Increasing Propagation Distance Research Articles

Accurate Range Modeling for High-Resolution Spaceborne Synthetic Aperture Radar.

Spaceborne synthetic aperture radar (SAR) is an advanced microwave imaging technology that provides all-weather and all-day target information. However, as spaceborne SAR resolution improves, traditional echo signal models based on airborne SAR design become inadequate due to the curved orbit, Earth rotation, and increased propagation distance. In this study, we propose an accurate range model for high-resolution spaceborne SAR by analyzing motion trajectory and Doppler parameters from the perspective of the space geometry of spaceborne SAR. We evaluate the accuracy of existing range models and propose an advanced equivalent squint range model (AESRM) that accurately fits the actual range history and compensates for high-order term errors by introducing third-order and fourth-order error terms while maintaining the simplicity of the traditional model. The proposed AESRM's concise two-dimensional frequency spectrum form facilitates the design of imaging algorithms. Point target simulations confirm the effectiveness of the proposed AESRM, demonstrating significant improvements in fitting accuracy for range histories characterized by nonlinear trajectories. The developed AESRM provides a robust foundation for designing imaging algorithms and enables higher resolution and more accurate radar imaging.

Theoretical and numerical study on transient acoustic wave propagation across ice layers in the Arctic Ocean.

The study of transient acoustic wave propagation across the Arctic Ocean ice layer provides theoretical guidance for the design of trans-ice acoustic communication systems. In this study, the Arctic Ocean was modeled as an ice-water composite structure, where the ice and water are regarded as an elastic solid and liquid, respectively. An analytical transient solution for acoustic wave propagation in this structure was derived using the eigenfunction expansion method. Further, the numerical procedures were presented and used to analyze the acoustic wave propagation characteristics across the ice layer. The results show that waveforms corresponding to the radial displacements are more severely distorted than the axial displacements. The amplitudes of the radial and axial displacements decreased rapidly with increasing propagation distance. The ice thickness had a greater impact on the radial displacement than axial displacement; the thicker the ice, the greater the distortion for both radial and axial displacements.

Overtaking Effects on Spherical Shock Wave Propagation in Metallic Mediums

The propagation of spherical shock waves through metallic mediums have been considered in this paper. The effect of overtaking disturbances in the propagation of spherical shock waves through solids, especially metals such as Aluminium and Copper, has been analysed by employing Chisnell-Chester-Whitham method. Under the influence of spherical shock waves flow variables have been obtained analytically as well as estimated numerically. Expressions for the pressure and the particle velocity behind the shock wave have also been obtained. It is found that shock strength in both the metals decreases as shock propagates through them. Shock velocity and modified shock velocity depict a decreasing pattern with increasing propagation distance. These observations suggest many industrial and medical applications of shock waves.

The impact of shallow waveguides on the spectral characteristics of modulated spectral lines in underwater radiated noise produced by ships

The modulation spectrum line spectrum of undersea radiated noise from ships is a significant characteristic in the identification of ship targets. This research presents an analysis of the impact of waveguide dispersion effect and demodulation bandwidth on the intensity of modulation line spectrum, based on the theoretical framework of square demodulation method. The physical mechanism underlying this influence is derived and discussed. The examination of collected data and numerical simulations demonstrates that the extracted modulated line spectrum intensity in an unobstructed environment effectively utilizes the carrier’s energy and generally does not exhibit any abnormal loss during propagation. However, in a shallow sea dispersive waveguide, as the propagation distance increases, a transimission loss anomaly of over 3dB occurs in the modulated line spectrum. The theoretical derivation yields implications that can offer theoretical backing for the reduction of vibration and noise in underwater targets, extraction of faint signals from high-order line spectra in the distant field, and identification of targets.

Discussion on the Discreteness of the Attenuation Parameters of the Peak Particle Velocity Induced by Blasting.

