Nonlinear optics with structured light

The interest in tailoring light in all its degrees of freedom is steadily gaining traction,driven by the tremendous developments in the toolkit for the creation,control and detection of what is now called structured light.Because the complexity of these optical fields is generally understood in terms of interference,the tools have historically been linear optical elements that create the desired superpositions.For this reason,despite the long and impressive history of nonlinear optics,only recently has the spatial structure of light in nonlinear processes come to the fore.In this review we provide a concise theoretical framework for understanding nonlinear optics in the context of structured light,offering an overview and perspective on the progress made,and the challenges that remain.

3D surface reconstruction based on binocular vision using structured light

A 3D surface reconstruction method using a binocular stereo vision technology and a coded structured light,which combines a gray code with phase-shift has been studied.The accuracy of the 3 D surface reconstruction mainly depends on the decoding of gray code views and phase-shift views.In order to find the boundary accurately,gray code patterns and their inverses are projected onto a human eye plaster model.The period dislocation between the gray code views and the phase-shift views in the course of decoding has been analyzed and a new method has been proposed to solve it.The splicing method is based on feature points.The result of the 3D surface reconstruction shows the accuracy and reliability of our method.

Three-Dimensional Measurement Using Structured Light Based on Deep Learning

Three-dimensional(3D)reconstruction using structured light projection has the characteristics of non-contact,high precision,easy operation,and strong real-time performance.However,for actual measurement,projection modulated images are disturbed by electronic noise or other interference,which reduces the precision of the measurement system.To solve this problem,a 3D measurement algorithm of structured light based on deep learning is proposed.The end-to-end multi-convolution neural network model is designed to separately extract the coarse-and fine-layer features of a 3D image.The point-cloud model is obtained by nonlinear regression.The weighting coefficient loss function is introduced to the multi-convolution neural network,and the point-cloud data are continuously optimized to obtain the 3D reconstruction model.To verify the effectiveness of the method,image datasets of different 3D gypsum models were collected,trained,and tested using the above method.Experimental results show that the algorithm effectively eliminates external light environmental interference,avoids the influence of object shape,and achieves higher stability and precision.The proposed method is proved to be effective for regular objects.

Filtering algorithm of line structured light for long-distance obstacle detection

Since unmanned ground vehicles often encounter concave and convex obstacles in wild ground,a filtering algorithm using line structured light to detect these long distance obstacles is proposed.For the line structured light image,a ranked-order based adaptively extremum median(RAEM)filter algorithm on salt and pepper noise is presented.In the algorithm,firstly effective points and noise points in a filtering window are differentiated;then the gray values of noise points are replaced by the medium of gray values of the effective pixels,with the efficient points' gray values unchanged;in the end this algorithm is proved to be efficient by experiments.Experimental results demonstrate that the proposed algorithm can remove noise points effectively and minimize the image blur,resulting into protecting the edge information as much as possible.

Robust structured light in atmospheric turbulence

Structured light is routinely used in free-space optical communication channels,both classical and quantum,where information is encoded in the spatial structure of the mode for increased bandwidth.Both real-world and experimentally simulated turbulence conditions have revealed that free-space structured light modes are perturbed in some manner by turbulence,resulting in both amplitude and phase distortions,and consequently,much attention has focused on whether one mode type is more robust than another,but with seemingly inconclusive and contradictory results.We present complex forms of structured light that are invariant under propagation through the atmosphere:the true eigenmodes of atmospheric turbulence.We provide a theoretical procedure for obtaining these eigenmodes and confirm their invariance both numerically and experimentally.Although we have demonstrated the approach on atmospheric turbulence,its generality allows it to be extended to other channels too,such as aberrated paths,underwater,and in optical fiber.

High Curvature Stripe Profile Extraction Algorithm of Line Structured Light Measuring System

In the line structured light measuring system, the accuracy of the process of laser stripe directly affects the measurement results. Therefore, the extraction algorithm for the laser stripe, especially the surface with high reflection and high curvature, is very important. The imaging principle of line structured light, the light intensity distribution law of laser stripe and the extraction algorithm have been studied, and a stripe profile extraction method based on real light intensity distribution has been proposed. In this algorithm, fast region of interest extraction, stripe width estimation, and adaptive filtering on the striped image are performed. Then the energy center of the stripe at the sub-pixel level is extracted. Finally, the low-quality center points are eliminated, and the context information is used to recover the missing central points. Simulated images generated based on the imaging principle of line structured light and real experimental images were used to evaluate the accuracy and repeatability of the proposed method. The results show that the method behaves excellently at the edges of high-curvature stripes;the maximum error is only 1.6 pixels, which is 1/10 of the classic Steger algorithm;the experiment repeatability is only 8.8 μm, which is 2.7 times that of the Steger method. Therefore, the proposed method improves the accuracy of object contour extraction, and it is especially suitable for contour detection of objects with high curvature.

