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Kinetics of fixation of phase holograms in LiNbO3
Abstract
Fixation of phase holograms in LiNbO3 is observable above 100 °C and is caused by transport of ions. From an analysis of the fixation kinetics a concentration of these ions of 2 × 1017 cm−3 and a diffusion constant at 180 °C of 5 × 10−13 cm2/s are determined. Si and OH ions suggested previously to be associated with fixation do not seem to be responsible for fixation. The development of fixed holograms by uniform illumination is based on a modulation of the optical absorption produced during fixation.Das Fixieren von Phasenhologrammen in LiNbO3 wird oberhalb 100 °C beobachtet und wird durch den Transport von Ionen verursacht. Aus der Analyse der Kinetik des Fixierens wird die Konzentration dieser Ionen zu 2 × 1017 cm−3 bestimmt und ihre Diffusionskonstante bei 180 °C zu 5 × 10−13 cm2/s. Si- und OH-Ionen, die bereits mit der Fixierung in Zusammenhang gebracht wurden, scheinen nicht für die Fixierung verantwortlich zu sein. Die Entwicklung fixierter Hologramme durch gleichmäßige Belichtung beruht auf einer Modulation der optischen Absorption, die beim Fixieren entsteht.
... The ability of a photorefractive material to erase through charge re-excitation also results in the undesired erasure of stored holograms during normal readout, and even gradual erasure in the dark through thermal excitation (173). Recorded holograms can be "fixed" -i.e. made semi-permanent and resistant to erasure during readout -by separate thermal (94,98,113,(191)(192)(193)(194)(195)(196)(197)(198) or electronic (178,179,199,200) processes. Unfortunately, this fixing process affects all the stored holograms within a volume simultaneously, and tends to be slow and cumbersome. ...
next generation data-storage is to use light to store information throughout the three-dimensional volume of a material. By distributing data within the volume of the recording medium, it should be possible to achieve far greater storage densities than current technologies can o#er. For instance, the surface storage density accessible with focused beams of light (without near--field techniques) is roughly 1/# (1, 2). With green light of roughly 0.5 micron wavelength, this should lead to 4 bits/sq. micron or more than 4 Gigabytes (GB) on each side of a 120mm diameter, 1mm thick disk. But by storing data throughout the volume at a density of 1/# , the capacity of the same disk could be increased 2000--fold, to 8 Terabytes (TB). It is interesting to note that the DVD disk standard exceeds this rough estimate of the areal density limit despite using light of slightly longer wavelength. However, no laboratory demonstration of volumetric storage to date has gotten closer than approxi
Holographic volume data storage has long been an intriguing subject of research interest. The main reason for this interest is the promise of enormous data storage capacities. In a long volume, storage of 1012 bits or more is possible using visible light (1). However, to date, all attempts at attaining and demonstrating a volume holographic data storage system of this density have fallen far short of this theoretical limit (2-6). Although materials limitations contributed to this shortcoming, of more fundamental importance, however, were the problems associated with the angular multiplexing schemes used to holographically access the volume of the storage medium (7-9).
Hydrogen is an important impurity in LiNbO 3 . During the crystal growth it enters the lattice, mostly in the form of OH ⁻ ions [1]. In photorefractive LiNbO 3 it is generally accepted that protons are the main mobile ionic entities giving rise to fixed volume holograms. The electronic space charge field which, via the electro-optic effect causes a refractive index modulation, induces proton migration above room temperature (140°C), well below the temperature at which electron thermal detrapping starts (> 180°C). The refractive index pattern arising from the proton space charge field can not be erased by illumination anymore, thus it is called a fixed pattern.
In holographic data storage, a photo-sensitive medium is exposed to the interference pattern that is generated when an object beam, with an input data page encoded in the spatial profile of the beam, is intersected by a second, coherent laser beam. The photosensitive medium replicates these interference fringes as a change in optical absorption, refractive index, or thickness. Data are retrieved from the medium by exposing it to light from just one of the beams, which is then diffracted from the stored fringe pattern to reconstruct the other beam, including all the information that had been in the input data page. For a material of sufficient thickness, a large number of interference patterns, each identified by a different grating vector, can be stored or “multiplexed” in the same volume element, with negligible crosstalk between the individual interference patterns. Multiplexing of a large number of pages in the same volume element of the recording medium can be accomplished in several ways—for example by varying the angle between object and reference beam or the wavelength of both beams. Given no other limiting factors, the number of holograms that can be multiplexed in one volume element is directly proportional to the product of the thickness of the medium and its refractive index—that is, materials with optical thicknesses of the order of several millimeters are desirable.
