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     

Polarization properties in two wave mixing with moving grating in photorefractive BSO crystals. Application to dynamic interferometry

The polarization properties of the volume gratings in photorefractive B₁₂SiO₂₀ crystals permit to increase the contrast and signal to noise ratio in the image plane of a large size vibrating object structure. The experiment is based on the nearly degenerate two wave mixing of a pump beam with a low intensity signal beam due to the object structure (moving grating recording with an externally applied field in the 110 crystallographic direction). For a correct orientation of the incident recording beam polarizations, the pump beam emerging from the crystal is linearly polarized, while the signal, after two beam coupling is elliptically polarized. The selective attenuation of the direct pump and scattered beams is obtained with a polarizer. Improved contrasts in the interferogram plane of the vibrating object are demonstrated.     

Volume polarization holographic recording in thick phenanthrenequinone-doped poly(methyl methacrylate) photopolymer

Volume polarization holographic recording in phenanthrenequinone-doped poly (methyl methacrylate) photopolymer is obtained. Photoinduced birefringence in a 2 mm thick sample is measured by a phase-modulated ellipsometry. The birefringence induced in this material by linearly polarized beam at 514 nm reaches 1.2×10(-5). In addition, ability for recording volume polarization grating using two different polarization configurations is demonstrated and compared. The experimental results show that the diffraction efficiency of the hologram reaches to ～40% by using two orthogonal circularly polarized beams.     

Polarization holographic gratings in an azobenzene side-chain liquid-crystalline polymer

Polarization holographic recording was investigated in a liquid-crystalline azobenzene side-chain polymer. A highly stable polarization holographic grating with the diffraction efficiency of 12.6% was written in the polymer film by two orthogonally linearly polarized 532 nm beams with equal intensities of 150 mW/cm². The polarization grating was studied with a 633 nm probe beam by surveying the conversion of the polarization state of the probe beam and measuring the dependence of diffraction efficiency on the polarization direction of the probe beam. It was found that the ± first-order diffraction efficiency and the polarization of the diffracted wave depended strongly on the polarization of the readout wave. The unusual diffraction properties of the recorded grating were attributed to the presence of the circular photoinduced birefringence. PACS 42.40.Ht; 42.40.Lx; 78.20.Fm     

Study on the rewritability of bisazobenzene-containing films in optical storage based on two-photon process

In this paper, we use a two-photon technique to record data points on bisazobenzene-containing films. The recording data points are evidently polarization dependent. It is found that the recording data points can be erased by the linearly polarized light whose polarization direction is perpendicular to that of the recording beam and then, in the same region, new data can be rewritten after erasing. In addition, the recorded data can also be erased completely by the circularly polarized light. Compared with linearly polarized beam, the circularly polarized light is a better choice for erasure as no main orientation is regained after erasing rather than reorientation.     

Polarization-controlled switching between diffraction orders in transverse-periodically aligned nematic liquid crystals

Transverse-periodic-oriented nematic liquid crystals (LCs) are a special type of optical axis grating that are capable of very high efficiency diffraction (theoretically, 100%) in thin layers of materials with thickness comparable to the radiation wavelength. In particular, they fully diffract linearly polarized input beam into circularly polarized +1st and -1st diffraction orders. We experimentally demonstrate switching between diffraction orders of such gratings when the polarization of the incident beam changes from right-circular to left-circular and vice versa with the aid of an electrically controlled LC phase retarder. Such a setup in which the diffraction efficiency and direction are controlled externally, without application of an electric field to the transverse-periodic grating, provides additional control opportunities and does not compromise the quality of the grating. The grating used in the experiment was 1.5 microm thick and had a period of 4 microm. The contrast ratio of switching between the +1st and -1st orders was as high as 267:1 for a He-Ne laser beam with a switching time of 6.6 ms.     

线偏光闪耀反射光栅衍射特性研究

通过数值方法对线偏光在Littrow装置中的理想导体闪耀光栅的偏振特性进行研究.用时域有限差分程序计算出任意偏振方向线偏光经过闪耀光栅衍射后的电磁场分布,提取衍射波的S偏振和P偏振分量,讨论了与光栅闪耀角和入射光波偏振角的关系.分析了在1530～1570nm光谱范围内,入射光波为线偏振波和园偏振波时,衍射光波的偏振色散特性与光栅闪耀角的关系,并给出了存在偏振色散的光栅闪耀角范围.这些分析和结果对工程设计具有一定的指导意义.     

Linearly polarized fiber laser using a point-by-point Bragg grating in a single-polarization photonic bandgap fiber

We present a narrow-linewidth, linearly polarized neodymium-doped fiber laser that incorporates a point-by-point Bragg grating inscribed into the core of a single-polarization all-solid photonic bandgap fiber. The Bragg grating was written within a single-polarization wavelength band of the fiber; thus, the Bragg reflection was polarized. This all-fiber laser produced 7.2 W, linearly polarized output with 25 pm FWHM and 19.6 dB polarization extinction ratio.     

