Phase Screen Approximation Research Articles

Interface stiffness identification of rough and weak bonded interface using developed ultrasonic reflection phase derivative spectrum

The thickness and interface roughness of coatings both affect the interface bonded quality. Existed ultrasonic testing methods based on traditional phase screen approximation or spring model assumption are difficult to simultaneously identify the interface roughness and stiffness of coating. This paper, a new method for integrated identifying coating thickness, interface roughness, and interface stiffness using developed ultrasonic reflection phase derivative spectrum (URPDS) is proposed. A phase-screen-approximated spring-model (PSASM) for ultrasound vertically propagating into rough and weak bonded interface is constructed. On basis of PSASM, a URPDS of coating/substrate structure is developed for identifying the interface stiffness and other parameters of coated parts. Cross-correlation analysis is used to eliminate the phase deviation of URPDS introduced by reference signal and initial phase of tested signal. Sensitivity analysis is used to determine the high-sensitivity regions of URPDS to interface roughness and interface stiffness. Genetic algorithm optimization is used to achieve integrated identification of coating thickness, interface roughness, and interface stiffness. The rationality of PSASM is verified through numerical simulation using a series of coating/substrate models with rough and weak bonded interface, and the relationship between the high-sensitivity regions and the high-precision measurement ranges of interface roughness Rq and interface stiffness Kn is clarified. Ultrasonic experiments are implemented on Nickel-coating samples and coated parts using plane wave probe. The coating thickness, interface roughness, and interface stiffness could be identified accurately, which shows that the proposed URPDS method can identify the interface stiffness of rough contacted dissimilar media or coated parts with rough interface.

Roughness correction method for the measurement of attenuation coefficient using contact transducers

The surface quality of a material significantly affects the accurate measurement of the ultrasonic attenuation coefficient. In this research, a roughness correction method for experimental calculation of the attenuation coefficient using contact transducers is proposed. Firstly, the losses due to the scattering from a rough interface are analysed. According to the mechanism of ultrasonic waves reflected from a thin layer between two solid media, the frequency-dependent reflection coefficient of the coupling interface involving a roughness parameter is derived based on the phase-screen approximation theory. Then, a compensation model is established to correct the error caused by the surface roughness. 304 stainless steel and 45 steel specimens with different levels of surface roughness are prepared and ultrasonic measurements are implemented using both the through-transmission and pulse-echo methods. The experimental results show that the proposed correction method can effectively eliminate the losses caused by surface roughness and improve the measurement accuracy of the attenuation coefficient.

Development of Singular Points in a Beam Passed Phase Screen Simulating Atmospheric Turbulence and Precision of Such a Screen Approximation by Zernike Polynomials

This article addresses two issues. Firstly, it was shown that if the initial phase of a Gaussian beam is specified by the sum of Zernike polynomials or by a screen simulating atmospheric turbulence, in the process of propagation, singular points appear in the wavefront of such a beam. With the use of numerical simulation, the dependence of the vortices number on the distortion characteristics and on the distance traveled by the beam was determined. The second problem analyzed in the article is the problem of a phase screen approximation by a series formed by Zernike polynomials. The carried out numerical experiments made it possible to determine the dependence of approximation accuracy on the screen parameters and on the number of polynomials entering the basis of approximation.

Efficient channel modeling of structured light in turbulence using generative adversarial networks.

We present a fast and efficient simulation method of structured light free space optics (FSO) channel effects from propagation through a turbulent atmosphere. In a system that makes use of multiple higher order modes (structured light), turbulence causes crosstalk between modes. This crosstalk can be described by a channel matrix, which usually requires a complete physical simulation or an experiment. Current simulation techniques based on the phase-screen approximation method are very computationally intensive and are limited by the accuracy of the underlying models. In this work, we propose to circumvent these limitations by using a data-driven approach for the decomposition matrix simulation with a conditional generative adversarial network (CGAN) synthetic simulator.

