小孔干涉仪滤波孔的研究

用惠更斯-基尔霍夫公式对小孔干涉仪的滤波孔进行计算及分析后指出:尽管滤波孔小于爱里斑,滤波孔的位置对参考波的质量仍有影响。在接收面上参考波位相有较大偏移处,往往伴有较大的光强变化(变弱)。给出实验结果,并进行了讨论。     

Evaluation of ultrasonic attenuation without invoking the diffraction correction separately

A method is presented for the ultrasonic attenuation measurement as a function of frequency using a broad band transducer. The diffraction correction has been introduced in the expression such that an algorithm may be developed for evaluation of attenuation directly. This eliminates the introduction of the diffraction correction curve separately. Results are presented for various types of aluminium alloys. The method is suitable for samples placed a distance further than three times the near field and which have a size much larger than the transducer.     

Simulation of the diffraction characteristics of a one-dimensional dual-periodicity superlattice

The diffraction characteristics of a dual-periodicity superlattice—a diffraction grating whose unit cell contains two different regions, one homogeneous and the other periodically nonhomogeneous—were numerically simulated. After choosing the second region to have sinusoidally varying permittivity, the widely used rigorous coupled-wave approach was applied to compute the reflection and transmission efficiency spectrums. An extensive parametric study revealed that the dual-periodicity superlattice exhibits anomalies in the long-wavelength regime which can be considered to be resonance anomalies (RAs), which are distinct from the Rayleigh–Wood anomalies. Also, double-periodicity superlattices exhibit RAs at higher wavelengths than single-periodicity superlattices.     

Analysis on diffraction properties of the transmission phase grating

Diffraction properties of the transmission phase grating are analyzed in terms of the Fourier optics theory. The analysis results show that the diffraction intensity distribution of the transmission phase grating is closely related to the optical thickness of the grating and the wavelength of the incident light, and not only determined by the grating's period and slit width. For monochromatic incident light, more than 80% of the diffraction energy will concentrate to the first diffraction order under certain conditions. For white incident light, the energy of the first order of diffraction spectrum may be much higher than other orders. Based on these results, some possible applications of the transmission phase grating are discussed.     

亥姆霍兹线圈磁场均匀范围的研究

推导了双环电流磁感应强度在直角坐标系中的解析式,将公式无量纲化,通过第一类和第二类完全椭圆积分计算场强的两个分量,从而计算合场强,画出亥姆霍兹线圈内外磁感应强度合场强曲面和三维均匀区域,发现磁场的均匀区域是"八爪"形状,并与双环电流不同距离的场强和均匀范围进行了比较.     

Computer studies of detection efficiency of fast-neutron spectrum based on PADC using the Monte Carlo method

A FORTRAN code for simulation of SSNTD detection efficiency is designed using the Monte Carlo method in this work. The theoretical basis for the fast-neutron spectrum measurement based on the assembly of PADC¹ detector+radiator+degrader is described. The main principle of this technique is that, the recoiled protons produced in the radiator by fast-neutrons can be slowed down in the degrader and only protons of energies within a limited range can be detected by PADC (proton energy within the PADC detection energy window). Therefore with a set of different-thickness degraders one can measure the fast-neutron spectrum. Studies show that the program can give reasonable results.     

Diffraction characteristics of optical vortex passing through an aperture-iris diaphragm

The diffraction of an optical vortex through an iris diaphragm which is a close approximation to a circular aperture has been investigated. The results are compared with those obtained from the diffraction of a Gaussian beam through the same aperture. In our findings the diffraction of an optical vortex and a Gaussian beam produce ball bearing sort of structure of darkness and brightness. The singularity of the vortex beam is found to be consistent even after the diffraction through the aperture. The presence of singularity at the centre of diffraction pattern of an optical vortex has been confirmed by interferometry. There is a good agreement between the experimental and numerical results. We propose that these results may find various applications in optical trapping experiments.     

Analysis of the diffraction by the gratings generated in the materials with a nonlinear response

Diffraction by a thick transmission phase grating being illuminated by a plane wave at the first, second and third Bragg angles is analyzed using the rigorous coupled-wave theory for the materials having a nonlinear response to the holographic illumination. It is shown that in the case of nonlinear holographic recording the two-wave approximation can be applied with the use of the effective value of the amplitudes of the refractive index modulation (n_1ef, n_2ef, and n_3ef). The applicability conditions of such approximation are defined. The analytical dependencies of n_1ef, n_2ef, and n_3ef on the amplitudes of the first, second and third harmonics of spatial frequency of the refraction index modulation amplitude n₁, n₂ and n₃ are derived and the criteria of their applicability are established here. The resulting expressions for the diffraction efficiencies are given for readout at three Bragg angles. The results of analytical and numerical methods are compared. The obtained formulas are used for the calculation of n₁, n₂ and n₃ of the holographic gratings generated in the photopolymer material.     

