Showing papers in "Optics and Laser Technology in 2020"

A novel chaos based optical image encryption using fractional Fourier transform and DNA sequence operation

Abstract: In this paper, we propose a new method for optical image encryption using fractional Fourier transform, DNA sequence operation and chaos theory. Random phase masks are generated using iterative Lorenz map and the plain image is transformed to a DNA matrix. This matrix is combined with the random phase mask and then transformed three times using the fractional Fourier transform. An Optical implementation of the encryption algorithm is proposed in our work. According to the experiment results and security analysis, we find that our algorithm has good encryption effect, larger secret key space and high sensitivity to the secret key. It can resist to most known attacks, such as statistical analysis and exhaustive attacks. All these features show that our encryption algorithm is very suitable for digital image encryption.

Review on residual stress in selective laser melting additive manufacturing of alloy parts

Abstract: The undesirable residual stress accumulated in the parts during the melting and solidification of the metal powder layer by layer retards the further application of the selective laser melting (SLM) process. This paper focuses on reviewing the recent illuminating achievements about physical modeling, experimental characterizing, and active adjusting of the residual stress in the parts fabricated by SLM. The advantages and disadvantages of the mainstream and emerging models or approaches are further analyzed. Based on the status and prospect of the relative techniques, a series of conceptual methods are discussed on mitigating residual stress to make some practical inspiration for developing a systematical residual stress balancing technique for SLM.

Hyperchaotic image encryption algorithm based on bit-level permutation and DNA encoding

Abstract: Many chaos-based image encryption algorithms using low-dimensional chaotic mapping and permutation diffusion structures have been proposed recently. However, low-dimensional chaotic maps are less secure than high-dimensional chaotic systems. Furthermore, the permutation process is independent of the plaintext and diffusion process. As a result, they are not very resistant to chosen plaintext attacks and chosen ciphertext attacks. In this paper, we propose a hyper-chaos-based image encryption algorithm that uses a 6-dimensional hyperchaotic system; the key stream generated by hyperchaotic system is related to the plaintext image. Then, bit-level permutation is employed to strengthen the security of the cryptosystem. Finally, DNA coding and operations are employed to change pixels. Theoretical analysis and numerical simulations demonstrate that the proposed algorithm is safe and reliable for image encryption.

Controlled alternate quantum walks based privacy preserving healthcare images in Internet of Things

Abstract: The development of quantum computers and quantum algorithms conveys a challenging scenario for several cryptographic protocols due to the mathematical scaffolding upon which those protocols have been built. Quantum walks constitute a universal quantum computational model which is widely used in various fields, including quantum algorithms and cryptography. Quantum walks can be utilized as a powerful tool for the development of modern chaos-based cryptographic applications due to their nonlinear dynamical behavior and high sensitivity to initial conditions. In this paper, we propose new encryption mechanism for privacy preserving Internet of Things-based healthcare systems in order to protect the patients’ privacy. The encryption/decryption processes are based on controlled alternate quantum walks. The proposed cryptosystem approach is composed of two phases: substitution and permutation, both based on independently computed quantum walks. Simulation results and numerical analysis of our data provide enough evidence to reasonably conjecture that our image encryption protocol is robust and efficient for protecting patients’ privacy protection.

One-pot synthesis of nanostructured CdS, CuS, and SnS by pulsed laser ablation in liquid environment and their antimicrobial activity

Abstract: We describe here a generic method for producing high-quality metal sulfide nanoparticles of a controllable and narrow size distribution in the nano-metric range. Metal sulfide nanostructures (CdS, CuS, and SnS) are synthesized by using nanosecond pulsed laser ablation in a liquid solution containing sulfur precursor in just one step (one-pot versatile synthesis method). The optical properties have been studied using UV–Visible spectroscopy showing the main characteristic transition peaks of metal sulfides. Transmission electron microscope (TEM) shows a uniform Smaller-sized and nearly spherical nanocrystals exhibited a cross lattice pattern. The structure study by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) reveal the purity and the spherical crystalline structure of all the prepared nano metal sulfides. After that, the antibacterial activity has been examined against four types of bacteria and unicellular fungi by using agar plate diffusion methods followed by studying the required time for killing. The tested microorganisms strain is showing that CuS nanoparticles have the highest antimicrobial activity.

