共查询到20条相似文献,搜索用时 15 毫秒
1.
J Tavakkoli D Cathignol R Souchon O A Sapozhnikov 《The Journal of the Acoustical Society of America》1998,104(4):2061-2072
A time-domain numerical model is presented for simulating the finite-amplitude focused acoustic pulse propagation in a dissipative and nonlinear medium with a symmetrical source geometry. In this method, the main effects responsible in finite-amplitude wave propagation, i.e., diffraction, nonlinearity, and absorption, are taken into account. These effects are treated independently using the method of fractional steps with a second-order operator-splitting algorithm. In this method, the acoustic beam propagates, plane-by-plane, from the surface of a highly focused radiator up to its focus. The results of calculations in an ideal (linear and nondissipative) medium show the validity of the model for simulating the effect of diffraction in highly focused pulse propagation. For real media, very good agreement was obtained in the shape of the theoretical and experimental pressure-time waveforms. A discrepancy in the amplitudes was observed with a maximum of around 20%, which can be explained by existing sources of error in our measurements and on the assumptions inherent in our theoretical model. The model has certain advantages over other time-domain methods previously reported in that it: (1) allows for arbitrary absorption and dispersion, and (2) makes use of full diffraction formulation. The latter point is particularly important for studying intense sources with high focusing gains. 相似文献
2.
有限振幅波的三维时域建模 总被引:2,自引:0,他引:2
通过对二维算法(Texas code)做扩展,建立了求解扩展版KZK(Khokhlov-Zabolotskaya-Kuznetsov)方程的三维时域有限差分算法。该算法先将KZK方程变换成TBE(Transformed Beam Equation),然后依次求解衍射(抛物线近似条件下)、热黏滞吸收、弛豫和非线性作用。数值仿真了圆形、矩形和方形阵列的非线性声场,与前人的结果相符,证明了算法的有效性,并分析了把弛豫吸收系数当成热黏滞吸收系数引入空气中的参量阵的模拟会带来的误差。 相似文献
3.
A three-dimensional time-domain algorithm,which is based on the augmented KZK (Khokhlov-Zabolotskaya-Kuznetsov) equation,is proposed to simulate the nonlinear field of the parametric array.First,KZK equation is transformed into TBE(Transformed beam equation). Then,the effects of diffraction(in parabolic approximation),thermoviscous absorption,relaxation, and nonlinearity are solved with finite difference methods.The numerical results of this code agree well with the theoretical and experimental results presented in previous studies, which demonstrates the validity of the three-dimensional algorithm.Using this code to calculate the nonlinear field of the parametric array in air,it is found that the small time interval is important to the accuracy of the simulation results of the difference frequency wave in the case of high sound pressure level,and the errors caused by taking relaxation absorption for thermoviscous absorption are influenced by the characteristic frequency. 相似文献
4.
Due to the lens-induced quadratic phase factor from converging lens, the beam-propagation method (BPM) is computationally expensive in simulations by the sampling theorem for lens-induced strongly focused beams propagating in nonlinear media. In the paper, we present a modified approach (MA) based on the separate envelope and lens-induced phase to only compute the envelope in the whole focal region. Then we demonstrate the correctness of MA analytically by the propagation of focused Gaussian beam and numerically by that of focused Super-Gaussian beam in linear media. Numerical simulations of strongly focused beams based on MA are in good agreement with those of BPM when propagating in Kerr media. The sampling requirement of MA which is similar with no lens-induced phase beam is lower than that of BPM which increases with the lens-induced chirp parameter and the propagation distance. Thus the computational efficiency can be greatly improved. 相似文献
5.
We demonstrate that a femtosecond laser pulse strongly focused in air can produce a highly symmetric damage pattern on glass. This damage pattern contains a series of near-perfect radial rings, with diameters much larger than the predicted focal spot diameter. These rings disappear when the experiment is conducted in vacuum, indicating atmospheric involvement. Surprisingly, the shape and size of the rings seem to be nearly independent of the shape of the generating laser beam, showing dramatic spatial mode cleaning. A “half moon” initial laser mode created by obscuring one side of the round beam produces rings of similar quality to those obtained with the unclipped beam. While spatial mode cleaning has previously been reported in filaments, this is the most dramatic demonstration of the effect that we are aware of. We argue that the effect is due primarily to ionization, in contrast to studies in longer filaments that attribute it to self-focusing. 相似文献
6.
Petrov NI 《Optics letters》2004,29(5):421-423
The reflection and transmission of tightly focused azimuthally and linearly polarized electromagnetic wave beams with subwavelength spot size at a dielectric interface are investigated. A substantial increase of the reflectance of a light beam that is normally incident from a higher-index medium to a lower-index medium and a decrease of the reflectance of a beam that is incident from a lower-index medium to a higher-index dielectric medium in comparison with the Fresnel reflectance are predicted. 相似文献
7.
Aleksandr Ya. Bekshaev 《Central European Journal of Physics》2010,8(6):947-960
A ray-optics model is proposed to describe the vector beam transformation in a strongly focusing optical system. In contrast
to usual approaches based on the focused field distribution near the focal plane, we use the beam pattern formed immediately
after the exit aperture. In this cross section, details of the output field distribution are of minor physical interest but
proper allowance is made for transformation of the beam polarization state. This enables the spin and orbital angular momentum
representations to be obtained, which are valid for any cross section of the transformed beam. Simple analytical results are
available for a transversely homogeneous, circularly polarized incident beam confined by a circular aperture. Variations of
the spin and orbital angular momenta of the output beam with change of the focusing strength are analyzed. The analytical
results are in good qualitative and reasonable quantitative agreement with the results of numerical calculations performed
for the Gaussian and Laguerre-Gaussian beams. The model supplies an efficient and physically transparent means for qualitative
analysis of the spin-to-orbital angular momentum conversion. It can be generalized to incident beams with complex spatial
and polarization structure. 相似文献
8.
