Jean-Claude Kastelik

Frequency and angular bandwidth of acousto-optic deflectors with ultrasonic walk-off

In the paper, bandwidth parameters of acousto-optical deflectors (AODs) are analyzed from the point of view of acoustical anisotropy. Equations for bandwidth and central frequency of AOD are derived for arbitrary propagation direction of ultrasound in optically uniaxial crystals. The phenomenon of bandwidth shift due to phase mismatch at the central frequency is studied theoretically and verified experimentally.

High dynamic range, bifrequency acousto-optic modulator

Multifrequency acousto-optic interaction is limited mainly by the two-tone, third-order intermodulation products. The tangential phase-matching (TPM) anisotropic interaction can greatly reduce the phase-grating intermodulations. We consider the two operating modes, high or low frequency, for the slow shear wave propagating in paratellurite with an angular tilt from the [110] axis. As two different angles lead to the same central operating frequency, we show that the low-frequency configuration allows one to greatly increase the dynamic range, compared with that obtained in the high-frequency mode. For experimental validation, two bifrequency acousto-optic modulators have been realized and tested. For a central frequency in the low-frequency configuration and a diffraction efficiency , the dynamic range is shown to be enhanced by 5 dB.

Simultaneous bandgaps in LiNbO3 phoxonic crystal slab

We study simultaneous photonic and phononic crystal slabs created in Z-cut lithium niobate membranes. Bandgaps for guided waves are identified using the three-dimensional finite element method (FEM). Three lattices are considered: the square, the hexagonal, and the honeycomb lattices. We investigate the evolution of band gaps as a function of geometrical parameters such as hole radius and membrane thickness. We show the existence of dual photonic and phononic bandgaps in the triangular lattice for suitable geometrical parameters and specific modal symmetries for both the elastic and the electromagnetic fields.

Image crosstalk reduction in stereoscopic laser-based display systems

Image cross talk is a problem specific to stereoscopic display, which influences the conspicuity of “ghost images” and the development of asthenopia. The higher the image cross talk, the lower the visual comfort for the observer, who may suffer from eye strain and headaches. The stereoscopic laser-based display system presented uses processing to provide command variables for the optical modulation device from the pixel intensities of both views of the stereo pairs. Two processing solutions, one minimizing the quadratic error and the other one canceling the image cross talk, are developed and compared using original evaluation tools. Finally, by comparing numerical results and the visual effects obtained with tests on stereo pairs, the presented results show objectively that the second processing solution is preferable.

Acousto-optic and electro-optic modulators for stereoscopic laser videoprojection

An original combination of acousto-optic modulators and electro-optic cells for stereoscopic laser videoprojection is presented. It is based on the separation by orthogonal polarizations of the two left and right images of a stereoscopic pair. As several video lines are scanned simultaneously, the acousto-optic modulators operate in multifrequency mode. The polarization states are controlled by the electro-optical devices. A practical bifrequency acousto-optic modulator using the slow shear acoustic wave in paratellurite has been designed, and an electrooptic device with four ammonium dihydrogen phosphate crystals has been implemented. Experimental results are reported.

Stereo-pair processing in a three-dimensional laser display system

The stereoscopic videoprojector developed at our laboratory is based on the simultaneous projection of the left and right images of a stereo-pair along two orthogonal polarization directions of a laser beam. The horizontal scanning system, which deflects four lines simultaneously, led to the design of a specific acousto-electro-optical modulation device using three independent command variables to control four pixel intensities. This introduces a degradation of the perceived images that we strive to minimize using the information redundancy in the images. In this paper, we first present the principle of operation and the optical modulation device. Then, an optimal solution for the computation of the command variables from the original stereo-pair pixel intensities is proposed and developed. The chosen mathematical criterion is the minimization of the quadratic error between the perceived and original pixel intensities. The study of the visual degradation introduced in the images by the proposed stereo-pair processing leads to the distinction of two kinds of phenomena: the mixing of the pixel intensities in the same image and the mixing of the pixel intensities between the left and right views. The simulations realized with reference images allow a quantitative evaluation of their relative proportions and of the final image quality.

Acousto-optic wide band optical low-frequency shifter

A wide bandwidth tunable optical low-frequency shifter is presented in this work. It is based on two acousto-optic devices operating in tandem. The relevant parameters of the specific configurations of acousto-optic interactions in paratellurite material are detailed. Results from numerical computations leading to the practical design parameters are given. The low-frequency shifter has been experimentally tested at a visible wavelength λ₀=514  nm and a high efficiency (>60%) has been measured. The tuning capability of the optical shifter covers a bandwidth Δf=26  MHz.

INDEPENDENT ACOUSTO-OPTIC SEPARATION OF THE TWO WAVELENGTHS OF A POLYCHROMATIC LIGHT BEAM

A special configuration based on two successive acoustooptic interactions in uniaxial crystals with two noncollinear acoustic waves is presented. It leads to two angularly split wavelengths from a multiline laser beam with independent acousto-optic efficiencies. General expressions of the angular deviation and the spectral bandwidth are derived. Since the application concerns the restitution of colored images, a novel combination of wavelengths using the blue primary at 458 nm is investigated for trichromatic color matching. Numerical computations have been drawn for paratellurite. A practical separator using the slow shear wave in paratellurite is designed, and experimental results for the independent separation of 514- and 458-nm wavelengths of an Ar laser are reported.

Acousto-optic couplings in a phoXonic crystal slab L1 cavity

Acousto-optic coupling mechanisms within simultaneous photonic and phononic crystal cavities are theoretically studied. The structure considered is a silicon slab drilled periodically with air holes arranged on a square lattice; the cavity is obtained by removing one hole from the ideal crystal structure. Two acousto-optic coupling mechanisms are taken into account: the classical photo-elastic effect and the opto-mechanic effect which arises from the moving boundaries. The coupling mechanisms are computed according to the finite element method. The results reveal that the mechanisms can sustain each other or act with counteracted effects.

Acousto-optic multifrequency modulation: linearization of the diffraction efficiencies

The global efficiency of multifrequency acousto-optic interaction is limited mainly by cross-modulation and multiple rediffraction effects. A study of the linearization of the N monochromatic light beams diffracted by N acoustic carriers is reported. It has been found that the introduction of another acoustic carrier allows the global acousto-optic efficiency to remain at the same level. The authors show that a suitable anisotropic interaction can greatly reduce the multiple rediffraction effects. The experimental results presented for a bifrequency paratellurite acousto-optic modulator, with a global efficiency up to 80%, agree well with the theoretical ones.