90°全息记录衍射效率分析与提高

对光折变全息记录特别是双中心全息记录中90°记录结构下较低的衍射效率进行了研究,采用局域衍射理论对90°记录结构的衍射进行了分析,表明在同样的折射率变化和2 mm的光束宽度的情况下,只有当折射率光栅振幅大于10^-4时,90°记录结构衍射效率才能够与小角度透射记录结构的衍射效率大致相当。针对环境干扰导致的干涉条纹振动影响光栅记录,提出了有效调制度概念,根据分析90°记录结构的干涉条纹间距很小,容易受外界环境干扰而导致低的折射率变化率,因此应采用主动条纹锁定系统。此外在双中心全息记录中,微观光学参量的优化对衍射效率影响也很重要。     

Formation dynamics of nonvolatile crossed-beam photorefractive gratings in doubly doped LiNbO3 crystals: Theoretical investigation

The formation dynamics of crossed-beam photorefractive gratings formed by the method of two-center holographic recording in doubly doped LiNbO₃ crystals is investigated in this paper based on the theoretical model combining the two-center band transport model with the two-dimensional coupled-wave theory. The numerical simulations are presented for two-center holographic recording crossed-beam photorefractive gratings in LiNbO₃:Fe:Mn crystals. The temporal and spatial evolutions of the refractive index modulation and the diffraction efficiency are shown. The spatial variation of the wave intensity is also presented.     

Spectral selectivity of holographic mirrors recorded in an attenuating medium

Numerical simulation of the optical properties of unslanted reflection holographic gratings (holographic mirrors) is made by the characteristic matrix method. The holograms are recorded in an attenuating medium but the reconstruction is that of pure phase structures in which the refractive index depends exponentially on the exposure. The dependence of the hologram spectral selectivity on medium absorption, amplitude ratio of the two interference waves and recording linearity is studied.     

含氟红敏光致聚合物全息材料的组成优化

用低折射率的含氟共聚物为光致聚合物全息材料的成膜物,以增加全息记录材料折射率的空间可调制程度。用溶液聚合法合成了低折射率的水溶性甲基丙烯酸2,2,3,3-四氟丙酯-甲基丙烯酸共聚物(折射率为1.441),以该共聚物为成膜物质,丙烯酰胺为单体、亚甲基蓝为光敏染料、三乙醇胺为引发剂配制了光致聚合物薄膜;经过对光致聚合物组成构成的优化,在空间分辨力为1580lp/mm时,光致聚合物薄膜的灵敏度约为19 mJ/cm2,衍射效率可达93%,两者均优于以聚乙烯醇(PVA)为成膜物质的光致聚合物材料。     

Permanent holographic recording in indium oxide thin films using 193 nm excimer laser radiation

Permanent holographic recording in sputtered indium oxide (InO_x) thin films is demonstrated, using ultraviolet radiation at 193 nm emitted by an ArF excimer laser. Steady-state refractive index changes of up to 5×10^-3 are calculated from the measured diffraction efficiency of a HeNe laser probe beam. The recorded gratings exhibit a dynamic behaviour that relaxes to a steady-state value that depends on the oxygen partial pressure used during growth and on the recording beam intensity. The observed behaviour is explained in terms of laser-induced structural changes. Received: 12 October 1998 / Accepted: 8 March 1999 / Published online: 4 August 1999     

Evanescent polarization holographic recording of sub-200-nm gratings in an azobenzene polyester

I demonstrate high-resolution polarization holographic recording with evanescent waves in a thin film of an azobenzene polyester deposited directly on the hypotenuse of a highly refractive prism. A spatial frequency greater than 7000 lines/mm and diffraction efficiency greater than 1% have been achieved. It was found that diffraction efficiency increases in the dark after the writing beams have been switched off. The biphotonic effect found in other azobenzene polymers, which converts cis states of the azobenzene to trans states followed by an ordering process that is due to aggregation, is proposed as the reason for this increase in diffraction efficiency.     

