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     

辅助紫光提高菌紫质全息衍射效率的实验和理论研究

由于菌紫质样品的饱和吸收特性，在全息记录中，当记录光强大于样品的饱和光强时，全息光栅透过率随记录光相位差的分布远离余弦型，因此衍射效率的稳定值很低.菌紫质样品在红光和紫光共同作用下存在着双光束互补抑制效应，紫光可以抑制红光的透过率，提高红光的饱和光强，使记录区域由非线性区移至线性区，从而使全息光栅透过率随记录光相位差的分布变为余弦型，可以有效地提高全息衍射效率.实验证明，辅助紫光大大提高了菌紫质样品全息衍射效率的稳定值.根据此原理，建立了三光束全息光存储系统，在红光记录全息图的同时加入辅助紫光，可以使全息图衍射效率及衍射像的像质得到提高. 关键词： 菌紫质 全息存储 衍射效率 饱和吸收     

Diffraction from angular multiplexing slanted volume hologram gratings

The diffraction characteristics of the volume holographic gratings made by multiexposures with angular multiplexing during its construction is investigated. Because of the reflection by the interface between the emulsion and the substrate or the air there is an extra unslanted periodic structure inside a slanted grating, it will affect properties of the slanted volume holographic gratings. When the angle between the surface of the substrate and the grating plane of the slanted grating is less than a certain value, an extra peak accompanying the main peak will appear in the diffraction spectrum. But, when the angle is larger than the certain value, one designed and expected main peak will disappeared while the extra peak is kept and observed. This phenomenon limits the capacity of the volume hologram for the application in wavelength-division multiplexing (WDM).     

Performance improvement of a holographic double-exposure interferometer by heating of the photorefractive recording crystal

A holographic double-exposure interferometer for quasi real-time analysis is realized, utilizing a photorefractive BTO:Cd crystal for hologram recording. Heating of the crystal reduces the time required to reach the steady-state working situation. Light-induced absorption losses are reduced by heating as well. Additionally, the diffraction efficiency is increased by this method. A sixfold increase in the light intensity of the reconstructed interferogram is achieved. Received: 2 December 1998 / Revised version: 28 January 1999 / Published online: 7 April 1999     

菌紫质同线偏振全息记录时再现光偏振方向对衍射效率的调制

当用相同偏振方向的物光和参考光在菌紫质薄膜上记录光栅时,再现光的偏振方向会影响其衍射效率.采用琼斯矩阵方法对此进行了理论分析,结果显示,再现光的偏振方向对衍射光的光强产生余弦调制;当再现光的偏振方向平行或垂直于记录光的偏振方向时,衍射光仍为线偏振光,其他情况下衍射光均变成椭圆偏振光.再现光偏振方向对衍射效率峰值的调制为正向余弦调制,对衍射效率稳定值的调制为反向余弦调制(与前者在相位上相差π).加入辅助紫光可抑制光栅的饱和,从而使得再现光偏振对衍射效率稳定值的调制由反向余弦调制变为正向余弦调制,并且提高了衍 关键词： 菌紫质 光致各向异性 衍射效率 琼斯矩阵     

A method for the evaluation of the grating envelope in volume holographic media

A novel method for measuring the three-dimensional spatial distribution of gratings in a volume holographic medium is described. In this method, the diffraction of a uniform probe light wave is measured as a function of the deviation (either in angle or wavelength) from the Bragg condition. The envelope function of the grating is obtained by solving the coupled wave equations inversely. Not only the amplitude but also the phase of the envelope function is obtained. This method allows for the depletion of the probe light wave in the hologram and is applicable to both dielectric (refractive index) and absorption gratings. The resolution and the maximum measurable depth in this method are discussed.     

Effects of applied DC/AC fields in electrical fixing process with SBN:60

We present experimental investigations of electrical fixing techniques in Ce:SBN:60. The effect of crystal fatigue on the diffraction efficiency of electrically fixed photorefractive gratings is studied. We observed that applying an ac field to the crystal eliminated crystal fatigue and improved diffraction efficiency. A controllable diffraction efficiency of a photorefractive grating is presented. A reproducible diffraction efficiency of up to 75% is obtained using a write-reveal grating technique with high-voltage pulses of opposite polarity. We also show that the diffraction efficiency can be controlled by varying the intensity of the writing beams incident to the crystal during the recording process. A method of determining the hysteresis loop using the domain-fixing technique is proposed. The dependence of the diffraction efficiency on the direction of the applied electric field while writing the hologram is studied. Our experiments show that, during writing, when the electric field is applied opposite to the c axis the grating can be successfully revealed with both positive and negative dc voltages. However, when the grating is written with a field parallel to the c axis, the grating can be revealed only with a field applied in the opposite direction. Received: 28 October 2002 / Revised version: 28 February 2003 / Published online: 14 May 2003 RID="*" ID="*"Corresponding author. Fax: +1-812/872-6167, E-mail: azad.siahmakoun@rose-hulman.edu     

