Search

Phase holography is a critical optical imaging and information processing technique with applications ranging from microscopy to optical communications. However, optimizing phase hologram generation remains a significant challenge due to the non-convex nature of the optimization problem. This paper presents a novel multiplane optimization approach for phase hologram generation to minimize the reconstruction error across multiple focal planes. We significantly improve holographic reconstruction quality by integrating advanced machine learning algorithms like RMSprop and Adam with GPU acceleration. The proposed method utilizes TensorFlow to implement custom propagation layers, optimizing the phase hologram to reduce errors at strategically selected distances. [ABSTRACT FROM AUTHOR]

Phase holography is a critical optical imaging and information processing technique with applications ranging from microscopy to optical communications. However, optimizing phase hologram generation remains a significant challenge due to the non-convex nature of the optimization problem. This paper presents a novel multiplane optimization approach for phase hologram generation to minimize the reconstruction error across multiple focal planes. We significantly improve holographic reconstruction quality by integrating advanced machine learning algorithms like RMSprop and Adam with GPU acceleration. The proposed method utilizes TensorFlow to implement custom propagation layers, optimizing the phase hologram to reduce errors at strategically selected distances.

The accuracy of ultrasonic nondestructive examination is limited by the limitation of near field diffraction. With the development of nearfield optical, angular spectrum method is introduced into the acoustic field, which provides a significant direction for the ultrasonic diffraction limit resolution detection. The main research of this paper is the transmission of near field ultrasound in thin workpiece and the law of the interaction of tiny flaws. The paper establishes the relationship between the longitudinal wave signal and the structure of the workpiece and the types of flaws. A method is found that whether there are flaws can be determined near field area by analysing the acoustic field characteristics of workpiece surface. Finally, comparing the calculation results with the finite element simulation, they verify each other, the method turns out to be correct in this paper. The model can also be used to improve the ultrasonic noise reduction algorithm and the extraction of minimal defect feature. It has a greatly practical significance. [ABSTRACT FROM AUTHOR]

Nowadays, structural colors successfully replace pigment‐based ones. Most of the newly proposed solutions rely on plasmonic systems, so speaking nanosized objects, exhibiting strongly size‐dependent optical properties. As a consequence, the fabrication requires high‐precision and time‐consuming technologies. A concept dealing with sub‐micronic surface features, easily reproducible on a large scale is proposed. The designed metasurface consists of randomly distributed holes serving both as cavity resonators and scatterers. The proposed solution grants an efficient control of the spatial distribution of diffuse light and offers a large palette of vibrant colors. The interferometric model and a two‐layer angular spectrum method provide a solid description of the physical origin of the phenomenon, with a high degree of agreement between simulations and experimental data. [ABSTRACT FROM AUTHOR]

In the rapidly advancing realm of mobile technology, under-display camera (UDC) systems have emerged as a promising solution for achieving seamless full-screen displays. Despite their innovative potential, UDC systems face significant challenges, including low light transmittance and pronounced diffraction effects that degrade image quality. This study aims to address these issues by examining degradation phenomena through optical simulation and employing a deep neural network model incorporating hybrid frequency–spatial domain learning. To effectively train the model, we generated a substantial synthetic dataset that virtually simulates the unique image degradation characteristics of UDC systems, utilizing the angular spectrum method for optical simulation. This approach enabled the creation of a diverse and comprehensive dataset of virtual degraded images by accurately replicating the degradation process from pristine images. The augmented virtual data were combined with actual degraded images as training data, compensating for the limitations of real data availability. Through our proposed methods, we achieved a marked improvement in image quality, with the average structural similarity index measure (SSIM) value increasing from 0.8047 to 0.9608 and the peak signal-to-noise ratio (PSNR) improving from 26.383 dB to 36.046 dB on an experimentally degraded image dataset. These results highlight the potential of our integrated optics and AI-based methodology in addressing image restoration challenges within UDC systems and advancing the quality of display technology in smartphones.

