在局域非线性响应的光折变晶体中的对称光扇

在具有非局域非线性响应的所有光折变晶体中几乎都能观察到光折变光扇效应．最近我们在具有局域非线性响应的光折变晶体（LiNbO₃∶Fe）观察到对称光扇．基于前向简并三波作用，从理论上研究了这种对称光扇的形成 关键词：     

Incoherent one beam recording in LiNbO3

We find a way to record and retrieve images in a photorefractive LiNbO₃ crystal using a single white light source instead of a coherent light source. According to the experimental results, we think this recording is not due to the recording of fanning gratings, but a variation of refractive index responding to the non-uniform illumination. We have also simulated the recording using the band transport model taking into account the photovoltaic effect. The simulation result agrees with the experimental results.     

光折变晶体中的稳态光扇

指出在高增益的光折变晶体内光束自身的多波混合是光扇形成的一个可能起因．通过光束自身的多波耦合作用，光能从低空间频率成分转移至高频成分上，从而形成宽角分布的扇形轮廓，所以放大噪声和放大光束中高频成分在光扇效应中都起重要的作用． 关键词：     

非相干背景光辐照对LiNbO3∶Fe与LiNbO3∶Fe∶In晶体中二波耦合的控制

理论分析和实验观测了非相干背景光辐照下掺杂LiNbO₃晶体中的二波耦合,发现非相干背景光能够在很大范围内灵活有效地控制信号光增益,抑制扇形效应,提高信噪比,缩短光栅的建立时间.入射光光强比一定时,适当地增大背景光光强可以使光栅响应时间减小一个数量级.尤其对于抽运光光强远大于信号光光强的情况下,上述作用更为明显. 关键词： 光折变 二波耦合 非相干背景光     

Field asymmetry of the dynamic gain coefficient in organic photorefractive devices

The performance of amorphous organic photorefractive materials in holographic two-beam coupling experiments in the typical tilted geometry was found to be asymmetric with respect to the applied electric field direction. For one field direction, light is coupled into the polymer layer and can be detected on the side of the devices. For the other, the originally Gaussian-shaped writing beams show a shoulder or even split into two. The strength of the asymmetry depends on the diameter of the beams writing the hologram. We demonstrate that this effect is due to beam fanning. As a result of the fanning, the apparent photorefractive gain coefficients take on unphysical values.     

Conical parametric scattering of a single extraordinary beam in BaTiO3 crystal

In this paper, the parametric scattering of a single extraordinary polarized beam of laser in BaTiO₃ photorefractive crystal has been investigated experimentally and theoretically. The resulting pattern consists of beam fanning, isotropic ring, and anisotropic one. Among all parts of scattering pattern, isotropic ring has not been studied as much as beam fanning and anisotropic ring, and there still are some differences in reports about it. Therefore, the study has mainly focused on this part. In this experimental configuration, isotropic ring is just visible in positive angles although the other parts of parametric scattering pattern can be visible from behind and in front of the crystal. In addition to steady state pattern in forward and backward directions, its transient behavior with the rotation of crystal has been studied. The results of experiments have been analyzed carefully, and their theoretical explanations have been presented based on the standard theory of parametric scattering in photorefractive crystals. It has been shown that this configuration corresponds to the so called parametric B-process scattering.     

Crystal orientation dependence of the SNR for signal beam amplification in photorefractive BaTiO3

In signal beam amplification by two-beam coupling in BaTiO₃ photorefractive crystals, beam fanning in the direction of the amplified signal reduces the signal-to-noise ratio (SNR). The dependence of the SNR and the signal beam gain on the crystal orientation are analysed using a HeNe laser. It is found that orientating the crystal for maximum gain gives poor signal-to-noise ratio. A compromise has to be made between the SNR and high gain for optimum signal amplification.     

One-beam recording in a LiNbO3 crystal

We propose a new way to record images in a photorefractive LiNbO3 crystal. This method involves only a single object light without any reference light. We believe that the recording is attained by fanning holograms that result from interference between the object light and its scattered light. Although volume gratings are involved, the recorded pattern can be viewed not only with a laser light beam that is incident over a certain angular range but also with white light.     

光折变两波耦合对光扇开效应的抑制作用

以多个两波耦合（ＴＷＭ）相位栅模型建立了考虑光扇开效应后的动态两波耦合方程，通过数值求解揭示了在大信号两波耦合对光扇开效应的抑制作用和在小信号下光束扇开损耗对两波耦合影响的规律，并进行了相应的实验研究。     

Image transmission through a thick dynamic distorter by the photorefractive fanning effect

A new method of one-way image transmission through a thick dynamic distorter without the need for a reference beam or four-wave mixing is demonstrated. In this method there are only one object beam and one sampling beam. The response time of the photorefractive crystal must be much longer than the fluctuation period of the dynamic distorter. Thus the crystal responds only to the time-averaged intensity pattern of a rapidly varying object beam. The images, reconstructed with high fidelity, are picked up through the photorefractive fanning effect.     

光扇效应对Ce∶KNSBN记录偏振组态的影响

在非同时读出条件下,采用Ar+ 514.5 nm单色激光为光源。以信号光为非寻常偏振光(e光),通过改变抽运光的偏振态,研究不同写入光偏振组态下Ce∶KNSBN晶体的两波耦合特性。结果表明     

Anisotropic conical scattering in photorefractive BaTiO3: diffraction efficiency measurements

The simultaneous presence of beam fanning and anisotropic conical scattering hinders the exact measurements on anisotropic conical diffraction. In this paper, a method is described for the measurement of diffraction efficiency of the gratings responsible for anisotropic conical scattering in a photorefractive BaTiO₃ crystal. Dependence of diffraction efficiency on the angle of incidence of the incident beam and the build-up processes of beam fanning and anisotropic scattering have been studied.     

