Effect of dopant composition ratio on nonvolatile holographic recording in LiNbO3:Cu:Ce crystals

The effect of dopant composition ratio on nonvolatile holographic recording in LiNbO3:Cu:Ce crystals is investigated experimentally. The results show that the dopant composition ratio affects the recording sensitivity and fixed diffraction efficiency by altering the UV light absorption characteristics of the crystals during nonvolatile, holographic recording. Increasing the dopant composition ratio of Cu and Ce leads to an increase in the absorption of UV light and further to an increase in the recording sensitivity and fixed diffraction efficiency. The UV light absorption characteristics of LiNbO3:Cu:Ce crystals and their roles in nonvolatile holographic recording are theoretically analyzed. The theoretical results are consistent with those of the experiments.     

氧化还原处理对Fe:LiNbO3晶体光折变性能及热固定效果的影响

在LiNbO3中掺进Fe2O3生长了Fe:LiNbO3晶体,并对晶体进行氧化、还原处理。利用二波耦合的方法,测试了不同还原处理的Fe:LiNbO3,晶体的指数增益系数、衍射效率及响应时间。测试了三种不同处理本热固定后的显影效率,记录了全息先栅的记录、热补偿及离子衰减过程。发现不同氧化还原处理的晶体在光折变性能和热固定效果上有不同的表现。     

Optimal conditions for thermal fixing of volume holograms in Fe:LiNbO3 crystals

We analyze and compare two typical recording and thermal fixing procedures of a volume hologram in a Fe:LiNbO(3) crystal (low-high-low procedure and high-low procedure). We consider the kinetics of the recording, compensating, and developing processes by taking into account the ratio of the conductivities between the protons and the electrons as a function of temperature. From the analysis the optimal environmental conditions (in terms of the fixing temperature and the compensation time) for each fixing procedure can be deduced for a crystal with given material parameters.     

Absorption characteristic and nonvolatile holographic recording in LiNbO3:Cr:Cu crystals

The absorption characteristic of lithium niobate crystals doped with chromium and copper (Cr and Cu) is investigated. We find that there are two apparent absorption bands for LiNbO3:Cr:Cu crystal doped with 0.14 wt.% Cr2O3 and 0.011 wt.% CuO; one is around 480 nm, and the other is around 660 nm. With a decrease in the doping composition of Cr and an increase in the doping composition of Cu, no apparent absorption band in the shorter wavelength range exists. The higher the doping level of Cr, the larger the absorbance around 660 nm. Although a 633 nm red light is located in the absorption band around 660 nm, the absorption at 633 nm does not help the photorefractive process; i.e., unlike other doubly doped crystals, for example, LiNbO3:Fe:Mn crystal, a nonvolatile holographic recording can be realized by a 633 nm red light as the recording light and a 390 nm UV light as the sensitizing light. For LiNbO3:Cr:Cu crystals, by changing the recording light from a 633 nm red light to a 514 nm green light, sensitizing with a 390 nm UV light and a 488 nm blue light, respectively, a nonvolatile holographic recording can be realized. Doping the appropriate Cr (for example, NCr = 2.795 x 10(25) m(-3) and NCr/NCu = 1) benefits the improvement of holographic recording properties.     

Thermal fixing of 10,000 Holograms in LiNbO3:Fe

We discuss thermal fixing as a solution to the volatility problem in holographic storage systems that use photorefractive materials such as LiNbO(3):Fe. We present a systematic study to characterize the effect of thermal fixing on the error performance of a large-scale holographic memory. We introduce a novel, to our knowledge, incremental fixing schedule to improve the overall system fixing efficiency. We thermally fixed 10,000 holograms in a 90 degrees -geometry setup by using this new schedule. All the fixed holograms were retrieved with no errors.     

Characteristics of recording and thermal fixing in lithium niobate

We present a theoretical model in which the band-transport equations and the coupled-wave equations are considered to study the two thermal-fixing methods (simultaneous fixing and postfixing) in Fe:LiNbO(3). We found that, in simultaneous fixing, the existing ionic-grating affects the writing of the electronic grating by reduction of the coupling gain, and the grating envelope of the fixed-index grating is quite uniform inside the photorefractive crystal in comparison with the method of postfixing. The resulting diffraction efficiency of the fixed-volume grating is dependent mainly on the initial intensity modulation of the two writing beams. A set of experiments is also presented.     

