Alain Maruani

Holographie conoscopique. Reconstructions numériques

RésuméL’holographie conoscopique est une nouvelle méthode d’’enregistrement d’hologrammes en lumière incohérente compatible avec les systèmes de numérisation et de traitement d’image. Les auteurs présentent le principe de base de Vholographie conoscopique, puis décrivent les systèmes expérimentaux mis en œuvre et enfin ils étudient les possibilités de traitement numérique des hologrammes conoscopiques et, en particulier les algorithmes de reconstruction.AbstractConoscopic holography is a new method for forming holograms in coherent light which may be a solution to the design of a simple holographic system compatible with microcomputers and numerical processing. We present the basic principles of conoscopic holography and experimental setup of conoscopic cameras. The paper focuses on the possibilities of numerical processing of conoscopic holograms and, more specially, we describe algorithms for numerical reconstructions.

Frequency multiplexed raster neural networks. 1: Theory.

In the first part of this paper we present frequency multiplexed raster (FMR) optical implementation of neural networks. A hidden difficulty for hardware (optical and electronic) implementation is that the dimension of the synaptic matrix is twice that of the input and output matrices or vectors. For 2-D images, which is we believe one of the greatest potentialities of neural networks, the synaptic matrix is 4-D and cannot be directly implemented in optics. We propose FMR as a method to fold this matrix into a 2-D format. In the second part of this paper we describe the system built in our laboratory showing the feasibility of FMR optical neural networks. The system is built from an optical input module, a fixed synaptic matrix coded on a transparency, a CCD camera, and a microcomputer which performs the thresholding and feedback operations. In a later stage the fixed matrix will be replaced by a programmable matrix.

Band-edge-induced Bragg diffraction in two-dimensional photonic crystals

Two-dimensional photonic crystals composed of two orthogonal volume diffraction gratings have been photogenerated in photopolymers. When the read beam is set at the Bragg angle, the diffraction efficiency of the transmission grating is strongly enhanced at the band edge of the reflection grating recorded in the material. Such a device provides Bragg operation and enhancement of the diffraction efficiency of the thin diffraction grating together with good wavelength selectivity. Such advantages could be interesting for optical signal processing.

Intracavity gain gratings.

Intracavity gain gratings are theoretically demonstrated to exhibit diffraction efficiencies that are 100 times larger than unity at pump powers substantially below the lasing threshold. Experiments performed using a Nd:YVO4 microlaser pumped below threshold by two interfering Ti:sapphire laser beams are described. Huge enhancement of the diffraction efficiency (5000X) and a large increase of the angular selectivity (10X) are demonstrated despite the angular reduction of the Fabry-Perot cavity finesse. Much better results are expected using gain gratings with larger areas or thinner cavities such as vertical cavity surface-emitting lasers. Such large fan-out values could be very interesting for applications to optical signal processing.

Uniform response high resolution tunable optical filtering using a grating-assisted acousto-optic device

A new filtering technique is proposed that associates the high dispersion of standard reflection gratings with the tuning speed of acousto-optic cells. Tuning is performed by adjusting the grating period so that the chosen wavelength is at Bragg resonance. In this way, the selected wavelength always experiences a maximum diffraction efficiency, ensuring good uniformity. Using a commercial acousto-optic modulator, a wavelength selectivity of 0.075 nm is demonstrated together with a uniformity of +/-0.2 dB on a tuning range of 2.2 nm corresponding to N = 30 resolvable frequencies. N > 600 could be obtained with an acousto-optic cell specially designed for this device.

Critical Properties of the 2D Ising Model: A Markovian Approach

We develop here an application of Markov Random Fields to the understanding and estimation of 2D Ising model properties with nearest-neighbor square-lattice and zero magnetic field. Using gauge invariance yields easily to the well-known phase transition properties of the model and to an analytical estimation of the spontaneous magnetization below phase transition. The estimated critical temperature derived from this model is more accurate than that of Mean Field Theory.

Gain grating in a Nd:YVO 4 microlaser

Intracavity gain gratings are theoretically demonstrated to exhibit diffraction efficiencies larger than unity at pumping powers below lasing threshold. Experiments performed using a Nd- YVO4 microlaser demonstrate huge enhancements of diffraction efficiency and angular selectivity.

Numerical analysis of a high-resolution fast tunable filter based on an intracavity Bragg grating

A fast tunable filtering technique is proposed associating a diffraction grating with an intracavity Bragg grating. The bandwidth and the tuning range of this filter can be easily adapted by changing the diffraction gratings orientation, or its period, and its response is uniform over the whole tuning range. A numerical simulation of the filter response to a Gaussian beam has been developed, and it fits the experimental results allowing a calculation of the performances that could be obtained with more specific elements. For example, using a commercial acousto-optic deflector would allow a separation of 500 frequencies. It would then be possible to have a tuning range of 100 nm with a bandwidth of 0.2 nm for optical telecommunications.

A fast tunable high resolution filter

We demonstrated experimentally the feasibility of a fast tuning high resolution filter with a good flatness. The device could easily be adapted to applications in spectroscopy or in DWDM communications (tunable lasers or filters for instance). Acousto-optic filtering has been extensively studied because its tuning rapidity makes it very useful for many applications. Nevertheless, its resolution is often not sufficient in the fields of telecommunication or spectrometry. The proposed grating-assisted acousto-optical filtering technique combines the high resolution of diffraction gratings (/spl sim/10 pm) and the tuning swiftness of acousto-optics (/spl mu/s).

Bragg thickness criterion for intracavity diffraction gratings

Summary form only given. The Fabry-Perot resonator enables a very sharp attenuation of diffraction order versus length especially for high finesse cavities. The Q/sub FP/ parameter defined here can be useful to find the best thickness/diffraction efficiency compromise in the design of holographic devices.