Vortex Filtering Research Articles

A Method for Fingerprint Edge Enhancement Based on Radial Hilbert Transform

Fingerprints play a significant role in various fields due to their uniqueness. In order to effectively utilize fingerprint information, it is necessary to enhance image quality. This paper introduces a method based on Radial Hilbert transform (RHLT), which simulates the vortex filter using the point spread function (PSF) of spiral phase plate (SPP) with a topological charge l=1, for fingerprint edge enhancement. The experimental results show that the processed fingerprint image has more distinct edges, with an increase in information entropy and average gradient. Unlike classical edge detection operators, the fingerprint edge image obtained by the RHLT method exhibits a lower mean square error (MSE) and a higher peak signal-to-noise ratio (PSNR). This indicates that the RHLT method provides more accurate edge detection and demonstrates higher noise-resistance capabilities. Due to its ability to highlight edge information while preserving more original features, this method has great application potential in fingerprint image processing.

Multi-order optical spatial vortex filters for simultaneous contour extraction of various parts of an object

We study the transformation of the analyzed object by a variety of spatial filters including bandpass, differential, radial filters, various types of vortex filters, and Hilbert filters. Advantages and disadvantages of different filters in terms of clarity and direction-invariance of edge extraction, as well as the energy efficiency are numerically demonstrated. Based on the results obtained, multi-order optical spatial vortex filters with different parameters are developed for simultaneously extracting contours of various object parts. We show that a multi-order filter makes it possible to form a set of images in one plane with a clearly defined contour, object corners and various parts of the contour. Numerical and experimental testing of 4- and 5-channel spatial vortex filters of various types was applied for test objects. A possibility of using the proposed filters to simultaneously highlight the edges of the entire image and various parts of the image in order to extract more features from the analyzed image is shown.

Short-wavelength-infrared upconversion edge enhancement imaging based on a Laguerre-Gaussian composite vortex filter

Edge-enhanced imaging by spiral phase contrast has proven instrumental in revealing phase or amplitude gradients of an object, with notable applications spanning feature extraction, target recognition, and biomedical fields. However, systems deploying spiral phase plates encounter limitations in phase mask modulation, hindering the characterization of the modulation function during image reconstruction. To address this need, we propose and demonstrate an innovative nonlinear reconstruction method using a Laguerre-Gaussian composite vortex filter, which modulates the spectrum of the target. The involved nonlinear process spectrally transforms the incident short-wavelength-infrared (SWIR) signal from 1550 to 864 nm, subsequently captured by a silicon charge-coupled device. Compared with conventional schemes, our novel filtering method effectively suppresses the diffraction noise, significantly enhancing image contrast and resolution. By loading specific phase holograms on the spatial light modulator, bright-field imaging, isotropic, amplitude-controlled anisotropic, and directional second-order edge-enhanced imaging are realized. Anticipated applications for the proposed SWIR edge-enhanced imaging system encompass domains such as artificial intelligence recognition, deep tissue medical diagnostics, and non-destructive defect inspection. These applications underscore the valuable potential of our cutting-edge methodology in furthering both scientific exploration and practical implementations.

Longitudinal encoding of focused vector vortex beams empowered by silicon metasurfaces

Vector vortex beams (VVBs) have attracted considerable applications due to their unique properties, such as vortex filtering, particle trapping, and quantum information. Recently, information coding methods based on metasurface strategies show excellent performance in remote sensing and communication, but the implementation of the longitudinal dimension of the optical field in information coding remains to be developed. Here, we provide a strategy for constructing all-dielectric terahertz (THz) metasurface designs for longitudinal codecs, with the individual focal planes carrying specific encrypted information. Benefiting from the joint modulation of the geometrical and propagation phases, the physical mechanism of spin-polarization multiplexing realizes the coherent superposition of multidimensional focused orbital angular momentum (OAM) modes. As a bridge between Poincaré sphere (PS) and the higher-order Poincaré sphere (HOPS), the proposed design produces a longitudinally controllable variation of OAM mode superposition states. A series of metasurface examples for generating specific coding sequences are characterized by manipulating topological charges and phase factors embedded within orthogonal circularly polarized channels. The longitudinal dimension, as a new coding degree of freedom, can be further expanded for meta-optical applications with the assistance of the VVB pattern.

