Wave-front analysis method of circular aperture sampling for collimation testing

A new type of wave-front analysis method for the collimation testing of laser beams is proposed. A concept of wave-front height is defined, and, on this basis, the wave-front analysis method of circular aperture sampling is introduced. The wave-front height of the tested noncollimated wave can be estimated from the distance between two identical fiducial diffraction planes of the sampled wave, and then the divergence is determined. The design is detailed, and the experiment is demonstrated. The principle and experiment results of the method are presented. Owing to the simplicity of the method and its low cost, it is a promising method for checking the collimation of a laser beam with a large divergence. © 2005 Optical Society of America.

LiNbO3：Cu：Ce非挥发全息记录掺杂组份比的优化

实验研究了掺杂组份比对LiNbO3：Cu：Ce晶体非挥发全息记录性能的影响。结果表明．在全息记录过程中，掺杂组份比通过改变晶体的紫外光吸收特性而引起全息记录性能的改变。增加LiNbO3：Cu：Ce晶体中Cu和Ce的掺杂组份比会导致晶体对紫外光吸收的增强，进而提高了全息记录灵敏度和固定衍射效率。在弱氧化处理的掺有CuO和Ce2O4的质量分数分别为0.085％和0．011％的LiNbO3：Ce：Cu晶体中．得到了最高的固定衍射效率ηf=32％和记录灵敏度S=0．022cm／J。

Improvement of sensitivity and refractive-index changes in LiNbO3 : Ce : Cu crystals with UV light

A nonvolatile recording scheme is proposed using LiNbO3:Ce:Cu crystals and modulated UV light to record gratings simultaneously in two centres and using red light to bleach the grating in the shallow centre to realize persistent photorefractive holographic storage. Compared with the normal UV-sensitized nonvolatile holographic system, the amplitude of refractive-index changes is greatly increased and the recording sensitivity is significantly enhanced by recording with UV light in the LiNbO3:Ce:Cu crystals. Based on jointly solving the two-centre material equations and the coupled-wave equations, temporal evolutions of the photorefractive grating and the diffraction effciency are effectively described and numerically analysed. Roles of doping levels and recording-beam intensity are discussed in detail. Theoretical results confirm and predict experimental results.

Sensitivity improvement by UV-light recording in LiNbO3 : Ce : Cu crystals

Modulated UV light is used to increase the sensitivity of the two-centre holographic recording. Inherent mechanisms of nonvolatile holographic recording in oxidized and reduced crystals are numerically analysed based on solving the two-centre material equations modilied for UV-Iight recording. Experiments verification is performed with an oxidized crystal and a reduced crystal, and the role of UV intensity on the sensitivity is presented.

Effect of microcosmic optical parameters of dopants on the signal-to-noise ratio in doubly doped LiNbO3 crystals

We propose a united theory that describes the two-center recording system by taking scattering noise into account. The temporal evolution of the signal-to-noise ratio in doubly doped photorefractive crystals is described based on jointly solving material equations and coupled-wave equations with the fourth-order Runge-Kutta method. Roles of microcosmic optical parameters of dopants on the signal-to-noise ratio are discussed in detail. The theoretical results can confirm and predict experimental results. (c) 2005 Elsevier GmbH. All rights reserved.

Improvement in refractive-index change in LiNbO3 : Ce : Cu by applying an external electric field

By jointly solving two-centre material equations with a nonzero external electric field and coupled-wave equations, we have numerically studied the dependence of the non-volatile holographic recording in LiNbO3:Ce:Cu crystals on the external electric field. The dominative photovoltaic effect of the non-volatile holographic recording in doubly doped LiNbO3 crystals is directly verified. And an external electric field that is applied in the positive direction along the c-axis (or a large one in the negative direction of the c-axis) in the recording phase and another one that is applied in the negative direction of the c-axis in the fixing phase are both proved to benefit strong photorefractive performances. Experimental verifications are given with a small electric field applied externally.

采用紫外光提高双掺杂铌酸锂晶体中全息记录的灵敏度和光栅强度

提出了一种在双掺杂铌酸锂晶体中用调制的双紫外光进行非挥发全息记录的方法。与通常的用紫外光敏化的非挥发全息记录相比，这种方法可以大幅度地提高光栅强度和记录灵敏度。联立双中心物质方程和双光束耦合波方程，数值分析了光栅强度和衍射效率随时间的变化并讨论了掺杂浓度和记录光强对紫外光非挥发全息记录机制下光折变效应的影响。研究发现，紫外光记录得到的深浅中心的光栅具有相同的相位，总的光栅（深浅中心光栅的叠加）强度为两光栅强度之和，固定过程中深中心的光栅得到增强；增大深浅中心掺杂的浓度可以提高光栅强度，增大记录紫外光的光强

Phase-step calibration technique based on a two-run-times-two-frame phase-shift method

A novel phase-step calibration technique is presented on the basis of a two-run-times-two-frame phase-shift method. First the symmetry factor M is defined to describe the distribution property of the distorted phase due to phase-shifter miscalibration; then the phase-step calibration technique, in which two sets of two interferograms with a straight fringe pattern are recorded and the phase step is obtained by calculating M of the wrapped phase map, is developed. With this technique, a good mirror is required, but no uniform illumination is needed and no complex mathematical operation is involved. This technique can be carried out in situ and is applicable to any phase shifter, whether linear or nonlinear. (c) 2006 Optical Society of America.

