Révision du processus de fabrication de coupleurs fusionnés à quelques fibres
2018
Ce memoire se concentre sur la theorie et les methodes de fabrication relatives a la fabrication
de coupleurs directionnels a plus de 2 fibres. Plus specifiquement, on s’interesse a faire la revision
du design de coupleurs doublement asymetriques dans le but de fabriquer des hybrides
optiques completement fibres. Ce coupleur est defini par sa symetrie transverse et sa symetrie
longitudinale. Sa coupe transverse, qu’on appelle structure ABAB, possede deux symetries
miroir, mais ne possede pas la symetrie de rotation par 90°. Sa coupe longitudinale est definie
par le fait qu’elle ne possede pas de symetrie de reflexion, contrairement aux coupleurs fibres
habituels. Sa structure qu’on qualifie de doublement asymetrique lui confere des proprietes
particulieres qui en simplifient la fabrication. Le coupleur doublement asymetrique est un
equipartiteur de puissance inconditionnel et une seule condition doit etre remplie lors de la
fabrication afin de garantir qu’il ait le comportement d’un hybride optique.
L’hybride optique doublement asymetrique possede une description mathematique simple
et elegante. Elle nous permet de definir une metrique experimentale, que nous appelons
l’integrant de l’hybride, permettant de determiner sans ambiguite le moment ou la fabrication
du composant doit etre arretee. Elle nous permet aussi d’ecrire sa dependance en longueur
d’onde de maniere simple et compacte.
La difficulte principale associee a la fabrication d’un coupleur a 3 ou 4 fibres est d’arriver
a fabriquer la preforme fusionnee qui precede l’etape d’etirage. Des essais de fabrication de
structures a 3 fibres ont montre qu’une technique utilisant des blocs de fusion specialises
permet d’obtenir un rendement en fabrication des preformes se situant entre 13 et 18%. Pour
augmenter ce rendement en fabrication, il sera necessaire de fabriquer des coupleurs plus
courts, qui permettront a leur tour de fabriquer des preformes fusionnees plus courtes.
Dans cette optique, nous avons demontre que des dispositifs faisant intervenir des fibres a
double gaine correctement parametrees nous permettaient de reduire radicalement la taille
de nos coupleurs fibres. Des essais d’effilages de fibre simple ont montre experimentalement
qu’il etait possible de reduire la taille d’un effilage adiabatique monomode d’un facteur 4,
et ce, en gardant les pertes d’effilages en dessous de 0,06 dB sur l’ensemble de la plage
λ∈ (630, 830) nm. Des calculs numeriques ont montre qu’il serait possible d’obtenir un
dispositif 20 fois plus court que son equivalent standard. Des calculs numeriques ont aussi
montre que des effilages simples de fibre a double gaine pourraient etre utilises pour effectuer
des transitions tres courtes a quelques modes. Ceci pourrait etre utilise pour fabriquer un
nouveau type de composant qu’on appelle De/multiplexeur de modes spatiaux.----------Abstract This thesis focuses on the theory and manufacturing methods for the fabrication of directional
fiber optic couplers with more than 2 fibers. More specifically, we are interested in reviewing
the design of doubly asymmetrical couplers in order to manufacture optical hybrids made
only from fiber optic. This coupler is defined by its transverse symmetry and its longitudinal
symmetry. Its transverse section, called ABAB structure, has two mirror symmetries, but
does not have rotation symmetry by 90°. Its longitudinal section is defined by the fact that
it does not possess a symmetry of reflection , unlike other fiber couplers. Its so-called doubly
asymmetrical structure gives it unique properties that simplify manufacturing. The doubly
asymmetrical coupler is an unconditional power splitter and only one condition must be met
during manufacture to ensure that it has the behavior of an optical hybrid.
Furthermore, the doubly asymmetrical optical hybrid has a simple and elegant mathematical
description. It allows to define an experimental metric, which we call the integrant of the
hybrid, that dictates when the tapering process must be stopped. Also, the doubly asymmetrical
optical hybrid has a wavelength dependence that can be written simply and has the
potential to be made achromatic.
The main difficulty associated with the manufacture of a coupler with 3 or 4 fibers is to
manufacture the fused preform. Production yields ranging between 13 and 18 % have been
achieved in essays intended to make 3 fibers fused preform. These essays have been conducted
using precisely made fusion blocks. To increase manufacturing yields even more, it is
necessary to reduce the length of the fused preform which can only be made by reducing the
total length of the coupler.
We minimize the length of our directional couplers by modifying the index profile of the
structure in order to reduce the coupling coefficient in between modes. This process is made
by studying the adiabatic power criterion of the structure. We have demonstrated that
by using properly parameterized double-clad fibers we were able to make radically shorter
adiabatic components. This applies to single fiber taper and directional couplers. We have
shown experimentally that single mode taper length can be reduced by a factor of 4, keeping
the excess losses below 0.06 dB in λ∈ (630, 830) nm. Numerical calculations have shown
that it would reduce the length of a single mode transition for a single fiber taper up to a
factor 20. Numerical calculations have also shown that multimode transition could also be
made shorter by using double clad fiber. In this case the single fiber taper is made using
a properly parametrized double clad fiber whose core contains many modes.
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