Phase shift upon resonant coupling into a thin corrugated optical waveguide

The coupling of light into a waveguide by a surface grating leads to the attenuation of the transmitted beam when the resonant modes of the waveguide are excited. These can be tuned by changing the incident angle or the wavelength. Upon excitation of a mode, the transmitted beam experiences a significant phase shift that can be measured and compared with that predicted by the Kramers-Kronig relations.     

Two-exposure quasi-common-path point diffraction interferometric phase microscopy using a four-step algorithm

A two-exposure nearly common-path point diffraction interferometric phase microscopy (IPM) is presented using polarization modulation and one-step grating shifting to implement quantitative phase imaging. The IPM is constructed by an improved Michelson configuration with a reflective grating, and its frequency spectrum generated by a circularly polarized object beam makes double copies through a beam splitter. One copy is low-pass filtered and reflected by a reflective pinhole mirror to create a reference beam, and the other copy is converted by a polarizer and then reflected by a reflective grating to achieve a 45° linearly polarized object beam. By the combination of a polarizing cube beam splitter with 45° tilted angle and a translation of the reflective grating with a π phase shift, four interferograms with π/2 phase shift can be obtained in two exposures. The standard four-step algorithm can then be used to reconstruct the phase of the specimen. The utility of the proposed method was demonstrated with measurements on a phase plate, cells and an oil drop.     

掺Yb光纤激光器阵列谱合成系统的光束传输模型及光束特性分析

建立了由掺Yb光纤激光器阵列、变换透镜、闪耀光栅和输出耦合镜组成的光束谱合成系统的光束传输模型.在考虑光栅角色散、光栅刻槽倾角误差和光栅衍射效率情况下,利用光线追迹法、衍射积分方法、光束非相干叠加原理和强度二阶矩方法,推导出高斯光束非平行倾斜入射到闪耀光栅的相位变化公式以及谱合成光束的光强分布解析表达式.分析了高斯光束非平行倾斜入射到光栅后,光栅角色散、光栅衍射效率和光栅刻槽倾角误差对掺Yb光纤激光器谱合成系统输出光束特性的影响.研究结果表明,谱合成光束具有与单根光纤激光器几乎相同的光束质量;光栅角色散对合成光束特性的影响可忽略;随着光栅刻槽倾角误差的增大,谱合成光束的光束质量明显变差;当光栅刻槽倾角误差较大时,必须考虑光栅衍射效率对合成光束特性的影响. 关键词： 掺Yb光纤激光器 非平行倾斜入射 光束谱合成 光束质量     

Beam and phase effects in angle-beam-through-transmission method of ultrasonic velocity measurement

The accuracy of a plane wave approximation for phase velocity measurements in isotropic and anisotropic material using the angle-beam-through-transmission method has been investigated numerically and experimentally. In this method the velocity is measured in different propagation directions as a function of incidence angle. The effect of two factors on the measurement accuracy have been discussed: intrinsic phase shift of the transmitted signal through a fluid-solid interface and beam diffraction due to the finite beam size of receiver and transmitter. It is shown that the interface-induced phase shift can introduce an error in time delay measurements of the shear wave after the first critical angle and that this time delay error can be accurately corrected for. Numerical results obtained by a time-domain beam model show that except at the critical angles, the finite width of the transmitter and receiver only affects the amplitudes of the transmitted signals and has almost no effect on the measured zero-cross time delay; therefore the plane wave approximation for obtaining phase velocity from the measured time delay data by this method and the plane wave interface-induced phase correction are fully applicable.     

