ±1.54 arcsec in the range of ±1000 arcsec. In addition, it is demonstrated that the system can be applied to the 3-DOF angle measurement of long-distance targets.

Influence and suppression of periodic contour error of tool path and velocity deviations on surface error.

When using multi-axis machines with a pneumatic tool to polish large-aperture optical surfaces, the paths generated by the computer numerical control system deviate from the desired ones. This causes periodic contour errors and surface ripples. In addition, because of the different machine layouts, the tool end velocity also can change. We introduce a multi-axis machine and analyze the surface error and power spectral density (PSD) of three commonly used paths (raster, spiral, and random path) in terms of the contour error using different position interpolation methods. A cubic polynomial is introduced to smooth the axis motion, and a velocity compensation method is considered to diminish the velocity deviation from the machine layout. The results show that the circular interpolation method exhibits a balanced performance in terms of both the contour error and the PSD for various paths. In addition, the optimization can be performed before G-code generation without affecting the characteristics of the original optimization system.

Three-degree-of-freedom autocollimator based on a combined target reflector.

As an angle measuring instrument, the traditional autocollimator has the ability to measure the two-degree-of-freedom angles, namely, pitch and yaw, but fails to measure the roll angle. In this study, we propose a novel autocollimator that can simultaneously measure the three-degree-of-freedom (3-DOF) angles. As a key component, a combined target reflector (CTR) is meticulously designed to split the collimated laser beam into two beams. The 3-DOF angle measurement is achieved by sensing the displacements of the two beam spots reflected from the CTR. The measurement principle and simulation analysis are presented in detail. Experiments are conducted to assess the performance of the proposed autocollimator, and the results indicate that it has an accuracy of better than 0.74 arcsec over a range of ${ \pm 200}\,\,{\rm arcsec}$±200arcsec, and it can be used for 3-DOF angular motion error measurement of a precision displacement stage.

Sparse aperture arrangement for periodic LED array sampling in a Fourier ptychography microscopy system.

In a traditional Fourier ptychography microscopy (FPM) system, multiple low-resolution (LR) intensity images have to be sequentially captured under variable illumination angles and then stitched together in the Fourier domain, which is time-consuming. This paper proposes a sparse aperture arrangement method in which the relationship between the spatial spectrum distribution and the light-emitting diodes array sampling pattern is investigated. Mathematical models of the proposed method are developed, and numerical simulations and optical experiments are demonstrated to verify its feasibility. The results show that the number of LR images could be decreased significantly without sacrificing image reconstruction quality. To the best of our knowledge, this is the first time that the sparse aperture arrangement has been exploited for FPM. The proposed method may provide new insights for improving the efficiency of the FPM platform and discovering more applications in digital pathology.

Developing on-machine 3D profile measurement for deterministic fabrication of aspheric mirrors: erratum.

This erratum adds a reference to the published paper, Appl. Opt.53, 4997 (2014)APOPAI0003-693510.1364/AO.53.004997.

Design of a free-form diffuser for the entrance optic to correct the cosine error in the photometer.

The entrance optic is an important part of the photometer head. It can match the directional response of the photometer to the cosine function. In this paper, an entrance optic consisting of a free-form diffuser, a shadow ring, and an integrating cavity is introduced. An iterative optimization algorithm is presented to design a free-form diffuser that exhibits better cosine response characteristics. Diffusers of different materials and sizes are designed in a simulation experiment. After a finite number of iterations, in the absence of the shadow ring, the integral cosine error of the free-form diffuser is less than 1%. The directional cosine error is less than 3% for incidence angles between 0° and 70°. After adding a shadow ring to correct the directional response of the incident angle greater than 80°, for incident angles between 0° and 85°, the cosine errors are typically less than 3%, except that the cosine errors of very few large incident angles are close to 5%. The experimental results show the effectiveness and robustness of the proposed method. In addition, the influence of an important percentage constant σ on iterative optimization is studied. It is found that the larger the parameter σ, the fewer the number of iterations, and the directional cosine error may be slightly larger but still acceptable. The wide range of values of σ further embodies the versatility and flexibility of the proposed method.

Reconstruction method based on the Hilbert fractal curve recovery sequence in a Fourier ptychography microscope.

