Conical Wave Research Articles

Tunable generator of radially and azimuthally polarized vortex Bessel beams based on the interference polarizer

The new optical system to generate azimuthally and radially polarized Bessel laser beams is proposed. The system is based on the transformation of the conical wave fronts when passing through interference polarizing plate. The polarization control characteristics occur when forming the beam by changing the divergence of the beam incident on the diffractive axicon. The radially and azimuthally polarized Bessel beams using binary phase axicon are experimentally obtained.

Coherence of Pseudo-Bessel Beams in a Turbulent Atmosphere

Coherent properties of nondiffracting pseudo-Bessel optical beams propagating in a turbulent atmosphere are studied theoretically. The solution of the equation formulated based on the paraxial approximation of the scalar wave equation for the second-order transverse mutual coherence function of the optical radiation field is analyzed. The behavior of the modulus and phase of the complex degree of coherence, coherence radius, and integral scale of the coherence degree of the Bessel-Gaussian optical beam and conical optical wave obtained by cone focusing of the optical beam by an axicon is studied as a function of optical beam parameters and characteristics of a turbulent atmosphere. Significant qualitative and quantitative distinctions between the studied coherence characteristics for cases of a Bessel-Gaussian optical beam and conical optical wave have been revealed. In general, under similar conditions of propagation in a turbulent atmosphere, the coherence of a conical optical wave is higher than that of a Bessel-Gaussian optical beam.

Shaping Bessel beams using source-integrated folded reflectarray.

This Letter proposes a highly integrated reflection-type approach to generate non-diffracting Bessel beams using a source-integrated folded reflectarray antenna (FRA). The FRA not only transforms spherical waves radiated by the feeding source to conical waves but also directly produces Bessel beams by specifying the phase distribution of reflectarray elements. To validate this approach, a millimeter-wave zero-order Bessel beam generator and a conventional collimated beam structure are analyzed, designed, fabricated, and measured. Good agreement among theoretical analysis, full-wave simulation, and experimental results demonstrate that the proposed FRA successfully generates non-diffracting Bessel beams and, moreover, the reflectarray elements are fully integrated with the feeding source in the same low-cost single-layered printed circuit board.

A Shape Optimization Method of a Specified Point Absorber Wave Energy Converter for the South China Sea

In this paper, a shape optimization method of a truncated conical point absorber wave energy converter is proposed. This method converts the wave energy absorption efficiency into the matching problem between the wave spectrum of the South China Sea and the buoy’s absorption power spectrum. An objective function which combines these two spectra is established to reflect the energy absorbing efficiency. By applying Taguchi design, the frequency domain hydrodynamic analysis and the response surface method (RSM), the radius, cone angle and draft of the buoy are optimized. Since the significant influence of power take-off system (PTO) on energy absorption, the optimal PTO damping under random wave conditions is also studied. The optimal shape is acquired by maximizing the energy absorbing efficiency. Four types of performance and the influence of each geometrical parameter are also obtained. In addition, the cause of the trend of performance as well as the effects of adjusting the input parameters are analyzed. This study can provide guidance for the shape optimization of multi-parameter buoys.

VUV Radiation of Xenon in a Fast Conical Discharge

We have studied the spectra of xenon in the range 10–17 nm emitted in a fast conical discharge (dI/dt ~ 1012 A/s). Cumulation of a converging conical shock wave leads to the appearance of a hot plasma region effectively moving along the axis of the discharge. Under conditions of a limited spectral resolution (λ/δλ ~ 150), we have identified main observed lines. Our analysis of the spectra has shown that the plasma contains Xe XI ions, which are not observed in the case of a discharge with a cylindrical shock wave (a capillary discharge) occurring in the same discharge circuit. We have studied the time behavior of the spectra with a resolution of 35 ns. It has been shown that a hot plasma region exists during the first 150 ns of the discharge, then the plasma is subjected to the action of matter eroded from the inner wall of the discharge cavity.

Coherence of a Bessel beam and a conic wave in turbulent atmosphere

Coherent properties of diffraction-free Bessel-like optical beams propagating in the turbulent atmosphere are theoretically studied. An analytical solution of the equation for the second-order transverse function of mutual coherence of the optical radiation field, which was obtained from paraxial approximation of the scalar wave equation, is analyzed. The behavior of the degree of coherence, coherence radius, and integral scale of the degree of coherence of the Bessel–Gaussian beam and conic wave obtained through conic focusing of a Gaussian beam by an axicon is analyzed for different beam parameters and characteristics of the turbulent atmosphere. Significant qualitative and quantitative differences are discovered between the studied coherence characteristics of the Bessel–Gaussian beam and the conic wave. In general, under identical propagation conditions in the turbulent atmosphere, the coherence of the conic wave is higher than that of the Bessel–Gaussian beam. The comparison of two spatial scales of the degree of coherence of optical beams reveals that the integral scale for diffraction-free beams is a more representative characteristic than the coherence radius. The integral scale of the degree of coherence of diffraction-free beams is more unequivocally connected with the conditions of propagation of optical radiation in the turbulent atmosphere.