By its very nature, the holographic-storage mechanism distributes the stored information redundantly throughout the recording volume.
The transient interaction between a refraction index grating and light beams during simultaneous writing and thermal fixing of a photorefractive hologram is investigated. With a diffusion- and photovoltaic-dominated carrier transport mechanism and carrier thermal activation (temperature dependent) considered in Fe:LiNbO3 crystal, from the standpoint of field–material coupling, the theoretical thermal fixing time and the space-charge field buildup, spatial distribution, and temperature dependence are given numerically by combining the band transport model with mobile ions with the coupled-wave equation. ©1999 John Wiley & Sons, Inc. Microwave Opt Technol Lett 23: 372–376, 1999.
Protons are identified as the mobile ions, which neutralize the electronic space charge fields during thermal fixing of holograms in LiNbO3:Fe. The protons form OH− centers and can be observed via vibrational absorption bands. In crystals with reduced OH− content holograms cannot be fixed thermally. In crystals containing fixed patterns the OH− concentration shows a spatial variation corresponding to the electric charge distribution.
The effective distribution coefficients of Fe and Si in LiNbO3 single crystals were determined using atomic absorption spectroscopy. For the determination of Si-concentration, the graphite furnace technique was used. In undoped crystals, the impurity levels of Si, Fe, Cu and Zn were measured. The influence of Si ions on the fixation process of phase holograms in LiNbO3-crystals is briefly discussed.
We investigate photorefractive media for which quasi-stabilized ionic gratings can be used to prolong readout lifetime. We use coupled-transport-mode theory to describe the coevolution of photorefractive gratings that arise from free-electron transport and ionic transport. We evaluate in detail the differences between low-temperature and high-temperature recording for typical conditions required by multiplex holography. We provide general normalized examples for simple diffusion transport and specific examples for photovoltaic LiNbO3. We introduce a common formalism to compare widely varying results present in the literature and to guide the materials and system development processes.
A hologram generally is formed in a photorefractive crystal by the interaction of the grating formulation that is due to the space-charge field that grows from the beam interference pattern and of the beam energy exchange that results from two-beam coupling in the grating. Analytic expressions for the grating recording formed by this interaction can be obtained only by simultaneous solution of the material equations and the coupled-wave equations. Using Moharam’s space-charge field function, we deduce both the exact steady-state analytic solution for two-beam coupling under any boundary-light modulation and constant-light excitation and the approximate analytic solutions with distance-dependent light excitation. Based on Kukhtarev’s idea of three-step calculation, the formulation of a hologram under any light modulation and excitation is further solved analytically. These steady-state analytic solutions understanding are useful in arriving at a clearer and more exact understanding of photorefractive two-beam coupling and holographic recording.
We propose a physical model to explain peculiarities of photorefractive recording and dark decay detected in LiNbO3 crystals within temperature ranges below and above 200 °C. Distinctive features of our description are proximity of the activation energies for protons and thermally excited electrons and their competitive contributions to the charge transport.
Using conventional volume-holographic angle multiplexing in an Fe:LiNbO3 crystal, we have developed a compact laser threat discriminator, intended for aircraft integration, that optically detects laser spatial coherence and angle of arrival while simultaneously rejecting incoherent background sources, such as the Sun. The device is intended for a specific type of psychophysical laser attack against U.S. Air Force pilots, namely, third-world-country exploitation of inexpensive and powerful cw Ar-ion or doubled Nd:YAG lasers in the visible spectrum to blind or disorient U.S. pilots. The component does not solve the general tactical laser weapon situation, which includes identifying precision-guided munitions, range finders, and lidar systems that use pulsed infrared lasers. These are fundamentally different threats requiring different detector solutions. The device incorporates a sequence of highly redundant, simple black-and-white warning patterns that are keyed to be reconstructed as the incident laser threat, playing the role of an uncooperative probe beam, changes angle with respect to the crystal. The device tracks both azimuth and elevation, using a nonconventional hologram viewing system. Recording and playback conditions are simplified because nonzero cross talk is a desirable feature of this discriminator, inasmuch as our application requires a nonzero probability of detection for arbitrary directions of arrival within the sensor’s field of view. The device can exploit phase-matched grating trade-off with probe-beam wavelength, accommodating wavelength-tunable threats, while still maintaining high direction-of-arrival tracking accuracy.