Ultrasonic diffraction grating spectroscopy and the measurement of particle size

The results of measurements using ultrasonic diffraction grating spectroscopy (UDGS) are found to be dependent upon the particle size of the slurry. This illustrates the emergence of a new technique for measuring particle size. Theoretical calculations are underway to describe and predict them as well. The ultrasonic grating surface is in contact with the slurry. The diffraction grating was formed by machining triangular grooves with a periodicity of 483 microm on the flat surface of an aluminum unit with send and receive transducers fastened to it. For this experimental configuration, the ultrasonic beam strikes the back of the grating at an incident angle of 30 degrees and produces a transmitted beam of spectral order m=1 in the slurry. The angle of this transmitted beam changes with frequency and, at the critical frequency of 3.47 MHz, it is located at an angle of 90 degrees. The receive transducer measures the reflected m=0 beam at an angle of 30 degrees. At a frequency slightly less than the critical frequency, the m=1 transmitted beam no longer exists and its energy is shared with all other beams. Therefore, due to energy conservation, the signal in the receive transducer exhibits a peak at the critical frequency. During the transition, the wave interacts with the particles of the slurry and ultrasound amplitude is reduced. Therefore, the peak observed by the receive transducer is reduced, compared to that for water. Data have been obtained for slurries of polystyrene spheres, ranging in size from 215 microm to 463 microm, and for weight percentages of 1-12%. Polystyrene spheres of different diameter show differing amounts of energy loss and thus, particle size identification is possible.     

超短脉冲激光光束被局域体全息光栅衍射的性质分析

利用二维耦合波理论,分析了超短脉冲激光光束被完全重叠型的局域体全息光栅衍射的时空变化性质,给出了衍射和透射脉冲激光光束沿光栅出射边界的强度时空分布。以LiNbO3晶体为例,数值研究了衍射光脉冲强度沿光栅出射边界的分布和脉冲波形的变化及光栅的总衍射效率受光栅二维尺寸、入射角度、光栅折射率调制度及入射脉冲的脉冲时域半峰全宽等条件的影响而变化的情况。与一维体全息光栅对超短脉冲激光光束衍射的性质,及此光栅对连续光衍射的性质作比较,给出了合理选择光栅参量及入射条件以在光栅出射边界上得到总衍射效率较大且分布较均匀的衍射光脉冲的方法。     

Ultrasonic diffraction grating spectroscopy and characterization of fluids and slurries

The ultrasonic diffraction grating is formed by machining triangular grooves, 300 microns apart, on a stainless steel surface. The grating surface is in contact with the liquid or slurry. The ultrasonic beam, traveling in the solid, strikes the back of the grating and produces a transmitted m=1 beam in the liquid. The angle of this beam in the liquid increases with decreasing frequency and the critical frequency FCR occurs when the angle is 90 degrees. At frequencies below FCR, this m=1 wave does not exist and its energy is shared with other types of waves. The signal of the reflected m=0 wave is observed and an increase is observed at FCR. This information yields the velocity of sound in the liquid and particle size.     

Diffraction testing of ultrasonically-produced phase gratings using a double beam illumination method

An optical far-field method is described for studying ultrasonically-produced phase gratings with very small phase variations. An ultrasonic grating is illuminated by two plane waves mutually inclined at an angle properly matched to the diffraction angle of the inspected grating. The expressions for the time-dependent irradiance in the far-field diffraction orders are derived. They enable the extraction of the Raman-Nath parameter of a progressive ultrasonic beam. The application of the technique to pulsed ultrasound beams is proposed.     

光栅角色散和制作误差对谱合成光束特性的影响

 考虑光束的发散、光栅角色散以及光栅制作过程中存在的刻槽倾角误差,应用光线追迹方法,建立了激光束入射到光栅时相位变化的计算模型。利用衍射积分方法和光束非相干叠加原理,推导出阵列光源各子光束经谱合成系统后合成光束的光强分布解析表达式。在此基础上,利用强度二阶矩方法,计算了合成光束的M²因子,并定量分析了光栅角色散和光栅刻槽倾角误差对谱合成光束特性的影响。研究结果表明:光栅角色散和光栅刻槽倾角误差的存在均会导致谱合成光束的光束质量变差,光栅刻槽倾角误差的影响比光栅角色散的影响更为明显。     

