Jean Lapierre

Analyse d’un coupleur bidirectionnel à fibres optiques monomodes fusionnées

The analysis of fused biconical single-mode fiber couplers shows that, as opposed to other types of single-mode fiber coupler, the coupling mechanism is not associated with evanescent waves and that the infinite cladding approximation is not valid. The two original claddings must be considered, after fusion and stretching, to form a single guide whose asymmetric excitation gives rise to the well-known beat phenomenon between two normal modes of the guide. For calculation purposes, this asymmetrically excited single guide is modeled into a pair of equivalent circular guides tangent to each other and having the same longitudinal profile as the real structure. The experimental results are compared with the theory.

Investigation of power oscillations along tapered monomode fibers.

An abruptly tapered fiber can be essentially divided into three different regions: one beating region in its central, quasi-cylindrical part, lying between two coupling regions, where the slopes are large. In this paper we report two experiments which help to visualize and analyze the beating region. We show that, in this region, there are principally two modes present, HE(11) and HE(12), both excited with large amplitudes.

Proprietes de polarisation des reflecteurs de Bragg induits par photosensibilite dans les fibres optiques monomodes.

Reflecting Bragg filters having some polarizing properties can be photoinduced in monomode optical fibers exposed to a linearly polarized laser beam. Their reflectivity does indeed reach a maximum when the polarization direction of a reading beam is parallel to the polarization direction of the writing beam. This behavior is independent of the relative angle between the polarization direction of the writing beam and the birefringence axes of the fiber. These polarizing properties disappear for long exposure times of the fiber to the writing beam. From these results we conclude that the photosensitization process is not uniform over a cross section of the fiber; and periodic modification of the refractive index is anisotropic and therefore a small photoinduced birefringence exists. We have verified that effect; this result gives more weight to the hypothesis that the photoinduced filters are essentially phase gratings.

Liquid-crystal-clad tapered fibers

The capability of changing the optical output of a tapered single-mode fiber by using a liquid crystal as an electrelectrooptical medium is shown.

Erratum: Photosensitivity effect in optical fibers: A model for the growth of an interference filter

A photosensitive property of short‐length single‐mode optical fibers has been shown to produce a periodical modification of the refractive index of the core when exposed to high‐intensity laser light. Using a model derived from the coupled‐mode theory, we calculate the space‐time variation of the interaction between two waves traveling in opposing directions that are responsible for the formation of an interference filter along the axis of the fiber. Our proposed model for the formation and growth of the filter is in good agreement with experiments; a characteristic photosensitivity coefficient is deduced.

Optical fiber probe to measure local void fraction profiles.

A fiber optic probe for measuring the local void fraction in a two-phase (air-water) flow has been developed. The liquid-gas and gas-liquid interfaces are detected by the normal reflections of IR laser light at the tip of a multimode fiber. The probe and its associated electronic circuitry were calibrated by using the averaged void fraction measured by a quick closing valve system. The void profiles obtained by this method were compared with those obtained by a conductivity local probe. The proposed optical system shows a better sensitivity at high void fractions.

Nematic liquid crystal clad tapered optical fiber with temperature sensing properties

We discuss the thermal and optical properties of the lyotropic liquid crystal decylammonium chloride/water/NH4Cl (DACl‐LLC) from −40 to 80 °C. Combined use of DACl‐LLC and highly tapered monomode optical fibers allows sensing of temperature (from room temperature up to 65 °C).

Infrared holographic interferometry

Infrared (10.6-μm) dynamic holograms are obtained on a recording medium consisting of a thin oil film on a glass substrate. Reconstruction with visible light (0.63 μm) permits copying the initial hologram on a photographic plate. Any subsequent displacement of the object modifies the corresponding reconstructed wave at the recording medium, thereby creating a characteristic visible interferogram. Analysis of the double image in copy holograms together with fringe localization and visibility did not show stringent setup conditions. Experimental results are well related to various objects and displacements while the long wavelength (≃ 10 μm) used offers the advantage of large displacements or wavefronts deformation measurability and considerably decreases the setup sensitivity to vibrations.

Fiber-optic integrated interference filters

The fabrication of interference filters in optical fibers is reported with new results indicating the possibility of growing several filters in the same fiber, together with the existence of an apparent saturation level that depends on the coupled power level.

Mesure de la respose en frequence de filtres de bragg dans une fibre optique par balayage thermique

Abstract We describe a method for the spectral analysis of Bragg filters in optical fibers based on the thermal expansion and index variation of the fiber. The method can only be equalled by the use of variable frequency lasers. A resolution of 30 MHz can easily be attained. The principle of the method is described together with the calibration procedure, and typical results are presented.