Dissipative optical flow in a nonlinear Fabry-Pérot cavity

We describe a classical nonlinear optical system that displays superfluidity and its breakdown. The system consists of a self-defocusing refractive medium inside a Fabry-Pérot cavity with a cylindrical obstacle. We have numerically solved for the transmitted beam when an incident plane wave strikes the cavity at an oblique angle. The presence of the incident beam pins the steady-state phase of the output, preventing the formation of vortices or time-dependent flow. When the incident beam is switched off, a transient wake of moving optical vortices is produced. This is analogous to the breakdown of superfluidity above a critical velocity.     

影响空间变化偏振矢量光束强聚焦后焦斑移位的因素

利用衍射积分公式数值模拟了不同阶次的空间变化偏振矢量贝塞尔高斯(BG)光束经过强聚焦系统后在焦点附近的强度分布。数值结果显示空间变化偏振矢量BG光束聚焦后在光束传播轴负方向存在焦斑移位现象,其焦斑移位的大小与光束的模阶次、光束宽度、波长以及局部偏振状态密切相关。矢量光束偏振态的变化依靠液晶相位延迟器(LCVR)的相位延迟角度来控制,由于LCVR的相位延迟角度可以在0～π之间连续变化,从而改变光束的局部偏振状态,间接实现了对矢量光束焦斑移位大小的实时控制,在光学微操纵领域具有潜在的应用价值。     

An ultrasound technique for monitoring the alcoholic wine fermentation

In this paper an ultrasonic technique, usually used in non-destructive-testing (NDT), is applied to the monitoring of alcoholic wine fermentation. This technique consists in placing a test tube, containing the analyzing fluid, between two matched ultrasonic piezoelectric transducers, one used as transmitter and the other as receiver. The transmitter generates an ultrasonic wave in the liquid sample which is received by the receiver; the attenuation and the delay of the received signal in respect to the transmitted one are used to characterize the testing fluid. As first experiments this technique was applied to some test solutions of saccharose in water, ethanol in water and of both solvents in water, in order to evaluate the method applicability and resolution. Best results are obtained by the propagation velocity measurement, with a resolution less than 2%. The method was then applied to monitor the wine fermentation: the propagation velocity in the must was measured every day and the results were related to chemical analyses. The obtained results show that the propagation velocity has the same behaviour of the saccharose concentration, putting in evidence the possibility of monitoring the process state by measuring the ultrasound propagation velocity.     

A free-space-based differential phase shift quantum key distribution scheme with higher key creation efficiency

Differential phase shift quantum key distribution scheme has been improved by introducing a passive path selection structure at the transmitter including two polarization controllers used to mark the photon pulses and a polarizing beam splitter to distinguish these pulses so that can make different time delay at the receiver. Theoretical analysis indicates that a free-space-based differential phase shift quantum key distribution system with this proposed improvement will have high efficiency with the utilization ratio of photons reaching 100% under ideal condition.     

A large ultrasonic bounded acoustic pulse transducer for acoustic transmission goniometry: modeling and calibration

A large, flat ultrasonic transmitter and a small receiver are developed for studies of material properties in acoustic transmission goniometry. While the character of the wave field produced by the transmitter can be considered as a plane wave as observed by the receiver, diffraction effects are noticeable near critical angles and result in the appearance of weak but detectable arrivals. Transmitted ultrasonic waveforms are acquired in one elastic silicate glass and two visco-elastic acrylic glass sample plates as a function of the angle of incidence. Phase velocities are determined from modeling of the shape of curves of the observed arrival times versus angle of incidence. The waveform observations are modeled using a phase propagation technique that incorporates full wave behavior including attenuation. Subtle diffraction effects are captured in addition to the main bounded pulse propagation. The full propagation modeling allows for various arrivals to be unambiguously interpreted. The results of the plane wave solution are close to the full wave propagation modeling without any corrections to the observed wave field. This is an advantage as it places confidence that later analyses can use simpler plane wave solutions without the need for additional diffraction corrections. A further advantage is that the uniform bounded acoustic pulse allows for the detection of weak arrivals such as a low energy edge diffraction observed in our experiments.     