双棱镜改变写入Bragg波长的相位掩模技术

将一种可旋转的双棱镜引入到相位掩模技术中以改变光栅的写入Bragg波长.在该系统中,光纤光栅是由来自可旋转双棱镜所形成的波长为248 nm的紫外干涉条纹写入的,其中,相位掩模被用作 1级衍射光的分束器,通过双棱镜的旋转可改变两写入光束的交叉角.为了初始化Bragg波长的参考值,双光栅的顶角由相位掩模的 1级衍射角和双棱镜的折射率确定.因为在～100 nm范围内两光束的非对称旋转对光栅周期的改变是5×10~(-4) nm,双棱镜引入的光栅的闪耀可忽略.当Bragg波长的移位为1 nm时,棱镜最大的旋转角为～1 degree,最小的旋转角是～2.4 min.与Talbot干涉仪中平面镜的旋转角～23 s/nm相比,该相位干涉仪中棱镜的旋转精度降低了2～3个数量级.     

利用一块相位掩模和两块棱镜改变光纤光栅的写入Bragg波长

一种相位掩模干涉仪被用于写入不同的Bragg波长的。在该系统中,光纤光栅是由来自两可旋转棱镜所形成的紫外干涉条纹写入的,其中,相位掩模被用作&;#61617;1级衍射光的分束器。并且,当两块顶角由相位掩模的&;#61617;1级衍射角和棱镜折射率确定的棱镜的底部相互平行放置时,该相位掩模给出了Bragg波长的参考值。当Bragg波长的移位为1 nm时,棱镜最大的旋转角为1 degree,最小的旋转角是~2.4 minute。与Talbot干涉仪中平面镜的旋转角~23 second/nm相比,该相位干涉仪中棱镜的旋转精度降低了2~3个数量级。替代了许多具有不同光栅周期的相位掩模,该可调谐相位掩模干涉仪仅用一块相位掩模和两块旋转棱镜就实现了写入具有不同Bragg波长的光栅。     

Holographic investigation of induced absorption

A holographic technique for studying induced absorption in transmitting media using a scheme with an additional phase measuring grating is described. A matrix analysis is conducted, and expressions for the induced absorption are derived from the asymmetry of the transmitted beam intensity variation. The dynamics of the induced absorption in a photochromic glass is studied by the standard single-beam diffraction method and a new dual-beam interference method. The influence of a residual absorption grating resulting from the measuring phase grating on the induced absorption dynamics is analyzed. Measuring data obtained by the two methods are shown to be in good agreement     

Strong and fast phase modulation for quantitative analysis of photorefractive gratings

A new approach for studying photorefractive gratings in two-wave mixing experiments by a phase modulation technique is presented. The introduction of a large-amplitude, high-frequency sinusoidal phase modulation in one of the input beams blurs the interference pattern and provides powerful harmonic signals for accurate measurements of the grating diffraction efficiency η and the output phase shift ϕ between the transmitted and diffracted waves. The blurring of the light fringes can be used to suppress the higher spatial harmonics of the grating, allowing a space-charge field with sinusoidal profile to be recorded. Although the presence of such a strong phase modulation affects the beam coupling in a rather complicated way, it is shown that for the special case of equal intensity input beams, the effect of the phase modulation on η and ϕ is reduced to a weakening of the coupling strength. The potentialities of the technique are illustrated in a study of refractive-index waves excited by running interference patterns in a Bi₁₂TiO₂₀ crystal. Expressions for the diffraction efficiency and the output phase shift are derived and used to match numerically calculated curves to the experimental data. The theoretical model is supported by the very good data fitting and allows the computation of important material parameters. PACS 42.40.Ht; 42.40.Kw; 42.70.Nq     

共振域矩形介质光栅的逆向设计

采用模态法研究了共振域光栅分束器及闪耀光栅等光学器件的设计原理及逆向设计方法,给出了-1级非偏振闪耀光栅的典型设计示例。研究表明,自准直角入射时,光束在光栅内传输将激发产生离散模,通过调节离散模透过光栅传输时的累积相位差,可对衍射光进行调控,实现偏振(非偏振)分束、偏振(非偏振)闪耀等功能。采用严格耦合波法对基于模态法逆向设计的-1级非偏振石英闪耀光栅的几何参数进行验证,计算结果显示两者非常吻合,证明了该方法的正确性。     

