Abstract: Physics of counterpropagating optical beams and spatial optical solitons is reviewed, including the formation of stationary states and spatiotemporal instabilities. First, several models describing the evolution and interactions between optical beams and spatial solitons are discussed, that propagate in opposite directions in nonlinear media. It is shown that coherent collisions between counterpropagating beams give rise to an interesting focusing mechanism resulting from the interference between the beams, and that interactions between such beams are insensitive to the relative phase between them. Second, recent experimental observations of the counterpropagation effects and instabilities in waveguides and bulk geometries, as well as in one-and two-dimensional photonic lattices are discussed. A variety of different generalizations of this concept are summarized, including the counterpropagating beams of complex structures, such as multipole beams and optical vortices, as well as the beams in different media, such as photorefractive materials and liquid crystals.
Abstract: Ince-Gaussian (IG) beams are a third complete family of solutions of the paraxial Helmholtz equation. While many applications of Hermite-Gaussian and Laguerre-Gaussian beams have been demonstrated for manipulation of microparticles, the potential of the more general class of IG beams has not yet been exploited at all. We describe the unique properties of IG beams with respect to optical trapping applications, demonstrate a flexible experimental realization of arbitrary IG beams and prove the concept by creating two- and three-dimensional, highly ordered assemblies of typical microparticles. The concept is universal and can easily be integrated into existing holographic optical tweezers setups. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3561770]
Abstract: We reveal a direct link between two fundamental wave phenomena in periodic media, Pendellosung oscillations and resonant coupling between spectral bands. We experimentally measure the power transfer between laser beams associated with the high-symmetry points in periodic and biased hexagonal photonic lattices. As a result, we demonstrate that Pendellosung oscillations dominate the dynamics of resonant interband transitions on a short propagation scale.
Abstract: We introduce a generalized approach to generate an elementary nondiffracting beam, whose transverse intensity is distributed corresponding to a two-dimensional kagome structure. Furthermore, we present an effective experimental implementation via a computer controlled phase controlling spatial light modulator in combination with a specific Fourier filter system. Intensity and phase analysis of the kagome lattice beam accounts for an experimental wave field implementation. Altogether, the examined wave field may be a fundament for the fabrication of large two-dimensional photonic crystals or photonic lattices in kagome symmetry using miscellaneous holographic matter structuring techniques. (C) 2011 American Institute of Physics. [doi:10.1063/1.3554759]
Abstract: We study experimentally and numerically the second-harmonic Cerenkov emission with two different characteristic azimuthal intensity distributions in strontium barium niobate with a random structure of chi(2) nonlinearity. We monitor in situ the Cerenkov emission during domain switching and show that a change of domain size and shape results in a fourfold azimuthal modulation of the Cerenkov cone. (C) 2011 Optical Society of America
Abstract: We present a general comprehensive framework for the description of symmetries of complex light fields, facilitating the construction of sophisticated periodic structures carrying phase dislocations. In particular, we demonstrate the derivation of all three fundamental two-dimensional vortex lattices based on vortices of triangular, quadratic, and hexagonal shape, respectively. We show that these patterns represent the foundation of complex three-dimensional lattices with outstanding helical intensity distributions which suggest valuable applications in holographic lithography. This systematic approach is substantiated by a comparative study of corresponding numerically calculated and experimentally realized complex intensity and phase distributions. (C) 2011 Optical Society of America
Abstract: We study numerically the effect of boundaries on Anderson localization of light in truncated two-dimensional photonic lattices in a nonlinear medium. We demonstrate suppression of Anderson localization at the edges and corners, so that stronger disorder is needed near the boundaries to obtain the same localization as in the bulk. We find that the level of suppression depends on the location in the lattice (edge vs corner), as well as on the strength of disorder. We also discuss the effect of nonlinearity on various regimes of Anderson localization.
Abstract: We investigate systematically the evolution of second harmonic generation in strontium barium niobate with different degrees of disorder of its chi(2) nonlinearity. These different degrees of disordered domain structures are achieved through electrically switching of the polarization at room temperature. The size and distribution of the domains change during the poling process and this in turn strongly affects the spatial distribution of the second harmonic signal. The degree of disorder can be determined by analyzing the angular distribution and wavelength dependence of the second harmonic emission patterns combined with measurements of the spontaneous polarization. We demonstrate evidence of the control of the second harmonic emission pattern by creating defined states of order, and successfully reproduce the resulting patterns theoretically. (C) 2011 Optical Society of America
Abstract: In a numerical study, we demonstrate the dimensionality crossover in Anderson localization of light. We consider crossover from the two-dimensional (2D) to the one-dimensional (1D) lattice, optically induced in both linear and nonlinear dielectric media. The joint influence of nonlinearity and disorder on Anderson localization in such systems is discussed in some detail. We find that, in the linear regime, the localization is more pronounced in two dimensions than in one dimension. We also find that the localization in the intermediate cases of crossover is less pronounced than in both the pure 1D and 2D cases in the linear regime, whereas in the nonlinear regime this depends on the strength of the nonlinearity. There exist strongly nonlinear regimes in which 1D localization is more pronounced than the 2D localization, opposite to the case of the linear regime. We find that the dimensionality crossover is characterized by two different localization lengths, whose behavior is different along different transverse directions.
Abstract: We present diverse reconfigurable complex 3D twister vortex superlattice structures in a large area embedded with tunable vortex spirals as well as dark rings, threaded by vortex helices. We demonstrate these tunable complex chiral vortex superlattices by the superposition of relatively phase engineered plane waves. The generated complex 3D twister lattice vortex structures are computationally as well as experimentally analyzed using various tools to verify the presence of phase singularities. Our observation indicates the application-specific flexibility of our approach to tailor the transverse superlattice spatial irradiance profile of these longitudinally whirling vortex-cluster units and dark rings. (C) 2011 Optical Society of America
Abstract: Computer-generated holograms displayed by phase-modulating spatial light modulators have become a well-established tool for beam shaping purposes in holographic optical tweezers. Still, the generation of light intensity patterns with high spatial symmetry and simultaneously without interfering ghost traps is a challenge. We have implemented an iterative Fourier transform algorithm that is capable of controlling these ghost traps and demonstrate the benefit of this approach in the experiment. (C) 2011 Optical Society of America
Abstract: We investigate discrete nondiffracting beams (DNBs) being the foundation of periodic and quasiperiodic intensity distributions. Besides the number of interfering plane waves, the phase relation among these waves is decisive to form a particular intensity lattice. In this manner, we systematize different classes of DNBs and present similarities as well as differences. As one prominent instance, we introduce the class of sixfold nondiffracting beams, offering four entirely different transverse intensity distributions: in detail, the hexagonal, kagome, and honeycomb pattern, as well as a hexagonal vortex beam. We further extend our considerations to quasiperiodic structures and show the changeover to Bessel beams. In addition, we introduce a highly flexible implementation of the experimental analog of DNBs, namely discrete pseudo-nondiffracting beams, and present locally resolved intensity and phase measurements, which underline the nondiffracting character of the generated wave fields.
Abstract: We demonstrate group-velocity control with an optically reconfigurable narrow-band filter at 1550 nm based on a volume-holographic Bragg grating. The filter is dynamically addressed to obtain different tunable magnitude and phase function by modification of the length, coupling strength, apodization, and phase discontinuities. We characterize the switching behavior, group-delay, electro-optical tuning, and Gaussian pulse propagation. (C) 2011 American Institute of Physics. [doi:10.1063/1.3600646]
Abstract: The class of rod-shaped bacteria is an important example of non-spherical objects where defined alignment is desired for the observation of intracellular processes or studies of the flagella. However, all available methods for orientational control of rod-shaped bacteria are either limited with respect to the accessible rotational axes or feasible angles or restricted to one single bacterium. In this paper we demonstrate a scheme to orientate rod-shaped bacteria with holographic optical tweezers (HOT) in any direction. While these bacteria have a strong preference to align along the direction of the incident laser beam, our scheme provides for the first time full rotational control of multiple bacteria with respect to any arbitrary axis. In combination with the translational control HOT inherently provide, this enables full control of all three translational and the two important rotational degrees of freedom of multiple rod-shaped bacteria and allows one to arrange them in any desired configuration.
Abstract: Experimental and numerical evidence of symmetry-breaking bifurcations of a circular dissipative soliton with additional boundary conditions in the feedback of a liquid crystal light valve are reported. By tuning the strength of the nonlinearity or the size of the additional boundaries, the circular structure breaks up into polygonal symmetries and the system exhibits multistability. The experimental results are confirmed by numerical simulations with different configurations of the polarizers thus demonstrating the universality of the phenomenon.
Abstract: Reliable methods for the optical acquisition of three-dimensional (3D) coordinates like the fringe projection technique or 3D laser scanning are sensible to object movements because they require the recording of a sequence of images. In contrast, techniques using the projection of a random pattern reduce the measurement time to a single exposure time. A method is presented which allows the 3D acquisition of a surface from a single stereo image pair by projecting a laser speckle pattern. The use of a laser enables a simple design of the projection device as well as a suppression of ambient light by narrow band-pass filtering. Corresponding image points are determined by a digital image correlation algorithm. In order to optimize the introduced method, the influence of various parameters on the number and accuracy of the determined 3D coordinates is analyzed on the basis of comparative measurements with the fringe projection technique. It is demonstrated that the introduced approach allows the acquisition of 3D data of skin surfaces.