Are Bessel beams resilient to aberrations and turbulence?

It is understood from the conical wave picture that Bessel beams may self-heal after certain opaque obstructions, but the extrapolation to transparent phase screens is not self-evident. Here we consider the propagation of Bessel beams through aberrated obstacles and show that the self-healing is not guaranteed, but rather a function of the severity of the aberration. Paradoxically, we explain why strong aberrations may show self-healing while weak aberrations will not, and highlight the parameters that influence this. Finally, we combine aberrations to pass the Bessel beam through turbulence, and debunk the myth that Bessel beams are resilient to such perturbations.

Narrow band vacuum ultraviolet radiation, produced by fast conical discharge

The article presents the experimental study of discharges in a conical cavity, filled with Ar at pressure 80 Pa. The electrical current driver (inductive storage with plasma erosion opening switch) supplies to the load electrical current pulse with growth rate about 1012 A s−1 and maximal value 30–40 kA. The convergent conical shock wave starts from the inner surface of the discharge cavity and collapses in ‘zippering’ mode. The pin hole camera imaging with MCP detector (time resolution 5 ns) have demonstrated the appearance of effectively fast moving compact plasma with visible velocity v = (1.5 ± 0.14) × 107 cm s−1. Plasma emits narrow band radiation in the spectral range of Rydberg series transitions of Ar VII, Ar VIII with quantum number up to n = 9 (wavelength about 11 nm). The intensity of radiation is comparable with the total plasma emission in the range 10–50 nm. Charge exchange between multiply charged Ar ions and cold Ar atoms of working gas is proposed as the possible mechanism of the origin of the radiation.

Analytical expressions for the borehole seismic signatures excited by an explosive source buried in an elastic half‐space

ABSTRACTBorehole guided waves that are excited by explosive sources outside of the borehole are important for interpreting borehole seismic surveys and for rock property inversion workflows. Borehole seismograms are typically modelled using numerical methods of wave propagation. In order to benchmark such numerical algorithms and partially to interpret the results of modelling, an analytical methodology is presented here to compute synthetic seismograms. The specific setup is a wavefield emanating from a monopole point source embedded within a homogeneous elastic medium that interacts with a fluid‐filled borehole and a free surface. The methodology assumes that the wavelength of the seismic signal is much larger than the borehole radius. In this paper, it is supposed that there is no poroelastic coupling between the formation and the borehole.The total wavefield solution consists of P, PP, and PS body waves; the surface Rayleigh wave; and the low‐frequency guided Stoneley wave (often referred as the tube wave) within the borehole. In its turn, the tube wave consists of the partial responses generated by the incident P‐wave and the reflected PP and PS body waves at the borehole mouth and by the Rayleigh wave, as well as the Stoneley wave eigenmode. The Mach tube wave, which is a conic tube wave, additionally appears in the Mach cone in a slow formation with the tube‐wave velocity greater than the shear one. The conditions of appearance of the Mach wave in a slow formation are formulated. It is shown that the amplitude of the Mach tube wave strongly depends on Poisson's ratio of the slow surrounding formation. The amplitude of the Mach tube wave exponentially decreases when the source depth grows for weakly compressible elastic media with Poisson's ratio close to 0.5 (i.e., saturated clays and saturated clay soils). Asymptotic expressions are also provided to compute the wavefield amplitudes for different combinations of source depth and source‐well offset. These expressions allow an approximate solution of the wavefield to be computed much faster (within several seconds) than directly computing the implicit integrals arising from the analytical formulation.

Conversion of a conical wave with circular polarization into a vortex cylindrically polarized beam in a metal waveguide

In this paper, we have developed a mathematical base for describing the propagation of waves in a metal waveguide in a cylindrical coordinate system. The transformation of a conical wave with circular polarization into a cylindrically polarized vortex beam is shown on the basis of the expansion of the field in vector cylindrical modes. The results of modeling, based on the expansion in plane waves, qualitatively agree with theoretical calculations.

