Dynamic studies of holographic gratings in dye-doped liquid-crystal films

The dynamic behavior of a holographic grating induced in a homeotropically aligned dye-doped liquid-crystal film is investigated. In the presence of an applied dc voltage, photoexcited azo dyes induce a photorefractive grating and then diffuse and are adsorbed onto cell substrates. The reorientation of liquid crystals as a result of adsorbed dyes leads to a phase grating that is phase shifted 90 degrees from the photorefractive grating. Competition of these two gratings induces two-beam coupling of the writing beams, initially transferring energy from beam 1 to beam 2 and then, after a pause, from beam 2 to beam 1.     

Effect of photoconductivity and dielectric constant of the photorefractive materials on two-beam coupling gain and phase-shifts for a single unidirectional photorefractive ring resonator

Photoconductive dependence of two-beam coupling between the pump beam and the signal beam in photorefractive materials have been analyzed in case of non-degenerate wave mixing under the undepleted pump approximation method. During the two-wave mixing in photorefractive materials, steady state amplification of the signal beam and oscillation characteristics of a single unidirectional ring resonator has been studied. The domination of the two-beam coupling gain over the combined absorption and resonator losses such as Fresnel reflections from the crystal and imperfect mirrors builds up unidirectional oscillation. The buildup of such an oscillation leads to a saturation of the gain, which can be explained in terms of the photorefractive phase-shift. The existence of this phase-shift between the photorefractive index grating and the illumination intensity pattern, which is of characteristic of the photorefractive effect, leads to an energy transfer between the two beams. For a single unidirectional ring resonators, the effects of photoconductivity of the materials, two-beam energy coupling coefficient, dielectric constant, crystal thickness, and material's absorption coefficient on amplification of the two-beam coupling gain and photorefractive phase-shifts of the signal beam have also been studied in detail. It has been found that amplification of the signal beam and phase-shift can be enhanced by taking the photorefractive crystal having higher photoconductivity and lower dielectric constant, which improves performance of the resonators.     

DMNPAA有机光折变材料的性能研究

通过双波耦合实验证明在一种新的有机聚合物光折变材料中双波耦合增益系数超过吸收系数。１０５μｍ厚的样品衍射效率可达到８６％，光栅形成的响应时间达到１００ｍｓ量级，双波耦合增益系数超过了２００ｃｍ＾－１（１），并引入了一种新的方法分析了其光折变机理。     

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     

Positive and negative feedback effects of four-wave mixing in photorefractive crystals with two active centres

Based on a two-level model of optical transitions, the problem of positive and negative feedback effects during simultaneous reading and writing of dynamic holograms in photorefractive crystals has been solved analytically. The diffraction efficiency and the gain coefficient are shown to be nonlinear functions of the readout beam intensity and are nonreciprocal with respect to the readout from the two input ports. The readout beam intensity value that affects a previously recorded grating is shown to depend on microscopic parameters such as the cross-section of photoionization, the recombination coefficient and the concentration of photorefractive centres. An analysis of the amplitude of the overall shared grating has shown that with increasing readout beam intensity the holographic grating thickness decreases. In this case, under negative feedback the grating amplitude decreases, while with positive feedback it increases. The possibility is shown of achieving (under positive feedback) a maximum value of diffraction efficiency and complete energy transfer in a finite thickness of the photorefractive crystal, in contrast to the infinite thickness required in the standard two-beam coupling case.     

High-efficiency volume hologram recording with a pulsed signal beam

We present an efficient photorefractive volume hologram recording technique with a pulsed signal beam and continuous reference-beam illumination. The grating envelope can be simply controlled by manipulation of the duty cycle of the signal beam. Thus, for any grating coupling strength and different initial reference-signal intensity ratios, the diffraction efficiency can be maximized with this technique and can be greatly increased in comparison with that of the conventional recording technique.     

Simple method for determining the gain coefficient of a photorefractive polymer film

We propose a simple method for measuring the gain coefficient of two-beam coupling for photorefractive polymer film. To measure the gain coefficient, we attached a phase grating onto the photorefractive polymer film and made an interference pattern between the transmitted and diffracted beams in the photorefractive film by using an illuminating single beam. The gain coefficient values measured at various external fields by our method showed good agreement with those measured by a standard two-beam coupling method. Compared to standard two-beam coupling, this method exhibited fairly improved stability of the output signal to external factors such as human sounds and airstreams.     