Local Thermal Fixing of a Photorefractive LiNbO(3) Hologram by use of a CO(2) Laser

A real-time, in situ fixing method by use of heating with a CO(2) laser beam is suggested for thermal fixing of a small local hologram in the bulk of a Fe:LiNbO(3) 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 CO(2) laser beam is short compared with the process by use of an oven, and the fixing efficiency is quite high.     

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 inthe shallow-level Fe centers, to develop the charge grating inthe deep-level Cu centers by transferring the charge grating in the Fe centers, and to fix only thecharge 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.     

Self-stabilized recording of fixed gratings at high temperature in LiNbO3:Fe

A high quality fixed holographic grating was recorded in a photorefractive LiNbO(3):Fe crystal at about 100 degrees C in a homemade temperature-controlled vacuum chamber. The recording was carried out using self-stabilization techniques with lambda=532 nm beams guided onto the crystal by polarization maintaining fibers. The diffraction efficiency of the fixed grating was eta=0.44 when measured in the recording setup using the same lambda=532 nm recording beams. A compatible eta was measured with lambda=633 nm in an auxiliary setup, and a 1 mrad angular Bragg selectivity at FWMH was estimated, thus demonstrating the uniformity and good quality of the fixed grating.     

Ti:Fe:LiNbO_3晶体的生长及其光折变性能的研究

采用熔体提拉法生长了不同掺杂浓度的Ti：Fe：LiNbO3晶体．研究了掺杂杂质离子浓度变化对晶体光折变性能的影响，测定了晶体经热化学还原处理前后的透射谱．用ESR方法证实，未经还原处理时，Ti：Fe：LiNbO3晶体中Ti离子以Ti4 形式存在．与Fe：LiNbO3和Ti：LiNbOa相比，Ti、Fe复合掺杂，通过电荷补偿效应，使未经还原处理的晶体中Fe2 增加，从而使光吸收增强；可以通过改变Ti、Fe掺杂浓度的方法来控制晶体中Fe2 离子的浓度，达到控制并改善晶体光折变性能的目的．本文还对Ti：Fe：LiNbO3晶体的全息性能进行了研究，测得Ti：Fe：LiNbO3晶体响应时间缩短，衍射效率高达90％以上．Ti：Fe：LiNbO3晶体是一种优质的光折变材料．     

Wavelength demultiplexing with layered multiple Bragg gratings in LiNbO3:Fe crystal

Dense wavelength division multiplexing (DWDM) is an important technology for expanding the capacity of optical network. The optical component based on the superimposed Bragg gratings shows that it can be used as one of advantageous multichannel components because of its excellent angle and wavelength selectivities. An optimized method for recording multiple Bragg gratings for wavelength demultiplexing in optical telecommunication band is proposed to achieve gratings with equal diffraction efficiency. A structure of three layers with twenty four gratings is demonstrated in a LiNbO(3):Fe crystal by employing the optimized recording method. Then an initial wavelength demultiplexing experiment based on the formed gratings is carried out in optical telecommunication C-band. The results obtained by measuring and analyzing the transmitted spectra of the fabricated gratings show that the diffraction efficiencies of the gratings are uniform. It is suggested that this kind of multiple gratings can be used for increasing the number of the demultiplexed wavelengths in recording medium with unit volume for WDM.     

Fe2O3的掺杂对TSSG法生长Zn:Fe:LiNbO3 晶体的结构和光折变性能的影响

本文利用TSSG方法生长了不同掺Fe2O3浓度的Zn∶Fe∶LiNbO3晶体,并对晶体进行了红外光谱、紫外光谱、晶格常数和居里温度的测试和分析.利用二波耦合实验测得的写入和擦除曲线,计算了晶体的写入时间常数、擦除时间常数和最大衍射效率.并讨论了Fe2O3的掺杂对材料结构和光折变性能的影响机制.     

Phase contrast using photorefractive LiNbO(3):Fe crystals

We theoretically and experimentally study phase contrast using a photorefractive LiNbO(3):Fe crystal sheet and realize the high-performance phase contrast operation using C-cut LiNbO(3):Fe crystal sheets in which the photovoltaic effect plays an important role. We estimate the maximum photovoltaic field in LiNbO(3):Fe using the phase contrast method.     

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 efficiency 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.     