The research on the attenuation law of blasting vibration has become the foundation and precondition of the effective control of blasting vibration damage. Aiming at the characteristics of low frequency, low velocity, and strong amplitude of the R wave, an improved wave component separation method based on R wave suppression is proposed. Combined with the measured vibration signals of a field test, the attenuation parameters of different types of waves in the propagation process of blasting seismic waves are studied. The analysis results show that, in the process of blasting seismic wave propagation, the attenuation parameters of different types of waves are significantly different. With an increase in propagation distance, the proportion of the different types of waves will also change. The study of attenuation law with only coupled particle peak vibration velocity often showed high discreteness. The fitting correlation coefficient and prediction accuracy of peak vibration velocity without distinguishing wave modes are lower than those induced by the P wave or R wave alone, which should be attributed to the conversion of dominant wave modes in blasting vibration at different distances.

Acoustic Source Localization in Composite Plates using sideband peak count – Index Technique

In this study the Non-Linear Ultrasonic Sideband Peak Count-Index (SPC-I) technique is used as the foundation for anovel approach towards acoustic source localization (ASL) in orthotropic composite plates. The SPC-I based technique proposed here does not require the signal attenuation information or any knowledge on the time of arrival of the signal. It should be noted that since individual sensors can have varying sensitivities, the signal attenuation measured from the recorded signal amplitude is not very reliable. In addition, it is not necessary to have any prior knowledge of the mechanical properties of the composite plate material. All these are achievable by attaching 25 sensors that are well-scattered on the surface of the plate. The signals that are generated by an acoustic source are recorded by these 25 sensors. The recorded signals are then analyzed to derive the SPC-I value for each signal. The calculated SPC-I values are run through an optimization algorithm to predict the acoustic source location. Such localization is possible because the composite plate is inherently a non-linear material. Hence, as the signal travels longer distances through a composite plate, the recorded signal should show increasing level of distortion due to material non-linearity and dispersion. This phenomenon manifests itself primarily as a consequence of signal scattering and frequency modulation. Because of this, the phenomena of increasing distortion in the signal with increasing propagation distance can be exploited and utilized to predict the location of the acoustic source by solely utilizing the SPC-I values. This acoustic source localization technique is experimentally verified on a Carbon Fiber Reinforced (CFR) composite plate of dimension 500 mm x 500 mm with a thickness of 1 mm. The experimental results confirmed the feasibility of the proposed technique.

Propagation properties of a partially coherent electromagnetic hyperbolic-sine-Gaussian vortex beam through anisotropic atmospheric turbulence.

Using the extended Huygens-Fresnel principle and the Rytov approximation, the analytical formula for the propagation of a partially coherent electromagnetic hyperbolic-sine-Gaussian vortex beam (PCEShVB) in anisotropic atmospheric turbulence has been theoretically derived. Detailed studies have been conducted on the evolution characteristics of the average intensity, the degree of coherence (DOC), and the degree of polarization (DOP) of the beam in turbulence. The results show that during propagation, the intensity distribution of the beam will exhibit a spiral structure, and the overall distribution of the light spots will rotate in a direction related to the sign of the topological charge. The DOC distribution of PCEShVB will display a pattern reminiscent of beam interference fringes with an increase in propagation distance, with the number of "interference fringes" greatly impacted by the hyperbolic sine parameter. Furthermore, PCEShVB with a large initial coherent length and hyperbolic sine parameter will increase the degree of separation of the spots and yield a large DOP. Finally, for the validation of the theoretical findings, the random phase screen method was employed to simulate the propagation of PCEShVB through anisotropic atmospheric turbulence. The studies revealed a consistent alignment between the simulation results and the theoretical predictions.