A mathematical model is developed to account for the fixing and developing behavior of holographic gratings in photorefractive materials such as LiNbO3. All transport processes for the carriers (electrons and protons) are considered. Moreover, unlike in previous studies, the thermal excitation of carriers is taken into account. Two possible experimental procedures that involve fixing during or after writing are theoretically described. The model is applied to simulate the kinetics, overall efficiency, and temperature dependence of the fixing process for LiNbO3:Fe, for which the most experimental information is available.
Light-induced scattering in a Ce:Fe:LiNbO3 crystal is highly temperature dependent. By considering the effect due to thermally activated ions in a photorefractive crystal, we have shown that the photoinduced space-charge field is neutralized by the activated ions. This neutralization in turn leads to a reduction of scattering noise in the crystal. By raising the crystal temperature, we have observed a reduction in scattering noise, which is consistent with our prediction. The signal-to-noise ratio (SNR) in a two-wave mixing amplification due to thermal activation effect is analyzed. Experimental demonstrations using a Ce:Fe:LiNbO3 crystal are provided in which we have shown that the SNR improves as the crystal temperature increases.
This article reviews the current knowledge on the main properties and applications of hydrogen in LiNbO3. The review is divided into three parts. The first is devoted to general properties such as methods of defect production and control, techniques used for their detection and characterization, as well as interactions with other lattice defects. The second part considers the central role played by OH- defects in the fabrication of optical waveguides by the protonexchange method. Conditions for the exchange process, the structure of exchanged layers, optical properties of exchanged waveguides and connection with relevant practical devices are examined. Finally, the third part discusses the production of permanent volume photorefractive gratings by the fixing process, where OH- ions are also considered to be the main responsible defects. In particular, the physics of the fixing process, current mathematical models and two major applications are discussed.
A photorefractive hologram can be fixed locally under a temperature nonuniform field by CO2 laser heating the LiNbO3. Here, on the basis of analyzing the relationship between the fixed space-charge field amplitude Esc and the fixing temperature Tf, two parameters, the localization coefficient Ψ and the average space-charge field amplitude E, are defined to evaluate the fixed local photorefractive hologram by CO2 laser heating the LiNbO3. The same analysis also can be used when the boundary conditions and the laser beam profile are changed. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 26: 89–93, 2000.
With emphasis on LiNbO3 the basic principles, the developments over four decades, and recent applications of the thermal fixation of photorefractive holograms are reviewed. Thermal fixing is the most promising method to realize optically stable memories in photorefractive media. The microscopic origin of thermal fixing is mainly caused by screening the electronic space-charges by protons thermally activated at about 120◦C. After fixing a diffraction efficiency comparable to that of untreated crystals is achieved and lifetimes of at least 2 years have been observed, whereas for special arrangements scientific sound estimates point to long terms as high as 100 years. A theoretical description is quite well established and will guide further improvements. Thermally fixed holograms have been tested in nonvolatile optical storage systems based on hologram multiplexing and are the basis in narrow-band devices with very high angular selectivity.
We treat the photorefractive effect in oxide crystals at elevated temperatures where charge compensation occurs in the absence of photoexcitation of the compensating species. These species can be either mobile ions or holes in the valence band. Two models are presented that take into account particularities of ion and hole transport. In the small-modulation approximation, solutions for the steady state and the dynamic evolution of the photorefractive effect are given. The maximum space-charge field Eq that can be reached depends on the effective number of electron traps in the crystal. However, in the steady state, while the component of the space-charge field that is due to electrons and the one that is due to the compensating carriers both approach the value Eq, an almost complete compensation of these two components occurs. The speed of compensation is slower for larger grating spacings than for smaller grating spacings and can be increased by applying an electric field. Applying an external electric field also produces a phase shift between the two gratings, therefore increasing the total space-charge field. Experiments performed in KNbO3 confirm the theoretical predictions and indicate that the ionic model is more appropriate for this crystal. Implications of these compensation effects for quasi-permanent
hologram storage are discussed.
