利用数字全息干涉术观察RuO2:LiNbO3晶体中畴反转的区域特性

利用马赫-曾德尔干涉光路和4f光学透镜系统，以部分畴反转的掺钌铌酸锂晶体(RuO₂:LiNbO₃)的透射光作为物光来记录全息图，并在数值再现过程对其进行频域滤波以实现物场波前信息的数值重建，检测出在一定电压作用下晶体内部折射率变化的二维分布.检测结果证实：晶体中发生畴反转的区域与发生电色效应的区域严格相符.数字全息干涉术非接触、无干扰、无破坏的优势在准实时监控、检测和分析铌酸锂晶体畴反转方面有很好的应用前景. 关键词： 畴反转 数字全息干涉术 电色效应     

Photorefraction and anisotropic light scattering in LiNbO3-Fe crystals

By holographic recording in LiNbO₃-Fe and LiTaO₃-Fe crystals a new light scattering effect has been observed with an optical indicatrix along the optical axis. The kinetics of the light scattering depends on the intensity, wavelength as well as the polarization of the incident light The holographic volume grating is created by the interference of an incident light and light scattered by crystal inhomogeneities.     

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.     

Suppression of the photoinduced light scattering in LiNbO3:Fe by redox treatment and incoherent homogeneous illumination

In this paper, the redox treatment and incoherent homogeneous illumination were tried for the suppression of the photoinduced light scattering in LiNbO₃:Fe. Meanwhile, the effects of both the redox treatment and incoherent homogeneous illumination on the holographic storage properties of LiNbO₃:Fe were also studied. Our experimental data show that the reduction treatment sacrifices the diffraction efficiency for the suppression of photoinduced light scattering in LiNbO₃:Fe. By contrast, incoherent homogeneous illumination with the wavelength of 460 nm is efficient to suppress the photoinduced light scattering without any tradeoff on the holographic storage performance of LiNbO₃:Fe. The superiority of homogeneous illumination over the reduction treatment was attributed to the untouched acceptor concentration under the homogeneous illumination.     

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.     

Enhanced nonvolatile holographic properties in Zn,Ru and Fe co-doped LiNbO3 crystals

A series of LiNbO₃ crystals doped with various concentrations of ZnO and fixed concentrations of RuO₂ and Fe₂O₃ have been grown by the Czochralski method from the congruent melts. The type of charge carriers was determined by Kr⁺ laser (476 nm) and He–Ne laser (633 nm). The results revealed that the holes were the dominant charge carriers at blue light irradiation. Dual-wavelength and two-color techniques were employed to investigate the nonvolatile holographic storage properties of Ru:Fe:LiNbO₃ and Zn doped Ru:Fe:LiNbO₃ crystals. The essential parameters of blue nonvolatile holographic storage in Zn:Ru:Fe:LiNbO₃ crystals were enhanced greatly with the increase of Zn concentration. This indicates that the damage resistant dopants Zn²⁺ ions enhance the photorefractive properties at 476 nm wavelength instead of suppressing the photorefraction. The different mechanisms of blue photorefractive and nonvolatile holographic storage properties by dual wavelength recording in Zn:Ru:Fe:LiNbO₃ crystals were discussed.     

Nonvolatile photorefractive holographic recording in Sc:Ce:Cu:LiNbO3

In this paper experimental studies of nonvolatile photorefractive holographic recording in Ce:Cu:LiNbO₃ crystals doped with Sc(0,1,2,3 mol%) were carried out. The Sc:Ce:Cu:LiNbO₃ crystals were grown by the Czochralski method and oxidized in Nb₂O₅ powders. The nonvolatile holographic recording in Sc:Ce:Cu:LiNbO₃ crystals was realized by the two-photon fixed method. We found that the recording time of Sc:Ce:Cu:LiNbO₃ crystal became shorter with the increase of Sc doping concentration, especially doping with Sc(3 mol%), which exceeds the so-called threshold, and there was little loss of nonvolatile diffraction efficiencies between Sc(3 mol%):Ce:Cu:LiNbO₃ and Ce:Cu:LiNbO₃ crystals.     

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.     

Polarization-anisotropic scattering lines in LiNbO3

Lines of light-induced scattering with extraordinary polarization from two ordinary polarized pump waves in LiNbO₃:Fe crystals are observed. A nonlinear mixing of four copropagating waves in the crystal with photovoltaic charge transport is shown to be the reason for parametric amplification of the seed radiation scattered from optical imperfections of the sample.     

White light interferometry test and analysis of LiNbO3 polarizer

White light interferometer can be used to measure the amplitude extinction ratio (ER) of polarizer and coupling distribution in fiber. A LiNbO₃ polarizer coupled with a polarization maintaining fiber and a silica planar waveguide at the two ends was measured using white light interferometer. According to the principles of optical coherence domain polarimeter (OCDP) technique, the test scheme is analyzed and presented to measure the ER of LiNbO₃ polarizer with its apparatus proposed correspondingly. By analyzing the interference intensity, both the ER of LiNbO₃ polarizer and its coupling crosstalk with optical fiber and waveguide are obtained. The results illustrate that the ER of a 5 mm-long LiNbO₃ polarizer is 71 dB and the crosstalk of the coupling points are around 40 dB. The results have good agreement with analysis.     

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.     

