有限厚度传导孔径对太赫兹脉冲的时空整形

太赫兹脉冲经时空整形后常被用在太赫兹光谱和成像上。用三维时域有限差分法模拟了矩形和圆形孔径的整形效应,模拟结果表明有限厚度矩形和圆形孔径对太赫兹脉冲能实现有效的整形和滤波。当矩形孔径长边垂直于入射波的偏振方向时,矩形孔径有较好的整形和滤波作用,当矩形孔径长边平行于入射波的偏振方向时,几乎没有明显的整形和滤波作用。圆形孔径具有比矩形孔径更加显著的整形和滤波作用。这些模拟结果用平面光波导理论可以得到很好的解释。     

Surface Emitting THz Wave Parametrical Oscillator Using MgO：LiNbO3

Room-temperature operation terahertz （THz） wave source is demonstrated using three MgO：LiNbO3 crystals which have a noncollinear arrangement. The experimental results show that the THz wave can be tunable from 0.8 THz to 3.0 THz, and the peak energy output is 103 pJ/pulse at 1.5 THz. The noncoilinear cavity configuration makes the THz beam have Gaussian-like spatial distribution, small divergence angle, perpendicularly eradiated from the crystal surface. The beam quality factor M2 is measured to be Mx^2 = 1.15, Mx^2 = 1.25 for characterizing the THz wave beam. Experiments also show that the THz beam can be focused by using a polyethylene lens, and the focal spot size is close to the diffraction limit.     

球透镜的衍射焦斑强度分布研究

根据国内外光通信耦合器件发展的趋势,研制了一种新型玻璃,依靠液体表面张力的作用,首次采用槽沉法来制备球透镜,得到了理想的效果。通过对直径为0.8mm的球透镜的焦斑强度分布进行研究,结果表明:当通光孔径分别为0.2mm,0.4mm和0.8mm时,球透镜的衍射焦斑强度的分布分别为0.2μm,0.4μm和1.0μm。因此,伴随着通光孔径的增加,球透镜的衍射焦斑强度分布增大,衍射的强度减小。     

聚焦毛细管在极紫外平行光管中的应用

采用聚焦毛细管将通用的极紫外光源引出法兰口之外,并控制光束发散角,以匹配平行光管孔径角。分析计算了直接用光源照亮靶盘的情形,光束不能充满有效主镜面上的直径;设计使用聚焦毛细管,使光束充满平行光管孔径角,并对极紫外平行光管有无毛细管两种状态进行测试。测试结果表明,采用聚焦毛细管后,扩大了光束的发散角,并且无毛细管的最大光子计数接近1×105个,加毛细管后最大光子计数能达到1×104个。测试数据说明极紫外聚焦毛细管的传输效率可以达到10%,能够满足极紫外平行光管的各项技术指标要求。     

Fabrication of double-aperture hololens imaging system: application to mechanics

Duffy suggested the use of double-aperture imaging system for the measurement of in-plane displacement component. In this arrangement, twin apertures are placed symmetrically over the imaging lens. This imaging system is used in speckle metrology to measure component of displacement parallel to the line joining the two apertures. The imaging lens used in such configuration need not be diffraction limited over the entire lens aperture, but only over the two small areas encompassed by the two apertures. In present work, compact holographic lenses have been used to generate the two apertures of Duffy's double-aperture interferometer. This imaging system has been used in speckle metrology to measure crack mouth opening displacement (CMOD) in an aluminium beam specimen having central edge crack subjected to three point bending. Experimental results are in good agreement with theoretical predictions. Present experimental investigation reveals that compact holographic lenses can be used advantageously to generate apertures of double-aperture speckle interferometer for its use in the measurement of in-plane displacement component.     

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.     

COS光束的零场截取性质

运用零场截取技术由COS光束得到COS芯(中央斑),数值模拟了COS芯通过不同的分数高斯光阑和分数高斯相位光阑后,在远场处的横向光强分布特性.结果表明,只要通过选取合适的光阑参量,COS芯传输后,在远场看来具有无旁斑的特性.     