The optimization of imaging accuracy and speed is a crucial issue in the development of computer-generated holograms (CGH) for three-dimensional (3D) displays. This paper proposes an optimized iterative algorithm based on the angular spectrum method (ASM) to achieve high-quality holographic imaging across multiple planes. To effectively utilize spatial resources for multi-image reconstruction and mitigate the speckle noise caused by the overlapping of target images, constraint factors are introduced between different layers within the same region. The seeking rule of the constraint factor is also analyzed. By utilizing both constraint factors and variable factors, the presented method is able to calculate phase holograms for target figure imaging at four different planes. Simulation and experimental results demonstrate that the proposed method effectively improves the overall quality of the different planes, thus holding great potential for wide-ranging applications in the field of holography. [ABSTRACT FROM AUTHOR]

The angular spectrum (AS) model is customized to calculate the spatial acoustic pressure field generated by a piston source and transmitted through a steel plate immersed in water, for normal beam incidence. A MATLAB program is developed for this specific problem combining use of Gauss quadrature and a generalized Filon method. The program calculates the pressure wave number spectrum generated by the piston source and transforms the wave number spectrum into the spatial domain. Convergence analysis show that the MATLAB program is far more efficient than the more traditional approach of using the fast Fourier transform algorithm to transform the pressure wavenumber spectrum into the spatial domain. The MATLAB program is published here and free for others to use. The methods and MATLAB algorithms are obtained by • Converting the original 2D AS model to a 1D model using cylindrical coordinates. • Combining use of Gauss quadrature and a generalized Filon method for more accurate pressure calculations compared with use of the fast Fourier transform. • Introducing adaptive numerical integration algorithms in MATLAB and error control parameters which are easy to use.

The optimization of imaging accuracy and speed is a crucial issue in the development of computer-generated holograms (CGH) for three-dimensional (3D) displays. This paper proposes an optimized iterative algorithm based on the angular spectrum method (ASM) to achieve high-quality holographic imaging across multiple planes. To effectively utilize spatial resources for multi-image reconstruction and mitigate the speckle noise caused by the overlapping of target images, constraint factors are introduced between different layers within the same region. The seeking rule of the constraint factor is also analyzed. By utilizing both constraint factors and variable factors, the presented method is able to calculate phase holograms for target figure imaging at four different planes. Simulation and experimental results demonstrate that the proposed method effectively improves the overall quality of the different planes, thus holding great potential for wide-ranging applications in the field of holography.

• FFT-based methods of optical propagation introduce artificial periodicity. • Transverse waves that should leave computational domain are reintroduced. • New sinc method preserves bandwidth and does not have periodicity artefact. • Error of sinc method does not depend on wavelength or propagation distance. We propose a method based on sinc series approximations for computing the Rayleigh-Sommerfeld and Fresnel diffraction integrals of optics. The diffraction integrals are given in terms of a convolution, and our proposed numerical approach is not only super-algebraically convergent, but it also satisfies an important property of the convolution—namely, the preservation of bandwidth. Furthermore, the accuracy of the proposed method depends only on how well the source field is approximated; it is independent of wavelength, propagation distance, and observation plane discretization. In contrast, methods based on the fast Fourier transform (FFT), such as the angular spectrum method (ASM) and its variants, approximate the optical fields in the source and observation planes using Fourier series. We will show that the ASM introduces artificial periodic boundary conditions and violates the preservation of bandwidth property, resulting in limited accuracy which decreases for longer propagation distances. The sinc-based approach avoids both of these problems. Numerical results are presented for Gaussian beam propagation and circular aperture diffraction to demonstrate the high-order accuracy of the sinc method for both short-range and long-range propagation. For comparison, we also present numerical results obtained with the angular spectrum method. [ABSTRACT FROM AUTHOR]

