Amplitude Of Wavefront Research Articles

Computational adaptive telescope imaging via self-interference digital holography

Conventional optical telescope only collect the amplitude of optical wavefront with incoherent light, while self-interference incoherent digital holography offers a unique method of coding both the optical wavefront amplitude and phase information by interference patterns. We developed an adaptive non-scanning three-dimensional (3D) telescopic imaging technique based on self-interference incoherent holography with a geometric phase lens (GP-SIDH). The phase recorded in the point spread hologram and the incoherent self-interference extended holograms are employed to detect spatial positions, correct optical aberration and extract edges by computational reconstruction. The 3D imaging and position detection performances of our proposed method are investigated. Both simulations and experimental results show that the proposed method exhibits better performances for different depth objects imaging and space position detection. Furthermore, this system can be easily modularization and implanted into any existing optical telescopic system owing to its simplicity and convenience. The proposed approach can be a powerful tool for the multi-objects long-working-distance imaging, which is more valuable for telescope application.

Focusing scattering light field with different states based on iterative algorithm

Transmitting optical information through scattering medium has broad application prospects in biomedical, aerospace and other fields. However, the light passing through the scattering medium will cause wavefront distortion and optical information blurring. Wavefront shaping technology uses a mathematical matrix to characterize the characteristics of scattering medium, which can achieve refocusing and imaging after light propagation through the scattering medium. It mainly includes optical phase conjugation, optical transmission matrix and wavefront shaping based on iterative optimization. However, the iterative wavefront shaping is considered to be a cost-effective method. Based on the wavefront amplitude modulation technology, the wavefront amplitude of the incident light is continuously adjusted by using the optimization algorithm to find the corresponding wavefront amplitude distribution that can maximize the light intensity in the target area. The system generates binary patterns implemented with digital-micromirror device (DMD) based on on-off state of micromirror, where “on” represents 1 and “off” refers to 0. The DMD has a high refresh rate and can achieve high speed wavefront amplitude modulation by using the iteration algorithm. In the experiment, the scattering medium is prepared with TiO<sub>2</sub>, water and gelatin, whose persistence times are controlled with the water-gelatin ratio (WGR). In addition, the Pearson correlation coefficient (Cor) curve obtained through 300-s-measurement under different WGR conditions, which shows that the greater WGR, the shorter the persistence time is. The experiment mainly studies the focusing of the spatial light through scattering media by wavefront amplitude modulation, and discusses the ability of point guard algorithm (PGA) and genetic algorithm (GA) to control the scattered light field with different persistence times in 64 × 64 segments. The experimental results show that the PGA can achieve higher enhancement factor and more uniform multi-point focusing than the GA after 1000 iterations in the scattering medium with the same persistence time. The relative standard deviation value is inversely proportional to the WGR value when multi-point focusing can be completed. We also demonstrate that GA can only achieve single-point focusing when WGR = 40, and it cannot accomplish multi-point focusing in self-made scattering medium. This study not only verifies a method to achieve focusing scattering light field, but also provides a new scheme for testing the performance of the iterative wavefront shaping.

Fast sampling based image reconstruction algorithm for sheared-beam imaging

Sheared-beam imaging (SBI) is an unconventional ground-based optical imaging technique. It breaks through the traditional optical imaging concept by using three coherent laser beams, which are laterally displaced at the transmit plane, to illuminate the target, reconstructing the target image from echo signals. However, the echo data sampling of the imaging system is still not fast enough to reconstruct the high resolution and clear image of the target when imaging the target that is at rapidly changing position and attitude. In order to solve this problem, in this work an image reconstruction method is proposed based on five-beam fast sampling. An emitted beam array arranged in the cross shape with a central symmetrical structure is proposed, and the encoding and decoding method of the imaging system are changed. With a single exposure, the echo signals carry more spectrum information of the target, and the number of reconstructed images can be increased from 1 to 8, which quickly suppresses the speckle effect of the reconstructed image. Firstly, the principle of the imaging technique based on fast sampling is presented. Then, an image reconstruction algorithm based on fast sampling is studied. Eight groups of phase differences and amplitude information of the target can be extracted from echo signals. The wavefront phases are solved by the least-squares method, and wavefront amplitude can be obtained by the algebraic operation of speckle amplitude. The target image is reconstructed by the inverse Fourier transform. The simulation results show that comparing with the traditional three-beam image reconstruction method, the sampling times of echo data needed to obtain the same quality image are reduced from 20 to 5 , which greatly reduces the sampling times of echo data and improves the sampling rate of echo data.