First principle calculations and mechanical properties of the intermetallic compounds in a laser welded steel/aluminum joint

Abstract: To analyze the IMC (Intermetallic Compound) properties and their effects on steel/aluminum welding, the equilibrium lattice constants, mechanical properties and electronic structures of the intermetallic compounds Fe3Al, FeAl, Fe2Al5, FeAl2, FeAl3 and Fe4Al13 were systematically calculated using the first-principle methods. The results show that the calculated elastic constants of the IMCs satisfy the mechanical stability conditions. Fe3Al and FeAl2 exhibit plastic characteristics; FeAl, Fe2Al5, FeAl3, and Fe4Al13 exhibit brittle characteristics; Fe-Al binary compounds have typical metallic properties; and the 3d bands of Fe contribute most significantly to the total density of states. In the vicinity of the Fermi level, the 3d bands of Fe contribute together with the bands of Al; the Fe-Al binary compounds have weak ionicity, relatively high hardness and high melting points; additionally, the effects of Fe-rich phases on the mechanical properties of the joints are superior compared to Al-rich phases. To verify the first principle calculations, T-joint laser welding experiments were conducted on 316L stainless steel and 6061 aluminum alloy sheets. The microstructure, reaction phases, fracture morphologies and mechanical properties of the welded joint were analyzed by optical microscopy, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and nanoindentation. According to the XRD and EDS analysis, Fe3Al, FeAl, Fe2Al5, FeAl2, FeAl3 and Fe4Al13 were formed; their properties and effects on the joint of these phases are consistent with the calculated results.

Effect of laser beam models on laser welding–brazing Al to steel

Abstract: Joining dissimilar metals Al to steel in butt configuration was successfully realized by laser welding–brazing process with different laser beam models (single laser beam, cross and in–line dual laser beams). Undercut occurred at the joint produced by single laser beam and satisfactory weld formations were obtained in the joints produced by cross and in–line dual laser beams. Dual laser beam was beneficial for the reduction of interfacial intermetallic compound (IMC) thickness and homogenization of interfacial IMC morphology. Numerical models were developed to calculate the thermal cycles along brazing interface. Calculation of interfacial thermal cycles revealed that lowest peak temperatures (1028 °C, 1036 °C and 990 °C respectively for top, middle and bottom regions), shortest time of interaction at high temperature (2.2 S) and smallest temperature gradients (1036–990 °C = 46 °C) were all obtained in the joint produced by cross dual laser beam and these all made contribution to its thinnest and most homogeneous interfacial IMC. Joint with the highest tensile strength was produced when the cross dual laser beam was adopted due to its satisfactory weld formation and most homogeneous of τ5.

Review of high sensitivity fibre-optic pressure sensors for low pressure sensing

Abstract: Fibre Bragg grating (FBG) pressure sensors show a great potential in replacing conventional electrical pressure sensors due to their numerous advantages. However, increasing their pressure sensitivity performance for low hydrostatic pressure measurement is still a challenge. This paper reviewed recent pressure sensitivity enhancement methods that could be divided into two groups, namely intrinsic and extrinsic. For the intrinsic enhancement method, this paper reviewed polymer FBGs, special fibre sensors, interferometric sensors, and special grating sensors. For the extrinsic enhancement method, polymer-based pressure transducers, diaphragm-based pressure transducers, and other structure-based pressure transducers were reviewed in detail.

Terahertz high and near-zero refractive index metamaterials by double layer metal ring microstructure

Abstract: We demonstrate the extensive improvement of the effective refractive index of designed metamaterials through the coupling effect of double layers in terahertz region. It is interesting that when the substrate thickness is decreased to small enough, the effective near zero refractive index can be realized. Both high refractive index and near zero refractive index at different frequency can be realized with designed structure. After optimizing geometric parameters of double-sided metamaterial, the effective ultrahigh refractive index with the peak value more than 100 can be obtained, and the bandwidth of near zero index can be achieved to about 2.0 THz. In order to verify the accuracy of our design, a single layer metallic ring metamaterial was prepared in experiment. The transmission was measured by using terahertz time domain spectroscopy at the frequency range from 0.1 THz to 2.5 THz. It is found that the theoretical design results are basically consistent with the experimental results.

