Photorefraction and anisotropic light scattering in LiNbO3-Fe crystals

By holographic recording in LiNbO₃-Fe and LiTaO₃-Fe crystals a new light scattering effect has been observed with an optical indicatrix along the optical axis. The kinetics of the light scattering depends on the intensity, wavelength as well as the polarization of the incident light The holographic volume grating is created by the interference of an incident light and light scattered by crystal inhomogeneities.     

用体全息术对皮秒光脉冲进行二维测量

本文提供一种测量两束相干的ps光脉冲的空间二维时间振幅相关函数的新方法.在此方法中,光脉冲是被记录在一种光致折变的体积全息介质中,而所记录的信息可以通过直接量度在介质中所形成的光栅的空间分布而读出.这种方法可的可行性已为实验证实,它是通过3.5ps倍频锁模Nd:YAG激光测量光致折变晶体LiNbO₃来实现的。     

Noise gratings in silver halide volume holograms

Diffraction efficiencies of 70% have been reported for planar gratings in silver halide emulsion. The main obstacle in increasing the efficiency is the granular structure of the recording material which causes scattering of the input light both at recording and at reconstruction. This study concentrates on the effect of noise gratings recorded when the incident beam interferes with its own scattered radiation. The experimental evaluation is divided into two parts. Firstly, one single beam is used for recording a holographic plate, and the transmitted intensity of the reconstructing beam illuminating the developed hologram is measured as a function of incident angle and wavelength. Dips in the measured intensity are associated with the reconstruction of a strong scattered beam. Secondly, a main grating is recorded by two incident beams, which also interfere with the field scattered by the grains and give rise to noise gratings. When a beam is incident upon the developed hologram at the same angle and wavelength as one of the recording beams then besides the desired beam the scattered radiation is also reconstructed. This may reduce the otherwise available efficiency by several percent.     

光折变晶体均匀多重全息图存储研究

本文推导了光折变晶体角度编码多重全息存储中写入均匀光栅时的曝光时间递推公式.分析了散射效应对光栅写入时间常数的影响和写入光耦合对光栅振幅的影响,给出了两种因素影响下曝光时间计算的修正递推公式.数值计算结果表明,按照这种修正公式计算所得多重存储中各幅全息光栅振幅不仅均匀性好,而且振幅相对较大,这种曝光方法有利于提高晶体的存储容量.实验中以递推公式所得时间进行曝光记录,在厚度为0.6mm的Fe:LiNbO₃晶体中采用角度编码很容易存储了30幅全息图.     

α-LiIO3单晶中离子输运引起的高强度准弹性光散射

激光束通过加c向直流电压的α-LiIO₃单晶时,发生三种准弹性散射:散射光电位移矢量有特定取向且与入射光电位移矢量方向不一致者,散射光电位移矢量与入射光电位移矢量方向相同而散射带垂直c轴者以及散射光电位移矢量与入射光电位移矢量方向相同而散射带对c轴倾斜者,本文详细研究了这三种散射的空间频谱、精细结构、结构敏感性、弛豫规律以及光散射和晶体内电流密度与局部电场的关系,发现光散射发生于晶体内电流密度较大的区域,本文对三种散射的起因给予了说明:离子导体在直流电压作用下空间电荷的分布和运动引起 关键词：     

Optical holographic memory using angular-rotationally phase-coded multiplexing in a LiNbO3:Fe crystal

We propose a new holographic memory scheme based on random-phase-coded multiplexing in a photorefractive LiNbO₃:Fe crystal. Experimental results show that rotating a diffuser placed as a random-phase modulator in the path of the reference beam provides a simple yet effective method of increasing the holographic storage capabilities of the crystal. Combining this rotational multiplexing with angular multiplexing offers further data-storage possibilities. The advantage of using post-image versus pre-image phase modulation of the object beam is demonstrated. Received: 15 August 2000 / Published online: 10 January 2001     

Suppression of the photoinduced light scattering in LiNbO3:Fe by redox treatment and incoherent homogeneous illumination