• Electric field of elliptical Gaussian vortex beam is rigorously formulated by using angular spectrum. • Distribution of the beam field and its evolution in propagation is studied numerically. • Effect of beam order and topological charge on the beam field is analyzed. Vortex beams with hollow elliptical ring structure have attracted broad interest in applications such as micro-particle trapping and manipulation. The paper investigates the dependence of the hollow elliptic ring structure of vortex beams on the beam order and the topological charge, while propagating in the near- and far-field zones. The electric field of the beam is formulated by using the angular spectrum method, which satisfies rigorously Maxwell's equation. The electric field, linear momentum and orbital angular momentum are calculated. Numerical results show that the hollow elliptical ring structure approximately remains unchanged over a finite propagation distance, accompanied by a rotation whose angle depends on the topological charge. In the far-field zone, the ring structure is broken, leading to two bright spots and several dark islands. It is found that the beam profile in the near-field zone is mainly related to the beam order but in the far-field zone is primarily affected by the topological charge. The results presented in the paper may be useful in predicting and explaining the interaction between the elliptic hollow Gaussian vortex beam and the micro-particles, such as the movement of micro-particles along the orbit and the rolling rotation. [ABSTRACT FROM AUTHOR]

A three-dimensional holographic image computation method based on angular spectrum layering is proposed that further optimizes the selection of hybrid constraint factors and rationalizes the allocation of spatial resources for multi-layer plane image reconstruction. By employing separately independent angular spectrum propagation processes, potential aliasing reconstruction errors during the image propagation process are circumvented. Through numerical simulations and optical experiments, our method was compared with the hybrid constraint iterative angular spectrum sequence algorithm in the reconstruction involving eight plane images, validating the effectiveness and feasibility of our approach. [ABSTRACT FROM AUTHOR]

A solid immersion Fresnel zone plate (SI-FZP) is designed, and its near-field diffraction characteristics are analyzed using the vector angular spectrum (VAS) method. The analytical VAS model of formulism for the electric field vector transmitted from a binary amplitude and/or phase circular SI-FZP is obtained for any polarization of incidence. Numerical calculations show that, when the solid film immersing a high-numerical-aperture FZP is truncated close to the FZP's focus in a single homogeneous medium, the focusing intensity in the vicinity of the SI-FZP–air interface can be improved dozens of times in magnitude for radially polarized incidence, compared with the FZP's far-field focusing in the single medium without an interface. The validity of the proposed analytical VAS model of formulism to analyze SI-FZP's diffraction problems has been affirmed by comparison with numerical results obtained by the finite-difference time-domain simulation. [ABSTRACT FROM AUTHOR]

Angular spectrum (AS) methods enable efficient calculation of wave propagation from one plane to another inside homogeneous media. For wave propagation in heterogeneous media such as biological tissues, AS methods cannot be applied directly. Split-stepping techniques decompose the heterogeneous domain into homogeneous and perturbation parts, and provide a solution for forward wave propagation by propagating the incident wave in both frequency-space and frequency-wavenumber domains. Recently, a split-step hybrid angular spectrum (HAS) method was proposed for plane wave propagation of focused ultrasound beams. In this study, we extend these methods to enable simulation of acoustic pressure field for an arbitrary source distribution, by decomposing the source and reflection spectra into orthogonal propagation direction components, propagating each component separately, and summing all components to get the total field. We show that our method can efficiently simulate the pressure field of arbitrary sources in heterogeneous media. The accuracy of the method was analyzed comparing the resultant pressure field with pseudospectral time domain (PSTD) solution for breast tomography and hemispherical transcranial-focused ultrasound simulation models. Eighty times acceleration was achieved for a 3-D breast simulation model compared to PSTD solution with 0.005 normalized root mean-squared difference (NRMSD) between two solutions. For the hemispherical phased array, aberrations due to skull were accurately calculated in a single simulation run as evidenced by the resultant-focused ultrasound beam simulations, which had 0.001 NRMSD with 40 times acceleration factor compared to the PSTD method. [ABSTRACT FROM AUTHOR]