Development and characterization of 316L/Inconel625 functionally graded material fabricated by laser direct metal deposition

Abstract: Functionally graded material (FGM) is a composite with innovative structure and function, which has the good overall performance and meets working requirements in harsh environments. FGM has been widely used in aerospace, biological, nuclear, and photoelectric engineering fields. Laser direct metal deposition (LDMD) is an advanced manufacturing method that is excellent at fabricating objects with optimized geometries and minimizing weight using far less material and energy. In this paper, FGM with the constitution varying from 100% 316L stainless steel to 100% Inconel625 alloy was successfully fabricated using LDMD technology. Grain morphology, composition, mechanical properties and abrasive resistance were obtained to investigate the microstructure and mechanical performance of FGM. With the Inconel625 content increasing, primary dendrite arm spacing gradually increased, and white second phases began to precipitate along dendrites boundary when the content of Inconel625 exceeded 80%. Micro-hardness gradually increased from 216.47 HV at the bottom of FGM to 355.7 HV at the top. With micro-hardness and the hard phase volume increasing, the wear rate of FGM declined and the wear resistance was improved. The fracture element analysis showed that a large number of small and uneven distributed second phases led to the graded material fracture and the tensile fracture mechanism was of typical micro-porous aggregation toughness fracture.

An automated early diabetic retinopathy detection through improved blood vessel and optic disc segmentation

Abstract: This paper presents an automated early diabetic retinopathy detection scheme from color fundus images through improved segmentation strategies for optic disc and blood vessels. The red lesions, microaneurysms and hemorrhages are the earliest signs of diabetic retinopathy. This paper essentially proposes improved techniques for microaneurysm as well as hemorrhages detection, which eventually contribute in the overall improvement in the early detection of diabetic retinopathy. The proposed method consists of five stages- pre-processing, detection of blood vessels, segmentation of optic disc, localization of fovea, feature extraction and classification. Mathematical morphology operation is used for pre-processing and blood vessel detection. Watershed transform is used for optic disc segmentation. The main contribution of this model is to propose an improved blood vessel and optic disc segmentation methods. Radial basis function neural network is used for classification of the diseases. The parameters of radial basis function neural network are trained by the features of microaneurysm and hemorrhages. The accuracy of the proposed algorithm is evaluated based on sensitivity and specificity, which are 87% and 93% respectively.

Design of novel superalkali doped silicon carbide nanocages with giant nonlinear optical response

Abstract: In this study, the electronic and nonlinear optical (NLO) properties of a novel class of superalkalis (Li2F, Li3O and Li4N) doped silicon carbide (Si12C12) nanocages are investigated by using density functional theory (DFT) calculations. Computational results reveal that these complexes are quite stable and superalkalis prefer Sitop position of the nanocage energetically to be chemisorbed. The doping of superalkalis effectively reduced the HOMO–LUMO energy gap and transformed Si12C12 nanocage from insulator to n-type semiconductor. More interestingly, these complexes exhibited significantly large first hyperpolarizabilities (βo) in the range of 2141–19864 au. This remarkable increase in first hyperpolarizability (βo) values is due to small transition energies ΔE, which comes from the corresponding charge transfer from superalkali to the nanocage. The NLO response of the superalkali-doped Si12C12 nanocage was much better to those of their alkali-metal-doped analogs. Moreover, frequency dependent hyperpolarizability calculations are performed in the range of 400–1600 nm including 532 and 1064 nm for commonly used lasers. The TD-DFT analysis reveals that these complexes possess enough transparency in the UV region which is required besides large NLO response for practical applications in the field of opto-electronics. This study will provide new insights for designing of novel NLO materials having useful applications in all-optical switching, wavelength conversion and harmonic generation.