Dynamically measuring the holo-information of light fields in three-dimensional space using a periodic polarization-structured light

Spatially structured light field has attracted great attention due to its novel properties and application potential in numerous fields.Among them,the most striking one is the polarization-structured light,known as the vector beam.Here,using a periodic polarization-structured light,we propose a method to dynamically measure the holo-information of light fields,including the amplitude,phase,and polarization distributions,in three-dimensional(3D)space.The measurement system is composed of a Mach-Zender interferometer involving a liquid crystal polarized grating in the reference arm,which is simple,stable,and easy to operate.Featuring the single-shot measurement,this method supports observing the dynamic variation of object light fields.The accuracy,3D polarimetry,and dynamic observation of this method are validated by measuring a calibrated quarter-wave plate,a vector vortex beam,a Poincarébeam,and a stressed polymethyl methacrylate sample.

Three-line structured light vision system for non-cooperative satellites in proximity operations

Adapter ring is a commonly used component in non-cooperative satellites,which has high strength and is suitable to be recognized and grasped by the space manipulator.During proximity operations,this circle feature may be occluded by the robot arm or limited field of view.Moreover,the captured images may be underexposed when there is not enough illumination.To address these problems,this paper presents a structured light vision system with three line lasers and a monocular camera.The lasers project lines onto the surface of the satellite,and six break points are formed along both sides of the adapter ring.A closed-form solution for real-time pose estimation is given using these break points.Then,a virtual structured light platform is constructed to simulate synthetic images of the target satellite.Compared with the predefined camera parameters and relative positions,the proposed method is demonstrated to be more effective,especially at a close distance.Besides,a physical space verification system is set up to prove the effectiveness and robustness of our method under different light conditions.Experimental results indicate that it is a practical and effective method for the pose measurement of on-orbit tasks.

Surface Reconstruction by Structured Light Projection

A rapid and practical method is proposed to reconstruct surface based on the linked structured light stripes which are produced by structured light projection.The subpixel points on a stripe are linked firstly one by one to form a stripe ensemble which is then transformed to a point ensemble in 3D space.The initial mesh with local optimization is generated by triangulating each two adjacent point ensembles.In order to obtain a better mesh,our improved edge flipping algorithm is employed to optimize the initial mesh globally.Because of employing the information of the linked structured stripes,our reconstruction algorithm is performed fastly.Moreover,the subpixel points on each stripe are already linked on the captured images such that they do not require the high sampling density.The experiments show that the proposed method constructs a surface rapidly and effectively.

A review of liquid crystal spatial light modulators:devices and applications

Spatial light modulators,as dynamic flat-panel optical devices,have witnessed rapid development over the past two decades,concomitant with the advancements in micro-and opto-electronic integration technology.In particular,liquid-crystal spatial light modulator(LC-SLM)technologies have been regarded as versatile tools for generating arbitrary optical fields and tailoring all degrees of freedom beyond just phase and amplitude.These devices have gained significant interest in the nascent field of structured light in space and time,facilitated by their ease of use and real-time light manipulation,fueling both fundamental research and practical applications.Here we provide an overview of the key working principles of LC-SLMs and review the significant progress made to date in their deployment for various applications,covering topics as diverse as beam shaping and steering,holography,optical trapping and tweezers,measurement,wavefront coding,optical vortex,and quantum optics.Finally,we conclude with an outlook on the potential opportunities and technical challenges in this rapidly developing field.

Generation of structure light in probe absorption spectrum via microwave-driven Y-type atomic system

Behavior of structure light is investigated by monitoring probe absorption using a microwave-driven Y-type atomic media configuration.The system under consideration is driven by one of the control vortex beams as well as an extra non-vortex control beam to ensure electromagnetically induced transparency.The significant aspect in the generation of structured light is the azimuthal phase-dependent modification for probe absorption.Further intensity distribution for absorption spectra is examined for simultaneously evaluating both the control vortex beams.We also go through the radial distribution of intensity for various orbital angular momentum values.Different modes of structural beams may be distinguished using the suggested approach.Our research gives us a way for rapidly transferring vortex wavefronts from control field to probe absorption profile.This could be useful in quantum information processing.