A pattern identification technique using the wavelet matched filter (WMF) based on the photorefractive (PR) crystal has been proposed. The essential merit of using the WMF is that it offers better signal discrimination than the conventional matched filter (CMF). Since a large number of WMFs can be synthesized in a PR crystal, fingerprint identification was chosen as the application. Simulated and experimental results are provided.
In this paper the thermal fixing of holographic gratings with K-vectors perpendicular to the crystallographic c-axis of LiNbO3 is considered in order to obtain information about anisotropy of the proton thermal diffusion in this crystal. Specifically, thermal decays of fixed holograms in particular crystallographic directions are measured and related with proton diffusion. The values obtained are compared with previous data of decays of fixed holograms with K-vector parallel to the c-axis. The results show a high anisotropy of the thermal diffusion of protons in lithium niobate crystals.
We develop a theory of high-temperature photorefractive phenomena in LiNbO3 crystals related to the problem of thermal fixing and storage of optical information. The theory covers the temperature range 20–300 °C relevant to the experiment. It is based on a systematic exploiting of small physical parameters, typical of the subject, and distinguished by general and simple expressions for characterization of the fixing, developing, and decay processes. It is shown that thermally excited electrons are competitive with protons (responsible for the charge compensation) within a wide high-temperature region 200–300 °C. They are not only responsible for the dark decay of information, but also for pronounced high-temperature peculiarities of the recording and relaxation processes. A good qualitative agreement between the theory and a great amount of accumulated factual data are obtained.
The stability of fixed volume phase holograms in lithium niobate is studied. The new measurement method is able to distinguish between electronic compensation and true erasure. It is found that at temperatures in the range of 70–120 °C both electronic thermal detrapping and ionic movement are active effects. The actual lifetime of fixed holograms is determined in this temperature range. From these measurements, we extrapolate a lifetime for the fixed hologram refractive index change of 3.7 years at room temperature. © 1997 American Institute of Physics.
In this paper we review four photorefractive devices which have been investigated and further developed in our laboratories. We discuss the fundamental behavior of a photorefractive Bi12SiO20 autocorrelator optimized for particle image velocimetry. The all optical correlator determines the velocities of particles in a cross section of a flow. We show that the autocorrelator is perfect for investigating both laminar and turbulent flows. Photorefractive materials can also be used for vibration analysis of diffusely scattering structures. We discuss recent work from a four-wave mixing interferometric setup with Bi12SiO20 and show that introducing a proper sinusoidal phase shift in one of the pump beams leads to a complete mapping of the vibrational phase and amplitude everywhere on the vibrating structure. Photorefractive materials are also promising candidates for optical storage devices. We discuss holographic storage in LiNbO3 and describe a detailed procedure to obtain a high signal to noise ratio in this device. Finally, we discuss the photorefractive interference filter which consists of alternating quarter-wave layers of high and low index. It has a unique high wavelength selectivity, and the reflectivity and center wavelength can be controlled dynamically by two external laser beams.
A narrow-bandwidth interference filter has been developed using volume holography in photorefractive LiNbO3 crystals. The device works as a Bragg reflector using a volume hologram recorded, fixed and developed in a LiNbO3 sample. With a 2 mm thick LiNbO3 plate the following performances have been achieved: a peak reflectivity above 32%, a full-width at half-maximum bandwidth below 0.05 nm, an angular field of view of about 3 degrees and a peak wavelength of 518.45 nm (for green Ar laser writing wavelength). These performances compare well with that exhibited by the conventional Lyot-Ohman and Solc filters. A complete explanation of the fabrication procedure is given, including practical details needed for producing such an efficiency with fixed holograms in LiNbO3.
Holographic gratings are written in copper-doped LiNbO3 crystals at temperatures of about 180 degrees C. At room temperature the gratings are fixed. The subsequent development process under homogeneous illumination is investigated by the microphotometric method. Measurements of the light intensity pattern at the exit face of the crystal reveal not only the change of the amplitude but also a phase shift of nearly pi . The dynamics of the development process can be fully understood by the assumption of a quasi-permanent modulated charge distribution and a second modulated charge distribution of optically excitable ions.