Near-field terahertz imaging with a dynamic aperture

By introduction of an optical gating beam on a semiconductor wafer, near-field terahertz (THz) imaging with a dynamic aperture has been realized. The spatial resolution is determined by the focus size of the optical gating bean and the near-field diffraction effect. THz imaging with subwavelength spatial resolution (better than 50mum) is demonstrated.     

高斯光束通过含失调窄缝光阑的失调光学系统的传输特性

 利用硬边窄缝光阑的近似展开式和适用于失调光学系统的广义衍射公式,得出了高斯光束经含失调窄缝光阑的失调光学系统传输的近似解析式。模拟结果表明输出光束场分布与光束参量、光阑尺寸、ABCD矩阵元、光阑失调量和光学系统失调量有关。针对给定的光学系统和高斯光束定量分析了各失调量对输出光束场分布的影响,结果表明:光阑横向位移、光学系统横向位移和角位移均引起垂直于z轴截面内明显的光强非轴对称分布。当光阑半宽度为1 mm时,光阑的衍射作用使腰斑半径为0.5 mm的高斯光束产生-1.586π～1.465π范围的相对相移,且光阑横向位移、光学系统横向位移和角位移均引起焦平面前后相对相移的迅速变化。随光阑宽度变小,各失调量对输出光束特性的影响越明显。     

拉盖尔-高斯光束傍轴度的变化

以拉盖尔-高斯(L-G)光束为例,详细研究了单色光束傍轴度的变化.结果表明,模指数、相对束腰宽度等光束参数的变化,通过近轴ABCD光学系统的传输,以及光阑衍射都会引起光束傍轴度的变化.但是,在自由空间中传输时,光束的傍轴度不变.对上述结果以数值计算例加以说明,并用光束傍轴度与远场发散角间的联系做了物理解释. 关键词： 傍轴度 远场发散角 拉盖尔-高斯光束     

晶体X射线劳厄衍射分束特性研究

本文基于X射线衍射动力学分析了劳厄晶体的分束特性,模拟了晶体吸收和入射光角发散对于透射光和衍射光摇摆曲线的影响,定量给出晶体衍射面内角调节范围和晶体加工厚度对于劳厄衍射分束比的调制.在实验中,采用分析晶体和分束晶体的消色散配置限制入射光角发散的影响,实现300μm厚Si(220)晶体面内角调节劳厄衍射分束的精确测量,并得到300μm, 400μm和500μm厚度Si晶体分束比的调节范围,实现了透射光和衍射光强度的定量调制.     

微透镜变化对半导体激光器光束匀化效果的影响

为了实现高均匀性的半导体激光器泵浦光源,研究了成像型光束积分器中微透镜的变化对泵浦光均匀性的影响。详细讨论了微透镜数值孔径与入射光束的角度匹配的问题。推导了高斯光束经成像型光束积分器的光场分布模型,分析了微透镜的边缘衍射对光斑均匀性的影响,明确了微透镜孔径大小的取值范围,并利用ZEMAX进行了系统仿真及实验验证。结果表明,经优化后的成像型光束积分器实现了不均匀性为8.11%的矩形光斑。     

Transmissive quasi-optical Ronchi phase grating for terahertz frequencies

A transmissive, square-wave Ronchi phase grating has been fabricated from the dielectric polytetrafluoroethylene to diffract an ~0.7 THz beam quasi-optically. When illuminated by a coherent, cw terahertz (THz) source, the spot separation of the ±1 diffractive orders and the diffraction efficiency were measured as a function of THz frequency and rotation angle. The grating performance depends sensitively on the refractive index, whose value can be measured with an accuracy limited by the fabrication precision. The use of these gratings for polarization-insensitive quasi-optical imaging and phased arrays is discussed.     