Enhancing light absorption for organic solar cells using front ITO nanograting and back ultrathin Al layer

To address the discrepancy between carrier collection and light absorption of organic solar cells caused by the limited carrier mobility and optical absorption coefficient for the normally employed organic photoactive layers,a light management structure composed of a front indium tin oxide(ITO)nanograting and ultrathin Al layer inserted in between the photoactive layer and the electron transport layer(ETL)is introduced.Owing to the antireflection and light scattering induced by the ITO nanograting and the suppression of light absorption in the ETL by the inserted Al layer,the light absorption of the photoactive layer is significantly enhanced in a spectral range from 400 nm to 650 nm that also covers the main energy region of solar irradiation for the normally employed active materials such as the P3HT:PC_(61)BM blend.The simulation results indicate that comparing with the control device with a planar configuration of ITO/PEDOT:PSS/P3HT:PC_(61)BM(80-nm thick)/ZnO/Al,the short-circuit current density and power conversion efficiency of the optimized light management structure can be improved by 32.86%and 34.46%.Moreover,good omnidirectional light management is observed for the proposed device structure.Owing to the fact that the light management structure possesses the simple structure and excellent performance,the exploration of such a structure can be believed to be significant in fabricating the thin film-based optoelectronic devices.

High-accuracy 3-D deformation measurement method with an improved structured-light principle

In this paper,a high-accuracy 3-D deformation measurement(HADM)method with structured light is proposed and applied to wing deformation measurement in wind tunnel experiments.The present method employs an arbitrarily arranged fringe projector and a perpendicularly placed camera.The exact phase-height mapping using the phase differences of the projected sinusoidal fringe patterns,as well as the spatial distribution of the fringe,is accurately derived.It not only presents high feasibility but also reduces systemic uncertainties arising from deviations between the ideal model and the real-world conditions.Meanwhile,a dynamic boundary process algorithm is proposed to reduce the measurement uncertainty caused by fringe fracture near the object boundary.It is calibrated that a high accuracy with the average measurement uncertainty of 0.0237 mm is achieved,which is less than 0.01%of the side length of 25 cm of the field of view.In the wind tunnel experiments,the 3-D deformations of the elastic wing,particularly the key geometric parameters such as wing tip position,angle of attack,and dihedral angle,are well reconstructed to provide an in-depth explanation for the aerodynamic characteristics.

Structured-Light-Field 3D Imaging System With Coaxial Projection

This paper presents a novel 3D measurement method for a light field camera(LFC)in which 3D information of object space is encoded by a microlens array(MLA).The light ray corresponding to each pixel of the LFC is calibrated.Once the matching points from at least two subviews exhibit sub-pixel accuracy,the 3D coordinates can be calculated optimally by intersecting light rays of these points matched through phase coding.Moreover,the proposed method obtains high-resolved results that exceed the subview resolution due to the virtual continuous phase search strategy.Finally,we combine the LFC and coaxial projection to solve the 3D data loss caused by shadowing and occlusion problems.Experimental results verify the feasibility of the proposed method,and the measurement error is about 30μm in a depth range of 60 mm.

Spatial light modulation for femtosecond laser manufacturing:Current developments and challenges

Since the invention of lasers,spatial-light-modulated laser processing has become a powerful tool for various applications.It enables multidimensional and dynamic modulation of the laser beam,which significantly improves the processing efficiency,accuracy,and flexibility,and presents wider prospects over traditional mechanical technologies for machining three-dimensional,hard,brittle,or transparent materials.In this review,we introduce:(1)The role of spatial light modulation technology in the development of femtosecond laser manufacturing;(2)the structured light generated by spatial light modulation and its generation methods;and(3)representative applications of spatial-light-modulated femtosecond laser manufacturing,including aberration correction,parallel processing,focal field engineering,and polarization control.Finally,we summarize the present challenges in the field and possible future research.

Lateral Performance for Wood-Frame Shear Walls–A Critical Review

Wood is a green material in line with the sustainable development strategy.From the excellent performance of engineering wood products,modern wood structures represented by light wood structures have gained more development opportunities.As an indispensable part of light wood structure systems,the wood-frame shear wall plays a vital role in the bearing capacity and earthquake resistance of light wood structure systems.This paper is focused on a review of the lateral performance of wood-frame shear walls and classifies the influencing factors in relevant experimental research into three categories,including internal factors such as shear wall structure,external factors such as test scheme,and other factors of material production and test process.Finally,the research prospects in this field were introduced based on the summary of the research status.This work can be a reference for further research on the lateral performance of wood-frame shear walls.