Modeling and Experimental Analysis of Roughness Effect on Ultrasonic Nondestructive Evaluation of Micro-crack

A high-precision evaluation of ultrasonic detection sensitivity for a micro-crack can be restricted by a corroded rough surface when the surface microtopography is of the same order of magnitude as the crack depth. In this study, a back-surface micro-crack is considered as a research target. A roughness-modified ultrasonic testing model for micro-cracks is established based on a multi-Gaussian beam model and the principle of phase-screen approximation. The echo signals of micro-cracks and noises corresponding to different rough front surfaces and rough back surfaces are obtained based on a reference reflector signal acquired from a two-dimensional simulation model. Further comparison between the analytical and numerical models shows that the responses of micro-cracks under the effects of different corroded rough surfaces can be accurately predicted. The numerical and analytical results show that the echo signal amplitude of the micro-crack decreases significantly with an increase in roughness, whereas the noise amplitude slightly increases. Moreover, the effect of the rough front surface on the echo signal of the micro-crack is greater than that of the rough back surface. When the root-mean-square (RMS) height of the surface microtopography is less than 15 μm, the two rough surfaces have less influence on the echo signals detected by a focused transducer with a frequency of 5 MHz and diameter of 6 mm. A method for predicting and evaluating the detection accuracy of micro-cracks under different rough surfaces is proposed by combining the theoretical model and a finite element simulation. Then, a series of rough surface samples containing different micro-cracks are fabricated to experimentally validate the evaluation method.

Physical meaning of the deviation scale under arbitrary turbulence strengths of optical orbital angular momentum.

The recently so-called deviation scale [Phys. Rev. A99, 013828 (2019)PLRAAN1050-294710.1103/PhysRevA.99.013828] bridges the connection between the result of the infinitesimal propagation equation (IPE) prediction and that of the single phase screen (SPS) approximation. Thanks to the multiple phase screen (MPS) approach, in this paper we elaborate the physical meaning of the deviation scale: the spatial accumulation of slight intensity modulation of incident orbital angular momentum (OAM)-carrying beam splits the original vortex into multiple individual vortices with a topological charge (TC) of +1 and regenerates the vortex-antivortex pairs with a TC of +1 and with a TC of -1, leading to a significant deviation between these two different results only when the disruption of this compound effect on the phase distribution of the incident OAM-carrying beam becomes more significant. Other than that, we also show that the appearance of the deviation scale cannot be predicted only by the Rytov variance, which can be predicted through the vortex-splitting ratio of the received optical field alone or with the help of the normalized propagation distance.

Numerical and analytical study for ultrasonic testing of internal delamination defects considering surface roughness

The improvement of detection accuracy and reliability of micro delamination defect is restricted by the rough surface. The flat bottom holes (FBHs) are frequently employed as reference targets to evaluate the sensitivity of ultrasonic testing for internal defects. A roughness-modified analytical model for ultrasonic testing of FBHs is established based on the principle of multi-Gaussian beam and phase-screen approximation. The signal of reference reflector is obtained from two-dimensional ultrasonic simulation model. The amplitude changes of echo signals and noises of FBHs with different diameters and depths under rough surfaces are presented. The analyses results indicate that the root-mean-square (rms) height plays a dominant role in the amplitude change of signals compared with the correlation length. The reflected wave amplitude of FBHs decreases nonlinearly with an increase of roughness whereas the amplitude of noise increases slightly. Subsequently, a method is proposed further combining the noise amplitude acquired from numerical simulation and the echo signal amplitude obtained by the analytical model, which is to predict and estimate the detection accuracy of internal defects under different surface roughness. The parallel experiments are performed on several samples with different roughness to validate the evaluation method.

Intensity fluctuations in ground-to-satellite laser beam transmission induced by atmospheric turbulence: reconsideration of strong fluctuation.