A novel chaotic image encryption algorithm based on extended Zigzag confusion and RNA operation

Abstract: In this paper, a novel chaotic image encryption algorithm based on extended Zigzag confusion and RNA operation is proposed. Firstly, a novel chaotic system is designed, which can be regarded as an improvement of traditional Logistic map and Sine map. Then it can be used to generate chaotic sequence combined with SHA 512 hash function. Secondly, extended Zigzag confusion scheme is proposed. It not can only process non square matrix, but also can choose from four directions to determine the position where encryption starts. This scheme can solve the problem of some elements keep in the same position in each encryption round. Thirdly, RNA operation is performed. In RNA computing, the choice of operators is determined according to the category of amino acids, and the coding rules of RNA matrix are completely controlled by chaotic sequence, which makes the results of operation more unpredictable. Finally, this encryption method can extend to color image. The data from experimental simulation and performance analysis show that this encryption algorithm has higher safety performance.

Terahertz phase imaging and biomedical applications

Abstract: Terahertz frequency (THz) radiation lies in between the microwave and infrared ranges. While it is strongly absorbed by water, it is nonionizing and has low possibility of causing tissue damage as it involves low energy photons. Recent technological progress in developing THz instrumentation, means that commercial THz systems are being produced with improving performance which are easier to operate and more reliable. THz phase imaging, an advanced imaging technology which combines the benefits of THz and commonly used phase imaging techniques, has recently received significant attention. In this paper, the current state of such imaging systems is reviewed. This review deals with both pulsed and continuous-wave (CW) imaging systems. Pulsed THz phase imaging is a coherent measurement, which includes terahertz pulsed imaging (TPI) based on femtosecond laser and holographic imaging in the time domain, both allow phase and amplitude information of the electric field to be recorded. CW THz phase imaging is mainly based on digital holography, interferometry and ptychography. These systems can obtain the complex amplitude by capturing diffraction patterns and applying numerical reconstruction techniques. Biomedical applications of such THz systems are highlighted.

Numerical simulation and experimental investigation on three-dimensional modelling of single-track geometry and temperature evolution by laser cladding

Abstract: Investigation of the temperature distribution law in molten pool during the laser cladding process and the varying principle of the forming parts’ cross section sizes to obtain optimum process parameters is essential to improve the precision of deposited parts. In this paper, a three-dimensional numerical single-track processing prediction model (STPPM) in laser cladding was established. The Gaussian distribution heat source was applied. Based on the element birth and death method, the transient temperature field and the geometry of the cladding could be calculated simultaneously by considering the temperature rise of the powder particles, laser energy attenuation and the material phase transition. With the prediction error less than 6.6% for track width and track height, approximately 3.1% for the maximum temperature, numerical simulation results of the track profile agreed well with the experiment results for 316 stainless steel material, indicating the validity of the STPPM. The effects of laser power and scanning speed on the geometry and temperature distribution of cladding were analyzed on the basis of the STPPM and experimentally proved. The results show that track height, width and the temperature in molten pool decreased accordingly with the increase of scanning speed. In addition, track width and the temperature in molten pool increased with the laser power while the track height stabilized. Research in this paper would contribute to minimizing the number of tentative experiments in laser cladding process and the experimental costs, meanwhile improving deposition efficiency during the processing.

Influence of laser power and scanning strategy on residual stress distribution in additively manufactured 316L steel

Abstract: Residual stress control in the metal components by additive manufacturing (AM) has been a major challenge. To mitigate this challenge, proper selection of AM process parameters is of great importance. In this study, we investigate the influence of laser power and scanning strategies on residual stress distribution in 316L steel by a metal AM process, namely, selective laser melting (SLM). Finite element simulation and experimental verification are conducted by using the identical process parameters and part geometry to ensure that the results are indeed comparable and can shed light on the challenging issue of residual stress control. With two levels of laser power (i.e., 160 W and 200 W) and two scanning strategies (i.e., stripe scanning and chessboard scanning), four process conditions are investigated. For all four conditions, both simulation and experiment show that the tensile residual stress in the area of interest (the center area of each layer) tends to gradually increase along the depth into surface. Also, the increase of laser power from 160 W to 200 W and the adoption of stripe scanning (instead of chessboard scanning) generally lead to the increase of tensile residual stress in the area of interest. The trends are also confirmed by both simulation and experiment. In addition, the laser power increase from 160 W to 200 W appears to have more significant effect, compared with the switch of two scanning strategies.