Cone-angle tunable second-harmonic generation at the edge of a cube uniaxial nonlinear crystal

A simple method to generate second-harmonic conical waves with tunable cone angles (apex angles) only using a cube uniaxial nonlinear crystal is proposed. The formation mechanism is based on the birefringence phase-matching among a plane fundamental wave, a spherical fundamental wave, and a conical second-harmonic wave. The cone angle in our demonstration of the method could be continuously adjusted from 0 to 30° by just rotating the crystal, which is the largest reported tunable range. Moreover, a tunable range of 0–68° could be achieved by using several crystals with different cut angles. The polarization and the longitudinal distribution of the generated second-harmonic wave are theoretically analyzed. The measured conversion efficiency was around 7%, and it decreased with the increasing of the cone angle.

Axis retrieval of a supersonic source in a reverberant space using time reversal

Localizing the axis of the Mach cone created by the supersonic displacement of a bullet in a reverberant environment is a challenging task, not only because of the high velocity of the moving source, but also because of the multiple wave reflections off of the walls. Although time reversal (TR) techniques allow static acoustic source localization in a reverberant space, they have not been explored yet on non stationary waves caused by supersonic displacements in urban canyons. The acoustic wave produced by a supersonic projectile has a conical wavefront and a N-shaped acoustic pressure signature. In this paper, this acoustic wave is reproduced using a line array of point-like sources (simulations) and loudspeakers (experiments). During the propagation of this conical wave in an urban canyon, the resulting pressure signals are measured using a time reversal array flush mounted into the ground. These acoustic signals allow to automatically retrieve with a high accuracy the location of the Mach cone axis using time reversal techniques. This inverse problem is solved using the maximization of a fourth-order statistical criterion of the backpropagated pressures. This criterion allows to estimate the intersections between the Mach cone axis and several vertical planes in the urban canyon. These estimations are then fitted to a 3D trajectory with a robust three dimensional interpolation technique based on the Random Sample Consensus (RANSAC) algorithm. This method allows to automatically retrieve the axis of the supersonic source with an angular accuracy of less than 0.5° and a misdistance of 0.5cm for both numerical simulations and experimental measurements.

Hypersonic dynamic stability of wave riders

PurposeThe purpose of this paper is to obtain close form expressions for the dynamic stability of conical wave riders with flat surfaces which could be equipped with lifting surfaces on its plain flat surface. Numerical simulation would require very large meshes to resolve flows at subscale level and the experimental evaluations would be equally difficult, requiring expensive measurement facilities with challenging procedures to secure such vehicles in confined test sections to obtain satisfactory wind on and wind off oscillations.Design/methodology/approachThe design method uses appropriate pressure fields using small disturbance theory, which, in turn, is perturbed using the unsteady shock expansion theory to recover suitable expressions for the dynamic stability behaviour.FindingsIt was observed that the dynamic stability of the standard half-cone-type wave riders with flat upper surfaces deteriorates with the axis position measured from the pointed apex reaching a minimum at around x/co = 0.666. The half-cone wave rider with flat upper surfaces is dynamically less stable than a pure cone.Research limitations/implicationsThe method is typically less accurate when the similarity parameter M∞θ ≤ 1 = 1 or if the angle of attack is not small.Practical implicationsWith renewed interest in hypersonic, future hypersonically would be designed as fast lifting bodies whose shapes would be close to the configurations of hypersonic wave riders, especially if they are designed to operate at upper atmosphere altitudes.Originality/valueThe analytic approach outlined in this paper for the evaluation of dynamic and static stability derivatives is original, drawing from the strengths of the small disturbance theory and shock expansion techniques. The method is particularly important, as there are no reported theoretical, numerical or experimental results in the literature.

Passive sensing of head wave propagation in the ambient noise field and its implications for geoacoustic inversion

Under certain conditions, the ambient noise field can produce and amplify head waves with unique propagation characteristics; these are detectable with a vertical line array and can be analyzed to extract geoacoustic information. Head wave phenomenon in the seabed can be observed using point sources, but these arrivals are usually very weak and difficult to detect ahead of the strong direct-path arrival. In contrast, surface-generated noise from wind and breaking waves are effectively a planar source that insonify the entire seabed at every angle and from every direction. Ambient noise head waves are conical waves that are generated when noise first reaches the seabed at the critical angle, and are amplified upon each subsequent interaction with the seabed. This interaction involves splitting the incident wave into water-borne and seabed-borne components, each separated in time by a fixed lag. A vertical line array can observe this phenomenon by cross-correlating beams steered upward and downward at the c...