A method allows the extraction of the recovery factor that maximizes the image contrast of OCT (optical coherence tomography) and/or OCTA (OCT angiography) of a living subject is proposed in this study. Due to the finite depth of focus in imaging optics, the volume OCT imaging suffers from blurriness in the lateral resolution. By utilizing the digital hologram method or angular spectrum method, the blurred image can be refocused. However, for in vivo OCT imaging, evaluation of the image focus is not easy, owing to the cloudy structure of the brain. In the proposed method, the blood flow signals were used as a guiding star to find the recovery factor. The propagation distance to a focal plane was automatically determined by evaluating the contrast of a cross-sectional OCTA image. The performance was examined though in vivo mouse brain OCT/OCTA imaging. The image singularity of the blood flow in OCTA was very effective at evaluating the contrast of the image. [ABSTRACT FROM AUTHOR]

• Digital holography of multiple-beam Fizeau interference (MBFI) holograms. • Optical phase reconstruction through the focal depth of optical fibers. • Collecting the optical phase map in progressive scanning process using angular spectrum method. • Optical phase magnification using the analysis of MBFI to several two-beam interference sets. Multiple-beam Fizeau interference (MBFI) is characterized by its ability to represent the fine features of phase objects. Single-shot digital holographic interferometry (SSDHI) is used for numerical focusing of the recorded holograms. In this work, the two methods are gathered and employed for solving the problem of studying the thick phase objects. The problem of thick phase objects, such as optical fibers, in MBFI is the full focusing of the fiber's optical phase in one-time single shot phase map. The proposed method is designed to collect the optical phase map due to the optical fiber through the focal depth. The main idea is based on the analysis of the MBFI to several two-beam interference sets. Each two-beam interference is treated by SSDHI method to get the optical phase at different planes of focus inside the object. Then, the resultant optical phase due to the optical fiber is collected from these planes of focus. Retrieving the optical phase maps of the multiple-beam holograms through the focal depth, for the first time, is possible using the angular spectrum method. In this case, a laser source with quite long coherence length is used as a light source. The proposed method is applied on two thick samples of, single mode and graded index, optical fibers to reconstruct the resultant optical phase maps through the focal depth. The reconstructed optical phase maps are determined for the harmonics of MBFI holograms' angular spectra. The method proves its ability to provide more accurate, numerical focusing and phase maps reconstruction. Where, MBFI analyses indicate the magnification of optical phase differences. [ABSTRACT FROM AUTHOR]

Phase-shifting microwave holography utilizes the antennas to record the hologram. In this technique, the antenna has to perform two-dimensional (2-D) scanning over the test area corresponding to each phase shift in the reference wave. A numerical three-step phase-shifting is proposed for recording the microwave holograms in which only a single 2-D scan over an area is recorded experimentally whereas two remaining phase shifts are introduced numerically. This method is applied for imaging of metallic objects to inspect the feasibility of the proposed method. The qualitative results exhibit its effectiveness while reconstructing the amplitude and phase of an object at a microwave frequency of 8.5 GHz (or λ = 35.29 mm). [ABSTRACT FROM AUTHOR]

A method allows the extraction of the recovery factor that maximizes the image contrast of OCT (optical coherence tomography) and/or OCTA (OCT angiography) of a living subject is proposed in this study. Due to the finite depth of focus in imaging optics, the volume OCT imaging suffers from blurriness in the lateral resolution. By utilizing the digital hologram method or angular spectrum method, the blurred image can be refocused. However, for in vivo OCT imaging, evaluation of the image focus is not easy, owing to the cloudy structure of the brain. In the proposed method, the blood flow signals were used as a guiding star to find the recovery factor. The propagation distance to a focal plane was automatically determined by evaluating the contrast of a cross-sectional OCTA image. The performance was examined though in vivo mouse brain OCT/OCTA imaging. The image singularity of the blood flow in OCTA was very effective at evaluating the contrast of the image.

