Broad-bandwidth semi-noncollinear optical parametric amplification in periodically poled LiNbO3 based on tilted quasi-phase-matched gratings

On the basis of noncollinear optical parametric amplification in periodically poled lithium niobate (PPLN) which is realized by quasi-phase matching (QPM) technology, we consider the possibility of semi-noncollinear phase matching between collinear and noncollinear geometries by tilting a PPLN-crystal's parallel grating at a sure angle. Numerical simulation with proper parameters shows that we can achieve a broader optical parametric amplification (OPA) bandwidth than that of noncollinear geometry. About 121 nm at a signal wavelength of 800 and 70 nm at a signal wavelength of 1064 nm under optimal conditions are obtained when the crystal length is 9 mm.

双掺杂LiNbO3晶体中光栅均匀性的优化研究

研究了非挥发全息记录中南于紫外光的强吸收而引起的光栅非均匀性，分析了这种非均匀性对光栅衍射效率的影响。结果显示，非均匀性致使光折变光栅的平均强度减弱，衍射效率降低。提出了采用两束等光强的敏化紫外光由晶体两侧入射的优化方案以改善光栅的均匀性，提高光栅的衍射效率。通过联立两中心带输运物质方程和双光束耦合波方程，进行了相应的理论模拟，并给出实验验证。结果表明双侧紫外光照射能够实现均匀性较好的光栅，是提高衍射效率的有效途径之一。

LiNbO3：Fe晶体光栅擦除特性

在光折变过程中，光波对掺杂中心的激发系数S是一个重要的物理量。为了求出光激发系数S对波长A的依赖，研究了在不同的擦除波长条件下，记录在光折变晶体LiNbO3：Fe中的光栅衰减特性。实验结果表明lnP（P为光擦除灵敏度）与擦除波长λ呈线性关系，即波长越短，光栅擦除越快，光擦除灵敏度越高。在单中心电子带传输模型的基础上，理论计算也证实了实验结果。利用带传输模型，给出了理论上光激发系数S对波长的指数依赖关系。同时微观量φμγ（φ为量子效率）也可由光栅擦除实验求出。

Ce：Mn：LiNbO3晶体紫外记录研究

采用紫外光作记录光在Ce：Mn：LiNbO3晶体中实现非挥发全息记录，灵敏度可达0．0803cm／J，衍射效率（固定）为5．07％，比采用红光为记录光，紫外光为敏化光的非挥发双中心记录方案均提高了50多倍。分析表明，采用紫外光作为记录光，深能级电子被激发比例极大提高，参与光折变过程的电子平均运动周期变短．提高了衍射效率和灵敏度；深浅能级电子光栅的同相位，使得固定空间电荷场变强。文中还研究了退火对记录性能的影响。

组合外加电场提高Fe：LiNbO3中90°记录体全息的衍射效率

针对90°全息记录结构的特殊性，在记录和读出阶段分别施加不同极性的外加电场，实现了Fe：LiNbO3晶体中高衍射效率的全息记录和读出。在库赫塔列夫（Kukhtarev）方程基础上引人两维耦合波理论，对组合外加电场提高掺铁铌酸锂晶体光折变特性的机理进行了探讨。实验和理论计算结果都表明这种分别在记录和读出过程施加不同极性组合的外加电场是在Fe：LiNbO3中实现高衍射效率90°记录结构体全息的有效技术方案。

LiNbO3：Fe：Ni晶体非挥发全息存储研究

采用三种不同的双光记录方案进行了LiNbO3：Fe：Ni晶体全息存储实验．详细研究了饱和衍射效率、固定衍射效率、动态范围和记录灵敏度，以及退火条件对记录的影响。结果表明，氧化LiNbO3：Fe：Ni晶体的饱和衍射效率、固定衍射效率和记录灵敏度比其他报道的双掺杂LiNbO3晶体高。结合掺杂能级图，理论分析了LiNbO3双掺杂晶体深陷阱中心能级的相对位置及其微观光学参量对全息记录性能的影响。LiNbO3：Fe：Ni晶体有望成为一种新的高效率非挥发全息存储材料。

LiNbO3：Ru晶体的光折变特性研究

采用丘克拉斯基法生长了单掺杂的LiNbO3：Ru晶体，并对其作氧化处理。实验研究了LiNbO3：Ru晶体二波混频条件下光栅记录特性和波长的依赖关系；并且研究了氧化处理对其透射谱和光栅记录特性的影响。研究结果表明，LiNbO3：Ru晶体的最佳记录波长为458nm；氧化处理会增加LiNbO3：Ru晶体对光的吸收，同时提高光栅记录的灵敏度和饱和衍射效率，表现了不同于LiNbO3：Fe晶体的氧化态依赖特性。分析认为，LiNbO3：Ru晶体的反常氧化态依赖特性可能是由于出现了电子通道效应的暗衰减机制，以及Ru存在多

双掺杂LiNbO3：Fe：Ru晶体的全息读出特性

为了解决读出过程中全息数据的擦除问题，研究了新型双掺杂LiNbOa：Fe：Ru晶体的全息读出特性。分析了双中心和单中心记录方案光栅的读出特性，并联立双中心物质方程和耦合波方程进行了模拟计算。结果表明，双中心记录所得到的饱和全息的读出时间常数远低于LiNbO2：Fe：Mn晶体的读出时间常数；单色光记录可以实现有效的全息，且其读出时间常数远大于记录时间常数，表现为准态非挥发读出。分析表明，这可能由于Ru的能级比Mn更靠近Fe，更易被红光激发，从而使得双中心记录所得饱和光栅的存贮持久性降低；单色光记录中红光能够