A novel method is proposed for non-contact measurement of the distribution of electrostatic charge on a surface based on scanning the sample surface with a focused high frequency acoustic beam to excite movement of the sample surface. An electric field is induced by exciting a charged film-like object, and an electric field sensor measures it instead of an electrostatic field. The focused ultrasound waves are generated by controlling individually the phase of each 285 airborne ultrasound transducers. 相似文献
9.
10.
V. E. Lobanov A. P. Sukhorukov 《Bulletin of the Russian Academy of Sciences: Physics》2008,72(1):10-12
The effect of parametric reflection of a signal beam from a focused reference beam has been investigated. It is shown that focusing reduces a pump power reflection threshold but causes certain distortions due to the reduction of the area of the parametric mirror and occurrence of its longitudinal curvature. These effects limit the width and angle of incidence of the signal beam. To decrease the sum wave power, it is proposed to introduce an additional dispersion mismatch. 相似文献
11.
Yunkyu Bang 《Physica C: Superconductivity and its Applications》1999,320(3-4):233-238
Assuming a phenomenological self-energy ImΣ(ω)|ω|β, (β=1), which becomes gapped below Tc, we derived a new gap equation. The new gap equation contains the effect of the kinetic energy gain upon developing a superconducting order parameter. However, this new kinetic energy gain mechanism works only for a repulsive pairing potential leading to a s-wave state. In this case, compared to the usual potential energy gain in the superconducting state as in the BCS gap equation, the kinetic energy gain is more effective to easily achieve a high critical temperature Tc, since it is naturally Fermi energy scale. In view of the experimental evidences of a d-wave pairing state in the hole-doped copper-oxide high-Tc superconductors, we discuss the implications of our results. 相似文献
12.
We describe the reflection of a strongly focused beam from an interface between two dielectric media. If the beam is incident from the optically denser medium, the image generated by the reflected light is strongly aberrated. This situation is encountered in high-resolution confocal microscopy and data sampling based on solid immersion lenses and oil immersion objectives. The origin of the observed aberrations lies in the nature of total internal reflection, for which there is a phase shift between incident and reflected waves. This phase shift displaces the apparent reflection point beyond the interface, similarly to the Goos-H?nchen shift. 相似文献
13.
14.
Rodriguez JP 《Physical review letters》1994,73(12):1675-1678
15.
Space-charge limiting currents for magnetically focused intense relativistic electron beams
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The self-consistent differential equations, which describe a
laminar-flow equilibrium state in a magnetically focused intense relativistic
electron beam propagating inside a conducting waveguide, are presented. The
canonical angular momentum, $p_{\theta }$, defined under the conditions at the
source, uniquely determines the possible solutions of these equations. By
numerically solving these equations, the space-charge limited current and
the externally applied magnetic field are obtained in a solid beam
and a hollow beam in two cases of $p_{\theta }=0$ (magnetically shielded source)
and $p_{\theta }=$~const. (immersed source) separately. It is shown that the hollow beam
is more beneficial to the propagation of the intense relativistic beam
through a drift tube than the solid beam. 相似文献
16.
A thin-film line array of thermoelectric detectors is applied for the measurement of intensity distribution of focused laser beams. The damage threshold of these detectors is relatively high. They can tolerate 50 μsec pulses with intensities as high as 3·105 W/cm2. High resolutions can be achieved by fabricating line arrays having a large number of thermoelectric junctions with distances as small as 5 μm between centers. 相似文献
17.
It is known that under certain circumstances the point of maximum intensity in a focused aberration-free wave is not at the geometrical focus, but is closer to the focusing lens. In the present note such a focal shift is analyzed for the case of a gaussian beam, focused by a thin lens, which fills a circular aperture of any prescribed radius, the waist of the beam being assumed to be located in the aperture plane. Many previously known results follow as limiting cases of our analysis. 相似文献
18.
详细研究了贝塞尔-高斯光束被无光阑透镜聚焦后的焦开关效应。推导出了轴上最大光强位置满足的一元三次方程和相对跃迁量公式,并用数值计算对理论结果作了说明。研究表明,当光束参数大于2时,聚焦贝塞尔-高斯光束会有焦开关现象出现。相对跃迁量随着光束参数的增大而增大,轴上归一化光强最小值随着光束参数的增大而减小,并且当光束参数大于3.9时,轴上归一化光强最小值为零。 相似文献
19.
Focal shift in focused off-axial Hermite-cosh-Gaussian beams 总被引:1,自引:0,他引:1
Based on the encircled-power criterion, the focal shift of off-axial Hermite-cosh-Gaussian beams passing through an apertured lens is studied and illustrated with detailed numerical examples. The dependence of focal shift on the beam and optical system parameters is discussed. 相似文献
20.
A closed-form propagation equation of Hermite-cosh-Gaussian beams passing through an unapertured thin lens is derived. Focal shifts are analyzed by means of two different methods according to the facts that the axial intensity of some focused Hermite-cosh-Gaussian beams are null and that of some others are not null but the principal maximum intensity may be located on the axis or off the axis. Optimal focusing for the beams is studied, and the condition of optimal focusing ensuring the smallest beam width is also given. 相似文献