大角带宽高衍射效率体全息光栅的设计和制备

高性能体全息光栅是全息波导的重要耦合元件,角带宽小、平均衍射效率不高是制约体全息光栅性能的重要因素。以不对称倾斜记录为出发点,设计并制备了大角带宽高衍射效率的体全息光栅。首先讨论在横电模式光和横磁模式光下体全息光栅的记录参数与其衍射效率的关系,找到平均衍射效率较高的记录参数范围,随后进一步分析在此范围内的记录参数与体全息光栅的角带宽的关系,从而确定获得大角带宽高衍射效率体全息光栅的最佳记录参数。实验结果表明:在参考光入射角度为25°、信号光入射角度为30°时,制备的体全息光栅的角带宽达到±14°,衍射效率为82%。     

Polarized-light-controlled holographic recording in an azobenzene-doped polymer film

A new polarized-light-controlled holographic recording based on the optical enhancement/restraint of self-diffraction has been demonstrated in an azobenzene-doped polymer film. It is found that a continuous variation of the polarization status as well as of the intensity of the pumping light results in a continuous variation of self-diffraction efficiency. The mechanism originates from the photo-induced anisotropy and polarization-dependent absorption. Both positive and negative replicas of an incident image were presented in real time by means of this incoherent–coherent optical conversion technique. Received: 30 March 2000 / Revised version: 28 September 2000 / Published online: 27 April 2001     

Investigations of As-S-Se thin films for use as inorganic photoresist for digital image-matrix holography

As-S-Se chalcogenide thin films are successfully employed in classical and dot-matrix holography as inorganic photoresists for obtaining a relief-phase hologram. However using these films for image-matrix hologram recording has not been studied due to some features of image-matrix technology. For the applied research of the optical properties of As-S-Se films an experimental device of digital image-matrix holographic recording based on 100 mW 405 nm semi-conductor laser and Spatial Light Modulator (SLM) has been created. The device has the following main parameters: 140 × 105 μm frame size; laser intensity during exposure 10 W/cm². With the help of this device diffraction grating and security holograms were recorded on As-S-Se thin films. The work reported herein presents results of an experimental study of how diffraction efficiency (DE) of the received relief-phase holographic gratings depends on an exposure and period. Diffraction grating profiles and speed of etching corresponding to different exposure doses are shown. Hologram samples with DE = 65% have been received which allows for using chalcogenide film as alternative to organic photoresists in applied dot-matrix and image-matrix holography.     

光强对(Cu,Ce)\:LiNbO3晶体非挥发性全息记录性质的影响

实验研究了 (Cu ,Ce)…LiNbO3 晶体用蓝光敏化红光全息记录的光强对记录灵敏度和饱和衍射效率的影响。敏化光增加了记录灵敏度 ,足够高的光强将导致灵敏度的饱和行为。记录光以亚线性的关系Ixr(x <1)增大记录曲线 η1/ 2 t初始记录阶段的斜率 ,因而降低了记录灵敏度。饱和衍射效率对记录光和敏化光光强的依赖关系表明了存在饱和衍射效率的极大值     

近化学配比LiNbO3晶体双色光全息记录的实验研究

实验用红光记录紫光敏化,研究了几种不同掺杂浓度和氧化还原状态的近化学配比铌酸锂晶体的双色全息记录特性,如其饱和衍射效率和记录灵敏度的变化规律。实验结果表明,晶体的饱和衍射效率随透射光强与紫外光强比值的增加先增加后减小,而记录灵敏度在不断减小。Mn含量相同时,Tb含量越高的晶体饱和衍射效率越高,而记录灵敏度越低。Tb含量相同时,含Mn比不含Mn的晶体的饱和衍射效率低,但记录灵敏度更高。还原晶体的饱和衍射效率和记录灵敏度均比氧化晶体高。记录光与敏化光的比值、掺杂浓度和氧化还原状态对晶体的全息特性产生影响。通过适当调整上述参数之间的关系,可对材料的全息记录特性进行优化。     

Design of High-Efficiency Diffraction Gratings Based on Rigorous Coupled-Wave Analysis for 800nm Wavelength

We report on the design of a high diffraction efficiency multi-layer dielectric grating with wide incident angle and broad bandwidth for 80Ohm. The optimized grating can achieve 〉 95% diffraction efficiency in the first order at an incident angle of 5° from Littrow and a wavelength from 77Ohm to 83Ohm, with peak diffraction efieieney of 〉 99.5% at 80Ohm. The electric field distribution of the optimized multi-layer dielectric grating within the gratings ridge is 1.3 times enhancement of the incidence light, which presents potential high laser resistance ability. Because of its high-effieieney, wide incident, broad bandwidth and potential high resistance ability, the multi-layer dielectric grating should have practical application in Ti：sapphire laser systems.     

Infrared holographic recording in lithium tantalate crystals by means of the pyroelectric effect

Infrared holographic recording in a two-step process is demonstrated in stoichiometric iron-doped lithium tantalate crystals. Through absorption of two intersecting infrared pulses (A = 1064 nm) a temperature grating and thus a modulated pyroelectric field build up. Free electrons, excited by homogeneous light of a shorter wavelength (lambda = 532 nm) drift in this field, and a phase hologram is stored that can be read nondestructively. The change in refractive index depends mainly on the absorption coefficient at the wavelength of the recording light and on the intensity of the infrared light. The proposed method may be extended to telecommunication wavelengths by choice of suitable dopants.     