LiNbO3：Ru：Fe晶体畴反转过程中的电色效应

观察到了双掺杂LiNbO3：Ru：Fe晶体中电色效应伴随畴反转而发生，且与畴反转一样也具有可逆性，两者相辅相成，畴反转导致了晶体变色，电色效应促进了畴反转，系统的实验结果证明了两者的相辅相成性．基于铌酸锂铁电微结构模型，简要解释了其机理．而且发现在极化过程中电色效应促使了畴核的形成，使之不同于非掺杂同成分比铌酸锂晶体的矫顽场大于击穿电场，用恒定直流电场代替脉冲电场也能实现畴反转，这将为周期性极化铌酸锂的制备提供一种新的技术改进．

Local thermal fixing of a photorefractive LiNbO3 hologram by use of a CO2 laser

A real-time, in situ fixing method by use of heating with a CO2 laser beam is suggested for thermal fixing of a small local hologram in the bulk of a Fe:LiNbO3 photorefractive crystal. For heating up to 100 degrees C-200 degrees C a volume with a shape similar to that of the laser beam a heat-guiding technique is developed. On the basis of the heat-transfer equations, different heating modes with or without metal absorbers for heat guiding-obtained by use of a continuous or pulsed laser beam are analyzed. The optimal mode may be pulsed heating with absorbers. On this basis experiments have been designed and demonstrated. It is seen that the fixing process with CO2 laser beam is short compared with the process by use of an oven, and the fixing efficiency is quite high. (C) 1998 Optical Society of America.

Nonvolatile holograms in LiNbO3 : Fe : Cu by use of the bleaching effect

We report our observation of a bleaching effect under an ultraviolet exposure in LiNbO3:Fe:Cu crystals. Two three-step recording-transferring-fixing schemes are proposed to record nonvolatile photorefractive holograms in such crystals. In the schemes two red laser beams and an ultraviolet illumination are used selectively to write the charge grating in the shallow-level Fe centers, to develop the charge grating in the deep-level Cu centers by transferring the charge grating in the Fe centers, and to fix only the charge grating in the Cu centers for unerasable read-out. Experimental results, verifications, and an optimal recording scheme are given. A comparison of the lithium niobate crystals of the same double-doping system of Fe:Mn, Ce:Mn, Ce:Cu, and Fe:Cu is outlined. (C) 2002 Optical Society of America.

Prescription for optimizing holograms in LiNbO3 : Fe : Mn

The photorefractive holographic dynamics of grating formation in photochromic doubly doped LiNbO3:Fe:Mn crystal is studied numerically and analytically in terms of the two-center model of Kukhtarev Et al. [Ferroelectrics 22, 949 (1979)]. The relations among the recorded and fixed space-charge fields and the doping densities, the oxidation-reduction states of the fields, and the intensities of UV-sensitizing and red recording beams are studied. Important conditions and effects are feued, and an optimal prescription for material doping and oxidation-reduction processing is suggested in which the crystal can be strongly oxidized and the Mn-doping density is smaller than the Fe-doping density. (C) 2000 Optical Society of America. OCIS codes: 050.7330, 190.5330, 090.2900.

Nonvolatile photorefractive holograms in LiNbO3 : Cu : Ce crystals

We report experimental and theoretical studies of nonvolatile photorefractive holographic recording in LiNbO3:Cu:Ce crystals with two illumination schemes: (1) UV light for sensitization and a red interfering pattern for recording and (2) blue light for sensitization and a red pattern for recording. The results show that the oxidized LiNbO3:Cu:Ce crystals can provide high, persistent refractive-index modulation with weak lightinduced scattering. The optimal working conditions and the prescription for doping and oxidation-reduction processing that yields the maximum refractive-index modulation are discussed. (C) 2000 Optical Society of America OCIS codes: 050.7330, 190.5330, 090.2900.

Effect of absorption on the diffraction efficiency in Fe : LiNbO3 crystal with 90 degrees recording geometry

In this paper the saturated diffraction efficiency has been optimized by considering the effect of the absorption of the recording light on a crossed-beam grating with 90 degrees recording geometry in Fe:LiNbO3 crystals. The dependence of saturated diffraction efficiency on the doping levels with a known oxidation-reduction state, as well as the dependence of saturated diffraction efficiency on oxidation-reduction state with known doping levels, has been investigated. Two competing effects on the saturated diffraction efficiency were discussed, and the intensity profile of the diffracted beam at the output boundary has also been investigated. The results show that the maximal saturated diffraction efficiency can be obtained in crystals with moderate doping levels and modest oxidation state. An experimental verification is performed and the results are consistent with those of the theoretical calculation.

Electrochromic effect in domain-inversion process LiNbO3 : Ru : Fe crystals

A reversible electrochromic effect accompanying domain-inversion during the electrical poling process in LiNbO3: Ru: Fe crystals at room temperature has been observed. In electrode area, both electrochromism and domain-inversion occur alternately, and electrochromism is also reversible during back-switch poling, which is experimentally verified and whose mechanism is briefly explained using a microstructure ferroelectric model. In addition, because of the enhancing elcctrochromic effect, different from the undoped LiNbO3 crystals, the coercive riled (21.0 kV/mm or so) measured in LiNbO3: Ru: Fe is lower than its breakdown field, thus providing a possible new technique for realizing the domain-inversion by constant electric field rather than a pulsed one.