Fine tuning of narrow-bandwidth interference filters based on volume holographic gratings fixed in photorefractive LiNbO3 has been demonstrated. A filter with a peak wavelength at 518.45 nm, peak reflectivity 32.5% and bandwidth of 50 pm was used. Filter tuning has been achieved both by varying the filter temperature and by applying an electric field. By varying the filter temperature between 35 and 60 degrees C a temperature tuning coefficient of 5.1 pm K-1 was obtained. The effect is induced by the temperature-dependent change of refractive index and by the lattice thermal expansion. By applying a direct current electric field, an electrical tuning coefficient of 5.9 pm kV-1 has been measured too. In this case the tuning mechanisms are the electro-optic and piezoelectric effects. On the other hand, with an applied alternating current electric field, a beam intensity modulation from 90% to 20% has been demonstrated.
The influence of Si concentration and temperature regimes on the visually observable properties of LiNbO3 single crystals grown by Czochralski method was studied. Si concentration measurements were carried out and minima in the radial and axial distribution curves have been established. An upper estimate is given for the equilibrium distribution coefficient of Si in LiNbO3. A direct correlation has been found between the degree of opacity and the Si concentration. The variation of the Grashof number during growth and a correlated modification of the structure of the concentration layer width, implying a position dependent fulfilment of the Hurle-Bardsley criterium for constitutional supercooling, accounts reasonably well with the visually observable properties of the opaque regions appearing in the course of crystal growth. The use of Si as an overall indicator for the characterization of growth regimes is discussed.
The macrosegregation of Al impurities has been studied in Czochralski grown LiNbO3 single crystals. Macrosegregation profiles determined by neutron activation analysis are presented and compared to predictions to the solutions of the neglect of parallel motion (NPM) model. A brief description of this model is given and a new formula is suggested for the interface segregation coefficient, k*, which accounts for different mechanisms of impurity incorporation due to the onset of criticalities. Optimization of model parameters led to fairly good agreement between observed and calculated profiles. Thus we have been able to give an upper estimate for the equilibrium distribution coefficient (k0) of Al. A numerical procedure was developed to assess the errors of the optimized model parameters.
Light-induced refractive index changes in electrooptic crystals like LiNbO3 may be used for the storage of volume phase holograms. To avoid this undesired erasure during read-out J.J. Amodei and D.L. Staebler have utilized a thermal fixing method. Up to now thermal fixing has been observed in Fe-doped electro-optic crystals only. These results pose the question whether a special Fe complex is necessarily involved. For this reaason we have chosen LiNbO4:Cu crystals for an investigation of thermal fixing processes. We start with the investigation of light-induced refractive index changes at room temperature. We have tried to obtain information on the migrating ions during thermal fixing. Furthermore we have investigated the angular selectivity of thermally fixed holograms.
Volume phase holograms in an electrooptic crystal like LiNbO3 must be fixed so that they are not destroyed upon reading. By heating up the crystal mobilized ions may partially compensate the electronic space charge field, and a subsequent development procedure fixes the hologram. A general theory of this thermal fixing process is developed and applied to LiNbO3:Cu. Charge transport by the photovoltaic effect, by drift in a space charge field and by diffusion is taken into account. It is unlikely that mobile protons alone are responsible for the fixing effect.Volumenphasenhologramme in einem elektrooptischen Kristall wie LiNbO3 müssen fixiert werden, so daß sie nach dem Lesen nicht zerstört werden. Die durch Aufheizen der Kristalle mobilisierten Ionen können partiell das elektronische Raumladungsfeld kompensieren und ein darauffolgendes Entwicklungsverfahren fixiert das Hologramm. Eine allgemeine Theorie dieses thermischen Fixierungsprozesses wird aufgestellt und auf LiNbO3:Cu angewendet. Ladungstransport durch den photovoltaischen Effekt, durch Drift in einem Raumladungsfeld und durch Diffusion werden berücksichtigt. Es ist unwahrscheinlich, daß bewegliche Protonen allein für den Fixierungseffekt verantwortlich sind.
