We propose a new stress-sensing technique based upon measurement of light scattering produced by an array of birefringent waveguides. When external stress is applied to the array of waveguides, their optical properties are modified via the photoelastic effect. Specifically, the intensity of light scattered by the optical waveguides is significantly affected by stress. In this work we analyse theoretically this stress-induced change. Our numerical simulations show that this effect provides a means to assess the strength and direction of the external force.

We develop a computational method for calculating the spatial profile of the electromagnetic field after scattering by an array of waveguides. Our formalism is very general and includes chromatic dependence, the influence of the array arrangement, and other effects such as the effect of stress. Our calculations of the amplitude and phase of scattered light provide valuable information about the features of the waveguides. These results can be applied to different areas of study, such as biological waveguides and fiber sensing.

We present a theoretical investigation of the nonlinear phenomenon of spatial frequency mixing in photorefractive materials. In particular, we study the kinetics of the second harmonics and the sum and difference (combinational) gratings when two photorefractive gratings are recorded in the material. The physical origin of the new gratings is extensively discussed. The formalism is applied to investigate multiple recording in LiNbO3 as a material relevant for applications. The influence of the multiple-recording method (either sequential or simultaneous) on the generation of second-order gratings is analyzed. We found remarkable differences in the kinetics of these gratings depending on the multiplexing procedure. Our theoretical predictions are in good agreement with a number of previously reported experimental results.

The inherent nonlinearity of the charge transport processes in photorefractive materials give rise to some new phenomena, such as generation of harmonics or grating cross talk, that have been extensively investigated for non-fixed gratings. Here, we investigate the corresponding phenomena taking place during the thermal fixing process. We study in detail the kinetics of the thermal fixing process for high light modulation initial recording-that results to be itself nonlinear. For single grating recording we obtain the amplitude of the first and second harmonic proton gratings and the final developed amplitude for the first harmonic. In turn, for multiple recording we focus our attention on combinational components. It is obtained that the key parameters that control nonlinear fixing are the proton concentration and grating K-vectors. Some preliminary experimental data are also presented.

The nonlinear combinations of gratings appearing in fixed multiplexed configurations during the recording as well as during the fixing of holograms in LiNbO3 has been investigated. We have obtained significant differences in comparison with the previously studied case of non-fixed holograms. Moreover, we have found that the relative strength of the combinational gratings depends on the particular experimental procedure used for thermal fixing. The first observation of fixed combinational gratings is also reported obtaining diffraction efficiencies of ∼0.01% for the sum and difference gratings. Our results should be relevant for applications involving fixed multiplexed holograms.

The nonlinear cross talk between two photorefractive gratings recorded in a BaTiO3 crystal has been investigated. Besides the photorefractive response described by the material equations, our theoretical analysis also includes the effect of beam coupling through the coupled-wave equations. We show that beam coupling has a strong influence on the nonlinear cross talk in BaTiO3. Depending on the initial conditions, it may lead to an increase or decrease of the nonlinear grating cross talk. Moreover, our experimental results provide experimental evidence of nonlinear cross talk in BaTiO3. Significant cross talk values, up to ∼20%, have been found experimentally. Furthermore, the experimental data are in good agreement with the theoretical predictions. The results have relevant implications in practical situations such as sequential recording of holograms for holographic storage.

The observation of diffraction spots from strong higher-order combinational gratings during sequential recording of two photorefractive holograms in a LiNbO3 crystal is reported. To the best of our knowledge, this is the first time that combinational photorefractive gratings of third and fourth order have been detected. The influence of the parameters of both intentionally recorded gratings on the amplitude of second-order combinational components (K-1 + K-2 and K-1 - K-2) has been investigated. We have found that the strength of these gratings essentially increases with the amplitude of the first grating recorded and with the modulation of the second grating recorded. However, the combinational gratings behave differently from the second-harmonic gratings in some respects. The results can be qualitatively explained in terms of the nonlinearities of band transport equations. (C) 1999 Optical Society of America [S0740-3224(99)00610-4].

The nonlinear interactions between gratings generated during multibeam photoreactive recording with one reference and N object beams have been investigated theoretically. It is shown that nonlinear cross talk between gratings, which is well known in three-beam recording, persists and even increases for the practical relevant case in which the number of object beams N is large. The magnitude of the cross talk depends on the particular grating, and it is significant for a range of intensities commonly used in many multibeam applications. Finally, it is shown that for large N(N > Fl)tbe nonlinear interactions strongly suppress most gratings, whereas specific gratings can be selectively amplified. (C) 1999 Optical Society of America [S0740-3224(99)00603-7]. OCIS codes: 190.5330, 090.7330.

