Conception et analyse de micro-résonateurs optiques pour la génération de peignes de fréquences

Clément Arlotti 1
1 LAAS-PHOTO - Équipe Photonique
LAAS - Laboratoire d'analyse et d'architecture des systèmes [Toulouse]
Abstract : Whispering-gallery -mode micro-resonators, whether in the form of disks, rings or racetracks, have become the key building blocks of many high-performance photonic components. The embodiments exploiting the III-V semiconductors are particularly attractive for they open the possibility of integrating active and passive sections together and therefore diversify the functionalities on the same photonic chip. Furthermore, the vertical integration of the resonator above its access waveguide(s) makes it possible to distribute the active and passive functions on distinct planes and makes the realization of the components easier by using better-controlled methods. A fabrication technique recently introduced in the team and based on the AlGaAs / AlOx technological platform, allowed us to realize, by means of simple steps, vertically-coupled microdisks. The performance of these components, however, is limited due to their architecture, complicated by their constitutive multilayer stack. The research carried out during this PhD thesis focused on the feasibility of emitting an optical frequency comb from these resonators. For this purpose, the components must be designed so as to present a sufficiently high quality factor while maximizing the power circulating in the cavity in order to be able to trigger the non-linear processes required for the comb generation. For a transverse single-mode component, the intracavity power is maximal when the system operates in critical coupling regime, i.e .when the losses inside the cavity are equal to external losses (or coupling losses). As a first step, we have developed a semi-analytical tool based on a modal expansion in order to carry out a broadband parametric study of the performances of vertically coupled systems. Up to now, this coupling layout has indeed been little studied, both theoretically and practically. Our generic model, based on the coupled mode theory (CMT) and the universal relations governing the spectral properties of coupled micro-resonators, reveals two theoretical conditions for obtaining an achromatic critical-coupling regime when the cavity and its access waveguide are phase-mismatched. We first applied it to the simulation of single- mode racetrack resonators made of Si3N4 / SiO2 since several studies have already demonstrated comb generation using this technological platform. Our work resulted in the design of phase-mismatched and technologically feasible structures with critical-copuling bandwidths being increased by one order of magnitude compared to the reference case of phase- matched waveguides. We subsequently initiated a numerical evaluation of frequency comb generation, based on the iterative resolution of the non-linear Schrödinger equation taking into account the variations of the spectral and dispersive properties of these racetracks. The generic model has finally been applied to AlGaAs / AlOx microdisks. For this purpose, we have introduced a criterion allowing an unambiguous implementation of CMT in the case of asymmetric couplers having a multi-layer separation zone. The results, in good agreement with experimental data, allowed us to better understand the limitations of the fabricated devices and to propose new structures AlGaAs / AlOx with improved performances. The experimental validation of the proposed designs for both the Si3N4 / SiO2 and AlGaAs / AlOx components is currently in progress.
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Clément Arlotti. Conception et analyse de micro-résonateurs optiques pour la génération de peignes de fréquences. Optique [physics.optics]. Université Paul Sabatier - Toulouse III, 2017. Français. ⟨NNT : 2017TOU30248⟩. ⟨tel-01681181v2⟩

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