Abstract: Complex reconfigurable 3D photonic quasicrystals (PQCs) are fabricated in nonlinear photorefractive strontium barium niobate by an optical phase-engineering-based single-step optical induction approach (see image). The approach demonstrates the embedded potential to use these structures as a reconfigurable platform for the investigation of advanced nonlinear light-matter interaction, or as templates fabricated in various photosensitive materials for photonic bandgap structures.
Abstract: Dielectrophoretic forces originating from highly modulated electric fields can be used to trap particles on surfaces. An all-optical way to induce such fields is the use of a photorefractive material, where the fields that modulate the refractive index are present at the surface. We present a method for two-dimensional particle alignment on an optically structured photorefractive lithium niobate crystal. The structuring is done using an amplitude-modulating spatial light modulator and laser illumination. We demonstrate trapping of uncharged graphite particles in periodic and arbitrary patterns and provide a discussion of the limitations and the necessary boundary conditions for maximum trapping efficiency. The photorefractive crystal is utilized as bottom part of a PDMS channel in order to demonstrate two-dimensional dielectrophoretic trapping in a microfluidic system. (C) 2010 Optical Society of America
Abstract: We realize an experimental control over the topological stability of three-lobe discrete vortex solitons by modifying the symmetry of a hexagonal photonic lattice optically induced in a photorefractive crystal. By continuously deforming the lattice wave in one transverse direction, we manipulate the coupling between lattice sites and induce or inhibit the reversal of soliton vorticity. (C) 2010 Optical Society of America
Abstract: We quantitatively investigate the axial imaging properties of dynamic phase-contrast microscopy, with a special focus on typical combinations of tracer particles and magnifications that are used for velocimetry analysis. We show, for the first time, that a dynamic phase-contrast microscope, which is the integration of an all-optical novelty filter in a commercially available inverted microscope, can visualize three-dimensional velocity fields with a significantly reduced optical sectioning depth. The depth of field for dynamic phase-contrast microscopy is extracted from the three-dimensional response function and compared with the respective values for incoherent bright-field illumination. These results are then used to perform a depth-resolved particle image velocimetry analysis of Hagen-Poiseuille as well as electro-osmotically actuated flows in a microchannel. (C) 2010 Optical Society of America
Abstract: We demonstrate slow and fast light at room temperature by dispersive phase-coupling in a photorefractive SBN:60 crystal. Non-degenerated wave-mixing is used to obtain delayed and amplified pulses with reduced or increased group velocity of v(g) = 0.2 cm s(-1) and v(g) = -1.3 cm s(-1), respectively. The gain spectrum is modulated by using multiple frequency shifted pumps simultaneously. The complete dispersion of the spectrum is determined via the phase modulation technique. We compare the experimental results to numerical simulations based on coupled-wave equations.
Abstract: We present tailoring of three dimensional light fields which act as light moulds for elaborate particle micro structures of variable shapes. Stereo microscopy is used for visualization of the 3D particle assemblies. The powerful method is demonstrated for the class of propagation invariant beams, where we introduce the use of Mathieu beams as light moulds with non-rotationally-symmetric structure. They offer multifarious field distributions and facilitate the creation of versatile particle structures. This general technique may find its application in micro fluidics, chemistry, biology, and medicine, to create highly efficient mixing tools, for hierarchical supramolecular organization or in 3D tissue engineering. (C) 2010 Optical Society of America
Abstract: Counter-propagating optical traps are widely used where long working distances, axially symmetric trapping potentials, or standing light waves are required. We demonstrate that optical phase-conjugation can automatically provide a counter-propagating replica of a wide range of incident light fields in an optical trapping configuration. The resulting counter-propagating traps are self-adjusting and adapt dynamically to changes of the input light field. It is shown that not only single counter-propagating traps can be implemented by phase-conjugation, but also structured light fields can be used. This step towards more complex traps enables advanced state-of-the-art applications where multiple traps or other elaborated trapping scenarios are required. The resulting traps cannot only be used statically, but they can be rearranged in real-time and allow for interactive dynamic manipulation. (C) 2010 Optical Society of America
Abstract: Dissipative solitons are self-localized states which can exist anywhere in a system with translational symmetry, but in real systems this translational symmetry is usually broken due to parasitic inhomogeneities leading to spatial disorder, pinning the soliton positions. We discuss the effects of semiconductor growth induced spatial disorder on the operation of a cavity soliton laser based on a vertical-cavity surface-emitting laser (VCSEL). We show that a refractive index variation induced by an external, suitably spatially modulated laser beam can be used to counteract the inherent disorder. In particular, it is demonstrated experimentally that the threshold of one cavity soliton can be lowered without influencing other cavity solitons making two solitons simultaneously bistable which were not without control. This proof of principle paves the way to achieve full control of large numbers of cavity solitons at the same time. (C) 2010 Optical Society of America
Abstract: Organization and patterning of zeolite L crystals with their unique properties such as their one-dimensional nano channel system is of highest topical interest with various applications in many areas of science. We demonstrate full three-dimensional optical control of single zeolite L crystals and for the first time fully reversible, dynamic organization of a multitude of individually controlled zeolite L crystals.
Abstract: We demonstrate a dynamic phase contrast microscope based on photorefractive holographic interferometry. This system allows us to quantify concentration changes during micro-mixing processes by measuring the change of the optical path length. Therefore, no labelling processes with dyes or tracers are needed. The obtained interferograms are evaluated by an intensity analysis of every pixel, resulting in a high lateral resolution of about one micrometre, and the possibility of real-time phase determination. Due to the interferometric nature of the method the phase measurement is unambiguous for optical phase changes in the interval of zero to p radians. We present the expansion of the phase measurement range by a two-wavelength method, allowing us to resolve concentration changes down to 4 x 10(-6) mol cm(-3) and quantify concentrations up to 10(-3) mol cm(-3). The result is an optimization of the resolution by a factor of two and of the dynamic range by a factor of six in comparison to the single-wavelength technique.
Abstract: We propose and demonstrate holographic phase contrast (HPC) as a new method to transfer a spatial phase distribution of arbitrary shape into a corresponding intensity pattern. A powerful application of HPC is the use in optical tweezers to dynamically control multiple traps like arrays or even more complex trapping geometries. Due to the image plane nature of HPC no hologram calculation is required and hence real-time control of complex tweezers configurations is possible. The inherent optical amplification by HPC can improve the fundamental limit in trapping power in optical tweezers that are based on common spatial light modulators.
Abstract: We report on the formation of spatial optical solitons in photorefractive media using picosecond laser pulses. Solitons are generated with laser pulses in the visible and infrared wavelength region using photorefractive strontium barium niobate. The dynamics of the soliton formation and the region of existence is studied in detail.
Notes: Spring Meeting of the Quantum Optics and Photonics Section of the German-Physical-Society, Darmstadt, GERMANY, MAR, 2008
Abstract: We report on the experimental observation of stable double-charge discrete vortex solitons generated in hexagonal photonic lattices created optically in self-focusing nonlinear media and show that single-charge vortex solitons are unstable in analogous conditions. Subsequently, we study, both theoretically and experimentally, the existence and stability of spatial vortex solitons in two-dimensional hexagonal photonic lattices. We demonstrate that the stability of the double-charge vortices is a consequence of the intersite power exchange in the vortex soliton, and we provide a simple stability criterion on the basis of the analysis of the corresponding discrete nonlinear model. We extend our analysis to the case of defocusing nonlinearity and show the inversion of the vortex stability properties resulting in the fact that single-charge vortices become stable while their double-charge counterparts are unstable.
Abstract: A nonlinear dynamic phase-contrast stereoscope has been developed for the measurement of all three velocity components of a microfluidic flow field. The stereoscope system captures simultaneously two images of different off-axis views of the same region of interest in a microflow, seeded with tracer particles. Two independent photorefractive two-beam coupling novelty filters, one in each stereoscope channel, are employed to enhance the contrast of tracer particle images. A subsequently applied particle tracking algorithm extracts the velocity information from the images, and in first experiments the axial velocity components could be determined with an error of less than 5%. Finally we report on the determination of the velocity field in a rectangular microchannel with a 170 mu m high microstep with the dynamic phase-contrast stereoscope.
Abstract: We experimentally investigate the formation of reconfigurable three-dimensional (3D) nonlinear photonic lattices in an externally biased cerium doped strontium barium niobate photorefractive crystal by a spatial light modulator-assisted versatile simplified single step optical induction approach. The analysis of the generated 3D nonlinear photonic lattices by plane wave guiding, momentum space spectroscopy, and far field diffraction pattern imaging is presented, which points to the embedded potential of these 3D structures as reconfigurable platform to investigate advanced nonlinear light-matter interaction in periodic structures. (C) 2009 Optical Society of America
Abstract: We investigate non-volatile holographic data storage in photorefractive lithium niobate crystals. Infrared picosecond laser pulses are used to write holograms after sensitizing the crystal with blue light from a cw-laser. The dependence of the dynamic range and the photoconductivity on the pulse intensities and the recording wavelength is investigated in detail. The results can be explained by a two-center model if the mean intensity of the laser pulses is considered. We demonstrate that several fixed holograms can be multiplexed by employing the wavelength multiplexing technique.
