Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Investigation on diffraction characteristics of continuous terahertz beam limited by hard-edge apertures

Hard-edge apertures are common devices in optical systems. The light beam would be diffracted while transmitting through an aperture. The diffraction characteristics of terahertz (THz) beam passing through two hard-edge apertures have been simulated based on the Fresnel diffraction integral function, and an approximate parallel beam has been obtained. The effect on the diffraction spot has been compared and analyzed by changing the aperture diameter and distance between the two apertures and the diffraction plane. Then the experimental results were presented, which basically accorded with the numerical results. It can be inferred that the diameter of THz beam diminishes and the divergence angle can be neglected after the restriction of two apertures. The method is preferably available in some experiments which require small beam diameter and divergence angle.     

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.     

声表面波在线性调频声栅上作Bragg衍射后的聚焦声场

本文应用准周期概念给出声表面波在线性调频声栅上Bragg衍射的透射函数。将一级衍射波的分布作为有限孔径内的已知分布。将求解透射区的一级声束的场分布问题作为求上述有限孔的衍射问题来处理。分析结果表明,对偏转声束来说,在Fresnel区存在一个Fraunhofer平面。线性调频声栅同时起两个作用:它既相当于一个等周期的声束偏转器,同时又是一个Fresnel透镜,在焦平面上的声场分布,是声栅出射平面上偏转声束幅度分布的Fourier变换。分析结果还表明:聚焦声轴随入射声波频率而平行移动。这一现象属首次发现,实验结果证实了上述理论分析预计。 关键词：     

缩减主光斑的衍射理论

结合理论分析和神光装置照明系统的设计,由于光的衍射作用,使通过照明系统的激光束入射到特定靶面成一光斑,为了缩减中央光斑大小,提出了一种新的系统孔径分割法,从光的衍射理论证明这种方法是有效的,使中央光斑缩小,缩减了中央光斑缓慢变化部分.     

高斯光束经环形光栅的衍射特性分析

基于傅里叶-贝塞尔变换计算高斯光束垂直入射环形光栅时的衍射远场分布,分析了其衍射远场光强分布的一般规律,并与平面波入射时的情况进行了比较.计算结果表明：当光栅半径为1.5倍高斯光束束腰半径时, 随着光栅环数的增加,中央亮斑半值全宽先减小后增大、中央亮斑所包含的功率占总功率的比值减小、中央主极大光强值减小,三者的变化趋势与平面波入射时的趋势一致;中央亮斑半径、次极大光强值变化趋势与平面波入射时的变化趋势不同.当环数小于5时,高斯光束经过环形光栅的衍射场光强变化无规律;当环数大于10后两种情况下衍射场光强变化都不明显;当环数趋于无穷时中央亮斑半径、中央亮斑半值全宽、次极大光强值趋于圆孔衍射(环数等于1)时的值,中央亮斑所包含的功率占总功率的比值约等于圆孔衍射时的1/2,中央主极大光强值约等于圆孔衍射时的1/4.     

Laser beam deflection by frequency shifting

A new type of beam scanner has been developed which utilizes frequency shifting of a laser beam followed by deflection of the beam in a dispersive element. Theoretical considerations involve the maximization of the frequency shift and number of resolvable spots. An experimental deflector has been constructed which employs a mode-locked 0.633 μm He-Ne laser, lithium-niobate frequency shifter, and a Fabry-Perot dispersion element. Scans of 15 resolvable spots have been obtained with a theoretical access time of 18.5 ns.     

Paraxial waves in the far-field region

By investigating the changes suffered by a paraxial beam propagating in the near-field and in the far-field regions, it has been found a set of wave equations valid for points gradually closer to the near field. A relevant expression for the validity of the far-field approximation is given from the paraxial Helmholtz equation. It is pointed out that the well-known Fresnel number associated with every transverse diffraction pattern can be interpreted as a magnitude that measures the relative standard deviation of the Fraunhofer pattern and a first-order field, thus reporting on an integral expression suitable for a general case. Finally, the Rayleigh range of the optical beam is deduced from the previously inferred Fresnel number, what has been applied for the cases of a spherical Gaussian beam and a uniform-illuminated circular aperture.     