Multi-modal physiological parameter for mapping ablation lesions

Abstract Funding Acknowledgements Type of funding sources: None. Background In addition to variety of force time indices, electrophysiologist use peak to peak voltage (Vpp) and or S wave on unipolar electrogram to assess RF lesions they deliver. This strategy ignores cardiac conduction speed, direction, and wave curvature, all of which are affected by delivered lesion. Electrode array catheters such as the Advisor HD Grid allow individual cardiac wavefronts from sites to be characterized simultaneously in two dimensions from which vectorcardiograms can be derived. This underappreciated attribute of electrograms when integrated into activation mapping may provide new insights into the electrophysiologic state of tissue including wavefront amplitude, direction and velocity. Objective We tested the hypothesis that intracardiac vectorcardiogram loops contain aspects of properties that may better detect lesions compared to Vpp reduction and that a novel multi-modal approach will provide additional insights on wavefront interaction with substrate. Methods In 5 ex-vivo Langendorff perfused swine hearts, 56-pole (7 x 8 configuration) electrode arrays were sutured to the anterior epicardial left ventricle. Unipolar electrograms were acquired with UHN mapping system during programmed stimulation from all cardinal directions before and after creating RF ablation lesions. Vector loop maps were derived from orthogonal bipole pairs to study wavefront organization and voltage. Isochrone maps were generated from unipolar local activation times. Vector loop eccentricity was defined for each 4-electrode group as a function of loop’s major and minor axis lengths. Low values denote less eccentricity (more circular). Results An illustrative example of vector, loop and isochrone maps before and after ablation is shown in figures 1 and 2. Pre-ablation conditions show activation from a point stimulation on the left side with slender vector loops indicative of a relatively homogeneous linear conduction. In contrast, post-ablation vector loops are much less eccentric near the ablated area and smaller reflecting attenuated amplitude. Interestingly, near border zone locations large round loops are observed, suggesting curved or nonlinear wave propagation. Figure 2 shows that vector loops recorded from scar areas were diminished in size following ablation with a mean Vpp of 3.76 vs 2.18mV (p=0.0001) and exhibited less mean eccentricity, 0.925 vs 0.848 (p<0.0001), with more round loops (eccentricity < 0.7) found after ablation (12% of sites vs 4% pre-ablation, p < 0.0001). Conclusions Significant changes in loop characteristics were observed even when ablation induced voltage attenuation was minimal. This suggests that vectorial loop eccentricity may provide information in addition to omnipolar Vpp. Wavefront propagation disruption near ablation border zones is thus potentially valuable to detect lesion gaps. This information along with wave amplitude and direction can now be integrated into a novel multi-modal map.

Wavefront Sensing by a Common-Path Interferometer for Wavefront Correction in Phase and Amplitude by a Liquid Crystal Spatial Light Modulator Aiming the Exoplanet Direct Imaging

We implemented the common-path achromatic interfero-coronagraph both for the wavefront sensing and the on-axis image component suppression, aiming for the stellar coronagraphy. A common-path achromatic interfero-coronagraph has its optical scheme based on a nulling rotational-shear interferometer. The angle of rotational shear can be chosen at a small angular extent of about 10 deg. Such a small angular shear maintains the coronagraphic contrast degradation known as the stellar leakage effect, caused by a finite stellar size. We study the phase and amplitude wavefront control by a liquid crystal spatial light modulator of reflection type which is used as the pixilated active adaptive optics unit. Therefore, adaptive optics perform a wavefront-correcting input toward a stellar interfero-coronagraph aiming at the direct exoplanet imaging. Presented here are both the numeric evaluations and the lab experiment stand to prove the declared functionality output.