Experimental Research of Dynamics and Macrostructure of Light Erosion Radiative Plasmodynamic Discharges

The investigations of dynamics and macrostructure (spatial and temporal distributions of charged particles) of optic-plasmadynamic discharges with light erosion mechanism of plasma formation and generation of accelerated gas–plasma flows of various chemical composition in the wide range of dynamic and energy & power characteristics are presented. The plasma was heated as a result of the thermalization due to shock interaction of high kinetic energy plasma flows with a gas medium, which works as a stopping barrier for such discharges. It was shown that the deceleration of the radially inhomogeneous flow at a deformable gas barrier has an essentially two-dimensional nature: a complex system of conical shock wave forms in a hypersonic plasma flow.

Near-infrared conical emission from 800 nm filament in air

We used a single shot imaging technique to register frequency-angular distribution at different positions along the filament of 55 fs 800 nm pulse focused with the numerical aperture of about 1/600 into air. We have revealed that the phenomena accompanying filamentation develop successively in the propagation direction. The conical emission in the visible comes first. Later in the propagation the conical emission in the infrared appears and develops down to 900 nm at least. Multiple robust light bullets become the secondary sources of the conical waves on the infrared side of the spectrum. The overall scenario of the nonlinear phenomena successive appearance in air is for the first time unified in one experiment in atmospheric gases and simulated with the carrier wave resolved in agreement with the experiment.

Overdriven Detonation and Its Application in Shaped Charges

Overdriven detonation (ODD) in high explosives can be generated by Mach reflection of conical detonation waves propagating quasi-steadily in a co-axial double layer cylindrical charge. The inner core of the charge consists of lower detonation velocity explosive with higher detonation velocity explosive for the outer core. The calculated pressures and detonation velocities in the ODD regime are compared with available results in the literature. The application of this technique to design a double layer shaped charge (DLSC) is numerically studied. It was discovered that the use of lower density-lower detonation velocity explosive in the inner core of DLSC can also yield similar results to those obtained with high density lower detonation velocity explosive. By analyzing previous experimental results and comparing with present simulations, it is demonstrated that ordinary shaped charges have some advantages over DLSC under certain conditions.

A dielectric rod antenna driven by a pulsed power system

A dielectric rod antenna was constructed for stimulating biological tissues. The antenna consists of three sections (a conical wave launcher, a cylindrical waveguide, and a conical emitting section). The simulation of the antenna in free space was confirmed with measurements of the return loss (S11) and the transient electric field. To reach the electric field threshold for tissue stimulation, a high voltage pulsed power system was designed and constructed to drive the antenna. Central to the pulsed power system is a transformer and an oil peaking switch. The transformer is an ultrafast, dual resonance pulse transformer with a linearly integrated primary (LIP) stage. It is capable of generating pulses with rise time <50 ns. The load for the transformer was an oil-filled transmission line that was connected to the antenna. An oil peaking switch was used to switch the line to produce high voltage picosecond transients. The electric field was measured in the near field at a distance of 4 cm. A 500 ps pulse with an amplitude of 600 V/cm was recorded for a 25 kV input from the pulse transformer. These results demonstrate the feasibility of the system and so future work will be focused on increasing the voltage and the electric field in order to stimulate biological tissues.

Spatiotemporal dynamics of underwater conical shock wave focusing

The paper presents an experimental study on spatiotemporal dynamics of conical shock waves focusing in water. A multichannel pulsed electrohydraulic discharge source with a cylindrical ceramic-coated electrode was used. Time-resolved visualizations revealed that cylindrical pressure waves were focused to produce conical shock wave reflection over the axis of symmetry in water. Positive and negative pressures of 372 MPa and $$-17$$ MPa at the focus with 0.48 mm lateral and 22 mm axial extension ( $$-6$$ dB) were measured by a fiber-optic probe hydrophone. The results clearly show the propagation process leading to the high-intensity underwater shock wave. Such strong and sharp shock wave focusing offers better localization for extracorporeal lithotripsy or other non-invasive medical shock wave procedures.

Filamentation-induced spectral broadening and pulse shortening of infrared pulses in Tellurite glass

Filamentation of infrared femtosecond pulses in Tellurite glass is reported, leading to the generation of a supercontinuum generation spanning from the visible up to 4μm. The angular distribution of the supercontinuum shows clear evidence of conical waves generation, in particular, in the visible region. Moreover, taking advantage of the spatio-temporal self-focusing effect occurring in the Tellurite glass, a twofold pulse shortening is demonstrated. Tellurite glass appears as a very convenient, versatile and promising medium for femtosecond nonlinear optics in the infrared region.