In this paper, the redox treatment and incoherent homogeneous illumination were tried for the suppression of the photoinduced light scattering in LiNbO₃:Fe. Meanwhile, the effects of both the redox treatment and incoherent homogeneous illumination on the holographic storage properties of LiNbO₃:Fe were also studied. Our experimental data show that the reduction treatment sacrifices the diffraction efficiency for the suppression of photoinduced light scattering in LiNbO₃:Fe. By contrast, incoherent homogeneous illumination with the wavelength of 460 nm is efficient to suppress the photoinduced light scattering without any tradeoff on the holographic storage performance of LiNbO₃:Fe. The superiority of homogeneous illumination over the reduction treatment was attributed to the untouched acceptor concentration under the homogeneous illumination.     

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.     

Denisiuk-type reflection holography display with sillenite crystals for imaging and interferometry of small objects

A novel setup for imaging and interferometry through reflection holography with Bi₁₂TiO₂₀(BTO) sillenite photorefractive crystals is proposed. A variation of the lensless Denisiuk arrangement was developed resulting in a compact, robust and simple interferometer. A red He-Ne laser was used as light source and the holographic recording occurred by diffusion with the grating vector parallel to the crystal [0 0 1]-axis. In order to enhance the holographic image quality and reduce noise a polarizing beam splitter (PBS) was positioned at the BTO input and the crystal was tilted around the [0 0 1]-axis. This enabled the orthogonally polarized transmission and diffracted beams to be separated by the PBS, providing the holographic image only. The possibility of performing deformation and strain analysis as well as vibration measurement of small objects was demonstrated.     

低噪音光折变体全息存储

光折变全息存储中,透过晶体的散射光会影响再现象的信噪比,本文基于两波耦合理论,对光折变存储中的散射噪音随光斑尺寸的变化作了简化计算和分析,结果表明减小辐照在晶体上的光斑尺寸能降低散射噪音,提高衍射效率和信噪比,采用球面参考波小光斑尺寸入射,在KNSBN光折变晶体中存储了傅里叶全息图,并得到了低噪音的再现象.     

Annular structures in photorefractive light scattering in a LiNbO3:Fe crystal

Ring-shaped photoinduced light scattering in pure and iron-doped lithium niobate crystals is found and studied under irradiation by a laser beam directed perpendicularly to the optic axis of the crystal. The scattering is observed in the directions of both reflected and transmitted pump beams. The structure and kinetics of the reported photorefractive scattering suggest that the phase-matching condition should be fulfilled in the vector interaction of the pump and scattered beams.     

Holographic recording in LiNbO3 using various polarized lights

Holographic recordings in a LiNbO₃ crystal using ordinary and extraordinary light in transmission-type geometry were performed. The system parameters of storage capacity (M/#), sensitivity, and signal-to-noise ratio were measured. Although extraordinary light will induce stronger fanning light (nonlinear enhanced scattered light) in LiNbO₃, our experimental results indicate that the system using extraordinary light is superior to that using ordinary light. Our demonstrations and results can help to optimize the holographic recording in some applications of volume holographic optical element (VHOE), such as wavelength division multiplexing (WDM) filters in optical communication.     

Study of near-infrared nonvolatile two-center holographic recording in LiNbO3:Fe:Rh crystal

The near-infrared nonvolatile holographic recording has been realized in a doubly doped LiNbO₃:Fe:Rh crystal by the traditional two-center holographic recording scheme, for the first time. The recording performance of this crystal has been investigated by recording with 633 nm red light, 752 nm red light and 799 nm near-infrared light and sensitizing with 405 nm purple light. The experimental results show that, co-doped with Fe and Rh, the near-infrared absorption and the photovoltaic coefficient of shallow trap Fe are enhanced in this LiNbO₃:Fe:Rh crystal, compared with other doubly doped LiNbO₃ crystals such as LiNbO₃:Fe:Mn. It is also found that the sensitizing light intensity affects the near-infrared recording sensitivity in a different way than two-center holographic recording with shorter wavelength, and the origin of experimental results is analyzed.     