Backflashover Performance Evaluation of the Partially Grounded Scheme of Overhead Lines With Fully Composite Pylons

A design of a fully composite pylon has been proposed for new-generation 400 kV transmission towers to save line corridors and to reduce visual impact. Correspondingly, a method of external down-leads is proposed to bring grounding potential to the shield wires, together with a plan that not all pylons are grounded called ‘partially grounded transmission lines’ (PGTLs). This paper investigates backflashover performance of a partial grounding scheme of overhead lines (OHLs) supported by composite pylons. The transient analysis was carried out in PSCAD based on Monte Carlo method. For OHLs with every pylon grounded, reducing footing resistance and soil resistivity can improve backflashover performance effectively, but for PGTLs, these two methods do not have obvious effect and increasing insulation distance has limited effect. When lightning strikes at PGTLs, overvoltage is mainly dependent on the distance to the nearest grounded pylon and a longer distance will cause overvoltage with larger amplitude and longer wave front duration. Therefore, backflashover rate also increases along with the distance to the nearest grounded pylon until reaching a value limited by the inceptive condition of flashover. A coefficient recommended by CIGRE TB 63 to estimate backflashover rate is discussed and modified when using in PGTLs.

Spatial domain sparse reconstruction algorithm of sheared beam imaging

Sheared beam imaging (SBI) is considered a computational imaging technique that transmits three sheared coherent laser beamlets for illumination, and a sensor array to receive the intensity of the speckle pattern reflected from the target. The SBI can be used to image remote objects through a turbulent medium with no need of any adaptive optics. However, while imaging low-orbit moving targets, the number of detectors of sensor array required by the receiving system of SBI is very large, and the development of sensor array is difficult and costly. In this work, a spatial domain sparse sampling technique is proposed for the SBI system through transmitting five laser beamlets to illuminate the target carrying more of its spectral information, which can reduce the number of detectors of the sensor array. Firstly, the principle of the sparse imaging technique is deduced. Then, a sparse reconstruction algorithm is studied. The phase difference and amplitude information of the target in the echo signal after sparse sampling can be extracted accurately by searching for the accurate positions of the beat frequency components. The wavefront phases can be demodulated by the least-squares method, and wavefront amplitude can be obtained by the algebraic operation of speckle amplitude. The reconstructed wavefront is used to formulate the two-dimension image of the target. Theoretically, without affecting the resolution, the number of detectors of the sensor array can be reduced to half of the traditional three-beam method, which breaks through the limitation that the detector spacing of sensor array is equal to the shear length of beamlet. From the simulation results, when the number of detectors of the sensor array is reduced by 50%, the proposed sparse reconstruction algorithm has almost the same quality as the reconstructed image with the traditional three-beam method.

Multi-Channel Near-Field Terahertz Communications Using Reprogrammable Graphene-Based Digital Metasurface

Digital metasurfaces have opened unprecedented ways to accomplish novel electromagnetic devices thanks to their simple manipulation of electromagnetic waves. However, the metasurfaces leveraging phase-only or amplitude-only modulation restricted the full-functionality control of the devices. Herein, a digital graphene-based metasurfaces engineering wavefront amplitude and phase are proposed for the first time to tackle this challenge in the terahertz (THz) band. The concept and its significance are verified using reprogrammable multi-focal meta-lens based on a 2/2-bit digital unit cell with independent control of 2-bit states of amplitude and phase individually. Moreover, we introduce a novel method to directly transmit digital information over multiple channels via the reprogrammable digital metasurface. Since these metasurfaces are composed of digital building blocks, the digital information can be directly modulated to the metasurface by selecting specific digital sequences and sent them to predetermined receivers distributed in the focal points. Following that, a multi-channel THz high-speed communication system and its application to build three-dimensional wireless agile interconnection are demonstrated. The presented method provides a new architecture for wireless communications without using complicated components of conventional systems. This work motivates versatile meta-devices in many applications envisioned for the THz frequencies, which will play a vital role in modern communications.