Abstract: We investigate the properties of angular momentum carrying vortex beams, reflected by a phase-conjugating mirror. It is shown that a self-pumped photorefractive phase-conjugating mirror is suitable to produce stable, high-fidelity phase conjugation of vortex beams. We prove that the topological charge of the vortex beam is maintained, and thus the angular momentum in the laboratory frame of reference is reversed, as it is expected by the time reversal property of the phase-conjugating mirror. The three dimensional interference pattern in front of the phase-conjugating mirror is studied and applications in optical traps are suggested. (C) 2009 Optical Society of America
Abstract: We report on the first observation of topologically stable spatially localized multivortex solitons generated in optically induced hexagonal photonic lattices. We demonstrate that topological stabilization of such nonlinear localized states can be achieved through self-trapping of truncated two-dimensional Bloch waves and confirm our experimental results by numerical simulations of the beam propagation in weakly deformed lattice potentials in anisotropic photorefractive media.
Abstract: We realize an absolute position control of drifting dissipative optical solitons by injecting an incoherent amplitude parameter gradient onto the nonlinear optical system. This allows for two-dimensional, arbitrary control patterns. The control of the soliton drift velocity is studied applying a periodic, hexagonally shaped modulation. The guiding of dissipative solitons by one- and two-dimensional parameter modulations is demonstrated. Furthermore, one-dimensional, line-shaped parameter modulations are designed to act as barriers for dissipative solitons, allowing implementations of position selectors for solitons. The interaction of dissipative optical solitons with barriers is studied for different barrier parameters.
Abstract: Synchronisation of spatiotemporal continuous disorder is realised in a Liquid Crystal Light Valve single feedback system with an incoherent, unidirectional master-slave-coupling scheme as excellent model system for synchronisation. Thus, complex states disordered in space and time were completely synchronised by using identical systems as master and slave. Thereby the impeding role of system differences is demonstrated in comparison to former experiments. A novel imaging method is introduced, in which the synchronisation process and effects like a time lag can be more easily characterised. [DOI: 10.2971/jeos.2008.08001]
Abstract: We utilize the finite time constant of a photorefractive optical novelty filter microscope to access full-field velocity information of fluid flows on microscopic scales. In contrast to conventional methods such as particle image velocimetry and particle tracking velocimetry, not only image acquisition of the tracer particle field but also evaluation of tracer particle velocities is done all-optically by the novelty filter. We investigate the velocity dependent parameters of two-beam coupling based optical novelty filters and demonstrate calibration and application of a photorefractive velocimetry system. Theoretical and practical limits to the range of accessible velocities are discussed. (C) 2008 American Institute of Physics.
Abstract: Overloaded phase codes for volume holographic data storage are introduced. In contrast to any previous phase-code design, overloaded phase codes enable multiplexing of a number of data pages that exceeds the number of utilized reference beams. In this way the achievable data capacity can be augmented. Overloaded codes are generated by extending multilevel phase codes based on the discrete Fourier transform. We demonstrate multiplexing of 70 analog pages by means of 64 reference beams. The analysis of reconstructed digital data pages suggests that a capacity gain of up to 15% is reasonable. (C) 2008 Optical Society of America.
Abstract: We present an efficient method for optical induction of photonic superlattices in photorefractive media via holographic multiplexing. By superimposing phase engineered periodic waves of different periodicities, incremental recording of one- and two-dimensional multiperiodic lattices is demonstrated. The induced structures are subsequently analysed in Fourier space as well as in real space to verify the existence of multiple band gaps in the linear transmission spectrum.
Notes: Meeting on Photorefractive Effects and Devices, Lake Tahoe, CA, 2007
Abstract: Bismuth tellurite is a photorefractive material for holographic data storage offering unique fixing capabilities. Important material and electro-optic properties obtained by four-wave-mixing and data storage experiments are reviewed and recent results concerning the applicability of bismuth tellurite for holographic data storage, including dynamic range, multiplexing capabilities and bit-error evaluations, are presented. Furthermore, it is demonstrated how the latest progress in growing Bi(2)TeO(5) made this crystal a candidate for durable holographic recording media.
Notes: Meeting on Photorefractive Effects and Devices, Lake Tahoe, CA, 2007
Abstract: We present a technique that enables true associative data search in phase-encoded volume holographic storage systems. The technique overcomes crucial shortcomings related to the only two methods proposed for associative searches in phase-encoded systems so far. An additional interferometric readout during content addressing is utilized to ascertain the cross-correlations between an input information and all data pages that are recorded by superposition in one location of the storage media. We present experimental investigations and thoroughly discuss the reliability of the technique. Under realistic conditions the inevitable normalization procedure, used to determine absolute correlation values, as well as the probability of small correlation values crucially affect the capabilities of associative search in phase-encoded holographic storage systems.
Abstract: Hybrid multinary block codes for implementation in page-oriented holographic storage systems are proposed. The codes utilize combined phase and amplitude modulations to encode input data. In comparison to pure amplitude-or pure phase-modulated block code designs hybrid multinary modulation coding allows us to augment the storage density at an unchanged error rate. Two different hybrid modulation code designs are introduced. Experimental implementation is thoroughly discussed, especially concentrating on readout concepts. Phase-resolved readout is accomplished by optical addition and subtraction, using an unmodulated reference page. Experimental results indicate that the overall error rate is usually dominated by errors related to amplitude detection. The study suggests that capacity gains of up to 31% or 47% are reasonable when utilizing phase modulations in conjunction with binary or ternary amplitude modulation.
Abstract: We control the emission properties of a broad-area vertical-cavity surface emitting laser by coupling it to an external feedback cavity containing a photorefractive crystal with an optically induced photonic lattice. The periodic modulation of the refractive index serves as a tunable filter and enables the dynamic suppression of unwanted spatial instabilities and modes, as originally suggested by Gomila et al. [Phys. Rev. Lett. 92, 253904 (2004)]. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2996257]
Abstract: We study experimentally two-dimensional periodic photonic lattices optically imprinted in photorefractive nonlinear media, and explore the effect of anisotropy on the induced refractive-index patterns. The orientation anisotropy is demonstrated by comparing square and diamond lattices, while the polarization anisotropy is shown to distinguish ordinarily and extraordinarily polarized light. In particular, the extraordinarily polarized lattice induces much stronger refractive-index modulation for the same conditions. Finally, we exploit the photorefractive anisotropy to generate a quasi-one-dimensional refractive-index pattern for the observation of two-dimensional solitons and corroborate these experiments by numerical simulations.
Abstract: We experimentally investigate pump beam frequency detuning as a control technique in a photorefractive single-mirror feedback system exhibiting spontaneous self-organized transverse pattern formation with a regime of multistability. Deterministic switching between bistable patterns is demonstrated experimentally. An experimental stability analysis uncovers the effect of frequency detuning on the thresholds of unstable modes. The interaction of independent modes is found to be responsible for the different pattern symmetries observed in this system. (c) 2007 Optical Society of America.
Abstract: We report on the experimental implementation of an external control for optical feedback solitons using incoherent spatial intensity distributions in a liquid crystal light valve (LCLV) optical single feedback system. The external control provides excellent experimental possibilities for static and dynamic control of the lateral positions of the optical feedback solitons which will be demonstrated. Particularly, the influence of different gradients onto the drift motion of spatial solitons is experimentally investigated in detail. In agreement with theoretical predictions, the drift velocity of the soliton increases according to the steepness of the gradient. Additionally, a completely incoherent addressing scheme including creation and erasure of feedback solitons is demonstrated for the LCLV setup. (c) 2007 American Institute of Physics.
Abstract: In biological fluid mechanics powerful imaging methods for flow analysis are required for making progress towards a better understanding of natural phenomena being optimised in the course of evolution. At the same time it is of crucial importance that the measuring and flow visualisation techniques employed guarantee biocompatibility, i.e. they do not distort the behaviour of biosystems. Unfortunately, this restricts seriously the measures for optimising the image generation in comparison to other flow fields in which no biological systems are present. As a consequence, images of lower quality leading to erroneous artefacts are obtained. Thus, either novel detection techniques that are able to overcome these disadvantages or advanced evaluation methods enabling the sophisticated analysis and description of flow fields are essential. In the present contribution, both areas are covered. A novel so-called neuronumerical hybrid allows to detect artefacts in conventional experimental particle image velocimetry (PIV) data of microorganismic flow fields generated by ciliates. The handling of artefacts is performed by the hybrid using a priori knowledge of the flow physics formulated in numerical expressions and the enormous potential of artificial neural networks in predicting artefacts and correcting them. In fact, the neuronumerical hybrid based on the physical knowledge provided by the Taylor’s hypothesis can detect not only spurious velocity vectors but also additional phenomena like a moving boundary, in the present case caused by the contraction of the zooid of a microorganism. Apart from the detection of the artefacts, a correction of the spurious velocity vectors is possible. Furthermore, a method to detect microscopic velocity fields based on nonlinear optical filtering, optical novelty filter (ONF) is presented. On the one hand, it can be employed to expose phase changes in flow fields directly from the nonlinear response and without additional tracers. On the other hand, it can be used to preprocess low quality images of flow fields loaded with particles and extract the motion of particles with an enhanced contrast. The flow fields obtained by the correlation based PIV method of the ONF filtered and unfiltered image sequences are compared and discussed.