Two-wave mixing and energy transfer in BaTiO3 application to laser beamsteering

Energy transfer between the two recording beams in a BaTiO₃ photorefractive crystal is analyzed as a function of the following parameters: incident beams ratio, spatial frequency, pump beam intensity. Exponential gain coefficients Γ ≈ 20 cm^-1 are reached for optimized holographic recording conditions. Application of the energy transfer to a new method of laser beam deflection is proposed.     

Light-induced scattering in SBN：Cr crystal under external electric fields and its suppression

In this paper, the enhancement of light-induced scattering in congruent SBN:Cr (Sr_{0.61}Ba_{0.39}Nb_2O_6:Cr) crystals in the presence of an externally applied electric field and its suppression are studied. If a coherent image is focalized in SBN:Cr crystal without applying external electric field, the output image will remain clear, because of the weak photorefractive effect in the crystal. When a field is applied properly along the crystal axis, markedly enhanced scattering from the signal beam and the output image dispersion can be observed due to the increase of the photorefractive two-beam coupling gain and the light-induced index change in SBN:Cr crystals. By introducing a coherent or incoherent beam with higher intensity the light-induced scattering can be suppressed through the erasure of scattering gratings. The difference between coherent and incoherent beam is that the former can also amplify the signal beam as the scattered light is removed, whereas the latter can only make the signal beam revert to its initial state. The results obtained under different experimental conditions are consistent with theoretical analysis.     

不同记录光强下辅助紫光对菌紫质薄膜全息衍射效率的影响

具有光致变色效应的菌紫质薄膜可用作可擦重写型全息记录介质. 在全息记录过程中,由于菌紫质薄膜对记录光的散射和反射引起记录光栅对比度下降,当记录光较弱时,这种影响较小,可以忽略;但当记录光较强时,散射和反射光对记录光栅的影响很大,必须考虑它们对光栅对比度的影响才能对实验结果进行合理的解释. 实验发现在不同的记录光强下辅助紫光对衍射效率动力学曲线的影响不同,当记录光强较弱时,加入辅助紫光可以提高衍射效率的稳定值、抑制峰值;而当记录光强较强时,加入辅助紫光除了提高衍射效率的稳定值外,还可以提高衍射效率的峰值. 关键词： 菌紫质 衍射效率 干涉条纹对比度     

c向直流电压作用下α—LiIO3单晶的光的异常散射的进一步研究

文献[1]发现激光束通过c向直流电压作用下的α-LiIO₃单晶时,产生一种电矢量有特定取向且不同于入射光电矢量方向的异常散射带。本文进一步研究了这种散射带的精细结构。确定散射光相对于入射光没有大于15MHz的频移。这种散射带的强度在透射光斑两侧分布不对称,一侧强,另一侧弱。我们发现两种绝对构型的晶体,散射带的强度分布正相反,D型晶体散射强的一侧,正是L型晶体散射弱的一侧。并研究了异常散射带位置和透射光斑的位置间的各种配置状况,发现它们取决于两个通光面相对于c轴的取向。 关键词：     

碱性品红光致聚合物薄膜的光致光衍射

本文研究了全息存储实验中以波长532 nm的半导体激光器作为记录和读出光源,碱性品红作为光敏剂的丙烯酰胺基光致聚合物薄膜的光致光衍射现 象.它可以考虑为透过全息干涉条纹的原始入射光和反射光,与来自光聚物中未曝光的不均匀表面和内部的散射光四波混频的结果.根据简并四波混频理论中的位相匹配条件对双光束全息写入时产生的衍射光锥,以及单光束辐照复现时产生的衍射光锥现象分别做出了理论解释;并且利用全息散射理论结合三角学知识对入射光的入射角与衍射光锥的锥角的依赖关系进行了深入讨论. 关键词： 全息 光致聚合物 衍射光锥 碱性品红     

Three-wave interference with participation of polaritons

Second-order interference with respect to the field strength during spontaneous parametric light scattering was studied under the conditions of absorption at the frequencies of idler (polariton) modes. An expression is derived that describes the scattered light intensity as a function of the scattering angle for an arbitrary shape of the nonlinear interaction region. The interference was experimentally studied for the light scattering on polaritons in a nonlinear crystal (lithium iodate, iodic acid), with a double slit placed into a pumping beam in front of the crystal.     

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