Shock Wave Detection for In-Process Depth Measurement in Laser Ablation Using a Photonic Nanojet

Three-dimensional micro- and submicrometer-scale structures exhibit unique functions that cannot be obtained with bulk materials. To create such three-dimensional microstructures with high precision and efficiency, we proposed laser ablation using a photonic nanojet. A photonic nanojet is an optical beam with both a small beam diameter and a large depth of focus, which is obtained by irradiating a dielectric microsphere using a laser beam. In this study, we proposed an in-process depth measurement method to improve the machining accuracy of laser ablation using a photonic nanojet. We focused on the propagation characteristics of the shock waves generated during laser ablation. Shock waves were generated at the deepest point of the machining area and reached the microspheres as the pressure decayed, showing that different machining depths exerted different pressures on the microspheres. The microspheres were displaced by the pressure of the shock wave, and the amount of displacement depended on the pressure. Therefore, microspheres can be used as probes for shock wave detection, and the machining depth can be determined by measuring the displacement of microspheres during photonic nanojet machining. In this study, the displacement of a microsphere was measured simultaneously during photonic nanojet machining using a confocal optical system. From the obtained microsphere vibration data, the effect of the shock wave pressure was extracted, and the displacement of the microsphere due to the shock wave was obtained. When the hole depth varied from 155 to 1121 nm, the displacement of the microspheres varied from 0.58 to 0.03 µm. The experimental results show that the displacement of the microspheres vibrated by the shock wave decreased as the machining depth increased. This was due to an increase in the shock wave propagation distance and a decrease in the pressure of the shock wave as the machining depth increased. In conclusion, in-process depth measurements are possible in laser ablation using a photonic nanojet with a microsphere as a probe to detect shock waves.

The Propagation Properties of a Lorentz–Gauss Vortex Beam in a Gradient-Index Medium

Based on the Huygens–Fresnel integral and ABCD matrix, the propagation equation for the Lorentz–Gauss vortex beam (LGVB) in a gradient-index medium (GRIN) is rederived. The evolution of the intensity and phase distributions of an LGVB through a GRIN medium are numerically calculated as a function of the gradient-index parameter with changes in the incident beam parameters. The results showed that the propagation path and intensity distributions changed periodically with increasing propagation distance. In contrast, phase distributions change at multiples of π/β or 2π/β, depending on whether the M values are odd or even, respectively. At the same time, the parameters of the gradient index determine the periodic values of the Lorentz–Gauss vortex beams during propagation, and as β increased, the period of evolution decreased. The Lorentz–Gauss vortex beam propagating through the gradient index will develop from a square beam to a Gaussian vortex beam more quickly with an increase of w0x=w0y. In addition, the topological charge affects the size of the dark spot at the center of the beam and the size of the beam, causing the phase distributions to change periodically in the medium. This study is beneficial for laser optics and optical communications.

“Cumulative effect” of second harmonic Lamb waves in a lossy plate

The second harmonic Lamb waves have high sensitivity to microstructural defects in materials and are therefore promising for incipient damage detection and monitoring of thin-walled structures. Existing studies have shown that the second harmonic Lamb waves can be cumulative with increasing propagation distance under the internal resonance conditions, which is conducive to nonlinear wave measurements in view of structural health monitoring. However, when propagating in a lossy structure with damping, the cumulative properties of the second harmonic Lamb waves are affected by energy dissipation and thus need to be re-examined. In this paper, a method for predicting the cumulative characteristics of second harmonic Lamb waves in damped plates is proposed. Instead of using material damping parameters which are difficult to obtain in practice, the proposed method relies on the attenuation patterns of Lamb waves at fundamental and double frequencies while taking into account the influence of the wave beam divergence. The proposed methodology is validated by finite element simulations and experiments. The results show that the cumulative second harmonic Lamb waves in the damped plate tend to increase and then decrease, and a “sweet” zone of relatively large amplitude can be predicted using the proposed method. The elucidation of the cumulative characteristics of the second harmonic Lamb waves provides guidance for effective system design for structural damage detection and monitoring applications.

Angular width and beam quality of a partially coherent standard Laguerre-Gaussian vortex beam in turbulent plasma.

