光折变聚合物中斩波调制对二波耦合增益系数的增强效应

通过二波耦合测量了光折变聚合物PVK:5CB:C60中的耦合增益系数,并利用斩波调制技术实现了二波耦合增益系数的增强.研究了最佳斩波频率及处于此频率下的耦合增益系数随外加电场、入射总光强、光栅波矢的关系. 关键词： 光折变聚合物 二波耦合 增益系数     

外电场对SBN：Cr晶体二波耦合特性的影响及应用

对固液同成分SBN：Cr(Sr0.61Ba0.39Nb2O.6：Cr)晶体在外加直流电场作用下的光折变二波耦合特性及其应用进行了实验研究，分析了晶体的二波耦合强度增益 系数随外电场变化的趋势，并给出了实验测量结果.发现在适当的外电场作用下，晶体的光折变二波耦合增益和响 应速度可以得到一定程度的提高.进一步的研究表明，这种电场响应特性有助于改善SBN：Cr晶体的某些应用性能.利用该晶体通过光折变二波耦合非线性放大原理实现光学图像边沿增强时，通过给晶体沿轴向施加适当的外电场，可进一步提高图像边沿增强效果；在基于光 折变边沿增强预处理的联合变换相关器中，适当的外加电场可进一步改善联合变换相关器的相关识别性能. 关键词： SBN：Cr晶体 光折变二波耦合 边沿增强 联合变换相关器     

高速调幅光束的光折变两波耦合的理论研究

对高速调幅光束的光折变两波耦合进行了理论研究，给出了复耦合系数为任意值时两波耦合方程的解析解，讨论了耦合系数对高速调幅光束的光折变两波耦合的影响。     

掺Ce,Fe系列LiNbO3晶体光折变效应光存储特性

研究了系列Ce：Fe掺杂以及不同后处理态(生长态、还原态和氧化态)铌酸锂晶体的透过率光谱和光折变全息存储特性。实验结果表明单掺Ce铌酸锂晶体具有较好的图像存储质量和较宽的透过率光谱范围,二波耦合增益相对较低;高掺杂铌酸锂样品的透过率光谱范围较窄,光折变二波耦合增益较低。晶体的后处理对铌酸锂样品的光折变特性影响明显,双掺Ce:Fe还原态铌酸锂晶体具有较高的二波耦合增益;氧化态样品具有较大的透过率光谱范围;还原态样品具有较大的光折变二波耦合增益特性。实验结果还表明在同种样品中难于同时获得大的二波耦合增益和图像存储质量。     

一般切割面的铋硅族氧化物光折变增益特性及动态光栅优化

利用耦合张量对光折变旋光，电光，压电及弹光铋硅族氧化物晶体中光波本征模的作用，在任意晶体切割面上求解了弱耦合的矢量波耦合方程, 并利用有效耦合概念处理了非旋光晶体 中的矢量波耦合问题. 进而计算旋光和非旋光材料中信号光的增益, 并分别比较各向同性耦 合和各向异性耦合对增益的影响. 对旋光材料优化增益，并把处理结果应用于BSO和BTO晶体 . 还讨论了压电和弹光效应对(110)，(111)及(112)切割面优化增益的影响. 关键词： 铋硅族氧化物 光折变 旋光效应 压电 弹光效应     

Cu∶KNSBN晶体光折变参量的温度特性研究

本文研究了Cu∶KNSBN晶体光折变材料参量的温度特性.用光折变理论分析了文献6给出的Cu∶KNSBN晶体两波耦合增益系数Γ随2θ在不同温度下的变化关系.得出了该晶体有效电荷密度Neff,有效电光系数和电子-空穴对抗因子乘积Rreff随温度的变化关系.     

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。     

掺杂的铌酸钾钠锶钡晶体的二波耦合增益系数的研究

对多种掺杂的铌酸钾钠锶钡(KNSBN)单晶样品进行了二波耦合测量,着重研究了二波耦合中各样品的增益系数Γ与两光束入射夹角2θ的关系,以及Γ与光强I的关系。用光折变理论公式拟合Γ-2θ关系的实验结果,求得不同样品的有效光折变电荷密度、有效电光系数和电子-空穴对抗率等参数值。采用一种模型,推导出测定与光强有关的增益系数的解析表达式,并应用于Γ-I关系的实验结果分析。测量结果显示,一些掺杂的KNSBN晶体具有很大的Γ值(>15cm^-1)。     

用自散焦介质观测Ce：BaTiO3晶体的自泵浦相位共轭频移

提出利用自散焦介质作为载体测量光折变晶体自泵浦相位共轭的微小频移的新方法，适宜用于区分二波耦合，四波辊频对频移的贡献，为研究光折变自泵浦相位共轭频移的产生机制提供了一种新的可能的途径。     

单模光纤中Raman光放大

本文报道在单模光纤中，背向受激拉曼散射（ＢＳＲＳ）对通信光的光放大研究。所用泵浦源为声光０开关Ｎｄ￣（＋３）：ＹＡＧ激光器，工作波长１．０６４μｍ。信号源为ＩｎＧａＡｓＰ半导体激光器，工作波长１．３０μｍ。在１．３０μｍ处实现了Ｒａｍａｎ光放大，增益达１９．４ｄＢ以上，增益系数为２．３×１０￣（－１２）ｃｍ／ｗ。     

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.     

Multiwave coupling in a high-gain photorefractive polymer

The characteristics of a new high-gain photorefractive polymer composite with a PNP chromophore are investigated. Competition between beam fanning and two-wave coupling (TWC) is predicted and verified experimentally. The intensity dependence of TWC gain is studied. Higher diffraction order and forward phase conjugation in a TWC geometry are observed and explained.     