Optimal conditions for the developing stage of fixed gratings in LiNbO3 have been theoretically determined. The analysis, that includes an external applied field is based on the well known band transport model. In particular, the role of the photovoltaic effect on the developing process has been studied in detail. Moreover, the applied field as well as an effective photovoltaic field are shown to be key parameters in order to optimize the developing efficiencies. The influence of other parameters such as grating wave vector, doping level and reduction state of the sample has also been analyzed.
The general properties of the nonlocal relationship between light intensity and bulk photovoltaic current density are studied within the linear response theory. As a result, only the even part of the response function is related to the bulk photovoltaic effect whereas the odd part is connected with diffusion of hot carriers. In addition, we present a calculation of the response function for a model of impurity centers in a ferroelectric crystal. We conclude that the characteristic decay length for the response function is neither the anisotropy length nor the mean-free path but the hot carrier diffusion length.
This Chapter has reviewed our present understanding of thermal fixing phenomena, the updated theoretical model, relevant experimental
data and techniques, and applications in photorefractive crystals, particularly LiNbO3. After describing the physical model for thermal fixing, the recent mathematical formulation of the model has been presented.
This has emerged as a coherent and clear picture from which particular models can be reliably developed to explain and predict
most experimental results, as well as to design long lifetime photorefractive devices. Common experimental methods and techniques
have been discussed in detail, emphasizing their advantages and drawbacks. Relevant experimental data are presented and discussed
to the light of the theoretical model, including those referred to photorefractive waveguides. Finally, applications of the
thermal fixing technique to practical devices have been presented for both bulk and waveguide geometries. Many of them exhibit
excellent performances with LiNbO3 and they are very close to the market. On the other hand, further improvements are needed with other materials before they
can be applied to real devices.
Lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) crystals have gained increasing interest during recent years because of their fascinating physical properties, such as piezoelectricity,
ferroelectricity, pyroelectricity, birefringence, electrooptical, and nonlinear-optic effects. These properties have been
successfully utilized for applications in various fields. Basic experiments on acoustic surface wave propagation, electrooptical
modulation, optical wave guiding, second harmonic generation, and pyroelectric detection have been performed with these crystals.
This contribution reviews the present knowledge on photorefractive thermal fixing, considering phenomena and their applications
to long-lifetime holographic devices. Most attention is devoted to LiNbO3, where a coherent and clear picture has recently emerged and most applications are implemented. Discussions an the physical
basis for thermal fixing as well as an the standard mathematical model are presented. Relevant experimental results an thermal
fixing are described and the capability of the model to reliably interpret them is emphasized. In addition, either bulk or
waveguide devices of potential interest in telecommunications are revised and discussed.
Photorefractive effect has been measured in LiNbO 3 :Fe at high temperature in the range 180–300 °C. At temperatures higher than 230 °C only transient effects can be observed. The kinetics of the diffraction efficiency is explained in terms of two kinds of mobile charges; electrons and protons. From the analysis of the final decay of the curves, an activation energy of 1.33±0.02 eV is obtained for the diffusion of protons in lithium niobate.
A new method to determine the proton concentration in LiNbO 3 is presented. The method is based on the measurement of the diffraction efficiency of a photorefractive grating in two situations. It is first measured after recording at room temperature, and second after saturation of the fixing process at a given temperature (about 150 °C). From only these two experimental data, the value obtained for the proton concentration in our sample is H 0 =(4.4±0.7)×10<sup>18</sup> cm<sup>-3</sup>. This value agrees, within the experimental error, with that obtained from the infrared absorption arising from the OH<sup>-</sup> stretching bond.
Wavelength multiplexed holograms using a Ce:Fe: doped LiNbO3 crystal with a visible-light tunable diode laser are reported. The advantages of wavelength multiplexed reflection type holograms are discussed. It is shown that the wavelength multiplexed holograms offer a more uniform selectivity over all the construction angles, as compared with the angularly multiplexed crystal holograms.
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.
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.
Macroscopic light patterns are thermally fixed in photorefractive iron- or copper-doped ${\mathrm{LiNbO}}_{3}$ crystals with different hydrogen concentrations. Spatially resolved absorption measurements reveal unambiguously that migrating hydrogen ions compensate for the space-charge fields at enhanced temperatures if the hydrogen concentration is large enough. However, in crystals with small hydrogen concentrations thermal fixing is also possible and another compensation mechanism appears. Partial depolarization of the crystals does not contribute to the charge compensation processes. The refractive-index pattern is developed by homogeneous illumination and measured by an interferometric technique. The development process is described considering the spatially modulated concentrations of filled and empty electron traps, which cause spatially modulated currents, even for homogeneous illumination.
