Far-field diffraction characteristics of a time dependent Gaussian-shaped pulsed beam from a rectangular mask with a circular aperture array

The diffraction characteristics of a broad-band light source from a rectangular mask with a circular aperture array in the far field are theoretically investigated with a rapid and precise mathematical derivation. We provide a rigorous treatment of a time dependent Gaussian-shaped pulsed beam passing through a rectangular mask with a circular aperture array and give more accurate and concrete criteria judging the timing for the spectral red shift, blue shift, and splitting of light distributions as well. In addition, the distortion of the space-time diffraction intensity distribution is also presented in detail. This study shows the mathematically analytic form for a pulsed beam incident on an array of small apertures, when the usage of lenses is not practical. Also, the energy distribution of the pulse in the frequency domain is presented to benefit the understanding of its applications in optics information systems.     

Spectrum apodization of a time-dependent Gaussian pulse incident on the apodized circular aperture in far-field

The diffraction spectral intensity characteristics of a time-dependent Gaussian pulse from an apodized circular aperture in the far field are studied both theoretically and numerically. It is shown that the apodization of the diffracted Gaussian pulse spectrum can be found. Also the red or blue shift of the spectral intensity maximum of the incident pulse is presented.     

Side-lobeless far-field spectrum of a short pulse from a circular aperture with Gaussian form of transmittance

The spectrum change of a Gaussian pulse in the far field is studied with the Fresnel diffraction integral when it is incident on an aperture with Gaussian form transmittance. It is found that the side-lobes of the diffracted spectral intensity, which exist in a normal circular aperture with unit transmittance, can be eliminated completely under such a condition. Also, the red shift and blue shift of the spectral intensity maximum of the incident pulse are presented.     

超短脉冲经线性小孔阵列衍射时的光谱开关特性

研究了超短高斯脉冲光束经线性排列的矩形光阑阵列衍射后的远场光谱奇变现象。基于菲涅耳积分公式,通过将光阑函数展开为有限项复高斯函数的叠加,得到了超短脉冲光束经线性排列的小孔光阑阵列衍射后的谱强度解析表达式,并对衍射场的光谱红移和蓝移现象进行了数值计算和分析,分析了小孔间距和光阑半径对光谱强度的影响。研究表明:在衍射场的某些观测点,即在调制函数为零的位置附近,存在光谱开关现象。     

Spectral shifts and spectral switches of a pulsed Bessel–Gauss beam from a circular aperture in the far field

The far-field spectral characteristics of a broadband-pulsed Bessel–Gauss beam passing through a circular aperture are studied. Based on the expansion of a hard aperture function into a finite sum of complex Gaussian functions and starting from the Fresnel diffraction integral, the approximate analytical expression for the spectral distribution of the diffraction field is derived. The “aperture dispersion”, that is, the red and blue shifts of the spectral intensity maximum and the spectral intensity shape distortion and distribution under different conditions are studied and illustrated with numerical calculations. It is shown that the far-field, on-axis maximum spectral intensity always shifts to the right (blue shift) and the amount of the spectral shift is dependent on the source bandwidth only. However, for the cases of off-axis, when the observation angle θ approaches a critical value, a rapid spectral switch occurs.     

Electrostatic potential penetration into a circular enclosure with circular apertures

Electrostatic potential penetration into a circular enclosure with circular apertures is investigated. An electrostatic boundary-value problem of the circular enclosure with circular apertures is solved based on the Hankel transform, mode matching technique, and superposition. The polarizability of the circular apertures is represented in a series and calculated in terms of the aperture geometry to understand the behaviors of potential penetration.     

Improvement of Laser-Beam Irradiation-Intensity Distribution Using Multi Lens Array and Edge-Shaped Plates

A new concept for improving the laser intensity distribution on an inertial confinement fusion target using a phase-controlled multi lens array has been proposed. Circular and hexagonal element lens apertures have been examined, and the circular aperture which eliminated azimuthally asymmetric intensity distribution was chosen. The diffraction fringe of the elementary lens was mitigated in a one-dimensional lens array using edge-shaped plates of a super-Gaussian phase profile. Uniform beam profiles were obtained using a one-dimensional array with 7 spherical lenses and a two-dimensional array with 37 lenses. An approximately flat-top intensity distribution was realized with the lens array with 37 lenses.     