孔径光阑限制下高斯光束的传输

对高斯光束在硬边孔径限制下的衍射进行了详细的理论研究,就不同口径的圆孔限制下高斯光束在菲涅耳衍射区和夫琅禾费衍射区的分布进行了理论分析,从而得到了孔径受限高斯光束的横向以及轴向的衍射公式,进而对高斯光束在不同衍射区域内衍射光场分布形状随孔径尺寸变化时的演化规律进行了数值计算,并对小口径光阑受限的高斯光束的衍射与平行光经同尺寸光阑的衍射进行了比较。结果表明在较小口径下,两者的分布基本一致。得到的孔径光阑限制下高斯光束的传输规律为高斯光束在自由空间光通信和光学超分辨中的应用提供了理论基础。     

Study on the approximative formula for the far-field of a Gaussian beam under circular aperture diffraction and its divergence

The approximative formula for the far-field diffraction of a Gaussian beam through a circular aperture is obtained by using the superposition of Gaussian beams instead of the aperture function, and the explicit expression for calculating the beam divergence is also gained. Using the formula, the influences of the aberrations on the far-field wavefront and the beam's divergence are researched, and the results show that the large aberrations badly affect the far-field wavefront and the divergence. It is suggested that the aberrations and the diffractions should be avoided when designing the transmitter.     

用位相共轭镜实现多模光纤束光束发散角压缩及光学不均匀性补偿

数值孔径为０．０７的多模光纤束与受激布里渊散位相共轭镜的组成的双光程装置中，输出光束的远场分布呈典型的二维列阵孔的径的衍射花样，输出光能分布在０．０２６ｒａｄ内，即由单根光纤芯径的衍射极限所决定的范围内，远小于由数值孔径所决定的高阶模相应的发散范围，并且偿了由纤束不均匀性带来的缺陷。     

Fabrication of apodized apertures for laser beam attenuation

To obtain maximum energy output from a high-power laser system it is necessary to carefully shape the input beam's cross-sectional intensity profile. This avoids diffraction-produced hot spots which can cause damage to the amplifier components. The intensity should be at a maximum in the central part of the beam and fall of gradually at the edges to avoid hard-edge diffraction effects. This kind of beam profile can be produced using an apodized aperture — a soft-edged aperture that may be constructed in various ways. This report describes a photographic rotating-film technique for fabricating apodized apertures from photo-sensitive glass plates.     

Cylindrical-sinc beam

Propagation of diffracted beams in free space has already been identified. One possible solution is derived from the Helmholtz wave equation and this solution is named as cylindrical-sinc beam. Therefore, cylindrical-sinc beam is a new beam type which can be obtained from Helmholtz equation. Diffraction properties of this new beam type were observed from an opaque aperture screen. Additionally, geometrical theory of diffraction is used to determine numerical values of diffracted fields. This new beam type which does not exist in the literature was observed by using these methods passing through an opaque aperture. The obtained expression was analyzed numerically. Simulation results of the beam depending on the length of aperture and distance to the observation point were added.     

Resonant excitation of intense acoustic waves in crystals

The resonant excitation of an intense elastic wave through nonspecular reflection of a special pump wave in a crystal is described. The choice of the plane and angle of incidence is dictated by the requirement that the excited reflected wave be close to the bulk eigenmode with its energy flow along a free boundary. The resonance parameters have been found for a medium with an arbitrary anisotropy. General relations are concretized for monoclinic, rhombic, and hexagonal systems. A criterion is formulated for an optimal selection of crystals in which the resonant reflection is close to the conversion one, when almost all of the energy from the incident beam of the pump wave falls into the near-surface narrow high-intensity reflected beam. Estimates and illustrations are given for such crystals as an example. The intensity of the reflected beam increases with its narrowing, but its diffraction divergence also increases with this narrowing. Nevertheless, the intensity of the beam can be increased by a factor of 5–10 at sufficiently high frequencies while keeping its divergence at an acceptable level. Amplification by two orders of magnitude can be achieved by compressing the beam in two dimensions through its double reflection.     

Radiation patterns of circular apertures with Gaussian illumination

We analyze the radiation pattern from circular apertures with Gaussian electric field distribution. Patterns are calculated using the Fraunhofer diffraction integral and we obtain a convenient expression for the FWHM beamwidth as a function of the edge taper. The aperture efficiency can be calculated for blocked and unblocked apertures in analytic form. The beam efficiency must be computed numerically, but can be obtained in a straightforward way from the beamwidth.