Photorefractive optical properties of volume phase gratings induced in sillenite crystals, when the grating vector lies on the (111) plane

We study the diffraction efficiency of volume phase gratings in Bi₁₂GeO₂₀, when the grating vector, lying on a (111) plane, is at an angle with respect to the crystallographic direction. An external bias field parallel to the direction is applied during recording, thus, the recording conditions depend on the grating vector orientation. The basic parameters determining the diffraction efficiency are the grating vector orientation, the rotatory power and the field-induced linear birefringence (primary and secondary). Analytic expressions for the diffraction efficiency have been obtained by taking into account all the above-mentioned parameters, provided that linearly polarized light is incident on the crystal. In this configuration, the influence of the secondary electro-optic effect (inverse piezoelectric and photoelastic effects) to the diffraction efficiency is actually stronger than the influence of the primary effect. Experimental results are given. Received: 3 January 2000 / Revised version: 6 June 2000 / Published online: 13 September 2000     

Thermal polarization properties of thick anisotropic diffraction holograms recorded in polymer-dispersed liquid crystals

The diffraction and polarization properties of thick anisotropic holographic gratings recorded in polymer-dispersed liquid crystals are studied experimentally and theoretically. Such gratings are anisotropic and sensitive to temperature variations and also exhibit a strong dependence of the diffraction properties on the incident radiation polarization because of the presence of liquid-crystal molecules. The dependences of the diffraction efficiency and diffracted beam polarization orientation on the diffraction grating temperature and incident beam linear polarization are obtained for the case of Bragg incidence. The same dependences are derived using the known experimental temperature curves of the liquid crystal permittivity. The experimental data are in agreement with the theoretical calculations. The feasibility of controlling the diffraction characteristics of the grating in a wide temperature interval is demonstrated.     

Holographic recording and retrieval of polarized light by use of polyester containing cyanoazobenzene units in the side chain

We examined the holographic recording properties of polyester containing cyanoazobenzene units in the side chain. It was revealed that, when a linearly polarized object beam with an arbitrary polarization direction was recorded, the retrieved beam had the same polarization state as that of the object beam, with a constant diffraction efficiency of 0.1. Moreover, two object beams with mutually orthogonal linear polarization could be recorded and retrieved separately from the same area. This is, to our knowledge, the first example of holographic polarization multiplexing. The retrieved images were found to resolve details of ~40line pairs/mm and a 40-mum -pitched checkerboard pattern. The resolution of the system is limited by the CCD cameras that are used. Polarization encoding to increase the storage capacity and (or) the signal-to-noise ratio of holographic data recording is also discussed.     

局域体全息光栅的衍射特性

基于三维耦合波理论，研究了两束有限宽度的任意偏振平面波干涉产生的局域体全息光栅的衍射问题。以单位均匀振幅的任意偏振平面波为例，给出了透射波和衍射波振幅的主分量和交叉分量的解析表达式，讨论了记录过程和再现过程中入射的参考光波的偏振角对透射波和衍射波振幅的影响。计算结果表明，记录过程中的偏振角越小，形成的光栅内的耦合越强，衍射效率越高，但光束质量越差；再现过程中不同的偏振角，由于入射波和衍射波的电矢量的不同方向的耦合强弱不同，透射波和衍射波振幅的主分量和交叉分量的振幅变化行为不同。     

Manipulation of the overall polarization orientation in the focal volume of high numerical objectives

We propose an approach for tuning the three-dimensional polarization of a focusing subwavelength spot by a high numerical aperture objective. The incident beams are composed of a radially polarized beam, an azimuthally polarized beam, and a linearly polarized beam with three different weighting factors, respectively. A specially designed adjustable amplitude angular selector is also inserted at the back aperture of the objective for tuning the polarization azimuthally. It is shown that any desired overall polarization orientation can be obtained. We calculated the overall polarization orientation in the focal volume. It is found that the polar angle of the overall polarization orientation can be arbitrarily tuned by the combination of a radially polarized beam and a linearly polarized beam with different weighting factors, and the azimuthal angle can be tuned by rotating the orientation of the linearly polarized beam azimuthally.     