Evolutionary-based image encryption using RNA codons truth table

Abstract: Symmetric image cryptography is a mechanism in which image pixels are encrypted into some meaningless format called cipher image. Only authorized users have access to the secret code for symmetric key encryption. In this paper a new symmetric image encryption method has been proposed using the concepts of ribonucleic acid (RNA) sequence and genetic algorithm (GA), called RNA-GA. The proposed method starts by generating specified number of initial cipher images using logistic map function. Then, the initial cipher images are converted to the corresponding one-dimensional binary sequences and relevant codons array using codons truth table. The codons array are then updated using encryption key and encryption RNA tables to form the initial population of genetic algorithm. Next, genetic algorithm optimizes the population using selection, crossover, and mutation operators. The results approve high resistance of the proposed method against well-known attacks and entropy of 7.9987 regarding standard image of size 256 × 256 after 30 repetitions.

Micro selective laser melting of NiTi shape memory alloy: Defects, microstructures and thermal/mechanical properties

Abstract: The use of micro selective laser melting (μ-SLM) enables product miniaturization, which is one of the megatrends in the metal processing industry and increasingly find its applications in biomedical and electronics industries. Among these, NiTi shape memory alloy (SMA) shows a great promise in functional micro-scaled components such as stent. There are inevitably some imperfections in SLM, but the imperfection formation in μ-SLM may not be the same as that in the conventional SLM. This work studies the imperfections in μ-SLM produced NiTi samples, with focus on defects, microstructure and thermal/mechanical behaviors. The effects of substrate material, laser-related process parameter and scanning strategy on defects such as porosity and cracks were analyzed, and a process window for “Scanning speed – Hatch spacing” was determined. Transformation peak was hardly detected in thermal behavior of as-printed and post heat-treated μ-SLM NiTi, resulting from microstructure inhomogeneity, Ti-rich impurity phases TiC1−xNx/Ti4Ni2Ox and precipitate Ti2Ni, which were introduced by powder preparation, μ-SLM or post heat treatment. The as-printed NiTi shows higher compressive strength and fracture strain than the post heat-treated samples, reaching 2796.57 MPa and 27.80% on average, respectively, but the plateau stress-strain stage is indistinguishable due to inhomogeneous and localized stress-induced martensite transformation. The brittle Ti2Ni phase was introduced in post heat treatment, leading to inhomogeneous microstructure and lower ductility. The underlying mechanisms revealed in these imperfections could serve as a guideline for defect control, process optimization, as well as post heat treatment methods for μ-SLM of NiTi alloy.

Current research and development status of dissimilar materials laser welding of titanium and its alloys

Abstract: Since its inception, laser beam welding as a high-quality fusion joining process has ascertained itself as an established and state of art technology exhibiting tremendous growth in a broad range of industries. This article provides a current state of understanding and detailed review of laser welding of titanium (Ti) alloys with corresponding dissimilar counterparts including steel, aluminium, magnesium, nickel, niobium, copper, etc. Particular emphasis is placed on the influence of critical processing parameters on the metallurgical features, tensile strength, hardness variation, percentage elongation and residual stress. Process modifications to improve dissimilar laser weldability by virtue of techniques such as laser offsetting, split beam, welding-brazing, hybrid welding and materials modifications by means of the introduction of single or multiple interlayers, fillers and pre-cut grooves are exploited. Detailed and comprehensive investigations on the phenomena governing the formation and distribution of the intermetallic phase, material flow mechanisms, their relations with laser parameters and their corresponding impact on the microstructural, geometrical and mechanical aspects of the welds are thoroughly examined. The critical issues related to the evolution of defects and the corresponding remedial measures applied are explored and the characteristics of fracture features reported in the literature are summarised in thematic tables. The purpose of this review is tantamount to emphasise the benefits and the growing trend of laser welding of Ti alloys in the academic sector to better exploit the process in the industry so that the applications are explored to a greater extent.