Nonlinear edge enhancement imaging based on Laguerre-Gaussian superimposed vortex filters.

Nonlinear reconstruction, which is based on the principle of cross correlation, is a commonly employed reconstruction technique in incoherent correlated digital holography systems. However, the modulation of phase masks in these systems is suppressed during the reconstruction process, resulting in an inability to express the characteristics of the phase masks. Consequently, achieving edge enhancement within these systems is constrained. We propose a nonlinear reconstruction method utilizing Laguerre-Gaussian superimposed vortex filters, which modulates the spectrum of the target during the reconstruction process. Experimental results demonstrate that this method performs well in reconstructing image edges for various phase-masked incoherent imaging systems and effectively suppresses noise. Additionally, this method enables directional edge enhancement.

Acoustic vortex filter based on tunable metasurfaces

In this paper, we present an acoustic vortex filter (AVF) based on tunable metasurfaces, which can selectively filter the incident multiplexed vortices that carry different orbital angular momentum (OAM). Our metasurface-based AVF is composed of an upper acoustic metasurface (UAM) and a lower acoustic metasurface, of which the intrinsic topological charge (ITC) can be tuned by mechanically rotating the UAM along its central axis. Due to the critical order of the propagating vortex modes in waveguide, controlling the ITC of the AVF allows for the selective filtering of incoming multiplexed acoustic vortex beams based on the sound vortex diffraction in phase-gradient metasurface, which endows the vortex filter the capability that let the incident vortex of specific OAM pass through it. In the following demonstration, both in theory and experiment, we design the AVF and effectively filter the acoustic vortices with two opposite topological charges by simply altering the orientation angle of the UAM. Based on this, we further demonstrate its application in asymmetric acoustic wave transmission. Our work offers an approach to selectively filter the incident acoustic vortex, which improves the capability to control the acoustic OAM via metasurfaces.

Robust Super‐Resolution Image Transmission Based on a Ring Core Fiber with Orbital Angular Momentum

AbstractSingle‐fiber imaging technology offers unique insights for research and inspection in difficult‐to‐reach and narrow spaces. In particular, ultra‐compact multimode fiber (MMF) imaging, has received increasing interest over the past decade. However, MMF imaging will be seriously distorted when subjected to dynamic perturbations due to time‐varying mode coupling, and the imaging of space objects via Gaussian beam will be relatively degraded at the edge due to insufficient contrast. Here, a robust super‐resolution image transmission method based on a ring core fiber (RCF) with orbital angular momentum (OAM) is proposed and experimentally demonstrated. The OAM modes propagating in the RCF form a series of weakly coupled mode groups, making the imaging system robust to external perturbations. In addition, a spiral phase plate is used as a vortex filter to produce OAM for edge enhancement, thus improving the image resolution. Furthermore, a few‐shot U‐Transformer neural network is proposed to enhance the resilience of the developed RCF‐OAM imaging system against environmental perturbations. Finally, the developed RCF‐OAM imaging system achieves biological image transmission, demonstrating the practicality of the scheme. This pioneering RCF OAM image transmission system may have broad applications, potentially revolutionizing fields such as biological imaging and industrial non‐destructive testing.