Tunable Metasurface With Dynamic Amplitude and Phase Control

Tunable metasurfaces are of great potential for the next-generation electromagnetic systems (e.g. beamforming systems due to the capabilities for ultra-fast reconfiguration speed and agile beam programming on the go). The fundamental aspects of tunable metasurfaces laid on their freedom in manipulating the phase and amplitude of the outgoing wavefront. However, the current research heavily focused on the phase manipulation while left the amplitude manipulation capabilities of the metasurfaces barely explored. To unlock the full potential of tunable metasurfaces, in this article, a novel tunable metasurface design with the capability to control both amplitude and phase of transmitted wavefront has been designed, simulated, and characterized experimentally. By incorporating individual phase and amplitude control modules under the guidance of equivalent circuit models, the proposed metasurface achieved the arbitrary phase and amplitude tuning while maintaining reasonably low physical profile. The measurement results of fabricated samples validated the design procedure and fulfilled the flexibility in terms of phase and amplitude tuning, also paved the way towards achieving more powerful dynamic metasurfaces.

Correction of water column height variation on 2D grid high-resolution seismic data using dGPS based methodology

Variations in the physical properties of water column usually impede exact water column height correction on high-resolution seismic data, especially when the data are collected in shallow marine environments. Changes in water column properties can be attributed to variation in tides and currents, wind-generated swells, long and short amplitude wave-fronts, or variation in salinity and water temperature. Likewise, the proper motion of the vessel complicates the determinability of the water column height. This study provides a less time-consuming and precise differential Global Positioning System based methodology that can be applied to most types of high-resolution seismic data in order to significantly improve the tracking and quality of deduced geological interpretations on smaller depth scales. The methodology was tested on geophysical profiles obtained from the German sector of the North Sea. The focus here was to identify, distinguish and classify various sub-surface sedimentary structures in a stratigraphically highly complex shallow marine environment on decimeter small-scale. After applying the correction to the profiles, the sea floor, in general, occurs 1.1 to 3.4 m (mean of 2.2 m) deeper than the uncorrected profiles and is consistent with the sea floor from published tide corrected bathymetry data. The corrected seismic profiles were used in plotting the depth of the base of Holocene channel structures and to define their gradients. The applied correction methodology was also crucial in glacial and post-glacial valley features distinction, across profile correlation and establishing structural and stratigraphic framework of the study area.

Bearing and range estimation with an exact source‐sensor spatial model

In sensor array processing literature, near-field bearing and range estimation algorithms generally use spherical wavefront to model only array sensors’ phase response (sometimes with Fresnel approximation) and assume equality in amplitude response. Ignoring the range dependent amplitudes, though facilitating the algorithmic development, will cause systematic estimation errors due to model mismatch. By taking the spherical wavefront amplitude into account, a new bearing and range estimation algorithm for locating multiple near-field sinusoid sources is presented. With the estimation of the near-field sensor array's response vector, closed-form formulas for bearing and range estimates are derived from its magnitude, or phase, or both. The problem of estimation ambiguity is discussed as well. Cramér-Rao bound is also derived to serve as a benchmark for performance study.

Riemann and Shock Waves in a Porous Liquid-Saturated Geometrically Nonlinear Medium

Within the framework of the classical Biot theory, the propagation of plane longitudinal waves in a porous liquidsaturated medium is considered with account for the nonlinear connection between deformations and displacements of solid phase. It is shown that the mathematical model accounting for the geometrical nonlinearity of the medium skeleton can be reduced to a system of evolution equations for the displacements of the skeleton of medium and of the liquid in pores. The system of evolution equations, in turn, depending on the presence of viscosity, is reduced to the equation of a simple wave or to the equation externally resembling the Burgers equation. The solution of the Riemann equation is obtained for a bell-shaped initial profile; the characteristic wave breaking is shown. In the second case, the solution is found in the form of a stationary shock wave having the profile of a nonsymmetric kink. The relationship between the amplitude and width of the shock wave front has been established. It is noted that the behavior of nonlinear waves in such media differs from the standard one typical of dissipative nondispersing media, in which the propagation of waves is described by the classical Burgers equation.