Abstract: We investigate the formation of fundamental discrete solitons and dipole-mode gap solitons in triangular photonic lattices imprinted in photorefractive nonlinear media. These lattices are strongly affected by the photorefractive anisotropy, resulting in orientation-dependent refractive index structures with reduced symmetry. It is demonstrated that two different orientations of the lattice wave enable the formation of fundamental discrete solitons in the total internal reflection gap. Furthermore, it is shown that one lattice orientation additionally supports dipole-mode solitons in the Bragg reflection gap. The experimental results are corroborated by numerical simulations using the full anisotropic model.
Notes: Spring Meeting of the Quantum Optics and Photonics Section of the German-Physical Society, Dusseldorf, GERMANY, 2007
Abstract: We report on the stabilization of inherently unstable counterpropagating photorefractive spatial solitons by the use of one- and two-dimensional photonic lattices. We numerically investigate the dependence of the instability dynamics on period and amplitude of the lattice and present experimental verification for the dynamic stabilization of the bi-directional soliton state. (C) 2007 Optical Society of America.
Abstract: We generate experimentally different types of two-dimensional Bloch waves of a square photonic lattice by employing the phase imprinting technique. We probe the local dispersion of the Bloch modes in the photonic lattice by analyzing the linear diffraction of beams associated with the high-symmetry points of the Brillouin zone, and also distinguish the regimes of normal, anomalous, and anisotropic diffraction through observations of nonlinear self-action effects. (c) 2006 Optical Society of America.
Abstract: We demonstrate theoretically and experimentally a novel type of localized beam supported by the combined effects of total internal and Bragg reflection in nonlinear two-dimensional square periodic structures. Such localized states exhibit strong anisotropy in their mobility properties, being highly mobile in one direction and trapped in the other, making them promising candidates for optical routing in nonlinear lattices.
Abstract: We propose a novel type of unitary matrix for phase-code multiplexed holographic memories, which could be quickly generated from geometric sequences. Our analysis shows that the phase-code matrices are unitary rather than orthogonal. The new matrices have complex elements. The order of unitary matrices can be any positive integer, so that we can accommodate the available spatial light modulators to obtain the maximum possible storage capacity. The cross-talk noises in phase-encoded memories with unitary matrices and with Hadamard matrices are of the same order of magnitude, which are much lower than those in holographic memories with wavelength multiplexing or angle multiplexing. (c) 2006 Optical Society of America.
Abstract: We predict theoretically and generate experimentally in photorefractive crystal two-dimensional self-trapped periodic waves of different symmetries, including vortex lattices-patterns of phase dislocations with internal energy flows. We demonstrate that these nonlinear waves exist with nonlocal nonlinearity even when the optically-induced periodic refractive index becomes highly anisotropic, and it depends on the orientation of the two-dimensional lattice relative to the crystallographic c-axis. (c) 2006 Optical Society of America.
Abstract: As the formation of spatial optical solitons in photorefractive mediad is governed by modification of the refractive index, every single solitons in complex configurations of solitons can act as a single waveguide for other light beams. In this article, we demonstrate guiding of amplitude-modulated beams in complex configurations of photorefractive solitons carrying information; we analyze the received signal in the kilohertz frequency range. The possibility of data transmission combined with waveguide couplers opens the route to all-optical networks.
Abstract: The cross-talk noise in phase encoded holographic memories employing unitary matrices is theoretically investigated. After reviewing some earlier work in this area, we derive a relationship for the noise-to-signal ratio for phase-code multiplexing with unitary matrices. The noise-to-signal ratio rises in a zigzag way on increasing the storage capacity. Cross-talk is mainly caused by high-frequency phase codes. Unitary matrices of even orders have only one bad code, while unitary matrices of odd orders have four bad codes. The signal-to-noise ratios of all other codes can in each case be drastically improved by omission of these bad codes. We summarize the optimal orders of Hadamard and unitary matrices for recording a given number of holograms. The unitary matrices can enable us to adjust the available spatial light modulators to achieve the maximum possible storage capacity in both circumstances with and without bad codes.
Abstract: We experimentally generate different types of two-dimensional self-trapped photonic lattices in a photorefractive medium and analyze the induced refractive index change using two different methods. One method gives the first experimental Fourier space analysis of both linear and nonlinear self-trapped photonic lattices with periodic phase modulation using partially spatially incoherent multiband excitation of the lattice modes. The other method utilizes the waveguiding properties of the lattice to achieve a real space analysis of the induced refractive index change. The results of both methods are compared.
Abstract: We experimentally observed a counterpropagating dipole-mode vector soliton in a photorefractive SBN:60Ce crystal. We investigated the transient formation dynamics and show that the formation process differs significantly from the copropagating geometry. The experimental results are compared with fully anisotropic numerical simulations and show good qualitative agreement. (c) 2005 Optical Society of America.
Abstract: We analyze theoretically and generate experimentally two-dimensional nonlinear lattices with periodic phase modulation in a photorefractive medium. The light-induced periodically modulated nonlinear refractive index is highly anisotropic and nonlocal, and it depends on the lattice orientation relative to the crystal axis. We discuss the stability of such induced photonic structures and their guiding properties. (c) 2005 Optical Society of America.
Abstract: We formulate an anisotropic nonlocal theory of the space charge field induced by. the coherent counterpropagating beams in biased photorefractive crystals. We establish that the competition between the drift and diffusion terms has to be taken into account when the crystal (c) over cap axis is tilted with respect to the propagation direction of the beams. We demonstrate that this configuration combines the features of both spatial soliton formation without energy exchange and two-wave mixing with energy exchange leading to pattern formation.
Abstract: We demonstrate that two-dimensional two-component bright solitons of an annular shape, carrying vorticities (m,+/- m) in the components, may be stable in media with the cubic-quintic nonlinearity, including the hidden-vorticity (HV) solitons of the type (m,-m), whose net vorticity is zero. Stability regions for the vortices of both (m,+/- m) types are identified for m=1, 2, and 3, by dint of the calculation of stability eigenvalues, and in direct simulations. In addition to the well-known symmetry-breaking (external) instability, which splits the ring soliton into a set of fragments flying away in tangential directions, we report two new scenarios of the development of weak instabilities specific to the HV solitons. One features charge flipping, with the two components exchanging angular momentum and periodically reversing the sign of their spins. The composite soliton does not directly split in this case; therefore, we identify such instability as an intrinsic one. Eventually, the soliton splits, as weak radiation loss drives it across the border of the ordinary strong (external) instability. Another scenario proceeds through separation of the vortex cores in the two components, each individual core moving toward the outer edge of the annular soliton. After expulsion of the cores, there remains a zero-vorticity breather with persistent internal vibrations.
Abstract: We report on the detailed experimental determination of the threshold for modulational instability in a photorefractive single-mirror feedback system using a Fourier control technique. Results are compared to analytical predictions and a disagreement for the experimentally significant multiple pattern region is found. Implications for the generation of nonhexagonal two-dimensional patterns are discussed.
Abstract: We report on the experimental observation of a dynamic instability in the interaction of counterpropagating self-trapped beams in a photorefractive strontium barium niobate crystal. While the interaction of copropagating spatial optical solitons exhibits only transient dynamics, resulting in a final steady state, the counterpropagating geometry supports a dynamic instability mediated by intrinsic feedback Experimental observations are compared with and found to be in qualitative agreement with numerical simulations. (c) 2005 Optical Society of America.
Abstract: Dynamical behavior of counterpropagating (CP) mutually incoherent vector solitons in a 5 x 5 x 23 mm SBN: 60Ce photorefractive crystal is investigated. Experimental study is carried out, displaying rich dynamics of three-dimensional CP solitons and higher-order multipole structures, and a theory formulated that is capable of capturing such dynamics. We find that our numerical simulations agree well with the experimental findings for various CP beam structures. Linear stability analysis is also performed, predicting a threshold for the modulational instability of CP beams, and an appropriate control parameter is identified. We attempt at utilizing these results to CP solitons, but find only qualitative agreement with the numerical simulations and experimental findings. However, when broader hyper-Gaussian CP beams are used in simulations, an improved agreement with the theory is obtained. (c) 2005 Optical Society of America.
Abstract: We demonstrate experimentally the existence of two transverse-dimensional counterpropagating (CP) incoherent spatial solitons in a 5 x 5 x 23 mm SBN:60Ce photorefractive crystal and investigate their dynamical behavior. We carry out numerical simulations that confirm our experimental findings. Substantially different behavior from the copropagating incoherent solitons is found. A symmetry breaking transition from stable overlapping CP solitons to unstable transversely displaced CP solitons is observed. We perform linear stability analysis that predicts the threshold for the splitup transition, in qualitative agreement with numerical simulations and experimental results.
