Visualized measurement of the acoustic levitation field based on digital holography with phase multiplication

By using digital holographic interferometory with phase multiplication, the visualized measurement of the acoustic levitation field (ALF) with single axis is carried out. The digital holograms of the ALF under different conditions are recorded by use of CCD. The corresponding digital holographic interferograms reflecting the sound pressure distribution and the interference phase distribution are obtained by numerical reconstruction and phase subtraction, which are consistent with the theoretical results. It indicates that the proposed digital holographic interferometory with phase multiplication can successfully double the fringe number of the interference phase patterns of the ALF and improve the measurement precision. Compared with the conventional optical holographic interferometory, digital holographic interferometory has the merits of quasi real-time, more exactitude and convenient operation, and it provides an effective way for studying the sound pressure distribution of the ALF.     

Real-time holographic interferometry using sillenite photorefractive crystals. Study and optimization of a transportable set-up for quantified phase measurements on large objects

We present the development of a holographic interferometer that uses a photorefractive crystal of the sillenite family as holographic recording medium. The aim of this work is to achieve a transportable and breadboard instrument, with a flexible design and which is user friendly for the quantified measurements of displacements on large scattering objects. The state of the art of the use of photorefractive crystals in holographic interferometry is presented. Based on the latter, a method that is the best suited to our mentioned objectives is chosen: the real-time interferometric technique associated with the crystal configuration exhibiting diffraction anisotropy. On this basis, we have studied and compared two imaging systems, we have experimentally determined the holographic figures of merit of three sillenite crystals samples, and, finally, we have shown the use of two measurement quantification methods. The result of these investigations is a certified and transportable holographic camera prototype containing all the necessary equipment for its operation, which can be easily adapted to various applications. Among these, we show its use in the detection of defects in composite structures, in the measurement of time-evolving deformations, and in the measurement of vibration modes. Received: 4 December 1998 / Revised version: 6 March 1999 / Published online: 12 April 1999     

Multiple-scattering speckle in holographic optical coherence imaging

We investigate the effect of multiple scattering on the image quality of holographic optical coherence imaging, which is a full-field coherence-domain imaging form of optical coherence tomography. The speckle holograms from turbid media and from multicellular tumor spheroids are characterized by high-contrast speckle on a multiply-scattered background caused by channel cross-talk. We quantify the multiple-scattered light that is accepted by the holographic coherence gate, and identify a cross-over from single-scattered to multiple-scattered light beyond 15 to 20 optical thicknesses. Speckle reduction relies on vibrating diffusers and on fast adaptive holograms in photorefractive quantum well devices. The high anisotropy factor for tumor tissue reduces multiply-scattered light contributions for biomedical tumor imaging.     

Relationship between temporal coherence and laser parameters in a two-longitudinal-mode HeNe laser

The relationship between the temporal coherenceg ⁽¹⁾ () and laser parameters - the output relative intensity (I ₁/I ₂) and the frequency width _h of the longitudinal mode in a two-longitudinal-mode HeNe laser - is studied experimentally. The experimental results show that both the relative output intensity and the frequency width of the longitudinal mode have a great influence on the temporal coherenceg ⁽¹⁾ (). Moreover, the spectral line-shape function of each longitudinal mode in a two-longitudinal mode HeNe laser is a Lorentzian function, with a frequency width of about 18.2 MHz. A model of the power spectrum with Gaussian amplitude distribution and equal frequency width of the longitudinal mode in a multi-longitudinal-mode gas laser is employed in the theoretical analysis. Good agreement between theory and experiment is obtained.     

Effects of recording wavelength on three-dimensional vector holograms in photoreactive liquid crystal composites

Effects of recording wavelength on the recently proposed (Sasaki, 2008) three-dimensional vector holograms, in which the optical anisotropy is three-dimensionally modulated, are presented experimentally and theoretically. The polarization states of the interference light are three-dimensionally modulated due to both the polarization interference and optical anisotropy in the recording medium. These spatial distributions of the polarization states and the resulting diffraction properties in the three-dimensional vector holograms are strongly dependent on the recording wavelength. Theoretical consideration based on the finite-difference time-domain (FDTD) method reveals the mechanism of the optical characteristics of the three-dimensional vector holograms recorded by various kinds of light sources with different wavelengths.     