Analytical formulas for the angular width and propagation factor of a partially coherent standard Laguerre-Gaussian (sLG) vortex beam through anisotropic turbulent plasma were derived based on the extended Huygens-Fresnel integral and the second-order moments of the Wigner distribution function. The evolution properties of the angular width and propagation factor of partially coherent sLG vortex beams propagating in anisotropic turbulent plasma were investigated numerically. The numerical results demonstrate the influence of the source and turbulence parameters on the normalized angular width and normalized propagation factor of the partially coherent sLG vortex beams. It can be observed that in a partially coherent sLG vortex beam with a lower beam order, topological charge, and wavelength, or a higher correlation coefficient of the source plane, and with increasing propagation distance and refractive index fluctuation variance or decreasing anisotropy parameters, the outer and inner scales of the turbulent plasma have a large angular width and propagation factor (i.e.,the beam quality is worse). The results of this study will be beneficial for applications in remote sensing and optical communications.

Unveiling detection probability for multi-Gaussian correlated anomalous vortex modes in maritime atmospheric turbulence

In this study, we employ the Rytov approximation to investigate the detection probability of orbital angular momentum (OAM) in multi-Gaussian correlated anomalous vortex (MGCAV) beams under non-Kolmogorov maritime atmospheric turbulence. Our results demonstrate that the OAM detection probability of a MGCAV beam is influenced by various factors, including beam parameters and the characteristics of maritime atmospheric turbulence. Specifically, an increase in propagation distance, beam order, and beam index, or a decrease in inner scale, spatial coherence width, and non-Kolmogorov parameter, leads to a decrease in the OAM detection probability. The phase characteristics of partially coherent vortex modes are affected by both atmospheric turbulence phase and initial random phase, resulting in reduced robustness compared to fully coherent vortex modes. Furthermore, a comparative analysis between Gaussian–Schell correlated anomalous vortex (GSCAV) beams and MGCAV beams reveals the superior resilience of GSCAV beams in mitigating the impact of maritime atmospheric turbulence. Moreover, specific combinations of beam order, topological charge, and beam waist, or the optimal beam width, yield maximum OAM detection probability or minimum scintillation. These findings provide valuable insights applicable to optical communication, particularly in scenarios above sea and ocean levels.

Modulating dual-wavelength optical fiber vector solitons

Optical fiber vector solitons have potential applications in the field of high-capacity optical fiber communications and have been widely explored in recent years. Here, we theoretically modulate dual-wavelength optical fiber vector solitons in an optical fiber system at a wavelength regime of 1 μm while considering the influence of group velocity dispersion. When the input dual-wavelength optical fiber vector solitons have the same two central wavelengths of 1057 nm and 1063 nm in orthogonal directions, the output modulated optical fiber vector solitons’ pulse shapes and optical spectra will maintain their peak intensities upon the change of the projection angle. When the two orthogonal central wavelengths of the input dual-wavelength optical fiber vector solitons are slightly different (1056 nm and 1062 nm in one polarization direction, 1058 nm and 1064 nm in the other direction), dual-peak pulse shapes appear and are accompanied by different wavelength peak intensities when the propagation distance increases. Our simulation results examine the out-cavity modulation of dual-wavelength optical fiber vector solitons and can be expanded to multi-wavelength optical fiber vector solitons’ modulation.

Ultrasonic visualization and quantitative analysis of internal defects in RTV coatings