A model and experimental study of fiber orientation effects on shear wave propagation through composite laminates

The strong elastic anisotropy of the discrete unidirectional plies in a composite laminate interacts sensitively with the polarization direction of a shear ultrasonic wave propagating in the thickness direction. The transmitted shear wave can therefore be used to detect errors in the ply orientation and stacking sequence of a laminate. The sensitivity is particularly high when the polarization directions of the shear wave transmitter and receiver are orthogonal to each other. To understand the interaction between normal-incident shear waves and ply orientations in a laminate, a complete analytical model was developed using local and global transfer matrices. The model predicted the transmitted signal amplitude as a function of polarization angle of the transmitter and time (or frequency) for a given laminate and input signal. To alleviate the experimental problems associated with shear wave coupling, electromagnetic acoustic transducers (EMATs) and metal delay lines were used in the angular scan of the transmitted signal. The EMAT system had the added advantage of being applicable to uncured composite laminates. Experiments were performed on both cured and uncured laminates with common layups for model verification. The sensitivity of the measured shear wave signals to fiber misorientation and stacking sequence errors was also demonstrated.     

Ultrasonic surface crack characterization on complex geometries using surface waves

The characterization of surface cracks on complex geometries using surface waves is investigated numerically and experimentally. The specimen geometry is implemented in a finite difference code by approximation of the contour using a Cartesian grid. In the experiments the out-of-plane surface displacement is measured by means of a heterodyne laser interferometer. Good agreement is shown by comparison of the calculated out-of-plane displacement with experimental results for both cracked and non-cracked specimens. The crack depth is measured down to a size of 0.7 times the surface wavelength using a time delay approach. The many Rayleigh pulses propagating after the crack can be separated from the other modes by a filtering procedure based on the surface wave propagation velocity. Only a detailed analysis of the scattering phenomenon using the simulation allows an identification of the transmitted pulse required for crack depth measurement. Application of the method to a specimen with a real fatigue crack shows a systematical error possibly due to the inclined crack profile.     

Analysis of the axial transmission technique for the assessment of skeletal status

Ultrasonic wave propagation in human cortical bone has been investigated in vitro using the so-called axial transmission technique. This technique, which relies on velocity measurement of the first arriving signal, has been used in earlier investigations to study bone status during fracture healing or osteoporosis. Two quasi-point-source elements, one transmitter and one receiver (central frequency 0.5 MHz), were used to generate a wide ultrasonic beam, part of which strikes the sample surface at the longitudinal critical angle, and to receive the signals reflected from the sample surface. The analysis of the field reflected from a fluid-solid interface for an incident spherical wave predicts the existence of a lateral wave propagating along the sample surface at a velocity close to the longitudinal velocity, in addition to the ordinary reflected wave and vibration modes. The transducer-sample and the transmitter-receiver distances were chosen such that the lateral wave is the first arriving signal. Validation of the measuring technique was performed on test materials and was followed by experiments on human cortical bones. Experimental results (arrival time and velocity) strongly suggest that the first detected signal corresponds to the lateral wave predicted by theory.     

How to measure the optical thickness of scattering particles from the phase delay of scattered waves: application to turbid samples

We present a method based on the optical theorem that yields absolute, calibration free estimates of the optical thickness of scattering particles. The thickness is determined from the phase delay of the zero angle scattered wave. It uses a heterodyne scattering scheme operating in the Raman-Nath approximation. The phase is determined by the position of Talbot-like modulations in the two dimensional power spectrum S(qx, qy) of the transmitted beam intensity distribution. The method is quite insensitive to multiple scattering. It is successfully tested to provide quantitative verification of the optical theorem. Exploratory tests on soft matter samples are reported to suggest its wide applicability to turbid samples.     

梯度特异材料表面对入射高斯光束的反射研究

最近人们[1-3]提出了一类反射式梯度特异材料表面系统,此系统可以高效地将入射波转换成被束缚于材料表面的表面波,这个系统成为连接行波和表面波的一座桥梁。以往人们研究梯度特异材料表面的反射特性时多是以入射平面波为主,这里我们结合实际,以入射的高斯光束作为研究对象,讨论梯度特异材料表面对其的反射特性,尤其是高斯光束的主入射角和束腰对反射行为的影响,得到了与平面波入射不同的反射现象。对于临界角为17.2°的梯度特异材料表面,束腰为4的高斯光束以主入射角分别为0°、10°、30°入射时,我们发现随着主入射角的变大,入射高斯光束转变为表面波的比例增大;而固定主入射角为30°,我们通过改变束腰,发现束腰越大,波矢分布越窄,越接近于平面波,转变为表面波的效率就越高。在数值模拟中,反射光束出现了明显的Goos-Hansen位移现象。同时还出现了类似光栅反射的特点,即分叉现象,这是由于模拟计算时网格不能无限小,梯度特异材料表面不能模拟为折射率连续变化的材料而应该是阶跃式变化的材料。由于实际制备的梯度特异材料表面是人工微结构的排列,因此微小的阶跃式变化的梯度特异材料表面更为实际。我们的工作对于利用梯度特异材料表面将入射高斯光束转变为表面波具有指导意义。     