Abstract: Due to modulation instability partially incoherent optical beams break up into stripe filaments in noninstantaneous media at a first threshold. We numerically and experimentally report on the formation of two-dimensional filaments at a secondary threshold, if increasing the nonlinearity further, which is due to material anisotropy. Particularly, we investigate the dependence of this secondary modulation instability on the coherence properties of the beam, using a photorefractive nonlinearity. From the measurement of modulation contrast in two dimensions and additionally using a Fourier method we quantitatively derive experimental thresholds for first and secondary onset of modulation instability and study how both onsets relate to the coherence properties of the beam. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: Under feedback extended nonlinear optical systems spontaneously show a variety of periodic patterns and structures. Control gives new insight into these phenomena and it can open the way for potential application of nonlinear optical structures. We briefly review methods to control localized states in single feedback experiments. Application of a Fourier control method allows to modify interaction behavior of the localized states. As a further approach we study a forcing method, using externally created light fields as additional input to the system. Recent experiments show that the forcing method enables to favor addressing positions for localized structures. We demonstrate static addressing and favoring of addressing positions. We extend the forcing method to a dynamic forcing scheme, which allows to move and reposition localized states. Additionally forcing is used to balance experimental imperfections.
Abstract: We investigate the characteristics of correlation signals accomplished by content addressing in a phase encoded volume holographic storage system under different realistic conditions. In particular, we explore two crucial cases with respect to the structure of the data. The first one deals with a scenario where only partial or defective data are available for content addressing. The second case takes similarities among the stored data sets into account, which significantly differ from their statistical correlation. For both the cases we provide, for the first time, a theoretical approach and present experimental results when employing phase-code multiplexing. Finally, we discuss the reliability of the employed methods.
Abstract: In this letter we present a technique which employs a photorefractive lithium-niobate crystal for novelty filtering. Due to the minimal trail formation exhibited by this novelty filter, it can be used for reliable quantitative phase measurement for time intervals of the order of a few hours. We present a simplified theoretical description of this filter based on a coupled wave theory [N. V. Kukhtarev, V. B. Markov, S. G. Odulov, M. S. Soskin, and V. L. Vinetskii, Ferroelectrics 22, 949 (1979); and ibid.22, 961 (1979)]. We also demonstrate the first experimental results of employing this device in the field of microfluid dynamics for measuring the concentration changes produced due to the mixing of two transparent liquids in a microchannel. (C) 2005 American Institute of Physics.
Abstract: A time-dependent model for the formation of self-trapped optical beams in photorefractive media by counterpropagating laser beams is analysed. It is shown that dynamically the beams may form stable steady-state structures or display periodic and irregular temporal behaviour. Steady-state solutions of non-uniform cross section are found, representing a general class of self-trapped waveguides, that include counterpropagating spatial vector solitons as a particular case. Two critical curves are identified in the plane of parameters, the first one separating vector solitons from the stable bidirectional waveguides and the second one separating stable waveguides from the unstable ones. Dynamically stable rotating beam structures are discovered that have no analogues in the usual steady-state theory of spatial solitons.
Abstract: We study numerically the counterpropagating vector solitons in SBN:60 photorefractive crystals. A simple theory is provided for explaining the symmetry-breaking transverse instability of these solitons. Phase diagram is produced that depicts the transition from stable counterpropagating solitons to bidirectional waveguides to unstable optical structures. Numerical simulations are performed that predict novel dynamical beam structures, such as the standing-wave and rotating multipole vector solitonic clusters. For larger coupling strengths and/or thicker crystals the beams form unstable self-trapped optical structures that have no counterparts in the copropagating geometry. (C) 2004 Optical Society of America.
Abstract: We describe different types of self-trapped optical beam carrying phase dislocations, including vortex solitons and ring-like soliton clusters. We demonstrate numerically how to create such nonlinear singular beams from the interaction of several fundamental optical solitons. Mutual trapping of several solitons can be regarded as a synthesis of ‘soliton molecules’, and it corresponds to a transfer of an initial orbital angular momentum of a system of solitons to a spin momentum of an optical vortex.
Notes: NATO Advanced Research Workshop on Singular Optics, Kiev, UKRAINE, JUN 24-28, 2003
Abstract: A wealth of periodic transverse patterns is observed in nonlinear optical, photorefractive single-feedback systems. It is the extension of a photorefractive medium in the direction of propagation of the laser beam that allows for the formation of stable hexagons, squares, rhombuses and dodecagons above the first bifurcation of the system. In this article we review the formation of these periodic patterns and present linear and nonlinear stability analyses that predict the type of pattern to appear. Amplitude equations for the interactions of unstable sidebands are introduced. We compare numerical and experimental results on two-dimensional transverse pattern formation. We present control methods for the invasive and non-invasive manipulation of different pattern states using a Fourier filtering technique. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Abstract: A photorefractive-based novelty filter microscope can be used for real-time phase measurement of phase changes, introduced by moving objects or microorganisms, from 0 to pi radians. In this article, we propose a method to extend the phase-measurement range over the entire 2pi radians and demonstrate its validity both experimentally and numerically. Our method makes use of the fact that the slopes of the calibration curve of the novelty filter microscope in the positive- and the negative-phase regions have opposite signs. By applying a known external phase trigger to the novelty filter system, the positive- and negative-phase regions can be differentiated.
Abstract: We suggest a novel type of composite spatial optical soliton created by a coherent vortex beam guiding a partially incoherent light beam in a self-focusing nonlinear medium. We show that the incoherence of the guided mode may enhance, rather than suppress, the vortex azimuthal instability, and we also demonstrate strong destabilization of dipole-mode solitons by partially incoherent light. (C) 2004 Optical Society of America.
Abstract: A time-dependent model for the generation of joint waveguides by counterpropagating light beams in photorefractive crystals is introduced. Depending on initial conditions and parameter values, the beams form stable structures or display periodic and irregular dynamics. Steady-state solutions nonuniform in the direction of propagation are found, representing a general class of self-trapped waveguides that include counterpropagating spatial vector solitons as a particular case.
Abstract: A photorefractive two-beam-coupling based novelty filter is sensitive to amplitude and phase changes. However, the phase sensitivity of this device has remained unexploited until now. In this paper we develop and demonstrate a method to overcome the effects of contour formation in novelty filter output and consequently employ the novelty filter as a quantitative real-time transient phase-measuring instrument. A microscope based on a two-beam-coupling novelty filter is implemented and is used for detecting as well as measuring phase changes introduced by moving homogeneous phase objects. The two-dimensional output from the microscope is analysed, and a method based on the intensity dependence of the photorefractive time constant is proposed to overcome the effects of contour formation in the context of real-time transient phase measurement. The phase transfer function of the novelty filter system is experimentally determined and a phase measurement resolution of at least lambda/20 at 532 nm is achieved.
Notes: 19th Congress of the International-Commission-for-Optics, FLORENCE, ITALY, AUG 25-30, 2002
Abstract: We study, both theoretically and experimentally, the scattering properties of optical dipole-mode vector solitons-radially asymmetric composite self-trapped optical beams. First, we analyze the soliton collisions in an isotropic two-component model with a saturable nonlinearity, and demonstrate that in many cases the scattering dynamics of the dipole-mode solitons allows us to classify them as “molecules of lightâ€-extremely robust spatially localized objects which survive a wide range of interactions and display many properties of composite states with a rotational degree of freedom. Next, we study the composite solitons in an anisotropic nonlinear model that describes photorefractive nonlinearities, and also present a number of experimental verifications of our analysis.
Abstract: We present a review of recent works on spatial optical solitons; in photorefractive media. We discuss fundamental properties of screening solitons and their interaction.
Abstract: We investigate the interaction of two-dimensional solitons propagating at small angles in a photorefractive crystal. We observe fusion of the beams when the intersecting angle is lower than some critical value. We measure the critical angle for fusion for different relative phase relations of the beams and demonstrate how this effect can be used to steer and switch the propagation of an additional optical beam.
Abstract: We discuss the creation of. an array of 9 x 9 photorefractive spatial screening solitons in a strontium barium niobate crystal. We investigate the waveguide properties of each channel with a beam of different wavelength and find that the waveguides guide the probe beam independently. A supplementary beam is used to influence the paths of the array solitons and to effectively combine two channels by use of mutual attraction of solitons. To our knowledge this is the first all-optical control of an array of photorefractive solitons. Furthermore, we show that in principle image procession is possible with parallel propagation of photorefractive solitons. (C) 2003 Optical Society of America.
Abstract: We investigate the durability of volume holograms in bismuth tellurite crystals, Bi2TeO5. This material enables self-fixing of holograms during the recording process rendering any further fixing technique unnecessary. Therefore the material overcomes the vulnerability of holograms which is a typical inconvenient property of inorganic storage crystals. The quality reduction of reconstructed analogue and digital data pages under permanent read-out is studied. In terms of digital page-oriented storage, the change of the bit error rate is investigated. For comparison, all experiments are also performed on iron doped lithium niobate crystals, LiNbO3:Fe.
Abstract: We introduce novel optical solitons that consist of a periodic and a spatially localized component coupled nonlinearly via cross-phase modulation. The spatially localized optical field can be treated as a gap soliton supported by the optically induced nonlinear grating. We find different types of these band-gap composite solitons and demonstrate their dynamical stability.
Abstract: Two-dimensional spatial solitonic lattices are generated and investigated experimentally and numerically in a Sr(x)Ba(1-x)Nb(2)O(6):Ce crystal. An enhanced stability of these lattices is achieved by exploiting the anisotropy of coherent soliton interaction, in particular the relative phase between soliton rows. The manipulation of individual soliton channels is achieved by the use of supplementary control beams.