A resampling ring-mask method in spatial spectral domain (RRM-SSD) is proposed for speckle reduction in one-shot digital holography. Spatial spectrum of a hologram with speckle is divided into several subspectra using the resampling ring masks. Different subspectra are individually used to obtain the corresponding reconstructed amplitude images by the angular spectrum method. With the random distribution of the speckle, averaging these uncorrelated amplitude images would successfully enable speckle-reduced reconstruction. No specific operations for hologram recording or complex process for reconstruction are required in this method. Comparison with the previous rectangular mask method in spatial domain proves the advantage of the RRM-SSD method for both speckle reduction and high-resolution reconstruction. Experimental results show that the RRM-SSD method is efficient and simple for speckle reduction, which can be widely used in holographic imaging. [ABSTRACT FROM AUTHOR]

Terahertz digital holography (THz-DH) has the potential to be used for non-destructive inspection of visibly opaque soft materials due to its good immunity to optical scattering and absorption. Although previous research on full-field off-axis THz-DH has usually been performed using Fresnel diffraction reconstruction, its minimum reconstruction distance occasionally prevents a sample from being placed near a THz imager to increase the signal-to-noise ratio in the hologram. In this article, we apply the angular spectrum method (ASM) for wavefront reconstruction in full-filed off-axis THz-DH because ASM is more accurate at short reconstruction distances. We demonstrate real-time phase imaging of a visibly opaque plastic sample with a phase resolution power of λ/49 at a frame rate of 3.5 Hz in addition to real-time amplitude imaging. We also perform digital focusing of the amplitude image for the same object with a depth selectivity of 447 μm. Furthermore, 3D imaging of visibly opaque silicon objects was achieved with a depth precision of 1.7 μm. The demonstrated results indicate the high potential of the proposed method for in-line or in-process non-destructive inspection of soft materials. [ABSTRACT FROM AUTHOR]

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

A low-complexity fast diffraction calculation algorithm is proposed for theoretically studying the Bessel–Gauss beam generated by reflective off-axis axicon. By carefully exploiting the non-uniform inverse discrete Fourier transform (NU-IDFT), complicated interpolation algorithm is avoided when calculating the diffraction propagation between two tilted planes. Since only the central diffraction field is calculated, the computational complexity is dramatically reduced. By simply shifting the spectrum in frequency domain according to the tilted angle, the algorithm can suit to different tilted angles without increasing the sampling points. At last, three different kinds of imperfect fabricated reflective axicons are studied, simulation results validate the accuracy and high efficiency of the proposed method, implying it is promising in studying the feature of the Bessel–Gauss beam generated by reflective off-axis axicon. [ABSTRACT FROM AUTHOR]

Gouy phase is the axial phase anomaly of converging light waves discovered over one century ago, and is so far widely studied in various systems. In this work, we have theoretically calculated Gouy phase of light beams in both paraxial and nonparaxial regime on two-dimensional curved surface by generalizing angular spectrum method. We find that curvature of surface will also introduce an extra phase shift, which is named as spatial curvature-induced (SCI) phase. The behaviors of both phase shifts are illustrated on two typical surfaces of revolution, circular truncated cone and spherical surface. Gouy phase evolves slower on surface with greater spatial curvature on circular truncated cone, which is however opposite on spherical surface, while SCI phase evolves faster with curvature on both surfaces. On circular truncated cone, both phase shifts approach to a limit value along propagation, which does not happen on spherical surface due to the existence of singularity on the pole. An interpretation is presented to explain this peculiar phenomenon. Finally we also provide the analytical expression of paraxial Gaussian beam on general SORs. By comparing the result with the exact method we find the analytical expression is valid under the approximation that beam waist and scale of surface are beyond order of wavelength. We expect this work will enhance the comprehension about the behavior of electromagnetic wave in curved space, and further contribute to the study of general relativity phenomena in laboratory.

In this paper, the approximate analytical expression of the Hankel transform of arbitrary order circular Airy vortex beam (CAVB) is derived, which enables researchers to solve the propagation characteristics of the family of CAVB via angular spectrum method. Moreover, based on the approximate analytical expression, the propagation characteristics of Fourier space modulated and azimuthally modulated CAVB are investigated. It is found that by modulating the CAVB via a Fourier space periodic function, multiple focal points can be generated during the propagation; and by modulating the CAVB via azimuthally grafting two spiral phases, the focal point of the beam will deviate from the center of the focal plane. These results provide a simple method for flexible regulation of the number and location of the focal points of the CAVB. • In this paper, the approximate analytical solution of the Hankel transform of arbitrary order circular Airy vortex beam (CAVB) is derived. • The propagation characteristics of Fourier space modulated and azimuthally modulated CAVB are investigated. [ABSTRACT FROM AUTHOR]