Relationship between coherence length and output power of a multi-longitudinal-mode Ar+ laser

The relationship between the temporal coherence (i.e., the coherence length) and the output power of a multi-longitudinal-mode Ar⁺ laser operated near the threshold current is studied. The experimental measurements show that the coherence length is a hyperbola as a function of the output power. A simple model of the multimode equal-amplitude power spectrum is employed in the theoretical analysis. Good agreement between theory and experiment is obtained. It is shown that for a multi-longitudinal-mode Ar⁺ laser, the product of coherence length and the output power is almost a constant. Mailing address: Department of Physics, Suzhou University, Suzhou, Jiangsu 215006, People's Republic of China     

Influence of auxiliary violet light on holographic kinetics at low and high recording intensities in bacteriorhodopsin film

The photochromic bacteriorhodopsin (BR) film can be used as a rewritable holographic recording medium. Due to the saturation absorption and the scattering and reflecting lights from the BR film, the grating contrast-ratio of the hologram is diminished during the hologram recording. When the intensity of the recording light is low, the influence of the saturation absorption and the scatter and reflection of BR film on the grating contrast-ratio is weak. But for the case of intense recording light, this influence is more serious. It is found that the influence of the auxiliary violet light on the holographic kinetics of diffraction efficiency is distinct under different recording intensities. At the low recording light intensity, the steady diffraction efficiency is increased and the peak diffraction efficiency is suppressed by the auxiliary violet light. But for intense recording light, the steady diffraction efficiency and the peak diffraction efficiency are both increased by the auxiliary violet light. Based on the two-state model of BR photochromism, we give a good theoretical explanation to the above phenomena.     

Associative recall in a volume holographic storage system based on phase-code multiplexing

We present two different techniques on how to realize a content-addressed holographic memory when using phase-code multiplexing, relying on simple intensity measurements rather than phase distributions. Theoretical and experimental results of associative recall in a phase-coded system designed for digital data storage will be presented and compared to the corresponding method when using angular multiplexing. Received: 2 August 2001 / Revised version: 20 September 2001 / Published online: 23 November 2001     

Design of optical filter for increased efficiency of wavelength converters based on fiber XPM

An optical filter, that remarkably increases the efficiency of fiber wavelength converters is proposed. Simulations show that required input powers are reduced by 50% and the maximum output powers are increased by a factor of three, when compared with equivalent systems. It is also shown that the proposed filter leads to back-to-back power penalties of 2 dB, increasing the dispersion tolerance of the converted signal by 25%. The filter’s practical implementation aspects of are assessed, showing that the main limitation results from highly demanding spectral characteristics.     

Nonlinear diffraction in gratings based on polymer–dispersed TiO 2 nanoparticles

In this letter we report a simple technique to produce volume holographic gratings based on photopolymerizable composites containing TiO₂ nanoparticles. Diffraction gratings with high refractive index modulation amplitude (up to 1.25 × 10⁻²) have been formed due to the periodic distribution of high refractive index nanoparticles in a low refractive index polymer matrix. The diffraction efficiency increases strongly on increasing the nanoparticle concentration. Taking the mixture with 10 wt.% TiO₂ nanoparticles, gratings with high diffraction efficiency, low level of scattering and high transparency in the visible-wavelength range have been obtained. This will ultimately lead to different applications of diffractive optical elements based on nanocomposites. The dependence of the gratings’ diffraction efficiency on the intensity of probe laser pulses at 1064 nm has been explored. It is shown that the nonlinear response of the gratings is attributed mainly to the nonlinear properties of the TiO₂ nanoparticles embedded in the polymer matrix. The mechanism of the grating formation and the reasons for the nonlinear behavior of the diffraction efficiency are discussed.     

基于干涉条纹图像对比法测量微位移

研究了基于迈克尔逊干涉条纹对比法测量微位移的实验。用He-Ne激光器、反射镜和分束镜组成的干涉光路,其中一个反射镜固定在被测物体上,通过被测物体的移动带动反射镜移动使干涉光路发生变化,从而导致干涉条纹的改变。在形成干涉条纹的位置利用线阵CCD采集干涉条纹图像,用序列图像对比的方法对图像进行处理和计算,得出被测物体的微位移。实验表明利用序列图像对比的方法测量微位移,方法切实可行、测量准确度高、测量的精度能够达到微米量级。     

Full-field optical coherence microscopy based on acousto-optic filtration of interference images

The problem of three-dimensional near-surface imaging of microscopic structures is discussed. Anew scheme of optical coherence microscope with detection in the spectral domain is proposed. It is based on a broadband light source and filtration of interference images by a tunable acousto-optic imaging filter placed in the detection channel. The applicability of the scheme for analyzing depth distributions of object properties is experimentally demonstrated.     