Abstract: Large two-dimensional spatial soliton arrays are generated experimentally and numerically in photorefractive media and their waveguiding properties at red and infrared wavelengths are demonstrated. An enhanced stability of solitonic lattices is achieved by exploiting the anisotropy of coherent soliton interaction and by controlling the relative phase between soliton rows. Manipulation of individual solitonic channels is accomplished by the use of separate control beams.
Abstract: We display rich spatial and temporal dynamics of light fields counterpropagating in a saturable self-focusing medium numerically, and analyze instabilities that counterpropagating solitons experience. An expression for the maximum length that the medium must not exceed for the solitons to be stable is derived and connected to the coupling strength of beam interaction. The instability can lead to periodic or irregular temporal dynamics of the light beams. By considering mutually incoherent counterpropagating beams, we show that differences to the copropagating case are due to the different boundary conditions.
Abstract: We report on the stable self-focusing of light for various degrees of spatial coherence. Since the diffraction of a partially spatial coherent light beam is mainly governed by the transverse correlation width, its self-focusing is generally non-trivial. Here, we systematically demonstrate that partially coherent solitons can be generated in a non-instantaneous nonlinear material when the external parameters are properly adapted to the degree of coherence of the light beam.
Abstract: We study the mutual trapping of two dipole-mode light beams in a medium with a nonlocal anisotropic saturable nonlinearity. It is shown that the incoherent attraction of perpendicularly aligned dipole-mode beams leads to the stabilization of their composite structure. Our numerical analysis gives a stationary solution that is stable only with respect to small amplitude modulations. The coupled solitary structure disintegrates in a well-defined way when the numerical perturbations or the experimental propagation length exceeds a certain limit. In this case a new and more robust, multicomponent solitary transverse light structure consisting of two dipole-mode vector solitons will be generated. (C) 2002 Optical Society of America.
Abstract: Bismuth tellurite - Bi2TeO5 is a new photorefractive material recently available for optical memory investigations. Analogue volume holograms of a two-dimensional test pattern were recorded in undoped Bi2TeO5 crystals by using a cw Nd:YAG laser at 532 nm. The quality, dark decay and durability during permanent reading of the image were studied and compared to those of the reference LiNbO3 :Fe crystals. The holograms in the two crystals were of comparable quality, and they were less vulnerable for strong laser exposure in Bi2TeO5 than in the LiNbO3 :Fe crystals by a factor of 50.
Abstract: We study (2 + 1)-dimensional multicomponent spatial vector solitons with a nontrivial topological structure of their constituents and demonstrate that these solitary waves exhibit a symmetry-breaking instability, provided their total topological charge is nonzero. We describe a novel type of stable multicomponent dipole-mode solitons with intriguing swinging dynamics. (C) 2002 Optical Society of America.
Abstract: We present theoretical and experimental investigations of the anisotropic character of the refractive-index modulation that is induced by a light beam propagating in a photorefractive strontium barium niobate crystal. Such a structure creates a so-called spatial screening soliton that is able to carry a second wave of a different wavelength and therefore can act as a waveguide. We show in numerical simulations as well as in experimental investigations the anisotropic property of refractive-index modulation. Furthermore, the noncircular shape of the induced waveguide is justified by the excitation of higher-order modes, which were found to be asymmetric in both transverse directions. Whereas in the direction perpendicular to the applied electric field the TEM01 and TEM02 modes can easily be excited, excitement of the TEM10 mode in the direction of the applied field is rather difficult. This effect can be explained by the constricted extension of the waveguide in this direction. (C) 2002 Optical Society of America.
Abstract: We study the existence and stability of multi-component spatial optical solitons in anisotropic nonlocal photorefractive media. For the case of three components, we find numerically the whole family of composite solitons with two perpendicular dipole components, and show their stability for a wide range of parameters. We confirm our theoretical results by an experimental observation of these novel types of composite optical solitons in a photorefractive strontium barium niobate crystal. (C) 2002 Elsevier Science B.V. All rights reserved.
Abstract: (2+1)-dimensional optical spatial solitons have become a major field of research in nonlinear physics throughout the last decade due to their potential in adaptive optical communication technologies. With the help of photorefractive crystals that supply the required type of nonlinearity for soliton generation, we are able to demonstrate experimentally the formation, the dynamic properties, and especially the interaction of solitary waves, which were so far only known from general soliton theory. Among the complex interaction scenarios of scalar solitons, we reveal a distinct behavior denoted as anomalous interaction, which is unique in soliton-supporting systems. Further on, we realize highly parallel, light-induced waveguide configurations based on photorefractive screening solitons that give rise to technical applications towards waveguide couplers and dividers as well as all-optical information processing devices where light is controlled by light itself. Finally, we demonstrate the generation, stability and propagation dynamics of multi-component or vector solitons, multipole transverse optical structures bearing a complex geometry. In analogy to the particle-light dualism of scalar solitons, various types of vector solitons can - in a broader sense - be interpreted as molecules of light.
Abstract: Experimental and theoretical studies of the photovoltaic shift in the position of the diffraction efficiency maximum of holograms recorded in LiNbO3 for the case of optimal electric field multiplexing are described. The experimental data are explained using a model in which the bulk photovoltaic field is excited in a crystal that is electrically connected with a low loading resistance. We suggest that the surface conductivity of the crystal can play an important role in the formation of this effective loading resistance.
Abstract: We report experimental observations of the mode dynamics in a weakly multimode photorefractive ring oscillator with induced astigmatism. First a gallery of transverse, higher-order Gaussian modes in the system is presented. Subsequently the dynamics of beating modes and the interactions between modes belonging to different bases are examined. The latter show the transition of modes between the three equivalent mode families experimentally. Finally regions of cavity symmetry leading to different spatial modes and mode dynamics are identified. (C) 2001 Optical Society of America.
Abstract: In this paper we show the guidance of a HeNe probe beam in photorefractive (2+1)D-solitons created by a beam of a frequency-doubled Nd:YAG laser in SBN. In the first part the development of a single soliton is shown in a time-resolved manner while the guided probe beam is found to follow exactly the movement of the soliton and even copies its shape. In the second part the guidance of a probe beam in interacting solitons is performed. When two (2+1)D-solitons propagate simultaneously different interaction scenarios can be observed, including the mutual exchange of energy. Using this effect, a guided probe beam can be divided effectively in two parts. Thus we present to our knowledge the first realization of a 1-to-2 waveguide divider using (2+1)D-solitons. (C) 2001 Elsevier Science B.V. All rights reserved.
Abstract: We report theoretical and experimental evidence for transverse modulational instability of two counterpropagating beams in a photorefractive medium with no external feedback. A frequency detuning is applied to one of the beams in order to drive the system to instability. We perform a linear-stability analysis that allows for detuning of the counterpropagating pump beams in addition to an additional frequency detuning of the generated sidebands relative to the main beams. The threshold condition for the general case of a complex photorefractive coupling constant is found, and instability is predicted for diffusion-dominated, drift-dominated, and mixed charge transport. We show that for the specific case of diffusion-dominated charge transport, transverse instability is always accompanied by a frequency shift of the sidebands. For frequency-degenerate pump beams the instability threshold is reached at a coupling-constant times interaction-length product of gammal = 5.25i. The threshold is lowered (raised) for small positive (negative) frequency shifts of one of the pump beams. The theoretical predictions were verified experimentally with a photorefractive crystal of KNbO(3). A modulational instability resulting in a spatially periodic roll pattern was observed for a certain range of positive frequency detunings. Measurements of the transverse scale of the structures and the relative sideband intensities were in agreement with the theoretical analysis. (C) 2001 Optical Society of America.
Abstract: We review the generation of the recently predicted multi-component spatial optical solitons in a saturable nonlinear bulk medium. We present numerical simulations for an effectively isotropic model and experimental results for a set of different combinations of a Gaussian beam co-propagating incoherently with a beam of a more complex internal structure, such as a higher order transverse laser mode. We discuss the different formation processes and the general properties of a variety of different dipole-mode composite solitons and expand our investigations to the generation of a quadrupole-mode composite soliton.
Abstract: We present two different techniques on how to realize a content-addressed holographic memory when using phase-code multiplexing, relying on simple intensity measurements rather than phase distributions. Theoretical and experimental results of associative recall in a phase-coded system designed for digital data storage will be presented and compared to the corresponding method when using angular multiplexing.
Abstract: Stable dipole-mode vector solitons in photorefractive (PR) crystals are studied, both in numerical simulations and in experimental investigation. Numerically exact multi-humped solitary solutions to the propagation equations of two incoherently coupled beams in PR media with an anisotropic nonlocal material response are found. A continuous set of dipole-mode vector solitons, ranging from the solitons with negligible dipole-mode contribution to the solitons with negligible ground mode contribution, is identified. The results are compared with experimental data. (C) 2001 Published by Elsevier Science B.V.
Abstract: We investigate the generation of higher-order optical vector solitons in two transverse dimensions in anisotropic nonlinear media consisting of an incoherent superposition of a Gaussian beam and a higher-order laser mode with a complex internal modal structure. We demonstrate both numerically and experimentally various examples of these stable self-trapped light structures and show that vortex modes carrying topological charge always decay into multiple-humped structures that remain self trapped during propagation. Furthermore. we demonstrate the mutual stabilization of a triple- and a double-humped transverse light structure leading to the formation of a two-dimensional vector soliton without a stabilizing fundamental Gaussian mode.