Nonvolatile holographic storage of near-stoichiometric LiNbO3:Cu:Ce with green light

The nonvolatile holographic storage of a near-stoichiometric LiNbO(3):Cu:Ce crystal with green light was investigated. With an increase in composition, improved nonvolatile holographic performance was obtained. The sensitivity S? of the near-stoichiometric LN(49.57):Cu:Ce crystal is 1 order of magnitude larger than the congruent crystal. And according to our research, Ce ions should be the deep centers of the LiNbO(3):Cu:Ce crystal.     

Pr:Fe:LiNbO3晶体光折变性能的研究

在Fe∶LiNbO3中掺进Pr6011生长Pr∶Fe∶LiNbO3晶体。对晶体进行氧化－还原处理。测试晶体的指数增益系数、衍射效率、响应时间、光电导和信噪比。测试结果Pr∶Fe∶LiNbO3晶体的响应时间缩短,信噪比提高,克服了Fe∶LiNbO3响应时间长,光电导和信噪低的缺点。以Pr∶Fe∶LiNbO3晶体作为存储元件实现了全息关联存储。     

Fe：Mn：LiNbO3晶体的生长及光学性能研究

采用坩埚下降法生长了掺杂Fe^3＋和Mn^2＋的LiNbO3单晶。晶体经极化处理后,用X-射线衍射仪测试了晶体的晶格常数,讨论了掺杂离子对晶格常数的影响。利用二渡耦合实验测试了晶体的指数增益系数、衍射效率和响应时间,结果表明：Fe：Mn：LiNbO3晶体的最大指数增益系数达到40cm^-1,Fe：Mn：LiNbO3的最大衍射效率分别为76％,二波耦合响应时间为2．5min。Fe：Mn：LiNbO3是一种性能优良的光折变晶体。     

Fixed holograms in iron-doped lithium niobate: simultaneous self-stabilized recording and compensation

We analyze the mechanisms leading to a highly diffractive fixed hologram in photorefractive Fe-doped lithium niobate crystals by simultaneous self-stabilized holographic recording and compensation at moderately high temperatures. We show that a partially compensated running hologram is produced during recording under this condition and discuss the performance of the process in terms of the operating temperature, the degree of oxidation ([Fe(3+)]/[Fe(2+)] ratio) of the sample, and the effect of the absorption grating arising from the spatial modulation of the Fe(2+) concentration produced during photorefractive recording. We experimentally measure the evolution of the uncompensated remaining hologram during recording and the evolution of the diffraction efficiency of the fixed hologram during white-light development and show that the maximum fixed grating modulation to be achieved is roughly limited by Fe-dopant saturation. A reproducible eta approximately 66% efficiency fixed grating was obtained on a sample exhibiting an otherwise maximum fixed eta approximately 3% when using the classical three-step (recording at room temperature--compensating at high temperature--developing at room temperature) process.     

Improved nonvolatile holographic storage properties in Zr:Ru:Fe:LiNbO3 crystal by blue light recording

Lithium niobate crystal triply doped with zirconium, ruthenium and iron was grown by conventional Czochralski method and its nonvolatile storage characteristics were investigated by means of two-wavelength technology, where blue and red beam were used as recording and readout light, respectively. In oxidized Zr:Ru:Fe:liNbO₃ crystal, short response time of 9.1 s and high recording sensitivity of 0.92 cm/J were demonstrated, together with high fixed diffraction efficiency of 44.9% maintained. These excellent properties were attributed to the elimination of intrinsic defects by ZrO₂ doping and effective excitation of electrons by blue light. Our experimental results indicated this doped LiNbO₃ crystal was an outstanding medium for holographic storage applications.     

Threshold effects for resistance to optical damage and nonvolatile holographic storage properties in In:Mn:Fe:LiNbO3 crystals

The threshold concentration for In2O3 was found in In:Mn:Fe:LiNbO3 crystals by measurement of the infrared spectra of the crystals. The resistance of the In:Mn:Fe:LiNbO3 crystals to optical damage is characterized by changes in photoinduced birefringence as well as by distortion of the transmitted beam pattern. The resistance increases remarkably when the concentration of In2O3 exceeds its threshold. The resistance to optical damage of a In(3.0 mol. %):Mn:Fe:LiNbO3 crystal is 2 orders of magnitude higher that of a Mn:Fe:LiNbO3 crystal. The dependence of defects on the resistance to optical damage of the In:Mn:Fe:LiNbO3 crystals is discussed in detail. Nonvolatile holographic storage was achieved for all crystals, and the sensitivity of the In(3.0 mol. %):Mn:Fe:LiNbO3 crystal is much higher than that of the others.