Application of laser ablation in adhesive bonding of metallic materials: A review

Abstract: Surface treatments have been widely applied to a variety of metallic materials in order to modify their physicochemical properties and to improve both strength and durability of adhesive-bonded joints. Among available surface treatment methods, laser ablation has the advantages of efficiency, environmental friendliness and easy industrialization. In this work, the principle of laser ablation and its effect on the physicochemical properties of metallic materials are reviewed. The mechanisms accounting for the improved bonding strength and corrosion resistance of the adhesive-bonded joints fabricated from laser-ablated metallic materials are reviewed and discussed. Bonding strength of laser-ablated adhesively bonded joints increases with the increase of roughness and wettability of the metallic adherend surface, and the surface roughness contributes to the improved joint strength only when the adhesive is capable to sufficiently wet the laser-ablated surface to form mechanical interlocking at the bonding interfaces. Although the laser-ablated surfaces are more active with lower polarization resistances and higher corrosion current, which suggests that the laser-ablated surfaces are more susceptible to corrosion, the increased roughness by laser ablation plays a crucial role in the improved corrosion resistance of laser-ablated adhesively bonded joints, as rough surfaces effectively retard the diffusion of corrosive solution.

A novel periodically tapered structure-based gold nanoparticles and graphene oxide – Immobilized optical fiber sensor to detect ascorbic acid

Abstract: This work presents a study, analysis, design, and characterization of a localized surface plasmon resonance (LSPR) based ascorbic acid (AA) sensor with improved sensitivity compared to those already reported. Various multi-tapered (four, five, and eight tapered) optical fiber sensors (OFSs) have been developed and characterized in this study. Along with these, different nanomaterials (NMs), such as gold nanoparticles (AuNPs) and graphene oxide (GO), are immobilized over the bare probe. Further, NMs immobilized probes are functionalized with ascorbate oxidase. The AuNPs are well studied in the past with the variation in size, shape and surface functionalization and have been found to be well-suited NMs for biosensing applications. On the other hand, GO-based material is equally promising in the nanostructure-based optical sensing due to their broad surface area, high electrical conductivity, good chemical stability, and excellent mechanical behavior. The performance parameters of the proposed sensor, such as sensitivity (8.3 nm/mM), correlation correlator (0.9724), and limit of detection (51.94 µM) are greatly improved over the previous designs. A wide range of characterization and validation of NMs immobilized structures are reported here. It indicates a great potential in the practical implementation of LSPR based OFSs for routine diagnostics.

Fast color image encryption scheme based on 3D orthogonal Latin squares and matching matrix

Abstract: To realize real-time image encryption, a fast color image encryption scheme by combining 3D orthogonal Latin squares (3D-OLSs) with matching matrix is proposed. The 3D-OLSs represent that each plane of two matrices must be Latin square and the corresponding planes of the two matrices must satisfy orthogonality. The matching matrix is to produce a matrix orthogonal with the 3D Latin square. In the permutation process, a new 3D permutation method with 3D-OLSs and matching matrix is devised. The proposed scheme could save encryption time to a certain degree, since the orthogonal Latin squares are defined over integers directly. In the diffusion process, to solve the diffuse problem between two planes in the 3D matrix, some matrices of the diffusion process are changed with three variables. Experimental results and security analyses show that the proposed color image encryption scheme has high security, fast speed and could resist various common attacks.

Polarization-sensitive terahertz resonator using asymmetrical F-shaped metamaterial

Abstract: A design of tunable terahertz (THz) resonator by using asymmetrical F-shaped metamaterial (AFSM) is presented, which is composed of Au layer fabricated on silicon-on-insulator (SOI) substrate. There are three designs of AFSM with different length of F-shaped microstructure, which are 60 μm, 65 μm, and 70 μm kept other parameters as constant. The electromagnetic response of tunable THz resonator exhibits the switch function for single-band resonance at transverse magnetic (TM) mode and dual-band resonance at transverse electric (TE) mode by changing the gap between AFSM microstructures. These characterizations of device can be used for a THz filter at TM mode and a THz switch at TE mode. To compare the proposed AFSM device with and without a gap, that can be switched in the range of 0.20–0.40 THz for single-band switching resonance at TM mode and dual-band switching resonance at TE mode, respectively. These resonances are ultra-narrow bandwidths with a highest Q-factor of 40 at TE mode and kept as stable at 20 at TM mode. Such results are very suitable to be used for an environmental sensor. To further enhance the flexibility of AFSM device, it is exposed on ambient environment with different refraction index for high-efficiency environmental sensor with a correlation coefficient of 0.9999. This study paves a way to the possibility of high-sensitivity of tunable THz metamaterial in filter, switch, polarizer, and other applications.