Dependence of gain and phase-shift on crystal parameters and pump intensity in unidirectional photorefractive ring resonators

The steady-state amplification of light beam during two-wave mixing in photorefractive materials has been analysed in the strong nonlinear regime. The oscillation conditions for unidirectional ring resonator have been studied. The signal beam can be amplified in the presence of material absorption, provided the gain due to the beam coupling is large enough to overcome the cavity losses. Such amplification is responsible for the oscillations. The gain bandwidth is only a few Hz. In spite of such an extremely narrow bandwidth, unidirectional oscillation can be observed easily at any cavity length in ring resonators by using photorefractive crystals as the medium and this can be explained in terms of the photorefractive phase-shift. The presence of such a phase-shift allows the possibility of the non-reciprocal steady-state transfer of energy between the two light beams. Dependence of gain bandwidth on coupling constant, absorption coefficient of the material’s cavity length (crystal length) and modulation ratio have also been studied.      

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.     

Field asymmetry of the dynamic gain coefficient in organic photorefractive devices

The performance of amorphous organic photorefractive materials in holographic two-beam coupling experiments in the typical tilted geometry was found to be asymmetric with respect to the applied electric field direction. For one field direction, light is coupled into the polymer layer and can be detected on the side of the devices. For the other, the originally Gaussian-shaped writing beams show a shoulder or even split into two. The strength of the asymmetry depends on the diameter of the beams writing the hologram. We demonstrate that this effect is due to beam fanning. As a result of the fanning, the apparent photorefractive gain coefficients take on unphysical values.     

Oscillation dependence of two-wave mixing gain for unidirectional ring resonator in photorefractive materials

Dependence of the coupling strength of two-wave mixing gain in photorefractive materials for the single unidirectional ring resonator on oscillation conditions has been analyzed in the strong nonlinear regime. In this regime, difference between the frequency of the pump beam and oscillating beam is proportional to the cavity-length detuning, which can be explained in terms of the photorefractive phase-shift. This phase-shift results due to slightly non-degenerate two-wave mixing that compensates for cavity detuning and satisfies the round-trip phase condition for the steady-state oscillation. The presence of such a phase-shift allows the possibility of the nonreciprocal steady-state energy transfer between the pump and oscillating beams. If the gain due to the beam coupling is large enough to overcome the cavity losses then the signal beam is amplified in the presence of material absorption. Such amplification is responsible for the oscillations. For the single unidirectional ring resonator, the effects of cavity-length detuning, energy coupling coefficient, crystal thickness of the material, reflectivity of the cavity mirrors and material's absorption coefficient on the frequency and intensity of oscillations have also been studied in detail. It has been found that for the smaller value of absorption coefficient (α) of the photorefractive crystal, the unidirectional ring resonator can oscillate at almost any cavity-length detuning (ΔΓ) whereas for the larger value of α oscillation occurs only when the cavity-length detuning is limited to small region (around ΔΓ=0). But reverse of the case is found for energy coupling coefficient (γ₀).     

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.     

掺杂CdS纳米粒子聚合物的光折变效应研究

研究了掺杂CdS纳米粒子的有机聚合物体系的光折变效应,体系中以CdS纳米粒子为光敏剂,聚乙烯咔唑(PVK)为载流子输运剂,4-(4-硝基苯偶氮)苯胺染料(DO3)为非线性生色团,9-乙基咔唑(ECZ)为增塑剂。反胶束法合成的CdS胶束颗粒采用紫外可见吸收光谱和透射电子显微镜(TEM)表征,结果表明,具有纳米尺寸和量子限制效应。研究了PVK-CdS薄膜的光电导特性,实验结果表明,CdS纳米粒子与PVK之间的电荷转移可以有效的提高PVK的光电导率。双光束耦合实验证明了该体系的光折变特性,在无外加电场下,获得二波耦合增益系数78.4 cm-1,分析表明聚合物薄膜具有强的光致取向增强效应;样品在两相干光束作用下,可建立折射率光栅,其衍射效率达到4.4%。     

Theoretical analysis of the effects of frequency detuning dependence of photorefractive two- beam coupling coefficient on gain and phase-shifts of the signal beam for a coupled unidirectional photorefractive ring resonators

Dependence of two-beam coupling gain and phase-shift of the signal beams on the frequency detuning for a coupled unidirectional ring resonators based on non-degenerate two-wave mixing in the photorefractive crystals have been investigated in details. The effects of various parameters characterizing the photorefractive medium such as frequency detuning, absorption strength, two-beam energy coupling strength and pump intensity of the external laser beams, on the two-beam coupling gain and phase-shift of the signal beams for a coupled UPRR have also been studied in details. It has been found that the photorefractive gain of the signal beam in the primary cavity of the coupled UPRR can be enhanced to the higher order by taking the lower value of the frequency detuning of the primary cavity which could exist at much lower value of the absorption strength of the crystal B. This higher value of photorefractive gains in the cavities are responsible for the strong coupling between the modes of the oscillations of the coupled UPRR.     

Crystal orientation dependence of the SNR for signal beam amplification in photorefractive BaTiO3

In signal beam amplification by two-beam coupling in BaTiO₃ photorefractive crystals, beam fanning in the direction of the amplified signal reduces the signal-to-noise ratio (SNR). The dependence of the SNR and the signal beam gain on the crystal orientation are analysed using a HeNe laser. It is found that orientating the crystal for maximum gain gives poor signal-to-noise ratio. A compromise has to be made between the SNR and high gain for optimum signal amplification.