This paper reports beam wave intensity fluctuations in uplink (ground-to-satellite) laser beam transmission caused by atmospheric turbulence. Intensity fluctuation in the strong region was apparently induced by uplink measurement in previous experiments. Statistical values of the uplink fluctuation were estimated by numerical calculation using moment equation analysis with thin phase screen approximation. The beam profile of the uplink, the scintillation index, and the covariance of the uplink intensity fluctuation were calculated using models of the refractive index structure constant. The generation of strong intensity variation was explained as the result of a speckle pattern on the receiving plane at the satellite produced by atmospheric turbulence when scanned along with trajectory of the satellite.

Features of light scattering on mosaic layers composed of structurally similar birefringent domains

ABSTRACTThe two-point generalized Mueller matrix method and phase screen approximation are applied to determine features of light scattering on mosaic birefringent layers composed of structurally similar domains. Two types of mosaic birefringent layers are considered: mosaic birefringent layers composed of identical non-chiral domains and statistically non-chiral layers composed of structurally similar (but generally not identical) domains. Analytical expressions for matrix operators that connect state vectors of the scattered and non-scattered components with the state vector of the incident beam are derived in terms of statistical structural characteristics of the layer. Based on the theoretical approach developed, some optical phenomena characteristic of mosaic birefringent layers, such as selective light scattering and inversion of circular polarization upon scattering, are explained. The effect of the degree of similarity of domains on the scattering properties of the layer is analyzed. Illustrative experimental examples are given.Abbreviations: LC: liquid crystal; NRPA layer: nematic random planar alignment layer; NPMB layer: non-planar mosaic birefringent layer; RIJM theory: rotationally invariant Jones matrix theory; SMB layer: simple mosaic birefringent layer; SNCM layer: statistically non-chiral mosaic layer

Scattering Patterns of Orthogonally Polarized Light Components for Statistically Rotationally Invariant Mosaic Birefringent Layers

Using the method of two-point generalized Mueller matrices and the phase screen approximation, we have studied the relationship between the shape of the angular spectra of linearly polarized components of light scattered by randomly inhomogeneous layers of a birefringent material and the correlation structural characteristics of the layers. For statistically rotationally invariant layers, the structural conditions for three types of scattering patterns of linearly polarized components have been revealed: (i) patterns invariant with respect to the azimuthal rotation by 180°, (ii) patterns invariant with respect to the azimuthal rotation by 90°, and (iii) patterns possessing a circular symmetry. For mosaic birefringent layers consisting of homogeneous fragments with different azimuthal orientations of their optic axes, a relationship between the correlation structural properties of the layer and the shape of the scattering pattern of polarized components has been determined. In particular, conditions for the observation of crosslike four-lobed scattering patterns in crossed and parallel polarizers have been determined and the structural characteristic that is responsible for the orientation of such a scattering pattern with respect to the direction of polarization of the incident light has been found. The inferences that were derived have been supported by experimental data and results of numerical simulations.

Ultrasonic roughness measurement based on scattering attenuation

Surface roughness measurement is very critical for the functional performance evaluation of machined parts. At present, two most widely used methods for roughness measurement are stylus profiler and optical scattering methods with disadvantages of slow measuring speed and measurable range limit. In this research, an approach for characterizing surface roughness of machined parts using one ultrasonic transducer is put forward, which is an alternative non-contact and efficient in situ measuring method. A single scattering event of ultrasonic wave on the rough surface is firstly analyzed with the consideration of the influence of local incident angle. Then, the theoretical relationship between the scattering attenuation coefficient of ultrasonic pulse-echo and the roughness level is derived based on Kirchhoff Approximation and Phase Screen Approximation. At last, a series of experiments were performed on nickel alloy samples with different levels of roughness to validate the established theoretical model. The measurement results demonstrated the feasibility and applicability of the proposed ultrasonic roughness measurement method.