Photorefractive effect in Fe-doped GaN

The photorefractive effect was observed in Fe-doped semi-insulating GaN. We measured the two-beam coupling constant and the grating formation time as a function of pump intensity at a wavelength of 458 nm in reflection geometry. The photorefractive gain coefficient was 0.39 cm⁻¹, and the grating formation time was 7 ms at a pump intensity of 1.0 W/cm². Besides the refractive index grating, the contribution of an absorption grating was also observed in a two-wave mixing experiment. The coupling constant of the absorption grating was negative and increased with the pump intensity. The origin of the absorption grating was attributed to light-intensity-dependent photochromism.     

Orientational gratings in dye-doped polymer-dispersed liquid crystals induced by the photorefractive effect

We report the observation of orientational gratings induced by the photorefractive effect in dye-doped polymer-dispersed liquid crystals. The photorefractive origin of the grating-induction effect is supported by the results of two-beam coupling experiments and by the possibility of erasing the grating by uniform illumination. For all the samples a stable memory effect was observed.     

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。     

Beam splitting, combining, and cross coupling through multiple superimposed volume-index gratings

We investigate the diffraction properties of multiple-superimposed index gratings from a vector synthetic viewpoint using coupled wave analysis with Bragg and non-Bragg readout. The vector synthetic gratings consist of two volume-index gratings with different grating wave vectors, which can be obtained in photorefractive materials, in optical fibers or in Bragg cells with two driving acoustic waves. This structure can realize beam splitting, beam combining, and beam cross coupling, etc. Analytic expressions for the diffraction efficiency as well as the amplitude of the waves involved are obtained. The results are presented and discussed.     

利用预置光栅的脉冲信号光自泵浦相位共轭特性研究

针对光折变晶体响应慢的问题,提出了一种预置光栅技术,该预置光栅可由另外的独立预置光源产生.通过对采用预置光栅的光折变晶体自泵浦相位共轭特性及其物理机制进行的实验研究证实,采用预置光栅技术可以大大缩短低重复频率、窄脉冲信号光的光折变自泵浦相位共轭的响应时间,因此对于低重复频率、窄脉冲光的自泵浦相位共轭应用有重要的实用价值.     

Achromatic multibeam coupling in KNbO3: Rb

We examine, both theoretically and experimentally, photorefractive coupling in KNbO₃: Rb with beams at several wavelengths in a dual grating achromatic arrangement. It is shown that, at normal incidence, the coupling gain in the multi-wavelength achromatic configuration is nearly equal to the gain of two beam coupling with a monochromatic source of the same intensity. The dependence of the achromatic gain on the angle between the bisector, of the pump and signal beams, and the normal to the crystal surface is also studied.     

反射型体全息光栅对超短脉冲激光光束衍射的性质

在光栅记录介质色散效应的影响下,拓展Kogelnik的耦合波理论研究了反射型体全息光栅对偏振方向垂直和平行于入射面的超短脉冲激光光束衍射的性质。结果表明对于记录在LiNbO3光折变晶体中的体全息光栅,在色散效应、光栅参量和入射条件的共同影响下,光栅对偏振方向垂直于入射面的超短脉冲激光光束衍射的光谱带宽大于对偏振方向平行于入射面的超短脉冲激光光束衍射的光谱带宽,在不考虑光栅介质色散效应的影响时,它们都变大。进一步给出了衍射光的光谱带宽及光栅的衍射效率随入射脉冲的光谱带宽与光栅的光谱带宽比值的变化情况。     

强外加电场与大调制度下光折变动力学光栅形成研究

强外加电场与大调制度在光折变效应的研究中已经得到了广泛应用。采用PDECOL算法,严格求解光折变带输运方程,得到外加电场时不同调制度下光折变晶体中随时间变化的空间电荷场、载流子浓度,并讨论了外加电场对它们的影响。通过将物质方程与耦合波方程联立数值求解,可得到光折变光栅形成过程中两波耦合增益系数以及光束条纹相位的变化。模拟结果表明,在强外加电场作用下,两束记录光之间的光强与相位耦合都得到了增强,而原有的解析式忽视了强外加电场与大调制度对空间电荷场相位耦合的影响,此时不再适用。同时发现折射率光栅与记录光束条纹均发生弯曲,并不再保持平行。     

