Structure and optical damage resistance of Zn:Mn:Fe:LiNbO3 crystals

The new non-volatile holographic storage materials, Zn:Mn:Fe:LiNbO₃ crystals, were prepared by Czochralski technique. Their microstructure was measured and analyzed by infrared (IR) transmission spectra. The optical damage resistance of Zn:Mn:Fe:LiNbO₃ crystals was characterized by the transmitted beam pattern distortion method. It increases remarkably when the concentration of ZnO is over a threshold concentration. Its value in Zn(7.0 mol%):Mn:Fe:LiNbO₃ crystal is about three orders of magnitude higher that in Mn:Fe:LiNbO₃ crystal. The photoinduced birefringence change was measured by the Sénarmont's method. It decreased with ZnO concentration increasing. The dependence of the defects on the optical damage resistance was discussed.     

Preparation and holographic storage properties of tri-doped Mg:Mn:Fe:LiNbO3 crystals

Doping MgO, MnO and Fe₂O₃ in LiNbO₃ crystals, tri-doped Mg:Mn:Fe:LiNbO₃ single crystals were prepared by the conventional Czochralski method. The UV-vis absorption spectra were measured and the shift mechanism of absorption edge was also investigated in this paper. In Mg:Mn:Fe:LiNbO₃ crystal, Mn and Fe locate at the deep level and the shallow level, respectively. The two-photon holographic storage is realized in Mg:Mn:Fe:LiNbO₃ crystals by using He-Ne laser as the light source and ultraviolet as the gating light. The results indicated that the recording time can be significantly reduced for introducing Mg²⁺ in the Mg:Mn:Fe:LiNbO₃ crystal.     

Study of near-infrared nonvolatile two-center holographic recording in LiNbO3:Fe:Rh crystal

The near-infrared nonvolatile holographic recording has been realized in a doubly doped LiNbO₃:Fe:Rh crystal by the traditional two-center holographic recording scheme, for the first time. The recording performance of this crystal has been investigated by recording with 633 nm red light, 752 nm red light and 799 nm near-infrared light and sensitizing with 405 nm purple light. The experimental results show that, co-doped with Fe and Rh, the near-infrared absorption and the photovoltaic coefficient of shallow trap Fe are enhanced in this LiNbO₃:Fe:Rh crystal, compared with other doubly doped LiNbO₃ crystals such as LiNbO₃:Fe:Mn. It is also found that the sensitizing light intensity affects the near-infrared recording sensitivity in a different way than two-center holographic recording with shorter wavelength, and the origin of experimental results is analyzed.     

Mg2+对Fe:LiNbO3晶体光折变响应时间的影响

在Fe:LiNbO₃中掺进3mol%和6mol%MgO,生长了Mg:Fe:LiNbO₃晶体.测试了Mg:Fe:LiNbO₃晶体抗光致散射能力、衍射效率、响应时间和光电导.推导响应时间与光电导之间的关系.在Fe:LiNbO₃晶体中掺进6mol%的Mg²⁺,它的抗光致散射能力比Fe:LiNbO₃晶体提高一个数量级,响应速度比Fe:LiNbO₃晶体提高四倍.     

Influence of Li/Nb ratios on defect structure and photorefractive properties of Zn: In: Fe: LiNbO3 crystals

A series of Zn: In: Fe: LiNbO₃ crystals are grown by the Czochralski technique with various ratios of Li/Nb = 0.94, 1.05, 1.20 and 1.38 in the melt. The Zn, In, Fe, Nb and Li concentrations in the crystals are analyzed by inductively coupled plasma (ICP) spectrometry. The results indicate that with increasing the [Li]/[Nb] ratio in melt, [Li]/[Nb] ratio increases and goes up continuously in the crystal, the segregation coefficients of both Zn and In ions decrease. The absorption spectra measurement and two-wave coupling experiment are employed to study the effect of [Li]/[Nb] ratio on photorefractive properties of Zn: In: Fe: LiNbO₃ crystals. It is found that the [Li]/[Nb] ratio increases, the write time is shortened and the photorefractive sensitivity is improved.     