In order to put in evidence the specular reflection and the non-specular reflection in the transient case, we have used a model for the study of the transient ultrasonic waves radiated by a linear phased array transducer in a liquid and reflected by a solid plane interface. This method is an extension of the angular spectrum method to the transient case where the reflection at the plane interface is taken into account by using the reflection coefficient for harmonic plane waves. The results obtained highlighted the different components of the ultrasonic field: the direct and edge waves as well as the longitudinal head waves or leaky Rayleigh waves. The transient representation of these waves have been carefully analyzed and discussed by the rays model. Instantaneous cartographies allowed a clear description of all the waves which appear at the liquid–solid interface. The obtained results have been compared to those obtained with a finite element method package. [ABSTRACT FROM AUTHOR]

An angular spectrum method (ASM) full-wave description of stress and energy density in a fluid-immersed plate for the optimization of leaky Lamb wave applications is presented. It models the case when leaky Lamb waves are generated by an external finite transmitter in the immersion fluid, and can calculate the associated stress, energy density, and other field variables within the plate. The normal component of the stress tensor and the energy density are compared against calculations in COMSOL with good agreement, but with some differences due to the two methods. The spatial field of the stress is analyzed using the angular spectrum (plane wave) representation of the stress, which is also used as a reference to exemplify the discrepancies between a pure plane wave approach in leaky Lamb wave applications and the spatial fields that accounts for diffraction and dispersion effects. Comments on the insight that the spatial fields within the plate may provide towards NDT/SHM applications are also given, along with a discussion on why the derivation and implementation of the ASM model is valuable when compared against a benchmarked, ready-to-use software such as COMSOL. • An ASM description of propagating waves inside a fluid-immersed plate is derived. • The model accounts for the generation of plate waves by an external source in fluid. • ASM calculations agrees well with COMSOL, but is much more efficient. • The ASM model can be valuable in optimization of applications in NDT and SHM. [ABSTRACT FROM AUTHOR]

We describe the experimental implementation of a virtual optical beam propagator system. This virtual propagator system allows the experimental study of beam propagation without physically moving any element. The approach uses a Fresnel diffraction algorithm (usually called the angular spectrum method) and its implementation on a spatial light modulator. We discuss the limits of the technique and provide a detailed description of the experimental procedures. Experimental results are included where we design a hologram capable of producing two patterns at two different distances, and we can change the effective plane of observation by changing the encoded propagation instead of by moving any element on the experimental system. [ABSTRACT FROM AUTHOR]

Non-destructive ultrasonic testing of steel structures provide valuable information in e.g. inspection of pipes, ships and offshore structures. In many practical applications, contact measurements are cumbersome or not possible, and air-coupled ultrasound can provide a solution. This paper presents air-coupled ultrasonic through-transmission measurements on a steel plate with thicknesses 10.15 mm; 10.0 mm; 9.8 mm. Ultrasound pulses were transmitted from a piezoelectric transducer at normal incidence, through the steel plate, and were received at the opposite side. The S 1 , A 2 and A 3 modes of the plate are excited, with resonance frequencies that depend on the material properties and the thickness of the plate. The results show that the resonances could be clearly identified after transmission through the steel plate, and that the frequencies of the resonances could be used to distinguish between the three plate thicknesses. The S 1 -mode resonance was observed to be shifted 10% down compared to a simple plane wave half-wave resonance model, while the A 2 and S 2 modes were found approximately at the corresponding plane-wave resonance frequencies. A model based on the angular spectrum method was used to predict the response of the through-transmission setup. This model included the finite aperture of the transmitter and receiver, and compressional and shear waves in the solid. The model predicts the frequencies of the observed modes of the plate to within 1%, including the down-shift of the S 1 -mode. [ABSTRACT FROM AUTHOR]