Investigation of optical photorefractive properties of Zr:Fe:LiNbO3 crystals

A series of Zr:Fe:LiNbO₃ crystals with various levels of ZrO₂ doping were grown by Czochraski technique. The optical damage resistance and photorefractive properties were deeply explored. The results showed that the ability optical damage resistance increased remarkably when the concentration of ZrO₂ is over threshold concentration, but which is lower than that of traditional damage resistant additive MgO. While, the holographic storage properties can be greatly enhanced by proper level of ZrO₂ doping in Fe:LiNbO₃. In terms of ions' site occupation model, the photo-damage resistant ability enhancement and the change of the photorefractive properties were discussed.     

Photorefractive response and optical damage of LiNbO<Subscript>3</Subscript> optical waveguides produced by swift heavy ion irradiation

The photorefractive behaviour of a novel type of optical waveguides fabricated in LiNbO₃ by swift heavy ion irradiation is investigated. First, the electro-optic coefficient r ₃₃ of these guides that is crucial in the photorefractive effect is measured. Second, two complementary aspects of the photorefractive response are studied: (i) recording and light-induced and dark erasure of holographic gratings; (ii) optical beam degradation in single-beam configuration. The main photorefractive parameters, recording and erasing time constants, maximum refractive-index change and optical damage thresholds are determined.     

LiTaO3 as holographic storage material

The optical storage properties of LiTaO₃:Fe are investigated and compared with those of the isomorphous compound LiNbO₃:Fe. Absorption, photocurrent, photoconductivity and holographic measurements are reported. In the case of photovoltaic writing similar results for LiTaO₃- and LiNbO₃-crystals are obtained. However, in the case of photoconductive writing using external electric fields LiTaO₃:Fe-crystals yield much better results due to large photoconductivity values. Considering the recording sensitivity and the extremely large storage time LiTaO₃:Fe turns out to be one of the most promising materials for photorefractive storage of volume phase holograms.     

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.     

Holographic digital data storage using phase-modulated pixels

We propose and demonstrate the use of phase images for holographic data storage. Use of phase images as input leads to uniform diffraction efficiency of multiplexed data pages. Use of binary phase-based data pages with 0 and π phase changes produces uniform spectral distribution at the Fourier plane. This in turn facilitates better recording of higher spatial frequencies. We experimentally demonstrate a phase-based holographic data storage system using shift multiplexing in a Fe:LiNbO₃ crystal, and use it for associative retrieval. Preliminary studies indicate high discrimination capabilities of phase-based holographic data storage system over the amplitude-based system in a content-addressable memory.     

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.     

Investigation of optical homogeneity and photorefractive properties of lithium-niobate single crystals by the Raman-spectroscopy and laser-conoscopy methods

Photorefractive properties and structural and optical homogeneity of (1) LiNbO₃:Cu crystals ([Cu] = 0.015 mas %) grown from a congruent melt, (2) nominally pure stoichiometric crystals grown from a melt with 58.6 mol % of Li₂O (LiNbO_3stoich), and (3) nominally pure congruent crystals (LiNbO_3congr) have been studied using the Raman-spectroscopy method with excitation in the UV, visible, and near-IR ranges; the laser-conoscopy method; and the electron paramagnetic resonance-spectroscopy method. In optically uniaxial LiNbO₃ crystals, a weak optical biaxiality has been revealed, which is attributed to an insignificant deformation of the optical indicatrix. This deformation can be caused both by the initial structural inhomogeneity of crystals and by the photorefractive effect. It has been shown that, under the action of light, charge exchange of copper cations Cu²⁺ → Cu⁺ takes place in the crystal LiNbO₃:Cu ([Cu] = 0.015 mas %). The LiNbO₃:Cu crystal exhibits photorefractive properties not only because of the occurrence of intrinsic defects with electrons localized at them, as is the case with the LiNbO_3stoich and LiNbO_3congr crystals, but also due to the charge exchange of copper cations under the action of the laser radiation.     

Spectral properties and quasi-phase-matched second-harmonic generation in a new active medium: optical superlattice Nd:MgO:LiNbO3

3 crystal – was grown in our Laboratory. The absorption and fluorescence spectra were measured and show some differences from the results of common Nd:MgO:LiNbO₃ with monodomain structure. Third-order quasi-phase-matched violet second-harmonic generation in the sample was demonstrated to characterize the quality of the crystal. The properties indicate that the periodically poled optical superlattice Nd:MgO:LiNbO₃ has great potential for constructing a blue or green light source through self-frequency-doubling operation using the maximum nonlinear optical coefficient d₃₃ of LiNbO₃ and the high-gain π polarization at the same time. Received: 22 August 1997/Revised version: 10 January 1998     

Reconstruction of extraordinary refractive index profiles of optical planar waveguides with single or double modes fabricated by O ion implantation into lithium niobate

A method named intensity calculation method (ICM), which is based on beam propagation method (BPM) and image processing, was carried out to reconstruct the extraordinary refractive index profile (RIP) of single-mode planar waveguide in lithium niobate (LiNbO₃), which was fabricated by multi-energy megaelectron-volt (MeV) O²⁺ ion implantation. In addition, it has been proved reasonable that the alternation of extraordinary refractive index induced by ion implantation into LiNbO₃ is mainly due to the degradation of polarization and reduction of material physical density. As a result, the possible extraordinary RIP of the double-mode planar waveguide could be reconstructed using BPM according to such a hypothesis and the calculated guiding mode values. The end-fire coupling and m-line arrangements were carried out to obtain the near-field modal patterns and dark-mode spectra of waveguides, respectively.