Photorefractive surface waves in a thin Bi12TiO20 crystal

A theoretical model considering total internal reflections of fanned beams from the side crystal surfaces and their coupling with the pump beam is proposed. The interaction of these beams results in the light intensity redistribution and in the generation of the photorefractive surface wave. An experimental observation of the light intensity self-concentration near the surface of a thin photorefractive Bi₁₂TiO₂₀ crystal is reported. It results in an acceleration of the photorefractive energy exchange process.     

Incoherent-to-coherent conversion by use of the photorefractive fanning effect

A dynamic incoherent-to-coherent converter using the photorefractive fanning effect is demonstrated in a BaTiO(3):Ce crystal, and high-quality positive replicas of the incoherent image are obtained with a resolution of 32line pairs/mm. In this method the incoherent image-bearing beam modulates the transmission intensity of the coherent beam directly, and no readout beam is required. When the intensity ratio of the incoherent beam to the coherent beam is smaller than 5, the transmission intensity of the coherent beam increases almost linearly with increased intensity of the incoherent beam.     

Multiwave coupling in a high-gain photorefractive polymer

The characteristics of a new high-gain photorefractive polymer composite with a PNP chromophore are investigated. Competition between beam fanning and two-wave coupling (TWC) is predicted and verified experimentally. The intensity dependence of TWC gain is studied. Higher diffraction order and forward phase conjugation in a TWC geometry are observed and explained.     

Dynamics of photorefractive response in sillenite crystals with double-ionized donor centers and shallow traps

The dynamics of the photorefractive response in the Bi₁₂TiO₂₀ crystal with double-ionized donor centers and shallow traps is analyzed. Consideration is given to the time dependences of the recording of a photorefractive grating in the absence of an external electric field, its storage in the dark, and its development upon the application of an external field and turning on of a reading light beam.     

Mutually pumped phase conjugate mirror using cross polarized photorefractive coupling

This paper presents a new proposal for a mutually pumped phase conjugate mirror (MPPCM) using cross polarization. In the cross polarized MPPCM, two light beams, whose polarizations are orthogonal to each other, incident on a photorefractive crystal, causing photorefractive interaction to generate phase conjugate waves with an orthogonal polarization to an incident beam. The use of Ar⁺ laser and BaTiO₃ crystal in an experiment for the cross polarized MPPCM found that diffraction efficiency for an extraordinary beam is about 20%. As opposed to a conventional parallel polarized MPPCM in which the use of a beam splitter is required to separate a diffraction beam, the cross polarized MPPCM can efficiently extract phase conjugate waves by means of a polarized beam splitter, demonstrating the advantage of substantially improved efficiency.     

Formation of spatial solitons and spatial shock waves in photorefractive crystals

An analytical model, which describes the drift and diffusion mechanisms for the formation of the nonlinear response (local and nonlocal nonlinearities) of photorefractive crystals on the microscopic level, is constructed. New types of stable self-consistent distributions of the light field intensity, i.e., spatial solitons, are found. The trajectories of their motion (self-bending) are calculated, and the possibility of observing a new nonlinear-optical effect in photorefractive crystals, viz., the formation of spatial shock waves, is demonstrated. The modulation instability appearing when plane waves propagate in photorefractive crystals is analyzed, and the characteristic spatial scales of the light field distribution formed as a result of self-interaction (fanning) are determined. The results of the analysis are confirmed by computer simulation data. Zh. éksp. Teor. Fiz. 111, 705–716 (February 1997)     

Influence of surroundings on photorefractive effect in lithium niobate crystals

In the paper results of the investigation of the influence of electric properties of the environment surrounding LiNbO₃ crystals on photorefractive effect induced in these crystals by Gaussian Ar⁺ laser beam with various intensities are presented. We show spatial and temporal dependences of changes of the refractive index obtained experimentally in LiNbO₃: Fe and LiNbO₃: Fe:Mn samples surrounded by media with different electric conductivities and different permittivities (water, air, water solution of CaCl₂). The space and time dependences of the refractive index changes induced by the Ar⁺ laser beam are observed by means of the Mach–Zehnder interferometer using light from HeNe laser. The experimentally obtained results are in a good agreement with those following from numerical calculations using the manifold mirroring method. The agreement between calculated and experimental results indicates that the polarization charge at the photorefractive crystal/surrounding medium boundary significantly influences the photorefractive process in the crystal. The experimentally observed slow spontaneous decrease of the refractive index change in a sample placed into a slightly conducting medium (air) after switching off the beam also indicates that the polarization charge in the sample's surroundings affects the photorefraction.     

Ce:KNSBN晶体光扇效应的入射光强度阈值特性研究

采用非同时读出条件下的两波耦合实验装置,以单束光入射Ce:KNSBN光折变晶体,系统研究了Ce:KNSBN晶体中光扇效应随入射光偏振态、入射光强度、光入射角的变化情况.结果表明异常偏振光入射晶体时光扇效应明显,且存在明显的入射光强度阈值特性,入射光强度阈值为38.2 mW/cm2;相同光入射角下,稳态光扇强度随入射光强度的增强而明显变大;对应相同的入射光强度,稳态光扇强度随光入射角θ的增大而增大,当θ为15°时到达峰值,而后随θ的增大而逐渐减小.同时对光扇效应的入射光强度阈值特性以及稳态光扇强度随入射光偏振态、入射光强度、光入射角的变化作出了相应的物理解释.