The angular spectrum (AS) model is customized to calculate the spatial acoustic pressure field generated by a piston source and transmitted through a steel plate immersed in water, for normal beam incidence. A MATLAB program is developed for this specific problem combining use of Gauss quadrature and a generalized Filon method. The program calculates the pressure wave number spectrum generated by the piston source and transforms the wave number spectrum into the spatial domain. Convergence analysis show that the MATLAB program is far more efficient than the more traditional approach of using the fast Fourier transform algorithm to transform the pressure wavenumber spectrum into the spatial domain. The MATLAB program is published here and free for others to use. The methods and MATLAB algorithms are obtained by•Converting the original 2D AS model to a 1D model using cylindrical coordinates.•Combining use of Gauss quadrature and a generalized Filon method for more accurate pressure calculations compared with use of the fast Fourier transform.•Introducing adaptive numerical integration algorithms in MATLAB and error control parameters which are easy to use., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2023 The Author(s).)

In the nondestructive evaluation domain, the reflection of ultrasonic waves at liquid-solid interfaces generates interesting and useful phenomena when the incident angle approaches the longitudinal critical angle or Rayleigh angle. In this paper, we have developed a model for the study of the transient ultrasonic waves radiated by a plane or a focusing transducer in a liquid and reflected by a solid plane interface. The method used is an extension of the angular spectrum method to the transient case where the reflection at the plane interface is taken into account by using the reflection coefficient for plane harmonic waves. The results obtained highlighted the different components of the ultrasonic field: the direct and edge waves as well as the longitudinal head waves or leaky Rayleigh waves. These waves have been carefully analyzed and discussed. Instantaneous cartographies allowed a clear description of all the waves which appear at the liquid-solid interface. [ABSTRACT FROM AUTHOR]

Abstract: Since many optical imaging systems suffer from a limited depth of focus, three-dimensional (3D) objects, or even flat objects tilted with respect to the optical axis, are imaged with only a portion of them in focus. Typically, to overcome this problem, in-focus areas are extracted from different images taken at different depths. The in-focus regions are thus merged together to build the extended focus image (EFI). In this work, we propose a quasi-automatic method for the EFI construction of tilted objects, extracted at once by 3D output of the Angular Spectrum Method (ASM) from a single digitally recorded hologram. Results show that our method can be effectively used for the correction of the anamorphism due to the reconstruction on a tilted plane with respect to the hologram one. [Copyright &y& Elsevier]

A computing simulation routine to model any type of circularly symmetric diffractive X-ray element has been implemented. The wavefield transmitted beyond the diffractive structures is numerically computed by the angular spectrum propagation method to an arbitrary propagation distance. Cylindrical symmetry is exploited to reduce the computation and memory requirements while preserving the accuracy of the numerical calculation through a quasi-discrete Hankel transform algorithm, an approach described by Guizar-Sicairos & Gutierrez-Vega [ J. Opt. Soc. Am. A, (2004), 21, 53-58]. In particular, the code has been used to investigate the requirements for the stacking of two high-resolution Fresnel zone plates with an outermost zone width of 20 nm. [ABSTRACT FROM AUTHOR]

Abstract: The modeling of parametric transmission by means of the spatial Fourier formalism (also called the angular spectrum method) is recalled. The secondary field can be estimated at any distance of the antenna. Various geometries of the transmitter can be also taken into account, including elongated antennas that feature a wide aperture in one plane. The relevance of the theoretical model with such geometry is tested with a linear array used as a parametric transmitter in an underwater application. Numerical estimations are compared with experimental measurements. In addition, the expected slight variation of the parametric efficiency with the relative phasing of the primary signals is experimentally observed. [Copyright &y& Elsevier]