Many Fourier ptychography microscopy techniques have been proposed to achieve higher recovery accuracy in the past few years, yet it is little known that their reconstructed quality is also dependent on the choice of recovery sequence, which is important for fast solution convergence during the Fourier ptychography reconstruction process. In this paper, we propose to use the Hilbert fractal curve, which is one of the most representative of classic space-filling curves, as a new kind of recovery sequence of mesh LED arrays and validate its effectiveness and robustness with both simulated and real experiments. Results show that the Hilbert fractal curve as the recovery sequence is a better choice for periodic LED arrays, compared with raster line, spiral line, and wave-shaped-curve three-recovery sequences.

Coherent noise reduction of phase images in digital holographic microscopy based on the adaptive anisotropic diffusion.

The suppression of coherent noise can produce higher-quality reconstructed images in digital holographic microscopy. A robust and effective phase coherent noise denoising algorithm is proposed in this paper that combines the anisotropic diffusion equation and the phase quality map. In order to accurately identify the noise and signal pixels, we introduce the phase quality map and edge detection to quantify the quality of the pixel information. In addition, a synthetic diffusion function is established to control the speed of the anisotropic diffusion process based on the quality coefficient. Several experiments have been carried out to validate the effectiveness of the proposed algorithm for coherent noise reduction. The results demonstrate that the proposed algorithm can reduce coherent noise and preserve edge details well.

Shape reconstruction based on zero-curl gradient field estimation in a fringe reflection technique.

A novel shape reconstruction method based on zero-curl gradient field estimation is presented in this paper. Zero-curl field estimation makes the most of curl information to obtain the ideal gradient data, and achieves the reconstruction with the quality map path integration method. In the estimation process, an algebraic approach is adopted to enforce integrability, which maintains the local information well. Moreover, we use the residual gradients of surface obtained from the Southwell zonal reconstruction algorithm as the raw gradient data in zero-curl field estimation, which has a stable tradeoff between smoothness and local shape confinement. The performance of the proposed method over antinoise capability is discussed and demonstrated by the simulations. The measurement experiment of an ultraprecision sphere mirror identifies the validity over general shapes, and the reconstruction results of hyperbolic surface with a local shape map demonstrate the better performance on local details retention. Therefore, this method performs well in handling complex objects with local mutation regions and high accuracy requirement of local information in practical measurement.

Removal of millimeter-scale rolled edges using bevel-cut-like tool influence function in magnetorheological jet polishing.

Subaperture polishing techniques usually produce rolled edges due to edge effect. The rolled edges, especially those in millimeter scale on small components, are difficult to eliminate using conventional polishing methods. Magnetorheological jet polishing (MJP) offers the possibility of the removal of these structures, owing to its small tool influence function (TIF) size. Hence, we investigate the removal characters of inclined MJP jetting models by means of computational fluid dynamics (CFD) simulations and polishing experiments. A discrete phase model (DPM) is introduced in the simulation to get the influence of abrasive particle concentration on the removal mechanism. Therefore, a more accurate model for MJP removal mechanisms is built. With several critical problems solved, a small bevel-cut-like TIF (B-TIF), which has fine acentric and unimodal characteristics, is obtained through inclined jetting. The B-TIF proves to have little edge effect and is applied in surface polishing of thin rolled edges. Finally, the RMS of the experimental section profile converges from 10.5 nm to 1.4 nm, and the rolled edges are successfully suppressed. Consequently, it is validated that the B-TIF has remarkable ability in the removal of millimeter-scale rolled edges.

High-precision rotation angle measurement method based on a lensless digital holographic microscope.

To accurately measure ultrasmall rotation angles, a robust and effective method based on lensless digital holographic microscopy is proposed in this paper. The method combines holographic microscopy, solid geometry, and 3D measurement, including holographic measurement and angle measurement processes. We can calculate the 3D shape by the angular spectrum algorithm and the least-squares phase-unwrapping algorithm in the holographic process. According to the relationship between the surface shape and rotation angles, the real-time rotation angles can be calculated. To validate the feasibility and practicability of the proposed approach, numerical noise simulations and experiments were performed. The measurement precision of rotation angle can reach 0.5″ in the range of 1000″ in this paper's experiments. The holographic method has high measurement precision and good stability. In addition, the compact small volume has great potential in small-angle sensor applications.

Large-aperture UV (250~400 nm) imaging spectrometer based on a solid Sagnac interferometer.