Implementing the edge enhancement with vortex filter in both linear and nonlinear optics

The edge enhancement technique, as an effective method to represent the boundary of objects, plays an important role in image processing. Among them, the vortex filtering, which is based on the radial Hilbert transformation, has been paid great attention due to its ability to achieve isotropic and anisotropic edge enhancement. Recent years have witnessed a growing interest in the nonlinear vortex filter to skillfully realize the visualization of the object edge under invisible light irradiation. In this paper, we start from reviewing the achievements have been made with the vortex filtering technique in linear optics, and then discussed the recent processes of the scalar and vector vortex filter in nonlinear optics. We hope that the nonlinear optical vortex filter can motivate some promising applications in biological edge imaging with visible light-sensitive specimens.

Creating Longitudinally Varying Vector Vortex Beams with an All‐Dielectric Metasurface

AbstractVector vortex beams have attracted considerable research attention owing to their unique optical properties, and have been explored in vortex filtering, photon entanglement, and the photonic spin hall effect. In this work, a universal all‐dielectric metasurface platform is proposed that can generate longitudinally varying vector vortex beams in the terahertz band. As a proof‐of‐concept demonstration, a series of metasurface examples generating evolution of topological charges and vector polarization states of generated vector vortex beams are characterized. The experimental results agree well with the simulations, fully verifying the feasibility of the proposed scheme. The proposed metasurface strategy may find applications ranging from optical traps to polarization optics.

Multi-tasking geometric phase element array based self-referenced vortex interferometer for three-dimensional topography.

Interferometry is a basic physical method to record and reconstruct the three-dimensional (3D) topography of a complex object. However, mainstream interferometers using two beams can be unstable in a volatile environment. Here, we present a self-referenced optical vortex interferometer employing multi-tasking geometric phase elements. Compared with conventional devices, the multitasking elements can enable vortex filters while deflecting the interference beams to achieve high mode purity in broadband. We use the proposed system to reconstruct the 3D topography of a sample while determining its surface elevations and depressions accurately and conveniently in one static interference pattern.

Robust Demultiplexing of Distinct Orbital Angular Momentum Infrared Vortex Beams Into Different Spatial Geometry Over a Broad Spectral Range

Multi-channel structured light with orbital angular momentum (OAM) can be applied in different applications. For example, OAM modulation and OAM multiplexing in fiber optics communications, high-dimensional quantum cryptography-based OAM states for transmitting secure information across free-space, and independent data streams through OAM beams multiplexed free-space optical links. Using a simple and efficient system consisting of a spiral phase element (SPE) and a multi-channel vortex filter (MVF), we have converted input Gaussian beams into multi-channel OAM-based vortex beams for infrared wavelengths. An SPE has been designed, which generates optical vortices with wavelength-dependent topological charge (including fractional values). The resulting complex fields are optically relayed on a binary MVF designed by modulo- $2\pi $ phase addition of multiple fork gratings with topological charges 1, 2, and 3 and azimuthal orientations. In this way, the MVF generates beams with different OAM states for different carrier waves with different angles and maps them at desired locations in the detector plane. In this study, both $3\times 3$ , as well as a hexagonal configuration, were used. Furthermore, the experimentally obtained OAM spectrum qualitatively agrees with the results of numerical simulations, thus verifying our approach. The presented approach opens a new pathway for developing an efficient multi-channel OAM beams generator designed for a specific wavelength and illuminated by an input beam of different wavelengths over a broad spectral range.

Development of a Technology for Dissolving a Homogeneous Mixture of Uranium and Plutonium Oxides Obtained by the Plasma-Chemical Method