Dependence of extinction cross-section on incident polarization state and particle orientation

This note reports on the effects of the polarization state of an incident quasi-monochromatic parallel beam of radiation and the orientation of a hexagonal ice particle with respect to the incident direction on the extinction process. When the incident beam is aligned with the six-fold rotational symmetry axis, the extinction is independent of the polarization state of the incident light. For other orientations, the extinction cross-section for linearly polarized light can be either larger or smaller than its counterpart for an unpolarized incident beam. Therefore, the attenuation of a quasi-monochromatic radiation beam by an ice cloud depends on the polarization state of the beam if ice crystals within the cloud are not randomly oriented. Furthermore, a case study of the extinction of light by a quartz particle is also presented to illustrate the dependence of the extinction cross-section on the polarization state of the incident light.     

Polarization-invariant grating based on a photoaligned liquid crystal in an oppositely twisted binary configuration

We demonstrate a polarization-invariant liquid-crystal (LC) grating in an oppositely twisted and mutually orthogonal configuration. The polarization-invariant LC grating with high diffraction efficiency is fabricated using a single-masking process with two-step exposure of a linearly polarized ultraviolet light. It is found that the zeroth- and the first-order diffracted beams are linearly polarized and perpendicular to each other irrespective of the incident polarization for the phase retardation of m pi (m=integer).     

Unidirectional transmission realized by two nonparallel gratings made of isotropic media

We realize a unidirectional transmission by cascading two nonparallel gratings (NPGs) made of isotropic, lossless, and linear media. For a pair of orthogonal linear polarizations, one of the gratings is designed as a polarizer, which is a reflector for one polarization and a transmitter for the other; another grating is designed as a polarization converter, which converts most of one polarized incident wave into another polarized transmitted wave. It is demonstrated by numerical calculation that more than 85% of the incident light energy can be transmitted with less than 1% transmission in the opposite direction for linearly polarized light at normal incidence, and the relative bandwidth of the unidirectional transmission is nearly 9%. The maximum transmission contrast ratio between the two directions is 62 dB. Unlike one-way diffraction grating, the transmitted light of the NPGs is collinear with the incident light, but their polarizations are orthogonal.     

Influence of light polarization on the diffraction efficiency of TIR holograms

Normally when forming a TIR hologram two interference gratings are stored in the recording medium. The object beam interferes with the incoming and with the reflected reference beam. An appropriate selection of the polarization orientations of the interfering light beams leads to the suppression of one of the grating systems. Even to reconstruct TIR hologram it is not all the same in which orientation the reconstructing laser beam is polarized. With the proper selection of the polarization orientations of the forming and reconstructing laser light the diffraction efficiency for TIR holograms can be optimized.     

大数值孔径光子筛偏振特性研究

根据角谱理论建立不同偏振照明条件下的光子筛矢量衍射模型。在此基础上,对入射光分别为线偏振光、径向偏振光、切向偏振光三种特殊偏振状态下的光子筛聚焦光强分布进行了模拟分析。研究结果表明,对于大数值孔径光子筛,入射光的偏振特性将对光子筛聚焦光强分布产生巨大影响。线偏振光将使聚焦光斑沿偏振方向拉伸,切向偏振光产生的聚焦光斑具有"中空"结构,而径向偏振光所产生的聚焦光斑呈较为规则的圆形,且其焦深优于线偏照明情况。在激光直写及高分辨成像等光子筛典型应用中采用径向偏振照明将进一步提高系统分辨力。     

线偏振高斯光束通过复合型衍射光栅的传输特性

提出一种集合浮雕和折射率周期分布调制的复合型衍射光栅,并利用角谱表示和严格的模式理论研究了线偏振高斯光束通过这种复合型衍射光栅的传输特性.数值计算结果表明,在光栅的同一透射深度处复合型衍射光栅光强的起伏频率要比浮雕型光栅光强的起伏频率小.最后使用复合型衍射光栅模型,研究了亚波长体积相位全息光栅的表面起伏和入射光束的束腰宽度对衍射效率的影响. 关键词： 复合型衍射光栅 亚波长体积相位全息光栅 高斯光束 严格模式理论     

涡旋光束在反射中的正交偏振特性研究

本文从角谱理论出发建立了涡旋光束在空气-玻璃界面反射时的傍轴传输模型, 并研究了反射过程中诱导产生的正交偏振效应. 当一水平偏振涡旋光束以不同角度入射时, 反射光束的正交偏振分量呈现出类似于一阶厄米-高斯模式的双峰强度分布, 而水平偏振分量强度分布呈现与入射光束相似的分布, 且只在布儒斯特角附近入射时才现出与正交偏振分量垂直的双峰分布. 对于任意线偏振入射光, 其正交偏振分量的偏振方向不再垂直于入射时的偏振方向, 而是与反射光束的中心波矢垂直, 此时正交偏振分量出现有趣的旋转特性, 其物理原因归结于任意线偏振光入射时所对应的水平与垂直偏振分量的反射系数不同. 最后进行了相关实验验证, 发现实验结果与理论分析符合得较好. 关键词： 正交偏振 涡旋光束 任意线偏振