Simultaneous diffraction of two finite waves at non-uniform dynamic volume reflection gratings

Based on the two-dimensional coupled-wave theory, the simultaneous diffraction of two finite waves with non-constant amplitude distribution at a finite non-uniform dynamic volume reflection grating is described. A coupled differential equation is derived and solved numerically with the aid of a two-dimensional Runge-Kutta method. The results for the chosen example of a planar volume reflection grating both for pure phase and absorption gratings show the dependence of the fidelity and diffraction efficiency on the normalized parameter grating strength, on the phase shift with respect to the interference pattern of the read-out waves and on the absorption modulation. In contrast to the transmission grating, a high fidelity of the enhanced beam can be achieved for reflection gratings. The numerical results are compared with the solution of the one-dimensional theory.     

Ce:KNSBN晶体衍射特性研究

在非同时读出条件下测量了Ce:KNSNB晶体两波耦合体光栅衍射效率随写入光强比、写入光偏振态和写入光夹角的变化关系,并与同时读出条件下衍射效率变化规律的测量结果进行了比较,发现二者基本一致.利用耦合波理论对实验结果进行了理论分析和拟合,拟合结果和实验数据较好的吻合.     

双棱镜结构中透射光束的古斯-汉欣位移

当入射角大于全反射临界角时,双棱镜结构中透射和反射光束的古斯-汉欣(Goos-Haenchen)位移具有饱和效应,并且只有波长数量级。利用稳态相位法研究了当入射角小于全反射临界角时双棱镜结构中透射光束的古斯-汉欣位移。研究表明,传播模式下透射光束的古斯-汉欣位移是空气层厚度、入射角和双棱镜折射率的周期性函数。当透射共振时,透射光束的古斯-汉欣位移可达入射波长的几十倍,与入射角大于全反射临界角的情况相比,透射光束的位移通过边界的相互作用具有共振增强效应;在非共振点处,对称结构中的反射光束具有与透射光束相同的古斯-汉欣位移。共振增强的透射光束的位移在光开关及光耦合器中具有潜在的应用。     

利用双光栅的多普勒频移测速度

衍射光的多普勒频移是与光栅运动速度有关的量.激光束依次通过动光栅、静光栅衍射,然后使1对光频不同、彼此平行又能部分重叠的衍射光束通过硅光电池进行光电转换,得到的信号电流的频率等于多普勒频移.信号电流经I/V转换、f/V转换和A/D转换后,由单片机测出电压,由此就可以确定动光栅的运动速度.最后对利用双光栅的多普勒频移测气垫导轨上滑块的速度进行分析和讨论.     

Contrast reversal with a point-diffraction interferometer with a carrier frequency

A modified point diffraction interferometer is presented, where interference is made in the first diffracted order of a composite grating. A 180° phase shift between reference and object beam is introduced to obtain a complete image contrast reversal.     

Theory of laser-induced amplitude and phase gratings including photoselection,orientational relaxation and population kinetics

The generation of amplitude and phase gratings in an absorbing medium is discussed, including the effects of light polarization, population kinetics and orientational relaxation. A theory is given which describes light diffraction by a combination of an amplitude and a phase grating. It is shown that the coherent coupling effect in transient absorption measurements is solely due to the amplitude grating. Weak absorption and a low triplet yield favour diffraction of the probing beam by the phase grating in triple-beam experiments. Phase grating experiments can be performed with highly soluble absorber dyes, the absorption maximum of which is far away from the laser wavelength. Due to photoselection, diffraction by an amplitude grating depends on the polarization of the probing beam while phase grating diffraction is insensitive to this. Perpendicular polarization of the two exciting pulses prevents the buildup of a phase grating and results in light diffracted by the amplitude grating that is proportional to the transient dichroism signal as known from the common absorption relaxation.