A plasma technology for the synthesis of mixed uranium and plutonium oxides UO2–PuO2 by denitration of mixed solutions of these metals has been developed at the Siberian Chemical Combine. The production process of U and Pu oxides includes the preparation of mixed initial solutions, their denitration in a plasma reactor with an inductive high-frequency discharge, and isolation of powders from the vapor–gas flow in vortex precipitators or filters with metal–fabric meshes. The particles of the obtained oxide powders possess the features of nanocrystalline and nanoscale materials containing crystalline and amorphous phases in the structure. The equipment for the implementation of this technology has been developed (Zenith plasma plant), on which mixed U and Pu oxides have been produced, and over 60 kg of oxides UO2–PuO2 with the plutonium content of 25 wt % has been synthesized, from which industrial fuel rods for the BN-600 reactor have been fabricated. After the operation of the fuel rods in the BN-600 reactor and corresponding exposure, the cores of the irradiated fuel rods should be dissolved for subsequent radiochemical treatment. The results of the studies on the search for the conditions of dissolution and effective solvents for mixed U and Pu oxides obtained by the plasma technology of decomposition of mixed solutions of uranium and plutonium are presented. It has been found that no complete dissolution of the synthesized oxides occurs in solutions of HNO3 and HNO3 with the addition of the reducing agents for tetravalent plutonium—ascorbic acid and hydrazine. Complete dissolution of the mixed oxides most effectively occurs in solutions of 10 M HNO3 with the addition of 0.25–1.0 g/L HF.

High-contrast anisotropic edge enhancement free of shadow effect.

We propose a Bessel-like composite vortex filter to perform high-contrast and power-controlled anisotropic edge enhancement with shadow-effect-free and low background noise. The background noise, which is commonly found and strongly decreases the filtered image quality in previous anisotropic vortex filters, is effectively reduced by suppressing the side lobes of the system point spread function, thereby increasing the image edge contrast to 0.98. The shadow effect is totally eliminated by keeping the radial symmetry of the filtering process, which makes edges sharper and improves image resolution. By introducing a weighting factor between two opposite vortex filter components, the power of edge enhancement becomes controllable. Numerical simulations and experimental results prove that the proposed filter achieves higher-contrast edge enhancement for both phase-contrast and amplitude-contrast objects.

基于涡旋滤波的图像边缘增强研究进展

As an important means of image processing, the edge enhancement techniques play an important role in amplitude-contrast and phase-contrast objects imaging. The vortex filtering techniques based on radial Hilbert transform have attracted much attention because it can achieve isotropic edge enhancement. However, the classical vortex filtering causes background noise and contrast reduction due to diffraction caused by central singularities and sharp edges. In recent years, many research groups have proposed new types of vortex filters for vortex filtering side lobe suppression. In addition, the isotropic and anisotropic edge enhancement techniques based on vortex filtering have also developed rapidly. In this paper, several methods for suppressing vortex side lobes were summarized in recent years, including Laguerre Gaussian amplitude modulation, Bessel-like amplitude modulation, and Airy amplitude modulation. What's more, from two aspects:scalar vortex filtering and vector vortex filtering, the isotropic and anisotropic edge enhancement methods and progress were reviewed.

EMCD with an electron vortex filter: Limitations and possibilities

We discuss the feasibility of detecting spin polarized electronic transitions with a vortex filter. This approach does not rely on the principal condition of the standard electron energy-loss magnetic chiral dichroism (EMCD) technique, the precise alignment of the crystal in order to use it as a beam splitter, and thus would pave the way for the application of EMCD to new classes of materials and problems, like amorphous magnetic alloys and interface magnetism. The dichroic signal strength at the L2, 3-edge of ferromagnetic Cobalt (Co) is estimated on theoretical grounds using a single atom scattering approach. To justify this approach, multi-slice simulations were carried out in order to confirm that orbital angular momentum (OAM) is conserved in amorphous materials over an extended range of sample thickness and also in very thin crystalline specimen, which is necessary for the detection of EMCD. Also artefact sources like spot size, mask tilt and astigmatism are discussed. In addition, the achievable SNR under typical experimental conditions is assessed.

Vectorial optical vortex filtering for edge enhancement

We propose to use a super-structured waveplate (called an S-waveplate) for vectorial optical vortex filtering, and experimentally demonstrate the radial Hilbert transform and selective edge enhancement. Based on the Jones calculus of polarization states and Fourier analysis, we calculate and analyze the point spread function of an optical 4-f system including an S-waveplate filter having the vectorial vortex of topological charge 1 (TC = 1). Numerical simulations and optical experiments demonstrate that a vectorial optical vortex filter can be used to implement selective edge enhancement with an analyzer before the output plane. The edge enhancement can be obtained even when the center of the filter is off-axis or the illuminating light is non-monochromatic.