Resonantly enhanced transmission of light through subwavelength apertures with dielectric filling

Using both analytical and numerical methods to study transmission of light through dielectric-filled subwavelength apertures in a real metal, we have found that a propagating mode can in principle exist inside a waveguide of arbitrary small size if a particular relationship between the dielectric constants of the cladding and filling materials at the incident frequency is satisfied. Practical transmission through a subwavelength aperture of finite depth can be enhanced when the depth is such that Fabry-Pérot-like resonances are excited. For 810 nm light incident on a silicon-filled 50-nm-diameter aperture in a 200-nm-thick gold film, we found that a normalized near-field intensity ratio of 1.6 at the exit can be achieved. This resonantly enhanced transmission phenomenon may be advantageously applicable to near-field scanning optical microscopy and single-molecule spectroscopy.     

Measuring the topological charge of vortex beams by using an annular ellipse aperture

We theoretically and experimentally investigate the Fraunhofer diffraction pattern of a vortex beam passing through an annular ellipse aperture. It is found that the pattern of the far-field diffraction intensity distribution exhibits some dark spots, which become clearer with increasing the value of the ellipticity factor of the annular ellipse aperture. The diffraction phenomenon is more obvious with increasing the value of the ratio of the inner long axis (or short axis) to the outer side of the annular ellipse aperture. The number of the dark spots in the Fraunhofer diffraction intensity distribution is just equal to the topological charge value of the measured optical vortex, and the centre of each dark spot is just a phase-singularity point. Based on this property, we can measure the topological charge of an optical vortex beam.     

半径随机的圆孔波纹锯齿光阑的衍射特性

研究了半径随角度的变化而随机变化的圆孔波纹锯齿光阑的衍射特性,提出并证明半径随机的圆孔纹波锯齿光阑能改善光束的近场分布和抑制光束中央部分的衍射调制。给出了衍射光轴上和横截面内光强分布的模拟计算结果,通过计算结果可以看出:通过半径随机圆孔波纹锯齿光阑后,衍射光横截面内填充因子比经过调幅型波纹锯齿光阑后的填充因子高,调制强度比经过调幅型波纹锯齿光阑低,并且半径随机锯齿光阑能在较大的空间范围内抑制轴上光强的衍射调制, 其可抑制的最远空间距离可达0.15 m。     

Optical-gyroscope application of efficient crossed-channel acoustooptic devices

An acoustooptic (AO) deflector/modulator using a single-mode crossed-channel waveguide of 2n type and an interdigital transducer is fabricated in aY-Z LiNbO₃ substrate. This module has shown a high diffraction efficiency. A 50% diffraction efficiency and a bandwidth of 13.4 MHz were obtained with 0.13 W of surface-acoustic wave power centered at 320MHz. Since the cross section of the channel waveguide and that of the optical fiber are comparable, the interfacting of the resultant acoustooptic devices with fiber optical systems would greatly simplified. In addition, the frequency-shifted optical beam can be conveniently used as a reference signal or local oscillator in heterodyne detection. Consequently, this cross-channel acoustooptic device should find a variety of applications in realizing an integrated-optics module with a 50–50% power slit, optical communication, and an optical fiber system. One of those applications, optical gyroscopes, is proposed by using this kind of crossed-channel AO device.     

Design and experiment of a large aperture digital beam deflector based on electro-optic crystal switch array

A large aperture two-dimensional (2D) digital beam deflector based on electro-optic (EO) crystal switch array has been proposed and experimentally demonstrated. The deflector employs cascaded beams splitting units, which consist of LiNbO₃ EO switches and polarizing beam splitters, and a deflecting prism array to produce a 2D array of beam positions with a wide field of view. Design, construction, and performance of a 3×3 rectangle scanning array with a clear aperture of 6.5 mm are described in detail. Transmission efficiency, half-wave voltage, and scanning speed are measured to describe the performance of the EO switches. The effect of deviation of the input light from the optical axis is analyzed and a technique to diminish this effect is also proposed.     