光电编码器单莫尔条纹测速方法

为了提高编码器的测速精度,研究了基于希尔伯特-黄变换的光电编码器单莫尔条纹测速方法。首先,利用AD采集编码器的单路莫尔条纹光电信号,并将信号序列通过EMD变换,滤除直流分量;然后,利用希尔伯特变换求出信号的相位变化,并通过差分运算求取信号的瞬时频率;最后,结合编码器的具体参数求取编码器的转速。实验结果表明:对某21位编码器进行测速实验,测速误差均方差由0.022 4 rad/s降低到0.013 4 rad/s。此方法测速稳定性高,抗干扰能力强,可用于速度精度要求较高的测速场合。     

New photopolymerizable holographic recording material based on polyvinylalcohol and 2-hydroxiethylmethacrylate (HEMA)

We have developed a new polyvinylalcohol-based photopolymeric holographic recording material. The recording is obtained by the copolymerization of acrylamide and 2-hydroxiethylmethacrylate (HEMA). Diffraction efficiencies near 70% are obtained with exposures of ≈65 mJ/cm² in materials of 110-μm thickness. The addition of HEMA increases the thickness of the film, which results in improved storage capacity for holographic optical storage. Received: 7 March 2002 / Published online: 2 May 2002     

干涉法测量光栅常数

讨论了一种测量光栅常数的新方法.这一方法基于通过横向移动光栅改变各级衍射条纹之间的相位关系,进而改变相邻衍射条纹之间的干涉效应,从而完成光栅常数的测量.同时给出了运用这种方法完成光栅常数测量的一般条件,并简单地同其他几种测量方法进行比较.     

一种基于光学干涉原理的图像分割方法

 介绍了基于光学干涉原理的图像分割理论算法。在计算机上模拟一个均匀相干光源、一个滤波器和一个接收屏。选择强度 相位的转换曲线，将输入图像的强度信息转化成伪相位信息，并显示在滤波器上。利用光源照明滤波器上的相位图，在接收屏上得到其干涉条纹。再通过选择阈值对干涉图进行分割得到二值图，从而实现输入图像的边缘分割。计算结果表明，该方法得到的图像轮廓，与Sobel 算子效果相当。与实验结果对比分析可得，改善硬件并实现良好的分割效果的关键在于：控制光源的相干长度，或增大液晶的点阵间隔，并利用CCD相机和计算机相连，对干涉图进行二值化。     

Double image encryption based on iterative fractional Fourier transform

We present an image encryption algorithm to simultaneously encrypt two images into a single one as the amplitudes of fractional Fourier transform with different orders. From the encrypted image we can get two original images independently by fractional Fourier transforms with two different fractional orders. This algorithm can be independent of additional random phases as the encryption/decryption keys. Numerical results are given to analyze the capability of this proposed method. A possible extension to multi-image encryption with a fractional order multiplexing scheme has also been given.     

Comparison of transient process between open and closed ladder-type systems with spontaneously generated coherence and incoherent pumping

We theoretically investigated transient response of open and closed three-level ladder-type atomic system with or without the spontaneously generated coherence (SGC) which could be satisfied with the help of an incoherent pumping. The existence of the SGC effect makes the open and closed system to be distinguished. We compared transient response of weak probe between open and closed system and found that transient properties exhibit different features by adjusting some related parameters, such as the relative phase between probe and coupling fields, the angle between two dipole moments.     

An improved quantum key distribution protocol based on second-order coherence

We improve the quantum key distribution protocol proposed by Pereira et al. [S.F. Pereira, Z.Y. Ou, H.J. Kimble, Phys. Rev. A 62 (2000) 042311], by employing the second-order coherence of optical fields, which can be easy experimentally measured with a Hanbury-Brown and Twiss intensity interferometer. It is shown that eavesdropping can be directly detected without sacrificing extra secret bits as test key. The efficiency of the improved system is enhanced greatly, since no secret bit needs to be discarded.     

马赫-曾德光纤干涉光谱测量系统的设计

光纤干涉光谱仪由于其体积小、分辨率高、抗电磁干扰和耐高温的特点,广泛应用于各种领域的物质检测中。设计了一种使用电动光纤延迟线实现光程差调制的基于Mach-Zehnder光纤干涉仪的光谱测量系统。介绍了系统的构成,推导了基于该系统的光谱计算公式以及分辨率计算公式。搭建了实验平台,对一宽谱光源进行测试,并采集其干涉信号,解调出了相应的光谱图。对结果进行分析,当最大光程差变为0.479 5 cm时,光谱分辨率为2.085 4 cm-1。电动光纤延迟线的性能在很大程度上决定了最终解调出的光谱的分辨率、精度,尤其是速度和延时时间的精度和线性度。