Non-centric cavity-based acoustic metasurface: Enabling full phase modulation

In this paper, an acoustic metasurface design is presented in which the transmitted phase may be tuned while preserving a high amplitude of transmission. To achieve full transmission with an arbitrary phase, the cavity is coupled with the coiling metamaterial to provide the desired level of impedance match. Furthermore, the effects of its geometrical parameters are investigated. The proposed structure exhibits an appropriate phase modulation in the desired phase frame (i.e., [0, 2π]) with high amounts of amplitudes. It is shown that this can be obtained by precisely designing the geometrical parameters even when the key parameters (i.e., the number of unit cells and their width and thickness values) are constant. Ultimately, three metasurfaces have been designed to redirect, split, and focus the transmitted wave while the resulted wavefront amplitude is preserved, enabling an efficient modulation platform.

Mitigation of the amplitude and celerity of dam break shock wave using combination of groyne and vertical pier

This paper aims to numerically investigate the impact of the combination of groynes and vertical piers on mitigating the amplitude and celerity of dam break wave-front. First, a robust numerical model based on the Finite Volume Method (FVM) with the Harten, Lax and van Leer (HLL) approximate Reimann’s solver’s technique is developed and validated with other research studies. Then many different scenarios including group of piers, group of groynes with different shapes and combination of groynes and piers are considered for dam break flow modeling. The final results showed that group of piers alone or group of groynes alone have no significant effects on the wave-front depth and celerity but, combination of L-head groyne with piers can decrease the amplitude and celerity of the dam break wavefront or flood wave. In this case, a blockage zone between the groynes and intensive backwater effects behind them were observed.

Sympathetic quantisation - A new approach to hologram quantisation

Spatial light modulators can typically only modulate the phase or the amplitude of an incident wavefront, with only a limited number of discrete values available. This is often accounted for in computer-generated holography algorithms by setting hologram pixel values to the nearest achievable value during what is known as quantisation. Sympathetic quantisation is an alternative to this nearest-neighbour approach that takes into account the underlying diffraction relationships in order to obtain a significantly improved post-quantisation performance. The concept of sympathetic quantisation is introduced in this paper and a simple implementation, soft sympathetic quantisation, is presented which is shown to improve mean squared error and structural similarity index error metrics by 50% for the considered case of single-transform algorithms.

Wide-Band Polarization Control of Leaky Waves on Anisotropic Holograms

Isotropic leaky-wave holograms have great potential in the synthesis of single-beam and multibeam radiations in the desired directions. However, conventional methods are based only on the phase synthesis of the object wave front and do not have the ability to control the polarization of waves. A solution to this problem is the use of anisotropic holograms that provide more degrees of freedom. In this paper, the aperture field estimation theory is proposed for the synthesis of anisotropic holograms. Its advantage is the accurate prediction of far-field wave front characteristics, including phase, amplitude, and polarization. This method is used in the design of two different types of holograms in order to generate the pencil beam and vortex beam radiations. The pencil beam hologram is designed to have a high polarization purity across the visible region. In addition, a surface-wave reflector is implemented for eliminating the unwanted backward modes and adding a scannability property from 16 to 19 GHz to the hologram with an appropriate level of cross-polarization across the bandwidth. In the second section, the theoretical equations for the synthesis of vortex beam hologram with circular polarization is developed. As an example, an anisotropic hologram with topological charge of $l=2$ is synthesized. Both theoretical and numerical results show that the anisotropic leaky-wave holograms are capable of generating orbital angular momentum (OAM) mode with the desired topological charge and polarization, which are simpler and more compact than other methods of generating OAM mode. For the proof of concept, a pencil beam hologram with a surface-wave reflector is fabricated and measured.