Incubation pit analysis and calculation of the hydrodynamic impact pressure from the implosion of an acoustic cavitation bubble

An experimental study to evaluate cavitation bubble dynamics is conducted. The aim is to predict the magnitude and statistical distribution of hydrodynamic impact pressure generated from the implosion of various individual acoustic cavitation bubbles near to a rigid boundary, considering geometrical features of the pitted area.A steel sample was subjected to cavitation impacts by an ultrasonic transducer with a 5 mm diameter probe. The pitted surface was then examined using high-precision 3D optical interferometer techniques. Only the incubation period where surface is plastically deformed without material loss is taken into account. The exposure time was adjusted in the range of 3–60 s to avoid pit overlapping and a special procedure for pit analysis and characterisation was then followed. Moreover, a high-speed camera device was deployed to capture the implosion mechanisms of cavitation bubbles near to the surface.The geometrical characteristics of single incubation pits as well as pit clusters were studied and their deformation patterns were compared. Consequently, a reverse engineering approach was applied in order the hydrodynamic impact pressure from the implosion of an individual cavitation bubble to be determined. The characteristic parameters of the cavitation implosion process such as hydrodynamic impact pressure and liquid micro-jet impact velocity as well as the hydrodynamic severity of the cavitation impacts were quantified. It was found that the length of the hypotenuse of the orthographic projections from the center of the pit, which basically represents the deformed area of the pit, increases with the hydrodynamic impact aggressiveness in a linear rate. Majority of the hydrodynamic impacts were in the range of 0.4–1 GPa while the corresponding micro-jet velocities were found to be in the range of 200–700 m/s. Outcomes of this study, contribute to further understanding the cavitation intensity from the implosion of acoustically generated bubbles and could certainly represent a significant step towards developing more accurate cavitation models.     

MTPO based potential function of the boundary diffraction wave theory

A novel potential function is introduced by using the modified theory of physical optics integrals for a perfectly conducting half-plane. The function is valid for arbitrary aspects of observation. The line integration of these functions gives the total scattered fields. The method is applied to the problem of diffraction of plane waves by an opaque half-plane for oblique incidence.     

Application of the diffraction trace formula to the three-disk scattering system

The diffraction trace formula derived previously and the spectral determinant are tested on the open three-disk scattering system. The system contains a generic and exponentially growing number of diffraction periodic orbits. In spite of this it is shown that even the scattering resonances with large imaginary part can be reproduced semiclassically. The nontrivial interplay of the diffraction periodic orbits with the usual geometrical orbits produces the fine structure of the complicated spectrum of scattering resonances, which are beyond the resolution of the conventional periodic orbit theory.     

Temporal diffraction characteristics of transmitted multilayer volume holographic grating illuminated by an ultrashort pulse

Based on the coupled wave theory of Kogelnik and Fourier optics, the time-domain diffraction characteristics of tramsmitted multilayer volume holographic grating (MVHG) under an ultrashort pulse readout are investigated. It is shown that the temporal diffraction characteristics depend not only on the numbers of the grating layers, but also on the thicknesses of the grating layers and buffer layers, grating period and the refractive index modulation. Furthermore, using group velocity dispersion we give explanation on the time-delay of diffraction pulse with respect to the center of the readout pulse. Results of our discussion may find applications in optical communications, pulse shaping and processing.     

Rigorous electromagnetic analysis of metallic cylindrical focusing micromirrors with high diffraction efficiency, achromatic aberration and long focal depth

In this paper, we propose a kind of metallic cylindrical focusing micromirrors (MCFMs) with parabolic profiles, and the focal performance is investigated based on rigorous electromagnetic theory and the boundary element method (BEM). Numerical results by the BEM show that the proposed MCFM possesses a high diffraction efficiency and axially achromatic aberration. In addition, through designing a modulating phase function, the MCFM holds long focal depth and high transverse resolution simultaneously. It is believed that the MCFM should have wide application prospects in micro-optical systems, such as micro solar concentrators.     

Effect of recording conditions on the anisotropic diffraction of volume holographic gratings

The anisotropic Bragg diffraction of the volume holographic gratings in photorefractive crystals are investigated based on the model of anisotropic coupled-wave theory. The effect of the initial intensity ratio and the recording angles of the two recording waves on the anisotropic Bragg diffraction properties is discussed. It is shown that both the ratio of the initial intensity and the incident angles of the recording waves are selective action for the anisotropic Bragg diffraction efficiency of the volume holographic gratings, while these two recording conditions are not selective action for the isotropic Bragg diffraction. Furthermore, the Bragg phase matching condition of anisotropic diffraction is analyzed when the recording angles change.     

Diffraction of spherical waves at an annular aperture in the use of the boundary diffraction wave theory: A comparison of different diffraction integral approaches

The integral expression for divergent spherical waves diffracted at an annular aperture is derived based on the theory of the boundary diffraction wave. The expressions for divergent spherical waves diffracted at a circular aperture and a disk, and the axial field are treated as the special cases of our general one. Numerical calculation results for axial and transversal intensity distributions are given to compare our results with the Kirchhoff diffraction integral, first and second Rayleigh diffraction integrals. As expected, our results are in agreement with those in the use of the Kirchhoff diffraction integral, but the computer time is reduced greatly by using the boundary diffraction wave theory. The four diffraction formulae are shown to be consistent for axial and transversal intensity distributions, if the source and observation points are positioned equally from the aperture, or the observation point is located enough far from the aperture. Otherwise, the mean value of the first and second Rayleigh diffraction integrals is equal to the result of the boundary diffraction wave theory.     

Experimental evidence of the phase reversal in diffraction patterns

In a diffracted wavefield occurs in patterns phase variations. The most familiar example is the Fraunhofer diffraction pattern through a single slit: the theory predicts that the side bands of the diffraction pattern alternate in phase. This paper deals with phase reversal phenomenon in diffraction patterns. Two interesting systems are recognized: the diffraction through a half plane and the diffraction through a single slit.