Volume holographic memory offers a compact means of information
storage through angle and wavelength multiplexing in three dimensions.
As laser warning systems often use both angle-of-arrival and wavelength
information in order to identify and/or discriminate among laser
threats, volume holographic memory structures are ideally suited for
consideration as laser warning receiver sensors. Holographic imaging in
volume phase media is examined from the perspective of coupled wave
diffraction theory for thick sinusoidal grating structures. Thermally
fixed, phase-written volume holograms are discussed and a novel
direction of arrival sensor element is proposed for laser warning
receiver applications
Holography using a specially doped LiNbO/sub 3/ crystal and a visible-light low-power laser diode is presented. Since the crystal has a wide range of spectral response, it would offer practical applications to interconnects, switching, massive storage, etc.< >
this article, I discuss the physical origin of these attractive technology features, and the components and engineering required to realize them. The strengths and weaknesses of available write--once and read--writeable storage media are discussed, including the crucial issue of achieving non--volatile readout from read--write media. Systems issues such as the major noise sources and avenues for defeating or finessing them are detailed, including the potentials and pitfalls of phase--conjugate readout and holographic storage on spinning media. The unique opportunities o#ered by using massively parallel optical correlation to instantaneously search through digital databases are then presented. I conclude by describing the current state of holographic storage research and development e#orts in the context of ongoing improvements to established storage technologies
Photocurrents in doped LiNbO 3 crystals are shown to be due to a bulk photovoltaic effect with saturation voltages in excess of 1000 V (∼10<sup>5</sup> V/cm). This effect accounts for the light‐induced index changes in LiNbO 3 . An explanation of the photovoltaic effect, based on the asymmetry of the lattice, is proposed.
Normally recorded phase holograms in undoped LiNbO3 can be made optically stable, i.e. fixed, by heating the crystal to roughly 100°C. A detailed study of the temperature dependence of this process, coupled with conductivity measurements, suggests a model in which the fixed holograms are associated with ionic charge patterns formed by drift of thermally activated ions in the electric field pattern of the original hologram. These ions would have an activation energy of ∼ 1.1 eV and a concentration in excess of 2 × 1013 cm-3 in fair agreement with previous measurements of ionic conductivity. The room temperature conductivity of the LiNbO3 used in our experiments is concluded to be less than 10-17 Ω cm)-1.
The photorefractive properties of LiNbO3∶Fe and LiNbO3∶Cu have been studied in combination with optical absorption-, Mssbauer- and EPR-measurements. The charge states of Fe in
successively reduced LiNbO3∶Fe have been investigated with respect to the influence on the photorefractive sensitivity and saturation value of the refractive
index change. The results of this experiment demonstrate clearly the close correlation between the concentration of Fe2+ impurities and the optical absorption band around 2.6 eV in LiNbO3∶Fe, which is known to give rise to an anisotropic charge transport upon optical excitation. The resulting photocurrents determine
the photorefractive sensitivity mainly in the initial state of halographic exposure. With increasing conversion from Fe3+ to Fe2+ the photorefractive sensitivity saturates and the saturation value of the refractive index change decreases remarkably.
In the case of LiNbO3∶Cu a similar behaviour of the photorefractive storage parameters after successive reduction treatments has been observed
qualitatively. However, in contradiction to LiNbO3∶Fe the Cu2+ centers cannot be related to the photorefractive sensitivity of LiNbO3∶Cu.
These results are discussed with respect to the predictions of two models concerning the microscopic nature of the photorefractive
process in doped LiNbO3.
A coupled wave analysis is given of the Bragg diffraction of light by thick hologram gratings, which is analogous to Phariseau's treatment of acoustic gratings and to the dynamical theory of X-ray diffraction. The theory remains valid for large diffraction efficiencies where the incident wave is strongly depleted. It is applied to transmission holograms and to reflection holograms. Spatial modulations of both the refractive index and the absorption constant are allowed for. The effects of loss in the grating and of slanted fringes are also considered. Algebraic formulas and their numerical evaluations are given for the diffraction efficiencies and the angular and wavelength sensitivities of the various hologram types.