Secure image encryption scheme using double random-phase encoding and compressed sensing

Abstract: A secure optical digital image encryption scheme with authentication capability is proposed using double random-phase encoding (DRPE) and compressed sensing (CS). Phase information of the plaintext image is obtained using DRPE and quantized to generate authentication information. Simultaneously, the plaintext image is compressed by CS and its measurements are quantized using the sigmoid map. Then the ciphertext image is obtained by permutation and diffusion after authentication information is embedded in quantified measurements. At receiving end, the authentication information is first extracted by inverse permutation and diffusion, and then the authentication image is obtained by inverse DRPE. Finally, the ciphertext image can be blindly authenticated using a nonlinear cross-correlation method with authentication image and reconstructed image. Experimental results demonstrate the effectiveness of our proposed scheme.

Influence of spatter particles contamination on densification behavior and tensile properties of CoCrW manufactured by selective laser melting

Abstract: Due to spattering particles forming during Selective Laser Melting, powder pollution may severely affect the quality of the fabricated components and cause defects inside the microstructure to degrade the mechanical properties. Therefore, investigations into the change in powder characteristics and related microstructure defects, tensile properties are needed. In this paper, the influence of powder re-using time on Selective Laser Melting-based fabrication of CoCrW alloy was studied. The spattering process and the powder pollution caused by spatter particles were observed via high-speed imaging. The densification mechanism and the evolution of the microstructure of the CoCrW printed components were analyzed. The mechanical properties of the components printed using powders polluted at a varying degree by the spatter were tested at different stages of the life cycle. Results indicate that the Selective Laser Melting-generated spatter particles look more complex than the virgin powder particles. Compared with laser irradiating on the substrate, much more spatter escaped from the molten pool as the laser irradiated on the powder; the powder showed a trend towards increased particle size diameter through repeated recycling; residual pores distribution state and size applied with the various powder usage times were analyzed, pores with the average size of 300 μm could be achieved when powder recycled for the sixth time. At last, it is concluded that the spatter severely affects the mechanical properties of the fabricated components, as the tensile strength, yield strength and elongation of CoCrW alloy was reduced from 1284 MPa, 855 MPa and 11.15% to 874 MPa, 689 MPa and 3.82%, respectively. This gives insights into the influence of powder features on Selective Laser Melting-fabricated components and into improvement in stability of Selective Laser Melting manufacturing processes by controlling spatter formation.

Artificial neural networks for nonlinear pulse shaping in optical fibers

Abstract: We use a supervised machine-learning model based on a neural network to predict the temporal and spectral intensity profiles of the pulses that form upon nonlinear propagation in optical fibers with both normal and anomalous second-order dispersion. We also show that the model is able to retrieve the parameters of the nonlinear propagation from the pulses observed at the output of the fiber. Various initial pulse shapes as well as initially chirped pulses are investigated.

Hot cracking, crystal orientation and compressive strength of an equimolar CoCrFeMnNi high-entropy alloy printed by selective laser melting

Abstract: Hot cracking, grains size, crystal orientation and compressive strength of an equimolar CoCrFeMnNi high-entropy alloy (HEA) printed by selective laser melting (SLM) were investigated. The CoCrFeMnNi HEA printed by SLM suffered from hot cracking no matter of the employed printing parameters. The preferred orientation of the printed sample was transformed in the order: 〈2 3 3〉→〈0 0 1〉→〈2 0 3〉→〈1 0 1〉, as the volumetric energy density (VED) rises. Also, the increased VED favors the grain growth and the increase of the grain orientation spread, owing to the presence of high temperature gradient and larger residual stress. The fracture strength increases with the VED, and the maximum compressive strength rises to 2447.7 MPa, but the elongation rate is still 77.6% due to the ultrafine microstructure. The results of this study have reference value for the preparation of HEA materials with controllable grain characteristics, crystalline texture and complex structures by SLM.

