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.     

光折变多重存储的栅自增强读出特性研究

研究了光折变重存储读出时折射率栅自增强特性与记录晶体厚度、记录时的光强比以及写入的折射率栅振幅之间的关系.结果表明记录晶体的厚度大于某一值,才有自增强效应;较小的参物光强比具有较大的自增强效果,能获得较大的读出衍射效率;自增强读出所能达到的最大衍射效率与写入的栅振幅大小无关,而读出时衍射效率增加的速率与写入的栅振幅成反比.     

光折变光栅在超短脉冲光照射下衍射的研究

利用Kogelnik单色光在体光栅中衍射效率公式推导出超短脉冲光读出稳态光折变体光栅时的衍射光频谱表达式,并在此基础上导出了脉冲光读出时的总衍射光强及总光强衍射效率.研究发现衍射光的频谱分布、总光强及衍射效率与光栅周期Λ、光栅厚度d和读出光脉冲宽度Δτ有密切的关系.通过调整这些参量,光折变光栅可以对入射的超短脉冲光实现不同程度的滤波作用.该结论可用于超短脉冲的整形研究.     

偏振控制的Ce:KNSBN类光纤光折变光开关

在Ce:KNSBN光折变类光纤晶体中,用不同偏振的读出光控制衍射光强的大小从而实现光折变光开关的开－关功能.实验结果表明e光读出时的光栅衍射效率是o光读出的310倍,而且衍射效率比不随写入光光强比的变化而变化.与块状晶体做了比较,并用理论进行了分析,在光折变类光纤中的衍射效率比是块状晶体的10倍,光折变类光纤晶体有着更好的光开关性能.     

基于多路复用体全息光栅的角度放大器设计

为设计基于多路复用体全息光栅的角度放大器, 建立了多路复用角度放大器(MAM)模型, 从效率均衡性和角度分布均匀性两个方面归纳了其设计规则; 研究了光刻过程中的误差对MAM性能的影响; 分析了实际发散光束对MAM性能的影响. 研究表明:控制光栅空间频率和光栅倾斜角可以实现需要的MAM角度分布, 控制光栅厚度和折射率调制深度可以实现MAM最佳衍射效率; MAM最大复用路数不超过10路; 增大光栅倾角或者记录光与工作光波长之比有利于抑制参考光角度误差带来的MAM出射角分布误差, 减小光栅厚度有利于抑制厚度误差与折射率调制深度误差对衍射效率的影响; 当远场发散角大于光栅角半宽时, 最佳衍射效率下降到50%以下且角度选择曲线失去局部最小值; 增大空间频率或者光栅厚度可以减小所需的折射率调制深度, 增多MAM可复用路数, 但是不利于效率均衡性设计和抑制发散光束的影响.     

基于多路复用体全息光栅的角度放大器设计

为设计基于多路复用体全息光栅的角度放大器, 建立了多路复用角度放大器(MAM)模型, 从效率均衡性和角度分布均匀性两个方面归纳了其设计规则; 研究了光刻过程中的误差对MAM性能的影响; 分析了实际发散光束对MAM性能的影响. 研究表明:控制光栅空间频率和光栅倾斜角可以实现需要的MAM角度分布, 控制光栅厚度和折射率调制深度可以实现MAM最佳衍射效率; MAM最大复用路数不超过10路; 增大光栅倾角或者记录光与工作光波长之比有利于抑制参考光角度误差带来的MAM出射角分布误差, 减小光栅厚度有利于抑制厚度误差     

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.     

The amplitude and phase of a transmission holographic grating due to four-wave interaction in a photorefractive medium

Based on a previously developed two-level model of four-wave interaction in photorefractive crystals, expressions for the amplitude and phase of a transmission holographic grating have been obtained. The analysis of these expressions points to the dependence of both the amplitude and the phase of the grating on the intensity of the readout beam.     

写入光偏振态对Ce:KNSBN晶体衍射特性的影响

采用非同时读出条件下晶体两波耦合实验装置,以单束光入射Ce:KNSBN光折变晶体,通过改变抽运光偏振方向和品体c轴的夹角φ.系统记录了不同φ角下的抽运光透射光强I',随时间的变化情况.实验结果表明,当φ≤30°时,基本没有光扇效应;实验研究了正交偏振光写入下Ce:KNSBN晶体的两波耦合动态过程,并与e光写入下两波耦合动态过程进行了比较,发现正交偏振光写入时光扇噪声得到了明显抑制,在相同的写入参量条件下光栅的衍射效率明显提高.     