Asymmetries in adaptive optics point spread functions

An explanation for the origin of asymmetry along the preferential axis of the point spread function (PSF) of an AO system is developed. When phase errors from high-altitude turbulence scintillate due to Fresnel propagation, wavefront amplitude errors may be spatially offset from residual phase errors. These correlated errors appear as asymmetry in the image plane under the Fraunhofer condition. In an analytic model with an open-loop AO system, the strength of the asymmetry is calculated for a single mode of phase aberration, which generalizes to two dimensions under a Fourier decomposition of the complex illumination. Other parameters included are the spatial offset of the AO correction, which is the wind velocity in the frozen flow regime multiplied by the effective AO time delay and propagation distance or altitude of the turbulent layer. In this model, the asymmetry is strongest when the wind is slow and nearest to the coronagraphic mask when the turbulent layer is far away, such as when the telescope is pointing low toward the horizon. A great emphasis is made about the fact that the brighter asymmetric lobe of the PSF points in the opposite direction as the wind, which is consistent analytically with the clarification that the image plane electric field distribution is actually the inverse Fourier transform of the aperture plane. Validation of this understanding is made with observations taken from the Gemini Planet Imager, as well as being reproducible in end-to-end AO simulations.

Simultaneous phase and amplitude aberration sensing with a liquid-crystal vector-Zernike phase mask.

We present an enhanced version of the Zernike wavefront sensor (WFS), which simultaneously measures phase and amplitude aberrations. The "vector-Zernike" WFS consists of a patterned liquid-crystal mask, which imposes a ±π/2 phase on the point spread function core through the achromatic geometric phase acting with the opposite sign on opposite circular polarizations. After splitting circular polarization, the ensuing pupil intensity images are used to reconstruct the phase and the amplitude of the incoming wavefront. We demonstrate reconstruction of the complex wavefront with monochromatic lab measurements and show in simulation the high accuracy and sensitivity over a bandwidth up to 100%.

A methodology for the digital reconstruction of an interferogram, a schlieren image, or a shadowgram from a single digital holographic recording.

This study presents an optical assembly and digital reconstruction method that is based on digital holography for characterizing transparent objects. The image-plane optical setup is based on a versatile Mach-Zehnder interferometer for the formation of controlled parallel fringe patterns, of tens micrometers of separations or less. The numerical reconstruction of the propagated wavefront by the hologram is performed by the Fresnel-Kirchhoff transform, and it is used of three ways: (a) interferometric phase reconstruction (as in interferometry) in the phase object position, (b) reconstruction of the wavefront amplitude at a determined distance from the object (shadowgraph technique), and (c) a composed digital reconstruction process of the wavefront amplitude, defined by a digital optical assembly for the selection of angular deflections produced by the phase object (schlieren technique). The resolving power of holographic reconstruction methodology proposed is determined by the sensitivity of hologram for detecting the interferometric phase. The fringe pattern of the hologram defines a minimum phase shift resolution of 0.15π rad. The scope of the technique is experimentally tested for a steady-state phase object.

Lensless phase-shifting point diffraction interferometer for spherical mirror measurement

We demonstrate a lensless phase-shifting point diffraction interferometer with a tiny pinhole to measure spherical mirror surface figures. The interferograms used to reconstruct the surface figure are formed without imaging optics. Fresnel diffraction calculations with a coordinate transformation are studied as a means of reconstruction method for wavefront amplitude and phase. The Radon transform is used to determine the distance from the tiny pinhole to the CCD target, which guarantees the accuracy of the diffraction calculations. Our lensless phase-shifting point diffraction interferometer not only retains the higher measurement precision by the ideal reference wavefront, but also overcome the fabrication and mounting limitation of the imaging optics. The simulations and experiments have validated the accuracy and feasibility of the spherical mirror measurement by proposed lensless phase-shifting point diffraction interferometer.