Ultra high-sensitivity and tunable dual-band perfect absorber as a plasmonic sensor

Abstract: In this paper, we demonstrate a dual-band tunable absorber coupled with a nanoscale metal-dielectric-metal (MDM) structure for sensing applications in the near-infrared spectral region. This structure exhibits a dipole resonance mode in absorbance and reflectance spectra which results in the enhancement of absorbance over a wide range of incident angles for TE polarization. Using a numerical and analytical study, the performance parameters of the structure including sensitivity (SS), the figure of merit (FoM) and quality factor (Q) are investigated by changing the incident polarization, geometrical parameters, filling dielectric and plasmonic metasurface material. Moreover, we study the dependence of the sensitivity as a function of plasmonic metasurface shape to demonstrate a better response compared with other methods. Results show that, in terms of the refractive index unit (RIU), an ultra-high sensitivity and tunable sensor can be designed with a maximum sensitivity of 1240.8 nm/RIU for a refractive index change of Δn = 0.0458. In the optimum design of the proposed dual-band absorber, a Q-factor and FoM equal to 123.45 and 44.5 are obtained. Furthermore, the proposed structure can be utilized for controlling the light propagation. By considering silver as a plasmonic metasurface, a slow down factor as high as 680 is obtained. Our work will be applied to future sensors capable of ultra-high sensitivity.

Prediction of solidification cracking by an empirical-statistical analysis for laser cladding of Inconel 718 powder on a non-weldable substrate

Abstract: This paper presents an empirical-statistical approach to predict solidification cracking during laser cladding of Inconel 718 powder on A-286 Fe-based superalloy. This approach is based on a linear regression analysis and empirical-statistical correlations between the key processing parameters (laser power, P; powder feed rate, F; and scanning speed, V) and the geometrical attributes of single laser cladding tracks. These correlations were used for the development of a processing map which assesses the effects of the geometrical characteristics on the solidification cracking and the required conditions to obtain crack-free clads. Scanning electron microscopy was used for microstructural characterization. Thermodynamic calculations using the non-equilibrium Scheil solidification model were also employed. The empirical-statistical analysis showed that the processing parameters directly associated with the height and angle of single laser cladding tracks are P 2 F V 2 and P 0.5 F V 1 , respectively. The processing map revealed that the dilution ratio is the governing macrostructural attribute required to avoid solidification cracking. Indeed, a substrate dilution ratio lower than 25% shifts the cladding composition to an alloy regime, which has lower susceptibility to solidification cracking. The role of this macrostructural feature in reducing the susceptibility of the fusion zone to solidification cracking is thoroughly discussed.

Effect of hot cracking on the mechanical properties of Hastelloy X superalloy fabricated by laser powder bed fusion additive manufacturing

Abstract: Nickel-based superalloys such as Hastelloy X (HX) are widely used in gas turbine engine applications and the aerospace industry. HX is susceptible to hot cracking, however, when processed using additive manufacturing technologies such as laser powder bed fusion (LPBF). This paper studies the effects of minor alloying elements on microcrack formation and the influences of hot cracking on the mechanical performance of LPBF-fabricated HX components, with an emphasis on the failure mechanism of the lattice structures. The experimental results demonstrate that a reduction in the amount of minor alloying elements used in the alloy results in the elimination of hot cracking in the LPBF-fabricated HX; however, this modification degrades the tensile strength by around 140 MPa. The microcracks were found to have formed uniformly at the high-angle grain boundaries, indicating that the cracks were intergranular, which is associated with Mo-rich carbide segregation. The study also shows that the plastic-collapse strength tends to increase with increasing strut sizes (i.e. relative density) in both the ‘with cracking’ and ‘cracking-free’ HX lattice structures, but the cracking-free HX exhibit a higher strength value. Under compression, the cracking-free HX lattice structures’ failure mechanism is controlled by plastic yielding, while the failure of the with-cracking HX is dominated by plastic buckling due to the microcracks formed within the LPBF process. The novelty of this work is its systematic examination of hot cracking on the compressive performance of LPBF-fabricated lattice structures. The findings will have significant implications for the design of new cracking-free superalloys, particularly for high-temperature applications.