Gérald Roosen

Volume hologram multiplexing using a deterministic phase encoding method

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.

Stimulated Raman scattering in an ethanol core microstructured optical fiber

We show that high efficiency stimulated Raman scattering can be obtained using hollow core photonic crystal fiber with the core filled with a low refractive index nonlinear liquid. This new architecture opens new perspectives in the development of nonlinear functions as any kind of nonlinear liquid media can now be used to implement them, with original properties not accessible with silica core fibers.

Detection of ultrasonic motion of a scattering surface by photorefractive InP:Fe under an applied dc field

The characteristics of an interferometric system based on two-wave mixing at 1.06 µm in photorefractive InP:Fe under an applied field for the detection of ultrasonic motion of a scattering surface are described. A theoretical analysis of possible configurations for the detection of small phase modulation in the undepleted-pump approximation is presented. Experimental assessment of the device for both cw and pulse regimes is performed: The sensitivity, the etendue, the response time, and the behavior under ambient vibrations or moving inspected samples are provided. This adaptive device presents many features appropriate for industrial inspection and compares advantageously with the passive confocal Fabry–Perot device that is now widely used.

Photorefractive detection of tagged photons in ultrasound modulated optical tomography of thick biological tissues

We present a new and simple method to obtain ultrasound modulated optical tomography images in thick biological tissues with the use of a photorefractive crystal. The technique offers the advantage of spatially adapting the output speckle wavefront by analysing the signal diffracted by the interference pattern between this output field and a reference beam, recorded inside the photorefractive crystal. Averaging out due to random phases of the speckle grains vanishes, and we can use a fast single photodetector to measure the ultrasound modulated optical contrast. This technique offers a promising way to make direct measurements within the decorrelation time scale of living tissues.

Potentialities and limitations of hologram multiplexing by using the phase-encoding technique.

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.

Transmission of time modulated optical signals through an absorbing photorefractive crystal

Abstract We solve the problem of the transmission of time modulated signals in a two-beam coupling photorefractive set-up using an absorbing material. We obtain an analytical expression that describes this phenomenon in absence of pump beam depletion due to beam coupling. We use this expression to analyse the influence of absorption on the photorefractive build-up. We also describe, using this model, the photorefractive beamsplitter response. It is also showed that the π 2 phase shift between illumination and index gratings is detrimental to a linear detection of the phase variation of the signal beam.

Light localization induced enhancement of third order nonlinearities in a GaAs photonic crystal waveguide

Nonlinear propagation experiments in GaAs photonic crystal waveguides (PCW) were performed, which exhibit a large enhancement of third order nonlinearities, due to light propagation in a slow mode regime, such as two-photon absorption (TPA), optical Kerr effect and refractive index changes due to free-carriers generated by TPA. A theoretical model has been established that shows a very good quantitative agreement with experimental data and demonstrates the important role that the group velocity plays. These observations give a strong insight into the use of PCWs for optical switching devices.

The TEM∗01 mode laser beam—A powerful tool for optical levitation of various types of spheres

Abstract We point out in this paper that a TEM∗01 mode laser beam is convenient to stably levitate either solid or hollow dielectric spheres or metallic ones. Except a sufficient incident flux, there is no stability condition required to levitate solid transparent spheres having a refractive index larger than the one of the surrounding medium. According to the beam and sphere diameters, the equilibrium position will be centered on the beam axis or off-axis. In the case of hollow dielectric or metallic spheres, the lateral stability requires specific conditions on sphere and beam diameters. These conditions are discussed and then experimentally shown by performing optical levitation of these various kinds of spheres either in vertical or in horizontal beams.

Observation and analysis of the fast photorefractive process in BSO

Abstract The dynamic behaviour of both diffracted beams in a two-wave mixing configuration, and induced photocurrent are investigated with nanosecond light excitation. The photorefractive Bi 12 SiO 20 (BSO) responds in the nanosecond time scale. The presence of thermally excited traps just under the conduction band must be considered. Attention should be paid to values for mobilities, lifetimes, etc. which characterize the material and could appear different from the values measured in experiments made with low power continuous laser beams.

Growth, spectroscopic and photorefractive investigation of vanadium-doped cadmium telluride

We present new results on the growth of semi-insulating vanadium-doped cadmium telluride crystals and their characterization by different optical techniques such as photoinduced current transient spectroscopy, absorption, photoconductivity spectra, and photorefractive wave mixing. Our joint research program aims at developing optimized crystals for efficient optical processing in the near infrared through the photorefractive effect.