Optical holographic memory using angular-rotationally phase-coded multiplexing in a LiNbO3:Fe crystal

We propose a new holographic memory scheme based on random-phase-coded multiplexing in a photorefractive LiNbO₃:Fe crystal. Experimental results show that rotating a diffuser placed as a random-phase modulator in the path of the reference beam provides a simple yet effective method of increasing the holographic storage capabilities of the crystal. Combining this rotational multiplexing with angular multiplexing offers further data-storage possibilities. The advantage of using post-image versus pre-image phase modulation of the object beam is demonstrated. Received: 15 August 2000 / Published online: 10 January 2001     

The effect of In doping on optical properties of Fe:LiNBO3 crystals

Fe:LiNbO₃ and In:Fe:LiNbO₃ crystals were grown by Czochralski method. The absorption spectra were measured to investigate their defect structure. The photo damage resistance and photorefractive properties were measured. The photo damage resistance of the In:Fe:LiNbO₃ crystal in which the In concentration is above the threshold value is one order of magnitude higher than that of the Fe:LiNbO₃ crystal. The mechanisms of the violet shift of the absorption edge and the enhancement of the photorefractive effect of In:Fe:LiNbO₃ crystals were investigated.     

掺锌LiNbO3晶体的生长及其光学性能

在LiNbO₃中掺进3mol%、5mol%、7mol%ZnO生长Zn:LiNbO₃晶体.测试Zn:LiNbO₃晶体的吸收光谱,研究Zn:LiNbO₃晶体吸收边紫移的机制.测试Zn:LiNbO₃晶体的红外光谱,研究Zn(7mol%):LiNbO₃晶体OH 吸收峰由3484cm^-1移到3530cm^-1的机制.测试Zn:LiNbO₃晶体倍频转换效率和相位匹配温度,研究Zn:LiNbO₃晶体倍频转换效率增强的机制.     

Forced oscillator model of dynamic spatial charge field in the reduced Zn:Fe:LiNbO3 crystal

Holographic data storage is promised to be the next-generation optical storage technology for many years. The Zn:Fe:LiNbO₃ crystal is studied widely because of its promising holographic storage properties. The forced oscillator model is used to explain the self-erasing phenomenon in the reduced Zn:Fe:LiNbO₃ crystals. It is showed that the total spatial charge field is dominated by two kinds of carrier with different respond time, which are electron and hole, respectively. The cooperative action of two kinds carrier induces that the total charge field non-monotonically varies with the recording time. The same diffraction efficiency of hologram with equal exposure energy is realized by the self-erasing property. The precision of the optical correlation recognition based on holographic storage will be improved.     

Defect structure and optical damage resistance of Zn:Fe:LiNbO<Subscript>3</Subscript> crystals

A series of Zn:Fe:LiNbO₃ crystals were prepared by the Czochralski technique with 0.015 wt. % Fe₂O₃ content and various concentrations of ZnO. The ultraviolet-visible absorption spectra and the infrared absorption spectra of the Zn:Fe:LiNbO₃ crystals were detected in order to investigate their defect structure. Their optical damage resistance was characterized by the photoinduced birefringence change and transmission facula distortion method. The optical damage resistance of the Zn:Fe:LiNbO₃ crystals increases remarkably when the concentration of ZnO is over its threshold concentration (more than 6.0 mol. %). The effects of defects on the optical damage resistance of the Zn:Fe:LiNbO₃ crystals are discussed in detail. Received: 25 October 2002 / Revised version: 6 January 2003 / Published online: 22 May 2003 RID="*" ID="*"Corresponding author. Fax: +86-451/2300-926, E-mail: zzxxhhdoctor@sina.com     

光折变晶体均匀多重全息图存储研究

本文推导了光折变晶体角度编码多重全息存储中写入均匀光栅时的曝光时间递推公式.分析了散射效应对光栅写入时间常数的影响和写入光耦合对光栅振幅的影响,给出了两种因素影响下曝光时间计算的修正递推公式.数值计算结果表明,按照这种修正公式计算所得多重存储中各幅全息光栅振幅不仅均匀性好,而且振幅相对较大,这种曝光方法有利于提高晶体的存储容量.实验中以递推公式所得时间进行曝光记录,在厚度为0.6mm的Fe:LiNbO₃晶体中采用角度编码很容易存储了30幅全息图.     