Effect of photoconductivity and oscillation frequency shift on the signal beam intensity in two beam coupling in photorefractive materials

A general theory of the two-beam coupling between a pump beam and a signal beam in photorefractive materials is presented. The coupled wave equations describing the non-linear two beam coupling are derived, based on Maxwell's wave equation. The coupled equation for the intensities of the two beams in the photorefractive crystals with the absorbing properties have been derived analytically. The intensity of the signal beam increases with the increasing crystal thickness, reaches a maximum and then decreases. The influence of energy beam coupling coefficient, oscillation frequency shift, crystal thickness, absorption coefficient and the input beam intensity ratio on the signal beam intensity have been studied in details. The effect of the photoconductivity of the materials on the intensities of the two beams in both the co-directional as well as contra-directional two beam coupling cases have been studied.     

Measurement of the spatial phase shift in high-gain photorefractive materials

The correct determination of the spatial phase shift ?(p) in photorefractive materials is crucial to the proper characterization of novel materials. It is shown that the grating translation techniques commonly used for the measurement of ?(p) need to be reevaluated for high-gain materials. Strong energy and phase coupling leads to nonuniform slanted gratings, which result in an apparent dependence of the phase shift of the beam ratio and the optical polarization. A revised theory is presented, and analytical solutions are obtained for the special case of ?(p)?pi/2 . Numerical solutions for arbitrary ?(p) are in good agreement with measurements in a photorefractive polymer.     

Enhancement of the diffraction efficiency of the hologram recorded in photorefractive media under the action of external electric field

Diffraction efficiency as a function of the applied electric field for the non-linear regime has been calculated by solving numerically the beam coupling equations. The refractive index variation used in the beam coupling equations was calculated directly from the material rate equations via the total space charge field. The diffraction efficiency of the holograms recorded in photorefractive media is not only a function of the applied external electric field but also a function of crystal thickness, diffusion field, reduced fringe contrast modulation ratio and absorption coefficient of the materials. The effects of these parameters on the efficiency of the holograms have been studied in details. In the absence of the external applied field, it is found that the diffraction efficiency of the holograms could be maximized for a thinner photorefractive crystal having lower absorption coefficient and higher value of diffusion field, which could exist at a much lower value of reduced fringe contrast modulation ratio of the index grating. More efficient holograms can be recorded in the presence of the externally applied electric field as compared to the case of no external field.     

透射型光折变体全息光栅对超短脉冲激光光束衍射的特性

在Kogelnik耦合波理论的基础上,考虑光栅记录介质的色散效应的影响,研究了光折变体全息光栅对不同偏振状态的超短脉冲激光光束衍射的性质,讨论了高斯型入射脉冲激光光束的谱宽与光栅的有效衍射谱宽之比不同时,衍射和透射光束的光谱宽度、时间宽度、波形和衍射效率的变化。结果表明,光栅的有效衍射谱宽受光栅参量及入射条件的影响,对衍射性质的影响很大,且在考虑光栅记录介质的色散效应时减小。当入射脉冲的偏振方向垂直于入射面时,光栅的有效衍射谱宽大于偏振方向平行于入射面的情形,衍射效率在入射脉冲宽度较大时小于偏振方向平行于入射面的情形;谱宽比较大时,衍射光束的时间分布曲线产生展宽和变形,且比偏振方向平行于入射面的情形展宽和变形得更加明显。     

Higher diffraction order photorefractive optical beam splitter

A higher diffraction order photorefractive (PR) optical beam splitter has been realized based on a PR higher diffraction order grating. In the experiments, the beam splitter was produced by two-wave coupling at a small incident angle in Fe:LiNbO₃ using the output from a He–Ne laser at 632.8 nm. An incident signal beam containing three different wavelengths (632.8, 532.0 and 488.0 nm) was split into multi-output beams by the beam splitter. The size of higher diffraction order is given and the influence of crystal thickness is discussed. Results show that the higher diffraction order PR optical beam splitter provides a practical method to split a multi-wavelength signal beam.