Anomalous behaviour of photorefractive grating erasure owing to existence of both negative and positive carriers

In LiNbO3:Fe, anomalous behaviour of grating erasure is observed with different wavelenghts, i.e. rapid grating erasure in the short wavelength range, which deviates from the results predicted by the electron transport band model. The deviation is related to the coexistance of electrons and holes in photorefraction, and charge-transfer process including electrons and hole has been proposed. The electron and hole contributions to photo-excitation coefficient S of the Fe centre on the wavelength.

Wavefront conversion by local volume holograms between cylindrical and plane waves with reconstruction at a different readout wavelength

Based on the two-dimensional coupled-wave theory, the wavefront conversion between cylindrical and plane waves by local volume holograms recorded at 632.8 nm and reconstructed at 800 nm is investigated. The proposed model can realize the 90 degrees holographic readout at a different readout wavelength. The analytical integral solutions for the amplitudes of the space harmonics of the field inside the transmission geometry are presented. The values of the off-Bragg parameter at the reconstructed process and the diffracted beam's amplitude distribution are analysed. In addition, the dependences of diffraction efficiency on the focal length of the recording cylindrical wave and on the geometrical dimensions of the grating are discussed. Furthermore, the focusing properties of this photorefractive holographic cylindrical lens are analysed.

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

敏化光波长对（Fe，Ni）：LiNbO3晶体全息记录性能的影响

分别采用514nm绿光、488nm蓝光和390nm紫外光作为敏化光，633nm红光作为记录光，详细研究了敏化光波长对氧化（Fe，Ni）：LiNbO3晶体全息记录性能的影响。结果表明：随着敏化光波长的逐渐减小，氧化（Fe，Ni）：LiNbO3晶体的非挥发全息记录性能逐渐优化，390nm紫外光是这三种敏化光中最优的敏化光。考虑敏化光的吸收，为了在双中心全息记录中获得最优的性能，应当选择合适波长的敏化光：一方面短波长敏化光能有效地敏化深中心；另一方面短波长敏化光的吸收太强（如对光折变效应无用的基质吸收），不能沿

双掺杂LiNbO3：Fe：Mn晶体局域光折变体全息光栅形成及衍射特性

将带输运模型与二维耦合波理论相结合，研究了双掺杂LiNbO3：Fe：Mn晶体中由两束有限宽度平面波干涉产生的局域光折变体全息的动力学机制及其衍射特性。采用三步法联立求解了双中心带输运物质波方程和二维耦合波方程。数值计算结果表明，局域光折变体全息光栅的空间电荷场在空间上呈不均匀分布。当光栅区域较小时空间电荷场较强，随着光栅区域的增大，空间电荷场急剧下降，并降低一个数量级。此外，在接近光束入射边界的区域，空间电荷场时空变化规律与一维无限大光折变体全息光栅的结果相似，随着光栅区域的扩大，边界效应对空间电荷场的影

正弦相位调制位移干涉测量技术中调制频率的优化选择

针对正弦相位调制（SPM）干涉测量技术用于位移测量时，调制频率对干涉信号相位解调的影响，提出一种调制频率的优化选择依据。通过对干涉信号的频谱进行分析，发现当被测位移幅度较小时，较小的调制频率即可满足相位解调的要求；而当被测位移较大时，必须相应地增大调制频率，才能获得比较准确的测量结果。模拟计算以及实验结果表明，被测位移信号的幅度每增大四分之一测量光源波长，调制频率需要相应增大4倍于被测信号频率的大小，才能满足正弦相位调制位移干涉测量技术信号处理的需要。

A four-frame phase shift method insensitive to phase shifter nonlinearity

A four-frame phase shift method and an associated algorithm using unequal phase steps are presented. The unique advantage of this method is that it becomes insensitive to phase shifter nonlinearity because of the performance of a special procedure, in which the phase shifts are shared out between the reference beam and the object beam. By this means, any phase shifter can work as long as one phase shift is accurately known. On the basis of the technique, a simple calibration method for the linear phase shifter is suggested. The influences of phase shifter miscalibration, detector nonlinearity and random noise on the algorithm are investigated, and the optimal phase shifts are given.