Nonlinear interactions between multiple gratings simultaneously recorded in a BSO crystal have been experimentally investigated. A forward four wave mixing configuration (one reference and three object beams) has been used to generate the gratings. As in previous experiments with only two object beams relevant cross talk effects have been found. The magnitude of the cross talk markedly depends on the relative intensity ratio between beams. Furthermore, it is demonstrated that enhancement of the diffraction efficiency may be achieved due to the nonlinear cross talk. The results can be well reproduced by theoretical calculations based on the band transport equations preserving their nonlinear terms.

crystal subjected to two counterpropagating pairs of pump beams. The model explains the characteristic features of the above effect and predicts a number of new peculiarities that we have found in additional experiments.

A numerical simulation for studying the kinetics of phase-conjugate beams generated by multigrating four-wave mixing has been developed. It has been applied to investigate the four-wave mixing process in BaTiO3 by use of realistic coupling coefficients. The role of the distinct gratings has been analyzed. Depending on the experimental conditions, constructive as well as destructive interference of the different contributions was obtained. Moreover, because of grating competition effects, rr-phase changes in the phase-conjugate beam can occur during the kinetics. For large values of the coupling strength and specific regions of the physical parameters, unstable four-wave mixing dynamics were obtained. The analysis of the results provides some criteria for predicting the phase conjugator's behavior in multigrating conditions. (C) 1998 Optical Society of America.

Recently, nonlinear coupling of simultaneously coherently recorded gratings in photorefractive BSO has been studied experimentally and theoretically [1]. This so-called nonlinear cross talk is induced through the nonlinear response of the photorefractive material. In applications involving multibeam interactions, such as photorefractive image amplification and holographic storage, the nonlinear cross talk may strongly affect the grating efficiencies. Previous interpretations of multibeam experiments in BaTiO3 are based entirely on beam coupling, which may lead to a disagreement between theory and experiments [2].

Photorefractive phase conjugate mirrors have been widely investigated in the past years. In particular, self-pumped phase conjugators are specially usefull for technological applications because of their self alignment capabilities and compactness.

We propose a physical model to explain peculiarities of photorefractive recording and dark decay detected in LiNbO3 crystals within temperature ranges below and above 200 degrees C. Distinctive features of our description are proximity of the activation energies for protons and thermally excited electrons and their competitive contributions to the charge transport. (C) 1998 Optical Society of America.

We develop a theory of high-temperature photorefractive phenomena in LiNbO3 crystals related to the problem of thermal fixing and storage of optical information. The theory covers the temperature range 20-300 degrees C relevant to the experiment. It is based on a systematic exploiting of small physical parameters, typical of the subject, and distinguished by general and simple expressions for characterization of the fixing, developing, and decay processes. It is shown that thermally excited electrons are competitive with protons (responsible for the charge compensation) within a wide high-temperature region 200-300 degrees C. They are not only responsible for the dark decay of information, but also for pronounced high-temperature peculiarities of the recording and relaxation processes. A good qualitative agreement between the theory and a gnat amount of accumulated factual data are obtained.

A two-dimensional time-dependent analysis is presented to describe photorefractive recording in a film with anisotropic transport parameters. The solution of the rate equations together with suitable boundary conditions on the film faces has been obtained by numerical methods for a parallel recording geometry. The recording curves very markedly depend on the anisotropy ratio ai=μz/μx (z being the axis perpendicular to the film faces), the grating period, and the ratio between film and buffer permittivities. For ai=1, the solution coincides with that previously obtained using an analytical approach. Approximate analytical solutions for ai≠1 have also been worked out and compared to the numerical results.

The kinetics of the phase conjugate beam generated by standard four-wave mixing in a BaTiO3 crystal has been investigated by means of an accurate numerical simulation. The transmission grating configuration has been considered and depletion of pumps was allowed. Special care has been taken in using suitable parameters for this material and consequently for the coupling coefficients γ and grating response times T. The influence on the reflectivity of the beam intensity ratios and coupling coefficients has been analysed. Oscillating as well as monotonic increasing kinetics has been found depending on the values of the coupling coefficients and the pump ratio. A comparison between the undepleted and depleted pump cases is performed finding that reflectivity oscillations are more pronounced for higher coupling coefficients and when the pumps were not depleted. Some comments on the origin of the oscillations will be presented.