Room temperature vulcanised (RTV) silicone rubber coatings effectively enhance the insulation properties of electrical equipment. However, RTV coatings are prone to internal defects caused by the coating process and the effects of aging during service, which can lead to debonding of the coatings. Internal debonding defects are challenging to detect and can ultimately lead to accidents due to a reduction in the insulation capacity of the equipment. To visualize the internal defect morphology of RTV coatings and quantify the defect size, an ultrasonic pulse-echo-based method for detecting and imaging debonding defects is proposed. The method involves the development of a finite element model to investigate how ultrasonic waves propagate in RTV coatings and the influence of ultrasonic probes and inspection conditions on defect echoes. Furthermore, an ultrasonic detection system specifically designed for RTV coating debonding defects is constructed. This system utilizes wavelet packets in the time-frequency domain to analyze the echo signals in both normal and defective regions. The three-dimensional reconstruction of the debonding defect morphology is accomplished by integrating ultrasonic echo amplitude and position information. Finally, the size of the debonding defects is quantified using an adaptive threshold segmentation method. The findings indicate that ultrasound waves reflected in RTV materials propagate as spherical waves, with the acoustic energy primarily concentrated near the acoustic axis. As the propagation distance increases, the sound beam disperses along the axis and extends beyond the transducer, resulting in a decrease in the sound field's directionality. The developed visual reconstruction method in this study offers the capability of three-dimensional visualization for defects present within RTV coatings, including their length, width, and depth. The accurate determination of defect size is achieved through the utilization of the adaptive threshold segmentation method, yielding an average error rate of 5.7 % across different defect types. In comparison, the maximal interclass variance method (OTSU) and the fuzzy C-means (FCM) method produced results with error rates of 9.8 % and 7.9 %, respectively. The research presented in this paper enables precise assessment of debonding defect severity and establishes a reliable foundation for on-site inspection, operation, and maintenance of RTV coatings.

Physics-informed deep filtering of ultrasonic guided waves for incipient defect inspection of large-scale square tube structures

For large-scale structure inspection, the traditional ultrasonic inspection methods are time-consuming and inefficient. Recently, many approaches based on ultrasonic guided waves have been developed. However, during the propagation of guided waves, as the propagation distance increases, the signal amplitude attenuates gradually, as a result, the reflected wave packets from the incipient defects are always seriously contaminated or even buried in noise, and the defect signatures cannot be effectively identified in this circumstance. To detect incipient defect as early as possible to ensure structure safety, it is very essential to suppress the background noise and enhance the reflected wave packet from defect. To address this issue, a physics-informed deep filtering method is proposed for ultrasonic guided waves enhancement. In the proposed method, a deep learning model is constructed and trained based on template signals of ultrasonic guided waves simulated by digital model with different parameters, and then generalized to enhancement of weak ultrasonic guided waves from practical cases. The simulation test and experimental results on large-scale square tube structures indicate that the proposed ultrasonic guided waves enhancement method is effective for early-stage defect inspection and outperforms the traditional guided waves enhancement approaches.

The effective beam width of a partially coherent rectangular mul-ti-Gaussian Schell-model vortex beam propagating in a turbulent plasma

Based on the derived analytical formulae for the optical intensity and effective beam width of a partially coherent rectangular multi-Gaussian Schell-model (RMGSM) vortex beam, we analyzed the evolution of the effective beam width of a partially coherent RMGSM vortex beam propagating in an anisotropic turbulent plasma using numerical examples. The numerical results demonstrated that the effective beam widths in the x and y directions were similar. In studies on the influences of the source parameters, one can see that an RMGSM vortex beam with a larger topological charge and beam waist or a smaller correlation coefficient σ evolves into a beam with a larger effective beam width. However, the influence of beam order was not evident. In addition, numerical examples proved that the effective beam width increases with increasing propagation distance and refractive index fluctuation variance or decreasing anisotropy parameter , outer scale , and inner scale . This study's results will be beneficial for applications in free-space optical communications.

Propagation Characteristics of a Partially Coherent Gaussian Schell‐model Array Vortex Beam in the Joint Turbulence Effect of a Jet Engine and Atmosphere