Focusing of scanning light beams below the diffraction limit without near-field spatial control using a saturable absorber and a negative-refraction material

We show that a scanning light beam can be focused below the diffraction limit without the control of moving near-field elements using the combination of two main components: a light-controlled saturable absorber, which creates seed evanescent components from the beam, and a layer of negative-refraction material, which amplifies the evanescent waves. Focusing to spots with a FWHM in the range of 0.2-0.3 lambda is predicted. For slightly off-resonant input beams, an intensity-dependent phase shift leads to smaller spots.     

Propagation characteristics of the two-dimensional off-axial Hermite-cosh-Gaussian beams through rectangular apertured and misaligned optical systems

By introducing a hard aperture function into a finite sum of complex Gaussian functions, an approximate analytical expression for the two-dimensional off-axial Hermite-cosh-Gaussian beams passing through a rectangular apertured and misaligned paraxially ABCD optical system has been derived. The results provide more convenience for studying their propagation and transformation than the usual way by using diffraction integral directly. Some numerical simulations are also illustrated for the propagation characteristics of a two-dimensional off-axial Hermite-cosh-Gaussian beam through a rectangular apertured ABCD optical system.     

连续相位变化毫米波衍射天线

基于菲涅尔原理及卡塞格伦天线设计方法,设计了一种口面直径为200 mm的卡赛格伦菲涅耳相位修正平面天线。天线采用连续相位修正方式,由一组同心菲涅耳相位修正圆环组成,与传统卡赛格伦抛物面天线相比,该天线具有平面化结构,大大减小了天线自身重量,天线辐射性能较离散相位衍射天线有大幅度提高。在95 GHz频率下,采用物理光学法进行仿真计算,并采用近场扫描系统进行了天线性能测试,天线3 dB波束宽度分别为0.95及1.05,天线实测增益为44.1 dB,天线口面效率为65%。     

Nonparaxial diffraction of vectorial plane waves at a small aperture

By using the vectorial Rayleigh diffraction integrals, a nonparaxial propagation equation of vectorial plane waves diffracted at a small rectangular aperture is derived analytically and some special cases are discussed. Numerical calculation results are given to illustrate the applicability and validity of our theoretical formulae. It is shown that for the apertured case the ratio of the aperture width and wavelength affects the beam nonparaxiality. The nonparaxial approach presented in this paper has to be used for diffracted plane waves if the aperture width is comparable with or less than the wavelength.     

高衍射效率衍射望远镜系统的设计/加工及成像性能测试

为提高衍射效率,设计并制作了口径为300mm的衍射成像系统.该系统的物镜是由一块四台阶位相型菲涅尔波带片通过激光直写套刻和Ar离子束物理刻蚀技术在石英玻璃基板上加工而成.测试了衍射物镜的衍射效率,实验结果表明:衍射物镜在波长632.8nm处的衍射效率为66.4%,达到理论值的82%.搭建了衍射成像系统光路,分别采用10μm星点孔与分辨率板,测试了系统的成像性能.实验测得星点像直径为44μm,分辨率板的极限分辨率达到84lp/mm,接近该系统的理论计算值,表明该衍射成像系统具有较好的成像性能.     

Ultrafast optical beam deflection in a pump probe configuration

Propagation of a signal beam in an Al Ga As/Ga As waveguide multiple-prism light deflector is theoretically investigated by solving the scalar Helmholtz equation to obtain the dependences of the temporal and spatial resolvable characteristics of the ultrafast deflector on the material dispersion of Ga As including group velocity dispersion and angular dispersion,interface reflection,and interface scattering of multiple-prism deflector.Furthermore,we experimentally confirm that,in this ultrafast beam deflection device,the deflecting angle of the signal light beam is linear with the pump fluence and the temporal resolution of the ultrafast deflection is 10 ps.Our results show that the improvement of the temporal and spatial resolvable performances is possible by properly choosing the structural parameters and enhancing the quality of the device.     

薄膜光栅耦合器的研究

在玻璃衬底上制备了周期为0.56μm的薄膜光栅耦合器,用复制法把光栅做在玻璃衬底上。实验中,我们观察到,激光束既可以从光栅的上面(空气区)或下面(衬底区)耦合于波导薄膜内,又可以通过光栅由波导薄膜中耦合出来。调节激光束入射角可以激发不同阶的波导模式。由实验测得的输入耦合效率达66.0±0.5％。 关键词：