Application of the wavenumber jump condition to the normal and oblique interaction of a plane acoustic wave and a plane shock

A kinematic approach is considered whereby the wavenumber jump conditions in conjunction with the appropriate dispersion relations is applied to the investigation of the normal and oblique interaction of a plane acoustic wave with a plane shock wave. For the normal interaction of an acoustic wave with a stationary plane shock a logarithmic shift in the wave spectra is obtained. For the normal interaction with a moving shock front it is shown that for shock Mach numbers above a critical value, the frequency of the transmitted wave becomes negative. This results in the fact that the crests of the transmitted signal arrive at a fixed observer in a reverse order to their generation. Finally, the oblique interaction of an acoustic wave with a stationary shock is considered. The “Snell's Law” for the transmitted wave is derived and two special angles of incidence are identified. The first is a no-refraction angle: i.e., the transmitted wave angle is the same as the incident wave angle. The second is a critical angle such that for incident angles greater than this critical angle there is no transmitted wave. A necessary and sufficient condition for the existence of a transmitted wave is derived in terms of the speed of sound and Mach number of the fluid and the frequency and tangential wavenumber component of the incident wave.The dynamics aspects of the interaction concerning the determination of the frequency independent transmission coefficients and shock displacements are determined for the simple case of the normal interaction with a moving shock as an illustration.     

计算电容中Fabry-Perot干涉仪测量位移的相位修正方法

计算电容是复现电学阻抗单位的基准装置, 利用计算电容值和量子霍尔电阻值可以准确计算出精细结构常数α. 计算电容的本质是通过高准确度地测量屏蔽电极的位移, 实现对电容量值的测量. 因此, 基于Fabry-Perot干涉仪的精密电极位移测量系统是计算电容装置中最为核心和关键的部分. 在Fabry-Perot干涉仪测位移过程中, 由于高斯激光束存在轴向Gouy相位, 该附加相位将会引起相邻干涉条纹对应位移的变化(大于或者小于λ/2), 导致位移的测量值与实际值存在偏差. 本文阐述了高斯激光场的传播特性, 利用高斯激光束在自由空间和透过薄透镜复振幅的变换关系, 建立了计算电容装置中Fabry-Perot干涉仪透射光束的传输模型; 通过对不同腔长的Fabry-Perot干涉仪透射光场相位的分析, 获得了高斯激光束轴向Gouy相位修正与传输距离的关系. 结果表明, 当腔长从111.3 mm移动至316.3 mm时, 在接收距离为560 mm的情况下, 高斯光束轴向Gouy 相位引起的位移修正的绝对值最小为0.7 nm, 其相对相位修正量|δL|/|ΔL| = 3.4×10^-9.     

多功能磁光子晶体太赫兹可调偏振控制器件

通过对磁场控制下的二维磁光子晶体太赫兹波偏振传输特性的研究,利用铁氧体磁光材料的磁导率随外磁场改变而变化的特点,设计了具有可控起偏、偏振分束和可调谐波片功能的光子晶体太赫兹偏振控制器件.利用平面波展开法和严格耦合波分析分别计算了光子晶体带隙位置和透过率随外磁场强度变化的关系,用时域有限差分法计算了场分布和相位.结果表明,该结构可以实现高偏振消光比的偏振起偏器和分束器,以及在1 THz附近-π-π相位范围的连续可调谐波片. 关键词： 太赫兹 光子晶体 铁氧体 偏振控制     

Measurement of absolute flow velocity vector using dual-angle, delay-encoded Doppler optical coherence tomography

Single-beam laser Doppler measurements of flow velocity are only sensitive to the velocity component parallel to the optical axis. We describe a simple modification to a standard Doppler optical coherence tomography (OCT) system using a single sample beam that provides velocity information from multiple angles within the beam. By introducing a glass plate midway into the OCT beam path, the sample beam is divided into several components, each with a different group delay and each providing a separate interferogram with its own effective Doppler angle. By combining the Doppler shift measured in each of these component interferograms, the flow velocity vector is fully determined.     

