Pierre Mathey

Influence of piezoelectricity on the photorefractive effect

We demonstrate that the spatially modulated electric field that is associated with a photorefractive grating generates stress and strain components with symmetries that are different from those induced by a uniform electric field. Therefore, because of piezoelectricity and the elasto-optic effect, the symmetries of the effective dielectric and electro-optic constants to be used to describe the photorefractive effect differ from the symmetries of the usual dielectric and electro-optic tensors. We derive analytical expressions to be used to compute these new constants from the measured clamped and unclamped dielectric and electro-optic coefficients. Experimental evidence is presented.

Refractive-index profile reconstructions in planar waveguides by the WKB inverse method and reflectivity calculations

Two formalisms used in index profile reconstructions are compared. We develop a novel formulation of the WKB inverse method and show that this method is not sufficient to recover the entire index profile in ion-implanted waveguides. A reflectivity calculation involving a matrix formalism solves the problem. The index profile parameters are adjusted to fit experimental mode indices. With this formalism, the reconstruction of the entire index profile is achieved. An application is shown in a BaTiO3 sample.

High performance of two-wave mixing in a BaTiO 3 waveguide realized by He + implantation

Two-wave mixing in a He+-implanted waveguide fabricated with a BaTiO3 substrate is investigated at 514.5 nm. The study is conducted for several directions of the grating vector relative to the optical c axis. The maximum gain of 58 cm-1 is far higher than the value of 15 cm-1 that is measurable in the same conditions in bulk samples. Taking into account the mode losses, we show that the experimental values for the gain are in good agreement with theory. The feasibility of photorefractive interactions between TE guided waves with different mode orders is demonstrated. The characteristics of the photorefractive effect in the waveguide are shown to be different from those in the bulk (opposite transfer direction, shorter response time), which suggests that the nature of the dominant charge carriers and the electro-optic properties are modified by the implantation.

Overcoming the trap density limitation in photorefractive two-beam coupling by applying pulsed electric fields

Abstract By applying pulsed electric fields to photorefractive crystals, the two-wave mixing gain is considerably increased. Unlike other enhancement techniques, the maximum gain is not limited by the crystal photorefractive trap density. However, because the maximum gain is obtained for a given fringe spacing of the interference pattern, the spatial resolution for image amplification is low.

Dynamics of novelty filtering and edge enhancement in cobalt-doped barium titanate

A compact photorefractive optical novelty filter is presented. Its two main components, the nonlinear photorefractive crystal and the spatial light modulator, are characterized. It is shown that, for this application, the involved BaTiO3:Co crystal is more efficient than a nominally undoped BaTiO3. It is demonstrated that the spatial light modulator alters the wave-mixing performance. The cut frequency of a cyclic event detection is measured versus the incident intensity. The agreement with previously developed theories about edge-enhancement characteristics is satisfactory.

Guiding properties and nonlinear wave mixing at 854 nm in a rhodium-doped BaTiO 3 waveguide implanted with He + ions

For the very first time to our knowledge, guided waves at 854 nm are observed in a BaTiO3:Rh waveguide fabricated by the technique of ion-beam implantation. The photorefractive interaction between two guided modes is demonstrated and characterized. The experiments revealed that the gain direction is reversed in the guiding layer in comparison with that in the bulk. A maximum gain of 24 cm-1 is achieved.

Unconventional beam amplification with photovoltaic and diffusion effects in a He + -implanted LiNbO 3 :Fe waveguide

Two-wave mixing at 514.5 nm is investigated in an x-cut LiNbO3:Fe waveguide twice implanted with helium ions. The energy transfer is studied in four configurations characterized by the orientation of the optical axis and the polarization of the input waves. It is shown that, in one arrangement, the kinetics of the wave mixing consists of two parts: a transient peak attributed to the photovoltaic effect followed by a slower decay toward the stationary state for which the classic diffusion mechanism is predominant. The appearance of the photovoltaic effect is unexpected in comparison with the results found for the bulk.

Performance evaluation of a photorefractive novelty filter for motion tracking and edge enhancement

Abstract The optimisation of a photorefractive novelty filter configuration based on two-beam coupling and energy depletion of the object bearing beam is conducted. The BaTiO 3 sample is characterised in terms of depletion efficency. The suitability of the system in motion tracking and edge enhancement is demonstrated. Two parameters, the contrast and the full width at half maximum of the detected edges, are used to characterise the information quality. The study is conducted for different object velocities and several beam intensities. The results obtained for edge enhancement are compared with those reached by data processing.

Coupling of ordinary and extraordinary waves in iron-doped BaTiO 3 by a grating recorded by spatially oscillating photovoltaic currents

Recording of a π/2-shifted photorefractive grating in iron-doped BaTiO3 by orthogonally polarized waves is reported. The unidirectional intensity transfer from ordinary to extraordinary waves is attributed to the photovoltaic process of charge redistribution. The sign of the antisymmetric component of the photovoltaic tensor is found, and an estimate of its absolute value is given.

Dynamics of four-wave-mixing oscillators with quasi-phase-matching

The effect of pump-wave misalignment on the oscillation spectra of a semilinear photorefractive oscillator is studied numerically and compared with the results of experiments performed with BaTiO{sub 3}:Co and KNbO{sub 3}:Ag,Fe crystals.