Propagation of a phase-locked circular dark hollow beams array in a turbulent atmosphere

The propagation of phase-locked circular dark hollow beams array in a turbulent atmosphere is studied. An analytical expression for the average intensity distribution at the receiving plane is obtained based on the extended Huygens-Fresnel principle. The effects of turbulence, dark parameter and beam order of the beams array on the intensity pattern are studied and analyzed. It is found that the intensity pattern of the phase-locked circular dark hollow beams array will evolve from a multiple-spot-pattern into a Gaussian beam spot under the isotropic influence of the turbulence. The intensity pattern of beam array with a larger dark parameter and beam order evolves into the Gaussian-shape faster with increasing propagation distance.     

Spectral shifts and spectral switches in diffraction of ultrashort pulsed beams passing through a circular aperture

Based on the Fourier transform method, a simple closed-form expression for the on-axis power spectrum of ultrashort Gaussian pulsed beams in diffraction at a circular aperture is derived, which permits us to study spectral changes both analytically and numerically. It is shown that for diffracted pulsed beams there exist spectral red and blue shifts, spectral narrowing, and spectral switches in the near field. The aperture diffraction plays an important role in spectral switching, but both the truncation parameter and bandwidth (or equally, Fourier transform limited pulse duration) affect the behavior of spectral switches.     

Simulation of circular array ultrasound transducers for intravascular applications

The beam shape of a circular array transducer that is commonly used in intravascular ultrasound catheters was investigated in linear mode of operation. For this use, a simulation program which can simulate the radio frequency (rf)-response of a number of scatterers has been developed. The program is based on the impulse response method, which is implemented in the frequency domain. Due to the unusual geometry of the transducer, the far field gets peculiarly shaped for large apertures. Instead of having a far field with its maximum intensity in a single lobe on the acoustical axis, the far field splits up into a dual-lobe far field with maximum intensity in two lobes off the acoustical axis. A formula is derived that predicts the occurrence of these beam shapes.     

Power broadening of the Na-D lines in a flame—I. Time-dependent spectral and spatial properties of a flashlamp pumped dye laser

Prior to power broadening experiments on the Na-D lines in a flame (described in Part II), we investigated the time-dependent spectral and spatial properties of a flashlamp-pumped pulsed dye laser (pulse duration about 800 ns, FWHM), with Rhodamine-6G in methanol as dye liquid and an interference filter combined with a Fabry-Pérot etalon as bandwidth-restricting elements in the cavity. An experimental set-up is described that permits measurements of time-dependent spectra with a time-resolution of 10 ns and a spectral resolution of 2.5 × 10⁵. Results are given for different concentrations of the dye solution and for two different bandwidth settings of the laser. In general the spectral profiles at different moments during the pulse are well described by Gaussian curves. The spatial profiles at different moments during the pulse are best fitted by Lorentzian curves. At small concentrations of the dye solution, the time behaviour of the shift and the bandwidth are in (qualitative) agreement with theoretical predictions, i.e. a narrowing of the bandwidth and a shift to longer wavelengths of the line centre.     

圆孔的菲涅耳衍射

得到了圆孔菲涅耳衍射光强分布的表达式,此式由贝塞尔函数的数列(或Lommel函数)给出,因此很容易利用普及的数学软件算出结果.并且利用CCD照相机量度了衍射光强分布,与运算的结果作了比较,也与Burch在1985报导的结果作了比较.     

部分相干啁啾光学相干涡旋晶格在生物组织湍流中的平均光强与光谱位移

本文研究了部分相干啁啾光学相干涡旋晶格在生物组织湍流中的平均光强和光谱位移,详细探讨了单色光场中的光学晶格结构和多色光场中的光谱快速跃迁特性.研究表明:在生物组织湍流中,光束从具有涡旋核的环形结构演变为具有暗区的周期阵列结构,最后呈类高斯图样.尽管晶格常数能调制光束结构,但它不影响光束在生物组织湍流中的光谱跃迁行为.光...     