Gradual edge enhancement in spiral phase contrast imaging with fractional vortex filters.

In the spiral phase contrast imaging, the integer spiral phase plate (SPP) are generally employed to perform the radial Hilbert transform on the object. Here we introduce fractional SPP filters, instead of the integer ones, to investigate the gradual formation of edge enhancement for pure phase objects. Two spatial light modulators are used in our experimental configuration. One is addressed to display the pure phase object of a five-pointed star, while the other serves as a dynamic filter of fractional topological charge Q. Of interest is the observation of the complete reversal of the edge and background brightness by gradually changing the fractional vortices from Q = 0 to 1. The experimental results were well interpreted based on the OAM spectra of fractional SPP, which indicates that the filtered output image can be considered as a coherent superposition of all possible images that are individually resulted from the integer OAM filtering. Besides, we show that the spiral phase contrast effect can still be observed in real time for a rotating three-leaf clover. Our results may find potential applications in the optical microscopic imaging.

Multi-step vortex filtering for phase extraction

A quantized version of a continuous spiral phase filter with unitary topological charge, here denominated multi-step spiral phase filter (MSSPF), is proposed to extract phase from rotated spiral interferograms. Spiral interferograms are usually obtained from phase objects by registering the interference of its vortex filtered complex amplitude with a reference complex amplitude. The structure found in this kind of interferograms, depend on the number of steps used in the MSSPF that usually are assumed with an infinite number of steps for the continuous spiral phase filter. Reducing the number of steps of the MSSPF affects the residual phase error obtained after the phase extraction method. This error is therefore analysed here using a numerical simulation of a Mach-Zender interferometer with a MSSPF and a reduced number of steps. It is shown that, for our proposed method of rotated spiral interferograms, a residual error persists as the number of steps is increased approaching the residual error reported for the phase extraction method of single-shot spiral interferograms. Furthermore, it is shown that this novel technique can be applied without further modifications for phase contrast measurement. Experimental results show similar performance of this phase extraction technique, when compared to the results obtained with a commercial interferometer and with the numerical simulations.

Mapping of phase singularities with spiral phase contrast microscopy

In spiral phase contrast (SPC) microscopy the edge-enhancement is typically independent of the helicity of the phase vortex filter. Here we show that for layered specimens containing screw-dislocations, as are e.g. present in mica or some crystallized organic substances, the intensity distribution in the filtered image acquires a dependence on the rotational direction of the filter. This allows one to map the distribution of phase singularities in the topography of the sample, by taking the intensity difference between two images recorded with opposite handedness. For the demonstration of this feature in a microscopy set-up, we encode the vortex filter as a binary off-axis hologram displayed on a spatial light modulator (SLM) placed in a Fourier plane. Using a binary grating, the diffraction efficiencies for the plus and minus first diffraction orders are equal, giving rise to two image waves which travel in different directions and are Fourier filtered with opposite helicity. The corresponding two images can be recorded simultaneously in two separate regions of the camera chip. This enables mapping of dislocations in the sample in a single camera exposure, as was demonstrated for various transparent samples.

Optical vortex filtering for the detection of electromagnetically induced transparency

We report on the realization of an optical filter based on an optical vortex mask designed to exclusively detect a weak coherent laser field in the presence of a much stronger, nearly spatially overlapping field. We demonstrate the performance of such an optical vortex filter to eliminate the strong control field and detect only a weak optical field's transmission under the conditions of electromagnetically induced transparency. The attractive feature of such a filter is its insensitivity to optical field frequencies and polarizations, which makes it applicable for a wide range of coherent processes.