Abstract: We show that manipulation and selection of optical patterns is possible by detuning the frequency of one of the two waves in counterpropagating photorefractive two-wave mixing with a feedback mirror. An analytical expression for the onset of modulational instability is derived for the general case of an arbitrary complex coupling constant, including a possible frequency shift of the generated sidebands. The influence of frequency detuning of the pump beams on the spatial scale of the resulting pattern and on the threshold value of the photorefractive coupling strength for modulational instability is investigated. The spatial scale and the frequency detuning of the generated sidebands were measured in an experiment using a photorefractive KNbO3 crystal. Comparison of theory and experiment show agreement for positive frequency detuning of the pump beams. For negative detuning we observe clamping of the spatial scale and no frequency shift of the sidebands. Possible explanations of this deviation between analytical results and experiment are discussed.
Abstract: We demonstrate the equivalence of the electric field and spectral multiplexing (SM) technique for holographic recording in photorefractive materials. The proposed technique of electric field multiplexing (EFM) provides a spectral selectivity of recorded holographic filters up to a few picometers. (C) 2001 Elsevier Science B.V. All rights reserved.
Notes: Symposium of the 2000 E-MRS-IUMRS-ICEM Spring Confernece, STRASBOURG, FRANCE, MAY 30-JUN 02, 2000
Abstract: The photorefractive properties of Bi2TeO5 crystals were measured with the wave mixing technique using the 532 nm output of a Nd:YAG laser for writing gratings. The saturation diffraction efficiency of the grating for the optimum crystal orientation and beam polarization configuration exceeded the 40%. Above a threshold write intensity, a remarkable fraction (10-20%) of the saturation diffraction efficiency remained stable for several days in the crystal. A weaker read beam of the same wavelength did not erase the grating. A two-dimensional holographic image was also written into the Bi2TeO5 crystals for the first time. (C) 2000 Elsevier Science B.V. All rights reserved.
Abstract: Theoretical and experimental investigations of electric field multiplexing and selectivity of reflection volume holograms in LiNbO3 are reported. Recording of at least five holograms is demonstrated. Equivalent spectral selectivity Delta lambda approximate to 4.5 pm for the case of three electrically tunable holograms and Delta lambda approximate to 9 pm for the case of five electrically tunable holograms are estimated.
Abstract: We report on the first experimental observation of a novel type of optical vector soliton, a dipole-mode soliton, recently predicted theoretically. We show that these vector solitons can be generated in a photorefractive medium employing two different processes: a phase imprinting, and a symmetry-breaking instability of a vortex-mode vector soliton. The experimental results display remarkable agreement with the theory, and confirm the robust nature of these radially asymmetric two-component solitary waves.
Abstract: We investigate the origin of dynamics of hexagonal pattern formation in a photorefractive single feedback system. By introducing an asymmetry, we induced different complex motion behaviour. A visualization method is presented and means to control the dynamics in the case of fixed external parameters are discussed. (C) 1999 Elsevier Science Ltd. All rights reserved.
Notes: WE-Herseus Seminar on Pattern Formation in Nonlinear Optical Systems, BAD HONNEF, GERMANY, JUN 23-25, 1997
Abstract: We report results on unidirectional oscillation in a photorefractive oscillator implemented with BaTiO3 that is actively stabilized. Dynamic spatial patterns and pattern competition were observed, depending strongly on the Fresnel number of the system. As the origin for spontaneous pattern alternation we identified variations in the cavity length that are in turn mainly due to temperature fluctuations. By varying the cavity length, we were able to adjust stable patterns of different mode families and control pattern dynamics as mode beats, optical vortices and their rotations and more complete pattern dynamics for higher Fresnel numbers. (C) 1999 Elsevier Science Ltd. Ail rights reserved.
Notes: WE-Herseus Seminar on Pattern Formation in Nonlinear Optical Systems, BAD HONNEF, GERMANY, JUN 23-25, 1997
Abstract: We present an experimental realization of an almost non-invasive stabilization and manipulation method of coexisting and unstable states of pattern forming systems. In a photorefractive single feedback system, a control path;is:used to realize amplitude and phase-sensitive Fourier-plane filtering, utilizing only a few per cent of the system’s intensity. By that means, we were able to stabilize desired but not predominantly excited patterns in parameter space regions where several patterns are present as coexisting or underlying solutions. Changing the phase of the control signal allows one to switch between different pattern states.
Abstract: We present a review of our recent theoretical and experimental results on the interaction of incoherent two-dimensional solitary beams in PR SBN crystals. We show that the inherent anisotropy of PR nonlinearity strongly affects the interaction between solitons. Theoretical and experimental results reveal that solitons interacting in a plane perpendicular to the direction of external biasing field always attract, whereas those colliding in a plane of the field exhibit anomalous behaviour. They may experience both attractive and repulsive forces, depending on their mutual separation. We also show that this anisotropy results in the complicated topology of soliton trajectories, featuring periodic collisions, prolonged mutual spiraling and collapse, depending on the initial conditions.
Abstract: Phase codes of Talbot array illumination, originally derived from Talbot effect for array illumination at fractional Talbot distance, are now suggested as a set of orthogonal phase codes for phase-encoded holographic multiplexing storage in volume-holographic media. The most attractive feature of the Talbot encoding method is that an arbitrary number of images can be phase-encoded while the previous Hadamard encoding methods can only encode the number of stored images, M = 2(n), and/or M = 4n (with n a positive integer). Thus the Talbot encoding method is highly interesting to take full use of the available pixel numbers of spatial light modulators and to achieve the maximum possible storage capacity of a phase-code multiplexed holographic memory. (C) 1999 Published by Elsevier Science B.V. All rights reserved.
Abstract: Self-bending of photorefractive solitons is caused by diffusion in photorefractive crystals and becomes an important effect when the beam size is in the range of the charge carriers diffusion length. In this paper we present an experimental and numerical examination of the beam bending dependence on relevant parameters such as the applied electric field and the beam intensity. We demonstrate that the bending dependence on the electric field in the low saturation regime has the form of a square function at low values of the field and becomes linear for higher values. For stronger saturation the curve gets the form of a square root function. The bending dependence on the beam intensity has a maximum at defined intensity. The experimental data are compared with numerical simulations, giving a good qualitative agreement. (C) 1999 Published by Elsevier Science B.V. All rights reserved.
Abstract: We report on the observation of a multiple-pattern stability region in a photorefractive single-feedback system. Whereas hexagonal patterns are predominant for feedback with positive diffraction length we show that a variety of stable non-hexagonal patterns are generated for certain negative diffraction lengths. For the same values of the control parameters square, rectangular, or squeezed hexagonal patterns are found alternating in time. Besides these pure states, we found a number of different mixed-pattern states. We review the linear stability analysis for this system and show that the special shape of the threshold curves in the investigated parameter region gives a first explanation for the occurrence of a multiple-pattern region.
Notes: Spring Meeting of the German-Physical-Society, UNIV HEIDELBERG, HEIDELBERG, GERMANY, 1999
Abstract: In this article, we demonstrate the capability of a two-beam coupling photorefractive optical novelty filter of detecting changes in the amplitude or phase of optical images. These changes may either be continuous or discrete in time. The performance of the two-beam coupling novelty filter is investigated and expressions for the output contrast corresponding to phase and amplitude changes based on a novel, simple interference model of two-beam coupling are derived. These expressions are verified by experimental results on the novelty contrast, revealing that the amplitude contrast is not described correctly by the commonly accepted coupled-wave theory. The novelty filter was applied to the detection of temporally continuous phase changes provided by a gas flow and moving microscopic objects. A novel scheme for image subtraction is also demonstrated, showing the novelty filter’s ability to detect temporally discrete changes.
Abstract: An experimental and numerical investigation of the dynamical, time-dependent effects accompanying the Formation and interaction of two-dimensional spatial screening solitons in a photorefractive strontium barium niobate crystal is performed. These effects include initial diffraction, collapse to the soliton shape, the oscillation of beam diameters, beam bending, and the rotation, twisting, and turning of soliton pairs. The dynamics of complex spiraling of two incoherent solitons is considered in more detail. [S1063-651X(99)05611-1].
Abstract: We present a control method for the minimally invasive manipulation of pattern states occurring in feedback systems with an optical nonlinearity. An interferometric feedback configuration is used to control spontaneously formed patterns by the method of Fourier filtering. This method is minimally invasive since the pattern forming feedback arm remains unchanged, while the energy removed by the filter or the control signal added to the system by the second control feedback arm tends to a small level when the desired pattern is obtained. Here, we present experimental results for a photorefractive two-arm feedback system, including switching from hexagons to rolls or squares by positive (in-phase) control and manipulation of the hexagon orientation by negative (out-of-phase) control. A comparison with results of a numerical simulation based on a thin saturable Kerr-slice model is performed. (C) 1999 Elsevier Science B.V. All rights reserved.