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     

高功率非相干光纤激光组束方法研究

提高非相干光纤激光组束功率需要较高的光栅衍射效率,通过理论分析和数值仿真,结果表明光栅衍射效率对组束中心波长不敏感,而随着组束波长带宽的增大而急剧减小。为了增加可参与组束的光纤激光器数目,同时确保较高的光栅衍射效率,应将光栅频率f控制在200~400mm-1,光栅厚度t在1~2mm,理论上可以获得10kW量级的组束激光。     

非相干光纤激光组束中光栅衍射效率的研究

提高非相干光纤激光组束的组束功率,需要增大光栅对组束激光的衍射效率。通过理论分析和数值仿真,结果表明对中心激光入射角偏移及组束光角偏移的精确控制是提高光栅衍射效率的光健,高的衍射效率对应较大的光栅周期和较小的光栅厚度。对于中心激光入射波长1060nm,应选择光栅频率200~400mm-1,对应的光栅厚度2~4mm。     

非相干光纤激光组束中光栅参数的确定

光栅衍射效率是非相干光纤激光组束系统的关键,理论分析和数值仿真表明光栅衍射效率随着光栅频率和光栅厚度的减小而增大。全面综合考虑光栅频率应该选取在200mm-1和400mm-1之间,光栅厚度在1mm到2mm之间。同时提出了一种不等间隔方法提高系统总体衍射效率,该方法较等间隔条件下衍射效率提高了0.1651。     

掺Yb光纤激光器阵列谱合成系统的光束传输模型及光束特性分析

建立了由掺Yb光纤激光器阵列、变换透镜、闪耀光栅和输出耦合镜组成的光束谱合成系统的光束传输模型.在考虑光栅角色散、光栅刻槽倾角误差和光栅衍射效率情况下,利用光线追迹法、衍射积分方法、光束非相干叠加原理和强度二阶矩方法,推导出高斯光束非平行倾斜入射到闪耀光栅的相位变化公式以及谱合成光束的光强分布解析表达式.分析了高斯光束非平行倾斜入射到光栅后,光栅角色散、光栅衍射效率和光栅刻槽倾角误差对掺Yb光纤激光器谱合成系统输出光束特性的影响.研究结果表明,谱合成光束具有与单根光纤激光器几乎相同的光束质量;光栅角色散对合成光束特性的影响可忽略;随着光栅刻槽倾角误差的增大,谱合成光束的光束质量明显变差;当光栅刻槽倾角误差较大时,必须考虑光栅衍射效率对合成光束特性的影响. 关键词： 掺Yb光纤激光器 非平行倾斜入射 光束谱合成 光束质量     

An Iteratively-Designed Binary Phase Grating for Flattop Beam Generation

A binary phase grating capable of shaping an incident Gaussian beam to a flattop beam in a Fourier domain is designed iteratively. It is essential in grating design to consider the phases of diffracted waves by the grating, in addition to their amplitude uniformity and diffraction efficiency. The optimized grating phase produces the in-phase diffracted waves that interfere constructively to form a uniform amplitude and phase profile. The dependence of the shaped profile on the illuminating amplitude distribution, the grating misalignment, and the lens focusing error is investigated by computer simulation and optical experiment.     

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     

Diffraction of an ultrashort pulsed beam with arbitrary polarization state from a volume holographic grating in LiNbO3 crystals

Based on a modified coupled wave theory of Kogelnik, we have studied the diffraction of an ultrashort pulsed beam with an arbitrary polarization state from a volume holographic grating in photorefractive LiNbO₃ crystals. The results indicate that the diffracted intensity distributions in the spectral and temporal domains and the diffraction efficiency of the grating are both changed by the polarization state and spectral bandwidth of the input pulsed beam. A method is given of choosing the grating parameters and input conditions to obtain a large variation range of the spectral bandwidth of the diffracted pulsed beam with an appropriate diffraction efficiency. Our study presents a possibility of using a volume holographic grating recorded in anisotropic materials to shape a broadband ultrashort pulsed beam by modulating its polarization state.     

Airy beam laser

A method to design lasers that emit an arbitrary beam profile is studied. In these lasers, output-coupling is performed by a diffraction grating that imposes a phase and amplitude distribution onto the diffracted light. A solid-state laser emitting beams with a two-dimensional Airy intensity profile is demonstrated both theoretically and experimentally. In this case, the diffraction grating adds a transverse cubic phase to the diffracted light. An Airy beam is obtained by performing optical Fourier transform of the out-coupled light. The laser beam profile and power characteristics are shown to agree with theory.     

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。     

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