Collinear diffraction of restricted light beams by nonmonochromatic sound

The collinear diffraction of Gaussian light beams by a two-frequency sound beam is investigated theoretically. A set of equations relating the amplitudes of transmitted and diffracted waves is derived in the case of third-order nonlinearity. The distributions of light intensities along an acousto-optical cell are calculated at different ratios between the radii of the light and sound beams. The transmission curves for detuned light waves and combination components of the diffracted waves are constructed at different ratios between the radii of the light and sound beams. The changes in the dynamic range of an acousto-optical device are calculated for different diffraction efficiencies.     

Collinear diffraction of a divergent light beam by ultrasound in a paratellurite crystal

The collinear acousto-optical interaction of a divergent light beam with ultrasound along the approximate [110] direction in a TeO₂ paratellurite crystal is investigated theoretically and experimentally. The collinear diffraction is studied at an ultrasonic frequency f ≈ 149 MHz under exposure of the crystal to an uncollimated laser light beam at a wavelength λ = 633 nm and at an angle of divergence as large as 4°. It is shown that the collinear diffraction along the direction forbidden for acousto-optical interactions of plane waves occurs only under conditions where the light beam is uncollimated and the diffraction efficiency increases with an increase in the divergence of the light beam. It is proved that the attenuation of an acoustic wave brings about a decrease in the diffraction efficiency and an increase in the transmission bandwidth of the device used. A model of the collinear acousto-optical filter based on a paratellurite crystal with an interaction length l = 2.7 cm is analyzed. The collinear acousto-optical filter is characterized by a high resolving power (～3000), a high diffraction efficiency (I₁/I₀ ≈ 0.8), and a large angular aperture (Δ? ≈ 4°). This makes collinear diffraction promising for use in acousto-optical filters based on paratellurite crystals.     

Weakly divergent ray and diffraction beams in oceanic waveguides

Conditions that should be satisfied by the sound velocity profile of an oceanic waveguide for the dependence of the ray cycle length on the ray phase velocity to contain smooth extrema are formulated. The extrema correspond to weakly divergent ray beams forming “caustic” beams. It is found that diffraction effects cause a considerable smoothing of the sharp extrema that occur in the dependence of the interference period of neighboring modes on their phase velocity. As a result, in addition to the weakly divergent ray beams, weakly divergent diffraction beams and the corresponding “diffraction” caustics can be formed.     

发散光束抽运的宽带光参量啁啾脉冲放大系统的理论研究

对发散光束抽运的光参量啁啾脉冲放大器的增益带宽进行了系统的理论研究.采用空间傅里叶变换和四阶Runge-Kutta算法，分别模拟了非衍射极限情况下的高斯光束和空间频谱为矩形的发散光束作为抽运光时的增益曲线.结果表明：不管是在可见光或是在近红外光谱区，用发散光束均可以明显地改善光参量啁啾脉冲放大系统的增益带宽.选取合适的发散角和抽运光强，可以获得高增益、宽谱带的信号光输出. 关键词： 光参量啁啾脉冲放大 增益带宽 发散角 高斯光束     

Discrete diffraction and waveguiding of optical beams in a cascade-induced lattice

Regularities of anisotropic discrete diffraction in a cascade-induced lattice formed by two crossed pump waves in a quadratically nonlinear medium have been studied. The transition from diffraction of a signal beam in free space to discrete diffraction with further capture into a waveguide in a defocusing lattice with an increase in the pump wave intensity has been traced. Diffraction-free propagation of a tilted beam is described.     

Collinear acousto-optic diffraction using two nearby sound frequencies

Collinear acousto-optic spectrometers with sound consisting of one frequency have a spectral transfer function with many good characteristics, such as a narrow bandwidth, ... the side lobes, however, are rather high. To reduce these side lobes, one can try to apodize the sound in the acousto-optic cell by using a sound wave consisting out of two frequencies. At certain times, this produces a more smoothened sound field in the cell than in the one-frequency case. The resulting transfer function will contain lower side lobes. Also, the influence of all the sound field parameters--the amplitude, frequency difference, and time--on the transmission function is studied. Out of these results, better-performing collinear spectrometers can be designed.     

Feature of kinematic diffraction of nonmonochromatic divergent X-ray beams in a crystal with a periodically strained lattice

Coherent scattering of a nonmonochromatic divergent beam of X-ray quanta in a crystal excited by an acoustic wave is described using a kinematic approach. The possibility of manifestation of a peculiar effect of amplification of the diffracted X-ray beam by the acoustic wave is predicted using this approach.     

A new method of coherent summation of laser beams by diffraction optical element

A new method of coherent summation of laser beams is proposed. Two-dimension numerical simulation based on the conjugate gradient algorithm shows that the combination efficiency is about 94.8%, and an error less than 2.4% with a 16-level phase quantized diffraction optical element. Error analysis has also been performed.     