Identification of the Velocity, Thickness, and Interfacial Roughness of Coating Using Full Time-Domain URCPS: Cross-Correlation-Based Inverse Problem

Aiming at characterizing interfacial roughness of thin coatings with unknown sound velocity and thickness, we derive a full time-domain ultrasonic reflection coefficient phase spectrum (URCPS) as a function of interfacial roughness based on the phase screen approximation theory. The constructed URCPS is used to determine the velocity, thickness, and interfacial roughness of specimens through the cross-correlation algorithm. The effect of detection frequency on the roughness measurement is investigated through the finite element method. A series of simulations were implemented on Ni-coating specimens with a thickness of 400 μm and interfacial roughness of 1.9–39.8 μm. Simulation results indicated that the measurement errors of interfacial roughness were less than 10% when the roughness satisfies the relationship of Rq = 1.6–10.0%λ. The measured velocity and thicknesses were in good agreement with those imported in simulation models with less than 9.3% error. Ultrasonic experiments were carried out on two Ni-coating specimens through a flat transducer with an optimized frequency of 15 MHz. Compared with the velocities measured by time-of-flight (TOF) method, the relative errors of inversed velocities were all less than 10%. The inversed thicknesses were in good agreement with those observed by optical microscopy with less than 10.9% and 7.6% error. The averaged interfacial roughness determined by the ultrasonic inversion method was 16.9 μm and 30.7 μm, respectively. The relative errors were 5.1% and 2.0% between ultrasonic and confocal laser scanning microscope (CLSM) method, respectively.

Картины рассеяния ортогонально поляризованных компонент света для статистически вращательно-инвариантных мозаичных двулучепреломляющих слоев

AbstractUsing the method of two-point generalized Mueller matrices and the phase screen approximation, we have studied the relationship between the shape of the angular spectra of linearly polarized components of light scattered by randomly inhomogeneous layers of a birefringent material and the correlation structural characteristics of the layers. For statistically rotationally invariant layers, the structural conditions for three types of scattering patterns of linearly polarized components have been revealed: (i) patterns invariant with respect to the azimuthal rotation by 180°, (ii) patterns invariant with respect to the azimuthal rotation by 90°, and (iii) patterns possessing a circular symmetry. For mosaic birefringent layers consisting of homogeneous fragments with different azimuthal orientations of their optic axes, a relationship between the correlation structural properties of the layer and the shape of the scattering pattern of polarized components has been determined. In particular, conditions for the observation of crosslike four-lobed scattering patterns in crossed and parallel polarizers have been determined and the structural characteristic that is responsible for the orientation of such a scattering pattern with respect to the direction of polarization of the incident light has been found. The inferences that were derived have been supported by experimental data and results of numerical simulations.

Quantitative characterization of the interfacial roughness and thickness of inhomogeneous coatings based on ultrasonic reflection coefficient phase spectrum

Aim at nondestructively characterizing the interfacial roughness of coatings, ultrasonic reflection coefficient phase spectrum (URCPS) as a function of interfacial roughness is derived based on the phase screen approximation theory [1]. For inhomogeneous coatings, the attenuation coefficient α(f) shows a non-negligible effect on the URCPS. The relationship of α(f) on frequency f is used to decouple the URCPS. The constructed URCPS is used to determine the interfacial roughness and thickness of specimens through a two-parameter inversion utilizing the cross-correlation algorithm. The effects of the coating inhomogeneity (such as porosity) on the roughness measurement are analyzed through numerical calculation. A series of simulations with interfacial roughness from 6.2 to 12.7 μm indicate that measurement errors of the thickness are all less than 8.0%. The relative errors of the measured roughness of models without porosity and models with the porosity of 3% are less than 11.8%. For the models with porosity 5%, when the roughness is larger than 10.3 μm, the relative error is still larger than 11.7%. Ultrasonic experiments were carried out on a tungsten carbide (WCNi) coating utilizing water immersion, flat transducer. The WC-Ni coating, with unknown interfacial roughness, was sprayed on a stainless steel using the high-velocity oxygen fuel (HVOF) method. Experimental results show that the interfacial roughness of specimen obtained by the proposed ultrasonic measurement are in good agreement with that of SEM observations, the absolute error of the measured roughness is less than 1.4 μm and the relative error was less than 11.0%.