Saturation effect in azobenzene polymers used for polarization holography

The saturation behavior of diffraction efficiency has been experimentally studied and theoretically modeled in azobenzene polyesters by recording polarization gratings with two beams of orthogonal circular polarization. The model is an extension of the phenomenological expression of Kakichashvili, derived from the rate equations for photoanisotropic materials, taking into account the effect of saturation with time and intensity. The saturation model of the polyester material can be used for the computer simulation of a polarization holographic data storage system under development. The simulation is based on fast Fourier transforms, taking the parameters of the recording material and the optical processes into account. Agreement between measured and simulated results proves the applicability of the saturation model as an efficient tool in the optimization of the system. Received: 16 September 2002 / Published online: 3 April 2003 RID="*" ID="*"Corresponding author. Fax: +36-1/463-4194, E-mail: pvarhe@math.bme.hu     

体全息光栅线性记录折射率分布的修正

在讨论非线性记录体全息光栅的光谱特性时,发现一些文献在线性记录时折射率分布的表达式有些不妥.重新讨论了线性记录情况下折射率分布的表达式,引入了光束比和总光强折射率调制系数,用耦合波理论计算讨论了不同光束比和总光强折射率调制系数对光栅衍射光谱特性的影响.计算结果表明,光栅衍射光谱主峰峰值随总光强调制系数的减小而略有降低,主峰峰值随光束比减小下降得不大,带宽随总光强调制系数和光束比的减小而明显变窄,次峰峰值随总光强调制系数和光束比的减小而明显减小.     

局域体全息光栅衍射的平面波与柱面波的波前转换研究

利用一维耦合波理论研究了入射方向相互垂直的一束平面波与一束柱面波干涉形成光折变局域体全息光栅的波前转换情况。给出了透射波和衍射波振幅的解析表达式。讨论了该局域体全息光栅的几何尺寸以及全息透镜的焦距对光栅衍射效率的影响。结果表明,随着光栅横向尺寸的增加,光栅的衍射效率也逐渐增加。然而,光栅的衍射效率却随着光栅纵向尺寸的增加而减小。全息透镜的焦距越长,光栅的衍射效率也越大。此外,分析了光栅的布拉格选择特性,该光栅具有非常好的角度选择特性。在平面波与柱面波干涉形成全息透镜的设计中,为了获得最优的衍射效率,应当根据要求合理地选择记录所用柱面波和设计光栅的几何尺寸。     

Investigation of the dynamic recording process of thick refractive index gratings in photorefractive crystals by the microphotometric method

Diffraction-efficiency measurements are compared with the microphotometric determination of the intensity pattern at the exit face of a LiNbO₃-sample during the holographic recording process. The special advantage of the microphotometric method is the possibility to determine phase shifts of the refractive index grating and of the light intensity pattern simultaneously and independently. Both quantities are very important for an understanding of dynamical effects during the holographic writing process. Furthermore, direct experimental evidence for the change of the modulation degree, i.e. the change of the contrast of the light pattern writing the grating, has been found. This change is caused by a Moiré-like effect which is closely related to the energy transfer between writing beams.     

Reconstruction of parasitic holograms to characterize photorefractive materials

Photoinduced light scattering is a serious drawback that limits the applicability of thick holographic recording media but provides valuable information on the recording medium. As long as there is no correlation between the scattering centers in the crystal, photoinduced light scattering may be explained to result from the interference pattern of the incident beam and the field scattered from a single point-like scattering center. The hologram of this ellipsoidally scattered wave field will have practically the same structure in the reciprocal space modified by a response function which reflects the anisotropic properties of the recording medium. We studied photoinduced light scattering in LiNbO₃:Fe, a model system for photorefractive materials. The transmitted intensity in the stationary state of the scattering process is investigated as a function of the reconstruction angle at different wavelengths and polarizations of the reconstructing beam. The experimental results are analyzed by a simple phenomenological model based on the Ewald construction and can be used to choose suitable conditions at which holographic scattering can be minimized as well as to extract some physical parameters of the crystal. Received: 27 September 2000 / Revised version: 1 December 2000 / Published online: 21 February 2001     

Hologram recording in glass and direct reconstruction using visible-wavelength laser beam

We propose a method of hologram recording in a glass plate by corona charging. The holographic recording materials used in this study are conventional soda-lime glass and azobenzene polymer film. A Fourier transform hologram on an azobenzene polymer film coated on the glass plate is recorded in the glass plate by corona charging. After removing the polymer film, the hologram recorded in the plate can be reconstructed using a visible-wavelength laser beam. The first-order diffraction efficiency of the hologram at a wavelength of 532 nm is 0.03%; the efficiency depends on the depth of the surface relief structure on the azobenzene polymer film and the corona-charging time. The hologram recorded in the glass plate has high environmental resistance.