Improvement of photorefractive properties in Hf:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios

Photorefractive properties of Hf:Fe:LiNbO3 crystals with various [Li]/[Nb] ratios have been investigated at 488 nm wavelength based on the two-wave coupling experiment. High diffraction efficiency and large recording sensitivity are observed and explained. The decrease in Li vacancies is suggested to be the main contributor to the increase in the photoconductivity and subsequently to the induction of the improvement of recording sensitivity. The saturation diffraction efficiency is measured up to 80.2%, and simultaneously the recording sensitivity of 0.91 cm/J is achieved to in the Hf:Fe:LiNbO3 crystal grown from the melt with the [Li]/[Nb] ratio of 1.20, which is significantly enhanced as compared with those of the Hf:Fe:LiNbO3 crystal with the [Li]/[Nb] ratio of 0.94 in melt under the same experimental conditions. Experimental results definitely show that increasing the [Li]/[Nb] ratio in crystal is an effective method for Hf:Fe:LiNbO3 crystal to improve its photorefractive properties.     

Holographic angle multiplexing combined with peristrophic multiplexing in a photorefractive LiNbO3 crystal

We have investigated angle multiplexing combined with peristrophic (rotational) multiplexing in a photorefractive LiNbO₃ crystal of cylindrical shape. The peristrophic multiplexing was achieved by rotating the recording medium while the angle multiplexing, by varying the incident angle of the reference beam. Angle multiplexing in synchronization with rotation of the sample made it possible to continuously record and retrieve holograms. In the combined multiplexing, holograms were stored at different peristrophic positions and thus their angular selectivity could be much improved in comparison to that of only angle-multiplexed holograms. The theoretical angular selectivity was derived, along with its dependence on the sample rotation. The experimentally measured selectivity was in agreement with the theoretical one. The angular selectivity of angle-multiplexed holograms was measured to be 0.03° in the absence of sample rotation and it became less than 0.0075° with a sample rotation by 5.4°.     

Defect structure and optical damage resistance of Hf:Fe:LiNbO3 crystals

A series of Hf:Fe:LiNbO₃ crystals were grown by the Czochralski technique with various doping concentrations of HfO₂. Their defect structures were analyzed by the UV-visible absorption spectra and infrared absorption spectra. The optical damage resistance of Hf:Fe:LiNbO₃ crystals was measured by the photo-induced birefringence change and the transmitted light spot distortion method. The results show that the optical damage resistance ability of Hf:Fe:LiNbO₃ crystals enhances remarkably with the HfO₂ concentration increasing when the HfO₂ concentration is lower than its threshold concentration (4 mol%). However, when the HfO₂ concentration exceeds its threshold concentration, the optical damage resistance ability of the crystals returns to decrease. This unusual behavior is explained by using the photovoltaic field produced in the crystals.     

Growth and photorefractive properties of near-stoichiometric In:Fe:Cu:LiNbO3 crystals

The near-stoichiometric LiNbO₃ crystal co-doped with In₂O₃, Fe₂O₃, and CuO has been grown from a Li-rich melt (Li/Nb = 1.38, atomic ratio) by the Czochralski method in air atmosphere for the first time. The OH⁻ absorption spectra were characterized to investigate the structure defects of the crystals. The appearance of the 3506 cm⁻¹ absorption peak manifests that the composition of the grown crystal is close to the stoichiometric ratio. The photorefractive properties were also measured by the two-wave coupling experiments. The results show that the near-stoichiometric In:Fe:Cu:LiNbO₃ crystal has a larger refractive index change, higher recording sensitivity and larger two-wave coupling gain coefficient than those obtained in the congruent In:Fe:Cu:LiNbO₃ crystal under the same experimental conditions. The material of near-stoichiometric In:Fe:Cu:LiNbO₃ crystal is a promising candidate for blue photorefractive holographic recording.     