Developing an ultraviolet (UV) imaging spectrometer is challenging due to a low level of incident power of photon flux, large chromatic aberration, and relatively low quantum efficiency of imaging sensor in UV waveband. In this paper, a large-aperture UV (250~400 nm) Fourier transform imaging spectrometer is presented for close-range hyperspectral sensing with high spatial resolution and decent spectral resolution. An advanced design based on a modified solid Sagnac interferometer working in UV waveband of 250~400 nm is introduced to improve the interferometric stability. A large-aperture and a reflective-transmissive filtering system are used to increase the spectral purity of the incident UV radiation, and air-spaced achromatic doublets are designed to address the chromatic aberration. The finished spectrometer has a spatial resolution of 23.44 µm on the target plane, a wavelengths resolution of 1.59 nm at 250 nm, and can provide approximately 59 wavelength samples over the waveband of 250~400 nm. The proposed imaging spectrometer acquires a hyperspectral data cube through push-broom scanning in a few minutes. Examples of UV hyperspectral imaging are demonstrated with a sample of resolution test chart, and a cotton sample with vitamin C (VC) and vitamin B6 (VB6) traces. Based on the analysis of spectra, monochromatic images, and k-Means clustering results, it can be concluded that the spectrometer is capable of UV hyperspectral imaging with excellent spectral accuracy, spatial performance, compactness, and robustness. The potential applications of the proposed instrument include materials analysis and traces detection with UV spectral characteristics.

Improvement of high-power laser performance for super-smooth optical surfaces using electrorheological finishing technology.

Laser-induced damage threshold (LIDT) is a key parameter for optical components heavily influenced by the surface roughness in high-power laser uses. Present polishing technologies often bring about directional micro waviness to the optical surfaces due to path effect. Roughness features of a K9 glass surface were studied in this paper. A new evaluating restriction for power spectral density specification was established, and the off-specification frequency contents were found out. Then the electromagnetic simulation of light field modulation was carried out, and the field enhancement factor reached 12.04, verifying the impact of these contents on the laser damage performance of optical components. To restrain the modulation effect by the textures, electrorheological finishing (ERF) technology was proposed, and the processing was undertaken on the K9 surface. Roughness data converged to minimal Ra 1.00 nm, and the angular spectrum decreased in expected ranges. ERF proved to be effective in eliminating the directional textures and restraining the light intensity modulation of the textures. As a result, the LIDTs of optical components can be improved by ERF processing.

Ductile mode grinding of reaction-bonded silicon carbide mirrors.

The demand for reaction-bonded silicon carbide (RB-SiC) mirrors has escalated recently with the rapid development of space optical remote sensors used in astronomy or Earth observation. However, RB-SiC is difficult to machine due to its high hardness. This study intends to perform ductile mode grinding to RB-SiC, which produces superior surface integrity and fewer subsurface damages, thus minimizing the workload of subsequent lapping and polishing. For this purpose, a modified theoretical model for grain depth of cut of grinding wheels is presented, which correlates various processing parameters and the material characteristics (i.e., elastic module) of a wheel's bonding matrix and workpiece. Ductile mode grinding can be achieved as the grain depth of cut of wheels decreases to be less than the critical cut depth of workpieces. The theoretical model gives a roadmap to optimize the grinding parameters for ductile mode grinding of RB-SiC and other ultra-hard brittle materials. Its feasibility was validated by experiments. With the optimized grinding parameters for RB-SiC, the ductile mode grinding produced highly specular surfaces (with roughness of ∼2.2-2.8 nm Ra), which means the material removal mechanism of RB-SiC is dominated by plastic deformation rather than brittle fracture. Contrast experiments were also conducted on fused silica, using the same grinding parameters; this produced only very rough surfaces, which further validated the feasibility of the proposed model.

Electromagnetic optimization of the integrated magnetorheological jet polishing tool and its application in millimeter-scale discontinuous structure processing.

To meet the special demands in small-scale discontinuous optical surface fabrication, the integrated magnetorheological jet polishing (IMJP) tool with multiple motion degrees is introduced in this paper. Four jetting models are implemented and investigated by means of the IMJP tool for practical manufacture. To ensure steady jetting in a long distance, ideal distribution characteristics of the magnetic field in the structure is proposed, based on electromagnetic theory. The magnetic field distribution is simulated subsequently using the finite element analysis method, and three key parameters in the IMJP tool structure are optimized through the simulations. The actual magnetic flux density is measured and spot polishing experiments are conducted in different standoff distances, verifying the effectiveness of the optimization. A processing experiment of a millimeter scale structure with milling tool marks located on a surface with nonuniform curvatures was conducted using the IMJP tool. The roughness of the polishing region converged to 4.86 nm Ra from a low initial quality after processing, and the tool marks have been efficiently removed. The experimental results reveal the reliability of the setup design and the remarkable roughness convergence ability of the IMJP tool for small complex structures.