Holographic formation of dynamic transmission diffraction gratings in a photopolymer material in the process of two-beam interaction at low contrasts

In the present study, the process of formation of holographic transmission diffraction gratings in photopolymer media is investigated at low contrasts with allowance for self-diffraction of recording waves. A theoretical model for the first spatial harmonics of the refractive index grating is constructed based on a solution of the coupled kinetic photopolymerization and diffraction equations. The temporal dynamics of amplitude and phase spatial profiles of harmonics, diffraction efficiency, and angular selectivity of recorded gratings is theoretically investigated. The calculated and experimental data are compared. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 35–42, November, 2006.     

番红花红T光敏感光致聚合物全息存储材料

制备了一种以番红花红T为光敏剂的新型全息存储材料,主要用Ar⁺激光器的514.5nm波长的绿光研究材料的全息特性.研究表明,该材料具有较高的衍射效率、曝光灵敏度和较大的折射率调制度,衍射效率近40%,灵敏度为7.22×10^-4cm²/mJ,折射率调制度为3.65×10^-4,同时,在读出时必须考虑布拉格偏移对全息存储的影响.在介质膜中存储了全息图像,再现图像较为清晰,说明该材料适合用作高密度全息存储介质. 关键词： 全息存储 光致聚合物 衍射效率 布拉格偏移     

Cu-diffused layers in LiNbO3 for reversible holographic storage

Some holographic recording characteristics of Cu-diffused layers fabricated in Rh-doped LiNbO₃ have been examined in comparison with those fabricated in undoped lithium niobate.The diffusion layers, whose thickness depends on the diffusion temperature, are found to be enormously susceptible to optically-induced refractive index changes and to be able to attain a high diffraction efficiency. A distinguished difference is found between the persistence of holograms stored in Rh-doped and undoped LiNbO₃.     

Grating spacing dependence of nonvolatile holographic recording with arbitrary charge transport lengths

Grating spacing dependence of nonvolatile holographic recording in doubly doped lithium niobate crystals is theoretically investigated allowing arbitrary charge transport lengths. It is shown that the nonvolatile refractive index modulation initially increases with increasing grating spacing, then a saturation behavior arises because of the dominant bulk photovoltaic effect. Although different charge transport length results in different nonvolatile refractive index modulation, the grating spacing dependence of nonvolatile holographic recording obeys almost the same rules for arbitrary charge transport lengths. The experimental results obtained by recording nonvolatile holograms in LiNbO₃:Cu:Ce crystals with different grating spacing are consistent with the theoretical analyses.     

Multi-wavelength holography in Bi12TiO20 crystals: Applications in refractometry

We present a recent development in holography with multimode, large free-spectral range (FSR) diode lasers in photorefractive sillenite crystals. A novel refractometry method based on this type of holographic recording in Bi₁₂TiO₂₀ (BTO) crystals is proposed. The holographic image of a prism-shaped transparent sample appears covered of interference fringes, and as the sample is properly translated, the fringes run along the holographic image. An expression providing the refractive index of the medium as a function of the sample displacement and the correspondent number of running fringes was derived. The refractive indexes of optical (BK7) glass, ethanol, hexan, cumene and aqueous solution of NaCl with different concentrations were measured in order to test the method. The obtained results are in good agreement with the ones reported in literature or measured by us using a commercial Abbe refractometre.     

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.     

非均匀反射体全息光栅的数值研究

本文运用矩阵方法详细分析了非均匀反射式全息光栅衍射特性.把全息光栅内部的折射率调制分布看作是一种空间非均匀折射率调制分布,在此基础上进行了数值计算,详细分析了几个影响其衍射性质的因子.基于该方法的数值模拟结果与耦合波理论和以前的实验结果能较好地符合.     