Collinear laser spectroscopy with reverse-extracted bunched beams at TRIUMF

Collinear laser spectroscopy with reverse-extracted cooled bunched beams was performed at TRIUMF with radioactive ion beams. Surface-ionized ^78,78mRb ions were injected into TITAN??s cooler-buncher, and reverse-extracted to the laser beam line. There they were neutralised and excited with a counter-propagating laser beam. The fluorescence signal from the D ₂ line was recorded with a photomultiplier tube. The cooler provided typically 50 ion bunches per second, with each bunch containing approximately 10⁵ ions, temporally distributed with a 1.6 ??s width. A hardware gate was set on the signal, suppressing the background by four orders of magnitude.     

Collinear laser spectroscopy at ionic beams of argon and chlorine

Metastable ions of argon and chlorine were accelerated to form a beam which was led antiparallel to dye-laser beam. When the laser beam was tuned to transitions from the metastables to higher states and the accelerating voltage was set to compensate for the remaining shift, fluorescence from the ions could be detected. Scanning the voltage excitation spectra of the ions could be obtained which allowed to determine the isotope shift of the lines the laser excited.     

Gaussian beams and minimum diffraction

I show that different legitimate measures of the amount of diffraction lead to contradictory conclusions concerning the beam profile that experiences minimum diffraction for a given beam width. In particular, when the Rényi entropy is used, the Gaussian beams no longer provide minimum diffraction, i.e., they do not have maximum quality.     

线偏振拉盖尔-高斯光束的远场发散特性

利用稳相法和矢量结构理论, 导出了线偏振拉盖尔-高斯光束的矢量结构项TE项和TM项在远场的解析表达式. 进而利用TE项和TM项的远场能流分布, 给出了TE项和TM项的功率占总功率比例的度量式,同时还给出了线偏振拉盖尔-高斯光束、TE项和TM项三者远场发散角的解析式以及三者远场发散角间的关系式. 所得到的公式不仅适用于傍轴情形,而且还适用于非傍轴情形. 通过数值计算, 分析了TE项和TM项在远场的功率占总功率的比例与参数f和模数间的依赖关系;还分析了拉盖尔-高斯光束、TE项和TM项的远场发散角随参数f、模数和线偏振角的变化关系.这一研究从矢量结构本性揭示了线偏振拉盖尔-高斯光束的远场发散特性, 丰富了对其传输特性的认识.     

Caustics and weakly divergent beams in oceanic waveguides

It is found that the weakly divergent beam formed around the reference ray with minimal cycle length does not stand out in intensity against the background of the total field. It is shown that, under the same conditions, another beam stands out in intensity: it is the caustic beam composed of the system of caustic lines, which are the envelopes of the family of rays forming the weakly divergent beam.     

Time-of-flight resolved transmission-grating diffraction of molecular beams

Diffraction of molecular beams of CH₃F, CHF₃, and metastable He^* and Ne^* from a 100-nm-period micro-fabricated transmission grating has been observed. Due to the finite velocity spread and the corresponding temporal coherence of the molecular beams, diffraction peaks of only the first few orders are resolved in total-intensity measurements, while higher order peaks are increasingly broadened and smeared-out. Combining time-of-flight (TOF) measurements with angular scans has allowed to observe TOF-resolved diffraction data which yield full resolution of all detected diffraction peaks. Here, this method has been applied to investigate atom/molecule-surface interactions. In general, it can be used to observe resolved diffraction patterns of a wide range of atomic and molecular beams whose broad velocity distributions prevent peak-resolution in conventional total-intensity measurements.Received: 3 September 2003, Published online: 21 October 2003PACS: 39.20. + q Atom interferometry techniques - 03.75.Be Atom and neutron optics - 07.77.Gx Atomic and molecular beam sources and detectors - 34.50.Dy Interactions of atoms and molecules with surfaces; photon and electron emission; neutralization of ionsW. Schöllkopf: Present address: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Abteilung Molekülphysik, Faradayweg 4-6, 14195 Berlin, Germany.R.E. Grisenti: Present address: Johann Wolfgang Goethe-Universität, Institut für Kernphysik, August-Euler-Str. 6, 60486 Frankfurt, Germany.     

Higher-order harmonics of limited diffraction Bessel beams

We investigate theoretically the nonlinear propagation of the limited diffraction Bessel beam in nonlinear media, under the successive approximation of the KZK equation. The result shows that the nth-order harmonic of the Bessel beam, like its fundamental component, is radially limited diffracting, and that the main beamwidth of the nth-order harmonic is exactly 1/n times that of the fundamental.     

Aberrated diffraction patterns of off-axis Gaussian beams

Fraunhofer diffraction patterns of off-axis Gaussian beams passing through an optical system having primary spherical aberration are investigated. The resultant diffraction patterns show a nearly elliptical form with a transverse focal shift which depends on the incident position of the Gaussian beams on the pupil and on their radius.     

Single-slit diffraction of the arbitrary vector beams

We present the single-slit diffraction of the arbitrary vector fields with different parameters m, n, and φ0theoretically and experimentally. The single slit covers the polarization singularity in the center and therefore the influence of the polarization singularity on the diffraction fringes is analyzed. The experimental results which agree well with the simulation results show that the total intensity of the diffraction field is related only to the topological charge m, but the polarization distribution of the diffraction field is related to all the parameters m, n, and φ0. Therefore, the diffraction patterns allow to determine all the parameters of the arbitrary vector fields.