Development of an In-Situ Laser Machining System Using a Three-Dimensional Galvanometer Scanner

In this study, a three-dimensional (3D) in-situ laser machining system integrating laser measurement and machining was built using a 3D galvanometer scanner equipped with a side-axis industrial camera. A line structured light measurement model based on a galvanometer scanner was proposed to obtain the 3D information of the workpiece. A height calibration method was proposed to further ensure measurement accuracy, so as to achieve accurate laser focusing. In-situ machining software was developed to realize time-saving and labor-saving 3D laser processing. The feasibility and practicability of this in-situ laser machining system were verified using specific cases. In comparison with the conventional line structured light measurement method, the proposed methods do not require light plane calibration, and do not need additional motion axes for 3D reconstruction;thus they provide technical and cost advantages. The insitu laser machining system realizes a simple operation process by integrating measurement and machining,which greatly reduces labor and time costs.

Estimation of the Scale of Artificial Reef Sets on the Basis of Underwater 3D Reconstruction

The creation of three-dimensional models from an unorganized set of points is an active research area in computer graphics.One of the purposes of this study is to explore the 3D reconstruction of a cube-type artificial reef(CTAR)set by linear structured light and binocular stereo vision technology in an underwater environment.The experimental setup is composed of two ca-meras in a stereo vision configuration.The alpha shapes method can be used to construct a surface that most closely reflects the arti-ficial reef set described by the points.A parameter study is conducted to assess the scales of the set(i.e.,usable volume,surface area,projected area,height,and base diameter)on the basis of 3D reconstruction.Experimental results show that the quality of 3D recon-struction in an underwater environment is acceptable for estimating the scale size of the CTAR set.According to the measurement of the scale sizing of the CTAR set,the relationships between the parameters of the CTAR set and the number of CTAR modules were determined.Moreover,the usable volume of the CTAR set can be estimated depending on the basis of the number of CTAR modules.

Phase Error Compensation of Three-Dimensional Reconstruction Combined with Hilbert Transform

Nonlinear response is an important factor affecting the accuracy of three-dimensional image measurement based on the fringe structured light method.A phase compensation algorithm combined with a Hilbert transform is proposed to reduce the phase error caused by the nonlinear response of a digital projector in the three-dimensional measurement system of fringe structured light.According to the analysis of the influence of Gamma distortion on the phase calculation,the algorithm establishes the relationship model between phase error and harmonic coefficient,introduces phase shift to the signal,and keeps the signal amplitude constant while filtering out the DC component.The phase error is converted to the transform domain,and compared with the numeric value in the space domain.The algorithm is combined with a spiral phase function to optimize the Hilbert transform,so as to eliminate external noise,enhance the image quality,and get an accurate phase value.Experimental results show that the proposed method can effectively improve the accuracy and speed of phase measurement.By performing phase error compensation for free-form surface objects,the phase error is reduced by about 26%,and about 27%of the image reconstruction time is saved,which further demonstrates the feasibility and effectiveness of the method.

3D Surface Digitization in Scientific Research and Product Development

This comprehensive review participates in the use of three different non-invasive surface scanning techniques directed in scientific research of medical, anthropology, archaeology forensic science, and product designing. 3D surface examining tools speak to a promising technique to provide reproducible data such as map the facial soft or hard tissues of a subject document skeletal remains, and trauma, generating 3D imitations of the components for documented and illustrative purposes while simultaneously holding exactness and unwavering quality. Three-dimensional imaging is rapidly turning into a vital tool for reconstruction and examination in scientific research. The final 3D mesh can be 3D printed or the digital version can be shared online with scientific researchers. This review manuscript highlights several studies utilizing non-invasive scanning techniques, depicts the pro and cons of the 3D scanning techniques, and different features of the scanners irrespective of the cost which would be helpful for future research work. A resourceful review was conducted using 7 databases;PubMed, CENTRAL, Science Direct, Scopus, Google Scholar, Research Gate, and IEEE Xplore from 2002 to 2020. Search terms were;“3D Laser scanning”, “Photogrammetry”, “Skeleton preservation”, “Documentation”, “Surface Scanning”, etc. Papers with quality work and related to the field of forensic science, anthropometry, 3D facial scanning, and product development were selected. From all the studies, 71 studies met the eligibility criteria, and other articles were excluded which were non-relevant, had duplicate records, and did not meet search criteria. This review provides in-depth understanding and discussions into methods, restrictions, and inferences from respective research publications.