To apply ultrasonic radiation force in precise manipulation for micro-components, a reconstruction technique for low-frequency sound field based on angular spectrum method (ASM) was adopted in the reconstruction for high-frequency sound field, and a high-precision reconstructing technique for high-frequency sound field was developed. First, the principle of reconstructing the sound field by ASM and four key factors on reconstruction precision were analysed. Second, the marginal Gibbs phenomenon and aperture replication effect were decreased by optimizing of the sampling interval and holographical aperture, and the signal-to-noise ratio (SNR) was increased by using the tapered filter to pretreat the signal data in wave number space. Then, the ill-posedness in reverse problem was eliminated by using a new kind of k-space filter to refine the transfer function. Finally, the reconstruction experiment of 1.75 MHz ultrasonic field was conducted by using the three-dimensional precise scanning system and needle-type hydrophone, and the experimental results validate the feasibility and efficiency of the method proposed in this study. [ABSTRACT FROM AUTHOR]

Investigations into Fourier beamforming for medical ultrasound imaging have largely been limited to plane-wave and single-element transmissions. The main aim of this work is to generalize Fourier beamforming to enable synthetic aperture imaging with arbitrary transmit sequences. When applied to focused transmit beams, the proposed approach yields a full-waveform-based alternative to virtual-source synthetic aperture, which has implications for both coherence imaging and sound speed estimation. When compared to virtual-source synthetic aperture and retrospective encoding for conventional ultrasound sequences (REFoCUS), the proposed imaging technique shows an 8.6 and 3.8 dB improvement in contrast over virtual source synthetic aperture and REFoCUS, respectively, and a 55% improvement in point target resolution over virtual source synthetic aperture. The proposed image reconstruction technique also demonstrates general imaging improvements in vivo, while avoiding limitations seen in prior techniques.

The near-field amplitude and phase characteristics of fractional optical vortex beam from a spiral phase plate fabricated on the tip of an optical fiber is reported here. A near-field scanning optical microscope is used to measure the intensity and phase structure of the fractional optical vortex beam as a function of propagation distance, Z and fractionality, l. The measured intensity-phase structure of the fractional optical vortex beam as a function of Z , from near- to far-field and l are complex and dominated by diffraction-related smoothening of the structure. Theoretical simulations carried out using the angular spectrum method matches well with the experimentally measured behavior. The trajectory of singular point in the vortex beam, as its charge is changed from l = – 1 to +1 in fractional steps, is simulated and measured using an interferometer setup. Such studies enable us to understand the complex nature of the fractional optical vortex beam in the near-field, its modifications during propagation to far-field, and as a function of fractionality. • Near-field measurement and analysis of fractional vortex beam from a spiral phase plate fabricated on the tip of an optical fiber is reported. • The amplitude and phase characteristics of the fractional vortex beams, simulated using the angular spectrum method, are found to match well with the experimental results. • The trajectory of singular point in the vortex beam, as the charge is changed from l = – 1 to +1 in fractional steps, by changing the wavelength of the input beam, is simulated and measured using an interferometer setup. • This study paves the way for understanding the complex nature of the fractional vortex beam both in the near-field and far-field. [ABSTRACT FROM AUTHOR]

Higher resolution has always been the goal of imaging. Resolution enhancement in most holographic imaging methods are achieved by collecting more object spectrum. As a computational imaging technology, the resolution of digital holography is determined by the utilized bandwidth in both optical recording and numerical reconstruction. There is little study on whether the collected spectrum is effectively utilized during reconstruction. In this work, we propose a resolution-enhanced digital in-line holography under spherical wave illumination. The collected object spectrum, which cannot be fully used by conventional methods, is effectively utilized by the developed band-extended angular spectrum method with improved computational efficiency. Numerical analysis and optical experiments have been performed to demonstrate that the proposed method can effectively enhance the imaging resolution of digital in-line holography by extension of the computational bandwidth. [ABSTRACT FROM AUTHOR]

This study investigates the validity of the Westervelt equation for focused transducers. The angular spectrum method is employed to analyze the second-harmonic acoustic field under the weakly nonlinear approximation. Although it is well known that the Westervelt equation is accurate for the case of quasi-plane waves, the present work demonstrates accurate solution for the highly focused case of a spherically-curved ultrasound transducer having an aperture angle of 80°. It is further found that the solution error is inversely dependent on the nonlinearity coefficient. [ABSTRACT FROM AUTHOR]