Yves Quiquempois

Photonic crystal fiber design by means of a genetic algorithm

A Genetic Algorithm (GA) is used to design photonic crystal fiber structures with user-defined chromatic dispersion properties. This GA is combined with a full vectorial finite element method in order to determine the effective index of propagation of the modes and then, the chromatic dispersion of structures generated by GA. This method proves to be a powerful tool for solving this inverse problem.

Modeling of the χ (2) susceptibility time-evolution in thermally poled fused silica

The dynamics of the second-order nonlinearity induced in a thermally poled Infrasil silica glass is experimentally and theoretically studied. 200 mum and 500 mum-thick samples have been poled for different durations varying from 1 minute to 100 minutes. After the poling process, the magnitude and the spatial distribution of the induced chi((2)) susceptibility have been characterized accurately with the layer peeling method. A two-charge carrier model with an electric field dependant charge injection is used to explain the experimental time-evolution of the chi((2)) profiles. A good agreement between experimental results and simulations is reported.

Improvements of solid-core photonic bandgap fibers by means of interstitial air holes

We report the fabrication, characterization and modeling of a solid-core photonic bandgap fiber with interstitial air holes between the cladding rods. The presence of these interstitial air holes leads to a great improvement of optical properties for this kind of fiber. Particularly, we demonstrate that confinement losses and bend sensitivity are substantially reduced. Our experimental results for this new solution are well supported by numerical results.

Time evolution of the second-order nonlinear distribution of poled Infrasil samples during annealing experiments

The spatial distribution of the second-order nonlinearity induced in thermally poled Infrasil silica samples is recorded after thermal annealing experiments. Two regimes have been studied: short and long poling durations. For short poling durations, the observations are in good agreement with a model where only one ion type recombines inside the depletion region. The nonlinear distribution and erasure observed for the other case are well explained by considering the addition of another positive-charged ion injected during the poling process. This second ion acts as a barrier during thermal annealing and reduces the mobility of the first one.

Suppression of discrete cladding mode resonances in fibre slanted Bragg gratings for gain equalisation

In a slanted Bragg grating, coupling between the fundamental guided mode and the counterpropagative cladding modes result in discrete resonances in the transmission spectrum. These resonances are a drawback when Slanted Bragg Gratings are used for gain flattening of fibres amplifiers. A new method based on a chemical etching of the cladding is proposed leading to an overlap of the resonances and a reduction of the amplitude of the modulation. This method can be applied for any value of photo induced modulation amplitude in the SBG.

Microbending behavior of large-effective-area Bragg fibers

We use a phase-sensitive optical low-coherence interferometry technique and camera imaging to identify microbend-induced mode couplings in Bragg fibers. Results show the importance of the quality of fabrication of Bragg fibers on their modal content when they are subject to microbends. Design strategies to improve the microbending robustness are identified.

Discrete focusing in an optical fiber with a two-dimensional square array of coupled waveguides

We report the experimental demonstration of discrete focusing inside a 60-cm-long optical fiber made of a 2D square array of coupled waveguides. The suitable input amplitude and phase distributions are imposed by using a spatial light modulator. Thus we demonstrate that focusing in a single core at the output by discrete propagation is possible despite some amount of transverse heterogeneities of the waveguide array.

Using advanced S2 analysis to measure mode coupling in a 2-LP-mode fiber

Mode coupling in a step-index 2-LP-mode fiber has been experimentally studied using advanced S2 analysis. A wire-mesh is used to locally induce micro-bending and it is demonstrated that coupling coefficients can be extracted.

Leakage loss analytical formulas for large-core low-refractive-index-contrast Bragg fibers

We propose analytical formulas for large-core low-contrast Bragg fibers that predict the leakage loss of their low-order core modes, whatever their polarizations are and whatever the number of rings in the cladding is, for the first time to our knowledge. We propose also a generalized analytical formulas, which encompasses the case of Bragg fibers with a nonperiodic cladding, by decomposing their cladding into its constitutive periodic elements, whose contributions are taken into account through a simple multiplication factor. These formulas are developed thanks to a perturbation approach and to the use of the asymptotic formulation of the field in the cladding and provide a good accuracy except in the vicinity of couplings between core modes and cladding modes.

Bending behaviors of all-solid silica large mode area Bragg fibers

Fibers used for high power delivery are designed to ensure single-mode operation (in order to guarantee good output beam quality), large effective areas (Aeff) and resistance to bend-induced distortions (in order to avoid non-linear effects). For simple step index fibers, the maximum Aeff of the fundamental mode that can practically be achieved at 1.06μm is ~350μm2. All-solid-silica Bragg fibers with large cores were proposed as an alternative solution for high power delivery through their fundamental core mode. These fibers consist of a low-refractive index core surrounded by a multilayer cladding that acts as a Bragg mirror. The loss spectrum of such fibers consists of a concatenation of several transmission windows separated by high-loss peaks. Here, we simultaneously study, for the first time (at our knowledge), the bending impact on Bragg fibers for the three critical properties required for high power delivery: large Aeff, single-mode propagation and low bend losses for the fundamental mode. Thanks to their specific guiding mechanism, Aeff as large as ~500μm2 at 1.06μm can be achieved in Bragg fibers, while maintaining single-mode operation and bend losses lower than 0.1dB/m. Our numerical results are validated by experimental measurements on a PCVD Bragg fiber with a 40μm diameter core.