Showing papers by "José A. Ferrari published in 2008"

Effect of size polydispersity in polymer-dispersed liquid-crystal films

Abstract: The electro-optical effect of size polydispersity in polymer-dispersed liquid-crystal (PDLC) films is investigated. Liquid crystal (LC) droplets of different sizes require different electric field amplitudes for director reorientation, which will produce a mismatch of effective refractive indices. This, in turn, will produce an increase in interdroplet scattering (i.e., decrease in the direct transmitted light) within a specific range of electric field amplitudes. We propose a phenomenological “two-size” model describing this enhanced “pseudo-off-state” achieved under application of an electrical field and present validation experiments. Our approach may be used to get more insight into the transmittance behavior of PDLCs affecting the contrast ratio.

Verdet constant dispersion measurement using polarization-stepping techniques.

Abstract: We present a novel method for measuring the Verdet constant dispersion. The proposed method involves spectral polarimetric measurements using three (or more) polarization steps. The procedure has formal similarity with the phase-shifting interferometry (PSI). Thus the Verdet constant in the desired spectral range can be retrieved using well-known PSI algorithms. Validation experiments are presented.

Magnetic Crosstalk Minimization in Optical Current Sensors

Abstract: We demonstrate that the magnetic crosstalk-induced errors associated with point-type optical (Faraday) current sensors working in three-phase electric systems may be minimized by properly designing the geometry of the system. We propose a novel architecture in which the optical path inside the magnetooptical material is parallel to the direction of the neighboring conductors, except in a small loop around the sensor head. This renders the crosstalk effect at least one order of magnitude smaller than in conventional Faraday sensor architectures.

Two-state model of light induced activation and thermal bleaching of photochromic glasses: theory and experiments

Abstract: The behavior of photochromic glasses during activation and bleaching is investigated. A two-state phenomenological model describing light-induced activation (darkening) and thermal bleaching is presented. The proposed model is based on first-order kinetics. We demonstrate that the time behavior in the activation process (acting simultaneously with the thermal fading) can be characterized by two relaxation times that depend on the intensity of the activating light. These characteristic times are lower than the decay times of the pure thermal bleaching process. We study the temporal evolution of the glass optical density and its dependence on the activating intensity. We also present a series of activation and bleaching experiments that validate the proposed model. Our approach may be used to gain more insight into the transmittance behavior of photosensitive glasses, which could be potentially relevant in a broad range of applications, e.g., real-time holography and reconfigurable optical memories.

Faraday current sensor using space-variant analyzers

Abstract: A Faraday current sensor using spatially inhomogeneous analyzers is presented. In the proposed optical architecture, the light waves coming from the sensor head travel in free space reaching a space-variant analyzer, and then they are acquired by a (one-or two-dimensional) photodetector array. This allows a full determination of the Faraday rotation induced by the electrical current, even in the case of a nonideal optical system (e.g., analyzer with poor polarization ratio). The proposed method resembles well-known techniques of phase-shifting interferometry. Validation experiments using linearly variant and azimuthal (radial) analyzers are presented and different detection schemes are compared.

Two-state model of light induced activation and thermal bleaching of photochromic glasses: erratum

Abstract: We correct some errors related to the symbols used in our recent paper [Appl. Opt.47, 3669-3673 (2008)APOPAI0003-693510.1364/AO.47.003669].