Effect of recording conditions on the anisotropic diffraction of volume holographic gratings

The anisotropic Bragg diffraction of the volume holographic gratings in photorefractive crystals are investigated based on the model of anisotropic coupled-wave theory. The effect of the initial intensity ratio and the recording angles of the two recording waves on the anisotropic Bragg diffraction properties is discussed. It is shown that both the ratio of the initial intensity and the incident angles of the recording waves are selective action for the anisotropic Bragg diffraction efficiency of the volume holographic gratings, while these two recording conditions are not selective action for the isotropic Bragg diffraction. Furthermore, the Bragg phase matching condition of anisotropic diffraction is analyzed when the recording angles change.     

非水溶性红敏光致聚合物全息性能的改善及其应用研究

为使非水溶性红敏光致聚合物材料的全息性能得到改善,采用复合光敏增感、复合引发和复合单体的方法反复对比实验,改善后体系的灵敏度为0.02 cm2/mJ,衍射效率高达83%,计算得到材料的最大折射率调制度为4.1×10-3.实验结果和理论分析表明,利用该材料制作的新型全息光栅具有损耗低、通道宽度窄、衍射效率高等优点,适于制作光通信中的波分复用器及各种光学元件.     

番红花红T敏化的新型光致聚合物全息存储材料

制备了一种以番红花红T为光敏剂的新型全息存储材料,实验结果表明,该材料具有较高的衍射效率、曝光灵敏度和较大的折射率调制度等,最大衍射效率约38.5%,折射率调制度为0.547×10－3,在存储介质膜中存储了模拟全息图像,再现图像有较好的保真度,说明该材料适合用作高密度体全息存储介质。     

Diffraction properties of ultrashort pulsed beams with arbitrary temporal profiles by a reflection volume holographic grating

The diffraction properties of reflection volume holographic gratings are studied when the gratings are illuminated by an ultrashort beam with arbitrary temporal profiles. The modified Kogelnik's theory of coupled waves is used. Simple analytical expressions for spectral and temporal distribution of the transmitted and diffracted are obtained. The dependences of the diffraction bandwidth, the Bragg selectivity bandwidth and the total diffraction efficiency of the reflection volume grating on the temporal profiles of the input ultrashort pulse are investigated. Study shows that the bandwidth of the reflection volume holographic gratings varies with the grating parameters more slowly than the transmitting volume holographic gratings. For three different temporal profiles, numerical results about diffraction properties of the reflection volume holographic gratings are given and compared with the transmitting volume holographic gratings.     

Influence of the characteristic curve of the material on the diffraction efficiency of holographic volume phase gratings

The behaviour of the diffraction efficiency of holographic gratings is discussed on the basis of the multiwave coupled-wave approach, taking into consideration the photoinduced change in the refractive index of the recording medium. The cases of Bragg and off-Bragg reconstruction in Bragg and mixed diffraction are considered. The influence of the higher harmonics of the refractive index modulation profile on the diffraction efficiency is shown.     

Influence of relative refractive index on the efficiency of total internal reflection diffraction gratings. Part I. s-Polarization

The influence of the relative refractive index on the efficiency of chromium diffraction gratings, working in total internal reflection with s-polarized light, is investigated. Calibrated aqueous solutions of sodium chloride with exactly known refractive indices at λ = 632·8 nm are used. The experimental results are explained by the modified characteristic wave method. A good quantitative agreement between theory and experiment within the measurement error is ensured for small changes (up to c. 10^-2) in the relative refractive index.     

Dynamic effects in photorefractive crystals at simultaneous read–write of holographic gratings

A two-level model of a simultaneous read–write of holographic gratings in photorefractive crystals has been developed for the nonstationary case at the applied electric field and unequality of recording beam intensities. The recurrent relationships both for the internal field amplitudes and for the amplitudes of the optical waves involved in interactions have been obtained. The use of a truncated power series expansion has allowed to obtain the expressions for diffraction efficiency and intensities of optical waves in nonstationary case. The obtained results were used for the analysis of simultaneous read–write dynamics in Bi₁₂SiO₂₀ crystals.