Spectral Intensity Distribution of a Time Dependent Gaussian Pulsed Beam From a Circular Aperture in the Near-Field Diffraction

The diffraction characteristics of a broadband light source from a circular aperture in the near-field are clarified theoretically via prompt and precise mathematical derivation. This paper provides a explicit way of treating in the case of time dependent Gaussian-shaped pulsed beam passing through a circular aperture and gives more accurate and concrete criteria for judging the timing of the spectral red shift, blue shift and splitting of light distributions. Furthermore, the results offer an understanding of the Gaussian beam quality of the output of an optical resonator in the near-field.     

Localization of Surface Plasmon Waves in Hybrid Photodetectors with Subwavelength Metallic Arrays

The spectral characteristics of the hole photocurrent in plasmon photodetectors based on Ge/Si heterostructures with Ge quantum dots combined with regular arrays of subwavelength apertures of various shapes in a gold film on the semiconductor surface are investigated. Dispersion relations characterizing the propagation of surface plasmon waves along the metal–semiconductor interface are determined from the dependences of the photocurrent on the angle of incidence of light. It is established that the plasmonic enhancement of the photocurrent in rectangular aperture array is suppressed as compared to that in circular and square aperture arrays. It is found that, in hybrid structures with rectangular apertures, there exists a range of wave vectors where the energy of surface plasmons is independent of the wave vector of incident radiation. The results are explained by the excitation of dipole modes localized at rectangular apertures with a large aspect ratio by light waves.     

Diffractive Focusing of a Gaussian Beam

The problem of diffraction of a spherical wave with Gaussian amplitude distribution on two infinitesimally thin and ideally reflecting screens with apertures on an optical axis is solved within the framework of the quasi-optical approximation. It is shown theoretically and experimentally that when a Gaussian beam illuminates such a type of bicomponent diffraction system with small Fresnel numbers in a near zone of the second screen, the effect of diffractive multifocal focusing of radiation is observed. In this case, the diffraction picture from the second screen in the focal planes represents the circular nonlocal bands of the Fresnel zones with a bright narrow peak at the center, whose intensity can exceed by six times the value of the incident wave intensity. The energy efficiency of diffractive focusing of Gaussian beams by the bicomponent diffraction system can be as high as 70%. The diffractive method proposed allows the focusing of wide-aperture beams without using classical refraction elements such as lenses and prisms, and it is applicable to both low-intensity and high-power radiation.     

Spectral characteristic of a pulsed hollow Gaussian beam passing through a circular aperture

The spectral characteristics of a pulsed hollow Gaussian beam passing through a circular aperture are studied. Based on the vectorial Rayleigh diffraction integrals, the analytical expressions of the spectra for the complex analytic signal representation and for the complex amplitude envelope representation are derived, respectively, and the comparison between them is made. The influences of the truncation parameter and the beam order on the spectral shifts and on the spectral switches are illustrated. It is shown that the spectrum for the complex analytic signal representation and that for the complex amplitude envelope representation are obvious different as the pulse shorter than an optical period.     

环形光栅的菲涅耳衍射

在圆孔的菲涅耳衍射的基础上 ,讨论环形光栅的菲涅耳衍射 ,并给出了计算机仿真结果     

Terahertz near-field imaging using subwavelength plasmonic apertures and a quantum cascade laser source

The first demonstration, to our knowledge, of near-field imaging using subwavelength plasmonic apertures with a terahertz quantum cascade laser source is presented. "Bull's-eye" apertures, featuring subwavelength circular apertures flanked by periodic annular corrugations were created using a novel fabrication method. A fivefold increase in intensity was observed for plasmonic apertures over plain apertures of the same diameter. Detailed studies of the transmitted beam profiles were undertaken for apertures with both planarized and corrugated exit facets, with the former producing spatially uniform intensity profiles and subwavelength spatial resolution. Finally, a proof-of-concept imaging experiment is presented, where an inhomogeneous pharmaceutical drug coating is investigated.     

高阶贝塞尔脉冲光束通过色散光阑透镜的传输特性

研究了高阶贝塞尔脉冲光束通过色散光阑透镜的光谱和时间空间特性.结果表明:脉冲光束在几何焦面上会出现光谱开关,透镜色散使产生光谱开关的临界位置向z轴移动;透镜一阶色散使几何焦面上峰值光强减小,并使脉冲波形产生显著时间移动和展宽,而二阶和高阶色散的影响较小.此外,光束阶数n和空间参量α也会影响横向光强分布.