Defect structure and nonvolatile hologram storage properties in Hf:Fe:Mn:LiNbO3 crystals

A series of Hf:Fe:Mn:LiNbO₃ crystals with various levels of HfO₂ doping were grown by Czochralski technique. The infrared spectra and ultraviolet spectra were measured and discussed to investigate their structure and defects. The optical damage resistance was characterized by the transmitted beam pattern distortion method. The nonvolatile two-color holographic recording experimental results showed that the recording speed was faster with the increase of HfO₂ doping concentration and at the same time little loss of nonvolatile diffraction efficiencies could be achieved.     

Investigation of nonvolatile holographic storage in LiNbO3:Mn:Ce:Fe crystal

We investigate the persistent holographic recording in triply doped LiNbO₃:Mn:Ce:Fe crystals at different oxidation/reduction states. The experimental results show that there is an optimum oxidation/reduction state, which results in the best dynamic range M/#. Compared with doubly doped LiNbO₃:Ce:Fe, we found that the nonvolatile diffraction efficiency and the best dynamic range M/# obtained in triply doped samples are larger than that obtained in doubly doped samples. The reason for the increase of the crystal about the nonvolatile diffraction efficiency and the dynamic range M/# was also explained.     

Effect of wavelength of sensitizing light on holographic storage properties in LiNbO3:Fe:Ni crystal

By sensitizing with 514 nm green light, 488 nm blue light and 390 nm ultraviolet light, respectively, recording with 633 nm red light, effect of wavelength of sensitizing light on holographic storage properties in LiNbO₃:Fe:Ni crystal is investigated in detail. It is shown that by shortening the wavelength of sensitizing light gradually, nonvolatile holographic recording properties of oxidized LiNbO₃:Fe:Ni crystal is optimized gradually, 390 nm ultraviolet light is the best as the sensitizing light. Considering the absorption of sensitizing light, to obtain the best performance in two-center holographic recording we must choose a sensitizing wavelength that is long enough to prevent unwanted absorptions (band-to-band, etc.) and short enough to result in efficient sensitization from the deep traps. So in practice a trade-off is always needed. Explanation is presented theoretically.     

Deterministic Phase Encoded Holographic Data Storage Using Lenticular Lens Array

This work presents a novel optical holographic encrypted data storage approach based on a phase encoding multiplexed scheme. In the proposed data storage scheme, patterns to be encrypted are stored holographically in a photorefractive LiNbO₃:Fe crystal using a lenticular lens array (LLA) sheet phase-encoded multiplexing. Experimental results reveal that rotating an LLA placed as a phase modulator in the path of the reference beam is a simple but effective method of increasing the phase addresses for holographic memory in a crystal. Combining this rotational multiplexing with two-axis rotating multiplexing provides further data storage and data encryption capacity.     

Nonvolatile holographic storage in triply doped LiNbO3: Hf, Fe, Mn crystals

It has been suggested to use LiNbO3:Fe,Mn crystal for solving the problem of information volatility during the read-out process with all-optical facilities,but the minute order response time is far from the requirements for the real-time information processing.We present the nonvolatile holographic storage properties of LiNbO3:Hf,Fe,Mn.The response time is shortened to 5.0 s,and the sensitivity S is enhanced to 0.22 cm/J in this triply doped crystal.The experimental results show that the HfO2 doping threshold is 5.0 mol.%.Thus it seems that we have found a useful tetravalent dopant for LiNbO3:Fe,Mn that can obviously improve the nonvolatile holographic recording sensitivity.