充水有限长圆柱薄壳声散射:Ⅱ.实验

从实验和理论上研究了充水有限长圆柱薄壳的声散射。实验采用指向性尖锐的宽带收发合置换能器,发射宽带调频信号和单频窄脉冲信号进行测量,所得结果分别在时域和频域上与理论计算结果相对比,证明其吻合程度很好。理论和实验的结果表明,内部充水圆柱壳在频域上增加了一组由内部流体负载引起的附加波的共振;而在时域上增加了一系列由于声波在壳体内来回反射产生的"回音廊"式回波。频率-角度图中"碗"形亮线出现分簇的原因解释为前几种模式"回音廊"式回波叠加的结果。      

A spiral wave front beacon for underwater navigation: transducer prototypes and testing

Transducers for acoustic beacons which can produce outgoing signals with wave fronts whose horizontal cross sections are circular or spiral are studied experimentally. A remote hydrophone is used to determine its aspect relative to the transducers by comparing the phase of the circular signal to the phase of the spiral signal. The transducers for a "physical-spiral" beacon are made by forming a strip of 1-3 piezocomposite transducer material around either a circular or spiral backing. A "phased-spiral" beacon is made from an array of transducer elements which can be driven either in phase or staggered out of phase so as to produce signals with either a circular or spiral wave front. Measurements are made to study outgoing signals and their usefulness in determining aspect angle. Vertical beam width is also examined and phase corrections applied when the hydrophone is out of the horizontal plane of the beacon. While numerical simulations indicate that the discontinuity in the physical-spiral beacon introduces errors into the measured phase, damping observed at the ends of the piezocomposite material is a more significant source of error. This damping is also reflected in laser Doppler vibrometer measurements of the transducer's surface velocity.     

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。     

Consequences of the Kramers-Kronig relations for light diffraction on thick gratings

Diffraction of monochromatic light on a grating leads to the attenuation of the transmitted beam of diffraction order zero. In the case of a thick grating the diffraction efficiency, and hence the effective attenuation coefficient, is a fast-varying function of the Bragg mismatch angle. According to Kramers-Kronig theory, the transmitted beam encounters a phase shift that also depends on the mismatch angle. This phase shift is measured for holographic gratings in a photoaddressable block copolymer and compared with analytical calculations.     

参考光束型激光多普勒测速仪的误差分析

阐述了参考光束型激光多普勒测速仪的基本原理,从理论上分析了原理公式近似、高斯光束干涉、激光线宽、探测器孔径、有限渡越时间、信号处理算法、空气折射率变化及角度测量等诸多因素对激光多普勒测速仪测量结果造成的误差,研究并给出了每一种影响因素所引起误差的理论公式,在每一种影响因素中具体讨论了各种光学参数对测量误差的影响并对部分参数进行实验研究。理论分析和实验表明:在实验中需要选择线宽较窄的激光器及孔径较小的光电探测器,而在选择光束光斑半径和信号光中心光线的方向与粒子运动方向的夹角时,既要考虑激光的质量和多普勒信号的强度,同时也需要考虑原理公式近似、探测器孔径的尺寸、有限渡越时间及高斯光束干涉等多种因素对测量结果的影响。     

Lamb waves beam deviation due to small inclination of the test structure in air-coupled ultrasonic NDT

In Lamb waves inspection, an air-coupled transmitter transducer is oriented at a specific angle such that it generates a pure Lamb mode which propagates along the structure and interacts with any existing defects. For this inspection system, amplitude losses appears when small inclinations of the tested structure occurs. An important factor which affects directly these losses has been observed, it consists of the Lamb waves beam (LWB) deviation due to this bad alignment. In this work, a simple expression of LWB deviation has been deduced. This expression includes the test structure angle, phase velocity of generated Lamb mode, and the phase velocity of waves propagating in the coupled medium. A(0) Lamb mode is generated and detected in 1 mm thick aluminium plate sample using 1 MHz PZ27 piezoelectric transducers of 20 mm of diameter. Experimental LWB deviation angles are measured for different inclination angles of the test sample. A comparative study is released with theoretical results. For 1 degree of misalignment in the aluminium plate inclination, and transducers separation distance of 35 mm, LWB deviation angle is around 7 degrees and the amplitude is reduced by around 11%. Then, for a large separation distance, we must move the receiver transducer to detect the deviated LWB. It is shown that, for both theoretical and experimental studies, the LWB deviation and its measured amplitude are very sensitive to the alignment of the tested structure with respect to the transmitter-receiver transducers plane. In metal plates it is most satisfactory to use A(0) mode compared with S(0) mode since it is easy to excite and has a large amplitude and small deviation beam angles.