Abstract: We investigate the interaction of mutually incoherent spatial solitons in photorefractive media with anisotropic nonlocal nonlinear response. We show that the photorefractive nonlinearity leads to an anomalous interaction between solitons. Theoretical and experimental results reveal that an incoherent soliton pair may experience both attractive and repulsive forces, depending on their mutual separation.
Abstract: We investigate experimentally the interaction of two-dimensional solitary beams in photorefractive strontium barium niobate crystal. We show that simultaneous collision of many solitons can result in complete annihilation of some of them. This effect depends on the relative phases of the solitons and may be useful for application in the formation of multiport waveguide junctions. (C) 1998 Optical Society of America.
Abstract: We investigate the temporal dynamics of transverse optical patterns spontaneously formed in a photorefractive single-feedback system with a virtual feedback mirror. The linear stability analysis for the system is reviewed and extended to the region of larger propagation lengths. The stationary patterns obtained experimentally are classified as a function of feedback reflectivity and feedback mirror position. Inserting masks into the feedback path permits pattern selection and control by Fourier filtering. When an asymmetry that is due to noncollinear pump beams is introduced, the otherwise stationary hexagons show several complex but periodic rotationlike motions. Furthermore, the competition of hexagonal and square patterns can be observed by the appropriate choice of feedback mirror position and coupling strength. The origin of this behavior is discussed. The temporal evolution of the patterns is illustrated by a method based on unfolding the angular distribution of the spots in the far field. (C) 1998 Optical Society of America.
Abstract: We present the design and realization of a compact volume holographic memory based on phase-coded multiplexing. Due to the use of nonmechanical reference beam changes in this multiplexing technique, rapid access of the stored data pages is achieved. Our system can reach a maximum storage capacity up to 480 data pages with a resolution of 640 x 480 pixels in a single interaction region of a photorefractive LiNbO3 crystal, We present the implementation of the system and first experimental results of both, high-resolution analog image storage and digital data storage. The storage of 180 data pages is demonstrated as well as the application of the system to real-time optical image processing.
Abstract: We present a review of our recent theoretical and experimental results on the interaction of two-dimensional solitary beams in photorefractive SBN crystals. We show that the collision of coherent solitons may result in energy exchange, fusion of the interacting solitons, the birth of a new solitary beam or the complete annihilation of some of them, depending on the relative phase of the interacting beams. In the case of mutually incoherent solitons, we show that the photorefractive nonlinearity leads to an anomalous interaction between solitons. Theoretical and experimental results reveal that a soliton pair may experience both attractive and repulsive forces; depending on their mutual separation. We also show that strong attraction leads to periodic collision or helical motion of solitons depending on initial conditions.
Abstract: We present an experimental realization of an almost noninvasive stabilization and manipulation method of coexisting and underlying states of pattern forming systems. In a photorefractive single feedback system, a ring control path is used to realize amplitude and phase-sensitive Fourier-plane filtering, utilizing only a few percent of the system’s intensity. We were able to stabilize desired but not predominantly excited patterns in parameter space regions where several patterns are present as underlying solutions. By positive (in-phase) and negative (out-of-phase) control, rolls could be excited in parameter regions where hexagonal structures are preferably stable.
Abstract: Spontaneous formation of optical patterns is studied in a system composed of a photorefractive phase-conjugating medium and a virtually internal feedback minor, In addition to ordinary regular hexagons, squares and squeezed hexagons are reported for the first time in optical pattern formation experiments, The angle of the generated sideband beams plotted as a function of the mirror position lies on two different curves, which can theoretically be explained by the contributions from two different regions of the medium separated by the virtual feedback mirror.
Abstract: The signal-to-noise ratio of the output of an adaptive holographic interferometer (AHI) based on a Bi12TiO20 crystal is investigated. We show experimentally that the sensitivity of an AHI using the non-Bragg orders of diffraction in a thin photoreactive material is more than an order of magnitude greater than that of an AHI employing two-wave mixing in photorefractive volume holograms. (C) 1997 Optical Society of America.
Abstract: We propose and demonstrate experimentally a method for utilizing a dynamic phase-encoded photorefractive memory to realize parallel optical addition, subtraction, and inversion operations of stored images. The phase-encoded holographic memory is realized in photorefractive BaTiO3, storing eight images using Walsh-Hadamard binary phase codes and an incremental recording procedure. By subsampling the set of reference beams during the recall operation, the selectivity of the phase address is decreased, allowing one to combine images in such a way that different linear combination of the images can be realized at the output of the memory. (C) 1996 Optical Society of America
Abstract: A numerical method to simulate the photorefractive phase conjugation process in rather general conditions, pump depletion and multigrating operation, has been developed. Previous theoretical works had been essentially devoted to study the steady state situation. With this numerical calculation we can analyze the time evolution of the phase conjugate beam when all gratings are operative. The response to time-modulated signals can also be simulated. Finally, the numerical method can be adapted to simulate the performance of self-pumped phase conjugators. The method has been applied to study the temporal behavior of phase conjugate signals in the standard four-wave mixing configuration in a BaTiO3 crystal.
Notes: Symposium on Photorefractive Materials, at the E-MRS 1994 Spring Meeting, STRASBOURG, FRANCE, MAY 24-26, 1994
Abstract: A general formalism for multiplexed holographic image storage is described which includes both phase and angular encoding techniques. A combination of both methods can easily be treated as well. We derive an expression for the spatial modulation of the refractive index after storage of all images, calculate the diffraction amplitude during readout and deduce the conditions for selective readout. For phase encoding, general rules for the construction of orthogonal phase codes are developped.
Notes: Symposium on Photorefractive Materials, at the E-MRS 1994 Spring Meeting, STRASBOURG, FRANCE, MAY 24-26, 1994
Abstract: We present the design, implementation and detailed investigations on the characteristics of an optical novelty filter based on two-wave mixing in photorefractive BaTiO3. The system is cabable on novelty filtering two-dimensional video images using a commercially available liquid crystal display. The setup was realized as a compact demonstrator device. A contrast of up to 40:1 and a limiting frequency of 1.6 Hz were obtained.
Notes: Symposium on Photorefractive Materials, at the E-MRS 1994 Spring Meeting, STRASBOURG, FRANCE, MAY 24-26, 1994
Abstract: A general theory of the resolution of arbitrary novelty filters, optical as well as electronic, including the only existing theory as a special case, is presented. Resolution is calculated for the two essential processes of object tracking, i.e. object localization and separation of different objects. Algorithms for the reconstruction of the original objects from the output image are developed. Contour generation, appearing in the novelty filtering of moving objects, is used as a tool to maintain the input resolution. The possibility of image processing and novelty filtering in the Fourier plane is analytically verified. The application of the theory to photorefractive optical novelty filters is briefly discussed.
Abstract: We propose and demonstrate experimentally a new, phase-locked refreshment method for reading out dynamic holographic photorefractive memories without destruction of the stored information. This refreshment method can be used for various multiplexing techniques as angular-, phase encoded- or wavelength multiplexing and for different recording schedules as sequential or incremental recording. In a first experimental realization, an image stored in a photorefractive BaTiO3-memory can be sustained during readout when it is fed back into the crystal using a double phase conjugation oscillator, realized with photorefractive BaTiO3 as well. Experimental results on the diffraction efficiency of the refreshed images, signal-to-noise ratio and optimal refreshing time are given as well as a demonstration of multiple refreshment implementing a series of refreshment cycles. Moreover, our method is experimentally compared to phase-locked hologram sustainment by redirecting both the image and the reference beam back into the memory via phase conjugation in photorefractive BaTiO3.
Abstract: The self-pumped BaTiO3 phase-conjugate mirror (SPPCM) is presented as a nonlinear system that exhibits temporal irregular fluctuations in the phase-conjugated output in dependence on one critical parameter - the beam input position. Qualitative examinations of the nature of these fluctuations, the Fourier transform, the autocorrelation function and the Poincare map, show signs of deterministic chaotic behavior. As a quantitative analysis method of nonlinear dynamics, the Grassberger-Procaccia analysis is carried out to compute a lower bound of the Hausdorff dimension of the temporal field. The fractal value of that dimension reveals the deterministic chaotic character of the irregular fluctuations. A variation of the beam input position as the control parameter drives the nonlinear system from regular behavior to deterministic chaos. Our analysis shows, how the fractal correlation dimension D2 changes from D2 = 1 to D2 = 8.5 depending on the critical input beam position. This allows conclusions on the theoretical model best suited to describe the system.
Abstract: The advantages and limitations of data storage in holographic materials by implementing a pure phase-encoding method of the reference beam are studied. We show that if deterministic orthogonal binary phase addresses are used, such a system is theoretically able to store as many images as the usual angular multiplexing method. However, we demonstrate that imperfections of available optical components generate optical noise and limit the storage capacity. We propose an improved recording technique to overcome some of these limitations.
Abstract: We present a novel phase encoding method for high capacity optical data storage in volume photorefractive materials. Using deterministic orthogonal phase codes in a reference-based multiplexing technique, we obtain a system that is able to retrieve multiple images with high diffraction efficiency without energy losses, adjustment problems or any time delay. The number of images which can be stored with that method is theoretically calculated and it is shown that image retrieval without any crosstalk is possible. Moreover, the method is experimentally demonstrated for a set of four orthogonal phase addresses. Actual limitations of the system are finally discussed.
