Suzanne Lacroix

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.

Tapered-fiber interferometric wavelength response: the achromatic fringe

In considering the wavelength-dependent transmission of tapered fibers we provide an observation of the fringe that shows the least response to variations in wavelength and that is, by definition, the achromatic fringe. This is a manifestation of the equalization of the HE11–HE12 group velocity occurring at V = 4.4. The fact that it was observed in a tapered single-mode fiber confers some particular features that are discussed and compared with those of other two-mode guides with regard to sensor applications.

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.

Circularly symmetric modal interferometers: Equalization wavelength and equivalent step determination for matched-cladding fibers

Abstract An LP01-LP02 modal interferometer fabricated from a standard matched-cladding telecommunication fiber is studied in the visible wavelength range. Its spectral response in the vicinity of the LP01/LP02 equalization wavelength is used to characterize the fiber by the determination of an equivalent step index profile.

Mesure de la birefringence lineaire des fibres optiques monomodes par une methode thermique.

The variation of internal stresses due to temperature changes in a monomode optical fiber induces a corresponding variation of the stress birefringence. This behavior is used to measure the degree of birefringence. Experimental results confirm the validity of the theoretical treatment.

Fibres unimodales effilées

RésuméL’aspect oscillatoire de la transmission des fibres optiques unimodales effilées est interprété. On décrit en outre les applications réalisées ou envisagées qui en découlent.AbstractOscillatory transmission of tapered single-mode fibres is explained. Several realized or proposed applications are also described.

Capteur de température interférométrique à fibre optique monomode

Abstract We describe the use of an optical fiber reflection two-wave interferometer as a temperature sensor. As it uses only one fiber this device is easy to set up. We calculate its sensitivity based on the temperature rate of change of the refractive index and length of the fiber, for the case of pure silica. The measured sensitivity, equal to 73 fringes/°C for a 1 m long fiber, is slightly higher than the theoretical value. This result is in agreement with the expected increase in the thermal expansion and thermo-optic coefficients of doped silica.

Réflecteur de Bragg induit par photosensibilité dans une fibre optique: modèle de croissance et réponse en fréquence

A theoretical model has been developed which describes the growth of a photo-induced Bragg filter in a single-mode optical fiber. This model is derived from the coupled wave theory and takes into account Fresnel reflections at both ends of the fiber. The frequency response of the filter is the result of the combined effect of both the distributed and localized reflections. It has been calculated at various instants of the growth process and for various phase conditions at the Fabry-Perot fiber interferometer.A theoretical model has been developed which describes the growth of a photo-induced Bragg filter in a single-mode optical fiber. This model is derived from the coupled wave theory and takes into account Fresnel reflections at both ends of the fiber. The frequency response of the filter is the result of the combined effect of both the distributed and localized reflections. It has been calculated at various instants of the growth process and for various phase conditions at the Fabry-Perot fiber interferometer.

Interféromètres modaux à symétrie circulate dans les fibres optiques

RésuméLa réponse spectrale d’une fibre quasi unimodale, dans le cas oùu les modes LP01 et LP02 sont mis en jeu, est étudiée en mettant en évidence les similitudes et les différences entre la transmission d’un tel interferometre bimodal et celle d’un interféromètre classique. Les applications envisagées sont, d’une part, la caractérisation des fibres (en particulier par la mesure d’une longueur d’onde d’égalisation) et, d’autre part, le développement de capteurs interférométriques tout fibre sans bras de référence.AbstractThe spectral response of a few-mode fiber in the case of LP01/LP02 excitation is studied showing the similarities and differences between the transmission of such a bimodal interferometer and that of a classical interferometer. The applications envisaged are, on the one hand, the characterization of fibers (in particular by measurement of an equalization wavelength), and on the other hand, the development of all-fiber interferometric sensors without the need of a separate reference arm.