Numerical simulation for echo characteristics of laser beams reflected by retro-reflectors in atmospheric turbulence

In this study, a split-step Fourier method (FFSM) was extended in the simulation of a double-passage laser propagation in atmospheric turbulence. The characteristics of the coherence length and intensity scintillation for the plane wave, spherical wave, and Gaussian beam reflected by the plane’s mirror and retro-reflector were numerically simulated and compared using a phase-screen approximation method. The properties of the echo waves from the mirror and retro-reflector, and that of the double-passage propagation without a target, were also presented. The results obtained in this study could be potentially used to detect the different characteristics of the echo fields of plane waves, spherical waves and Gaussian waves reflected by different targets, so as to obtain the target characteristics. In addition, the study results could also provide theoretical and technical guidance for the application of laser target detection and recognition.

Synthesis Of Ultrasound Field Sources Based on Phase Screen Approximation

Here is proposed the method for synthesizing the sources of an acoustic field on the basis of an approximation of the phase screen. The technology of manufacturing ultrasonic phased arrays providing the formation of a field of a given distribution is proposed. An experimental setup has been developed for the formation of a vortex field at a distance of 10 cm.

Experimentally observed decay of high-dimensional entanglement through turbulence

The evolution of high dimensional entanglement in atmospheric turbulence is investigated. We study the effects of turbulence on photonic states generated by spontaneous parametric down-conversion, both theoretically and experimentally. One of the photons propagates through turbulence, while the other is left undisturbed. The atmospheric turbulence is simulated by a single phase screen based on the Kolmogorov theory of turbulence. The output after turbulence is projected into a three-dimensional (qutrit) basis composed of specific Laguerre-Gaussian modes. A full state tomography is performed to determine the density matrix for each output quantum state. These density matrices are used to determine the amount of entanglement, quantified in terms of the negativity, as a function of the scintillation strength. Theoretically, the entanglement is calculated using a single phase screen approximation. We obtain good agreement between theory and experiment.

Analysis of Wave Front Fluctuations in Strong Turbulence Using Thin Phase Screen Approximation

Analysis of Wave Front Fluctuations in Strong Turbulence Using Thin Phase Screen Approximation

Theoretical analysis of quantum ghost imaging through turbulence

Atmospheric turbulence generally affects the resolution and visibility of an image in long-distance imaging. In a recent quantum ghost imaging experiment [P. B. Dixon et al., Phys. Rev. A 83, 051803 (2011)], it was found that the effect of the turbulence can nevertheless be mitigated under certain conditions. This paper gives a detailed theoretical analysis to the setup and results reported in the experiment. Entangled photons with a finite correlation area and a turbulence model beyond the phase screen approximation are considered.

Signal Loss of Reflected and Transmitted Ultrasonic Waves Induced by Double Rough Interfaces

A simple analytical model evaluating the signal loss of reflected and transmitted ultrasonic waves induced by double random rough interfaces was presented based on the Phase-Screen approximation (PSA). By expressing the phase correlation factor of the acoustic beam doubly transmitting through rough interfaces into the function of the flaw depth and the incident angle, the model overcomes some limitations and inadequacy subjected by those models which use only the phase-screen approximation to evaluate the signal loss of waves induced by the single rough interface. The signal attenuation will decrease with the increase of the beamSs incident angle for the flaw near the upper interface, while the signal attenuation is not sensitive to the incident angle, as the flaw is far from the upper interface. The results indicate that the influence of the angle factor for the waves doubly transmitted through the rough interface on the phase correlation will only operate near the upper interface. The experimental results show that the theoretical prediction is in good agreement with experimental measurements. Therefore, the model proposed in the paper is suitable for the real-time ultrasonic non-destructive testing of rough flaws under the rough surfaces in materials.