The conditions for the incorporation of OH− ions on O– ion lattice sites and on interstitial sites are derived from the Li/Nb ratio and an anti-Frenkel disorder of the LiNbO3 crystals. OH− ions substituting O– ions cause an absorption band at 2.865 μm and have a mobility of v(OHo−) = = 400 KT−1 exp (−13120 K/T) cm2/Vs. OH− ions on interstitial sites show the vibrational absorption band at 2.875 μm. Their mobility is higher than that of the OHo− ions: v(OHi−) = = 670 KT−1 exp (−12540 K/T) cm2/Vs. The absorption bands at 2.865 and 2.875 μm are connected with very weak absorptions at 2.248 and 2.255 μm, respectively. The results are discussed and compared with those for Ba2NaNb5O15 crystals.Die Bedingungen für den Einbau von OH−-Ionen auf O–- und Zwischengitterpltzen werden aus dem Li/Nb-Verhltnis und einer Anti-Frenkel-Fehlordnung der LiNbO3-Kristalle abgeleitet. OH−-Ionen auf O–-Pltzen verursachen eine Absorption bei 2,865 μm und haben eine Beweglichkeit v(OHo−) = 400 KT−1 exp (−13120 K/T) cm2/Vs. OH−-Zwischengitterionen absorbieren bei 2,875 μm und haben eine höhere Beweglichkeit als die OHo−-Ionen: v(OHi−) = 670 KT−1 X X exp (−12540 K/T) cm2/Vs. Die Absorptionsbanden bei 2,865 und 2,875 μm werden von sehr schwachen Absorptionen bei 2,248 bzw. bei 2,255 μm begleitet. Die Ergebnisse werden diskutiert und mit denen für Ba2NaNb5O15-Kristalle verglichen.
Partial reversal of spontaneous polarization in iron-doped LiNbO3 crystals takes place at room temperature in an external field of 104 V cm−1, as proved by the Barkhausen effect. Similarly, the formation of an internal photorefractive field of 104 V cm−1 leads to partial ferroelectric switching in the illuminated region of crystal. The dependences of the number of Barkhausen jumps on the external and internal fields are in close agreement.In Eisen-dotierten LiNbO3-Kristallen findet teilweise Umkehr der spontanen Polarisation bei Zimmertemperatur in einem äußeren Feld von 104 V cm−1 statt, wie durch Barkhauseneffekt nachgewiesen wird. In ähnlicher Weise führt die Bildung eines inneren photorefraktiven Feldes von 104 Vcm−1 zu partiellem ferroelektrischem Schalten im belichteten Teil des Kristalls. Die Abhängigkeit der Zahl der Barkhausensprüngen vom äußeren und inneren Feld befinden sich in enger Übereinstimmung.
A mobile ionic species fixes thick‐phase holograms in Fe‐doped LiNbO 3 crystals at temperatures between 100 and 200 °C. Results are given which show that Si ions are mobile at 200 °C in these crystals.
This paper describes the results of an investigation into techniques for obtaining erasure resistant holograms in electro‐optic crystals. The most successful approach made use of thermally activated ionic drift during or after recording. The samples are heated for about 30 min at 100°C to obtain optically nonerasable holograms with as much as 50% diffraction efficiency in LiNbO 3 or in doped Ba 2 NaNb 5 O 15 .
Single‐crystal lithium niobate has been used as a holographic storage medium. The material undergoes a change in refractive indices upon exposure to suitably intense light thus allowing it to act as a pure‐phase, volume‐holographic medium requiring no processing. The holograms formed have high diffraction efficiencies and are thermally erasable. The high resolution obtained suggests that such material may be useful in high‐capacity, changeable optical information storage, processing and display devices.
After defining photoferroelectrics as photosensitive and nonlinear electrooptic materials, we report electrical control of holographic diffraction efficiency in PLZT ceramics and electrical fixation and erasure of holograms recorded in SBN single crystals. On the basis of experimental data, these effects are explained in terms of the local field amplitude, computed as the sum of the applied field and the photoinduced space charge field and high nonlinear electrooptic effects associated with ferroelectric phase transitions.