AbstractThis work investigates the joint effects of jet engine exhaust‐induced turbulence and atmospheric turbulence on the propagation of a partially coherent Gaussian Schell‐model Array (GSMA) vortex beam. Using the two‐process propagation method, analytical formulae are derived for the cross‐spectral density, spectral density, degree of coherence, and beam width of the considered beam. The results show that the considered beam takes different shapes; when the spatial coherence is large, the spectral density of the GSMA vortex beam takes an elliptical shape, whereas when the spatial coherence is smaller, the spectral density remains a Gaussian shape. The evolution profile of the degree of coherence weakens gradually when the propagation distance, topological charge, and turbulence strength increase. Moreover, the profile of the degree of coherence takes the Gaussian profile when the propagation distance is longer or turbulence atmospheric is stronger. Furthermore, the results reveal that the corresponding beam spreads faster with a larger propagation distance, lower spatial coherence, and high‐strength turbulence. This study also concludes from the results that the beam is affected more when its propagation is near the jet engine exhaust, which means that this latter has a significant impact.

Experimental investigation of the evolution and energy transmission of a type-a internal solitary wave packet over a gentle slope

Laboratory experiments were conducted to investigate the evolution characteristics of a type-a ISW packet propagating over a gentle slope. The elevation type-a ISW packet was generated by the gravity collapse of stratified density fluid, and its transformation process over the gentle slope was measured by use of the multichannel conductivity-probe arrays. The energy transmission and loss of the ISW packets during its evolution over the gentle slope were further analyzed. The experimental results showed that the evolution and energy transmission of the type-a ISW packet over the gentle slope is closely related to the amplitude of the leading wave in the wave packet. The conversion of the wave packet type is caused by the variations in amplitudes of the leading and tailing waves in the type-a ISW packet due to the topography effect. The distance between the adjacent waves in a type-a ISW packet gradually increases with the increasing propagation distance on the slope, as well as its increase rate is also enlarged. The wave speeds of both the leading and tailing waves always decrease with the propagation distance. The energy loss of the whole ISW packet increases with both the incident wave amplitude and the propagation distance.

Second-order coherence and polarization characteristics of partially coherent radially polarized cosh-Gaussian vortex beams in a turbulent atmosphere

The second-order coherence and polarization characteristics of partially coherent radially polarized cosh-Gaussian vortex (PCRPChGV) beams in atmospheric turbulence are investigated based on the extended Huygens–Fresnel principle. The analytical propagation formulas of cross-spectral density matrix, average intensity and degree of polarization (DOP) for PCRPChGV beams in turbulence are derived. The influences of initial correlation length, cosh factor and turbulence intensity on average intensity, on-axis intensity and DOP of PCRPChGV beams are examined numerically. Results show that with increasing propagation distance, the PCRPChGV beam evolves from the initially separated double-lobe spot into overlapping double-lobe spot, and Gaussian spot. The corresponding intensities of the two Cartesian components of light field rotate markedly around the beam axis in a unique manner. Initial correlation length and cosh factor have an obvious influence on the position and size of normalized on-axis intensity peak, respectively. In addition, PCRPChGV beams in moderate turbulence have strong anti-turbulence ability.

Roadways fracture response under varying dominant blasting frequencies and lateral pressure coefficients

Dominant frequency of blasting waveform attenuates with increasing propagation distance, leading to diverse diffractive effects on a roadway located in different sublevels. To investigate the roadway fracture characteristics in response to varying blasting wave frequencies and geo-stresses. Numerical simulation and experiments were performed by using discrete element methods and acoustic emission (AE) techniques, respectively. Roadway response differences in terms of microcracks propagation regions were primarily investigated, to reveal the relationships between damage-prone regions and the lateral pressure coefficient and blast waveform frequencies. The results showed that the excavation damage zone (EDZ) and the types of microcracks developed are primarily influenced by the principal stress direction. Low-frequencies blast waves contribute significantly to the overall damage to the surrounding rock around the roadway, while high-frequencies blast waves mainly damage the sides of the facing blast. Under the coupling effect of principal stress and blast wave, microcracks are more prone to generate in regions coupling with high-stress concentration regions induced by principal stress and disturbed regions diffracted by blast waves. With the results from this study, the roadway support should consider the difference in damage-prone regions triggered by blasting waves with varying dominant frequencies under different lateral pressure coefficients.