Raphaël Jamier

Simplified hollow-core photonic crystal fiber.

A simplified design inspired from kagomé-lattice fiber reduced to one layer of air-holes was proposed demonstrating the anti-resonant core guiding capability. Two large low-loss windows were measured (minimum attenuation <0.2dB/m) with acceptable infrared bend losses.

Generation and confinement of microwave gas-plasma in photonic dielectric microstructure.

We report on a self-guided microwave surface-wave induced generation of ~60 μm diameter and 6 cm-long column of argon-plasma confined in the core of a hollow-core photonic crystal fiber. At gas pressure of 1 mbar, the micro-confined plasma exhibits a stable transverse profile with a maximum gas-temperature as high as 1300 ± 200 K, and a wall-temperature as low as 500 K, and an electron density level of 10¹⁴ cm⁻³. The fiber guided fluorescence emission presents strong Ar⁺ spectral lines in the visible and near UV. Theory shows that the observed combination of relatively low wall-temperature and high ionisation rate in this strongly confined configuration is due to an unprecedentedly wide electrostatic space-charge field and the subsequent ion acceleration dominance in the plasma-to-gas power transfer.

Low-loss singlemode large mode area all-silica photonic bandgap fiber.

We describe the design and characterization of solid core large mode area bandgap fibers exhibiting low propagation loss and low bend loss. The fibers have been prepared by modified chemical vapor deposition process. The bandgap guidance obtained thanks to a 3-bilayer periodic cladding is assisted by a very slight index step (5.10-4) in the solid core. The propagation loss reaches a few dB/km and is found to be close to material loss.

Inner cladding microstructuration based on symmetry reduction for improvement of singlemode robustness in VLMA fiber

Very large mode area, active optical fibers with a low high order mode content in the actively doped core region were designed by removing the inner cladding symmetry. The relevance of the numerical approach is demonstrated here by the investigation of a standard air-silica Large Pitch Fiber, used as a reference. A detailed study of all-solid structures is also performed. Finally, we propose new kinds of geometry for 50 μm core, all-solid microstructured fibers enabling a robust singlemode laser emission from 400 nm to 2200 nm.

Highly dispersive large mode area photonic bandgap fiber.

An all-silica photonic bandgap fiber composed of a low-index core surrounded by alternating high- and low-index rings allows us to achieve a large mode area (500 microm(2)) and large chromatic dispersion. Sharp resonances from the even Bragg mode to odd ring modes theoretically lead to 20,000 ps/(nm km) chromatic dispersion when large bends are applied. By nature, sharp resonances are sensitive to inhomogeneities along the fiber length. Under experimental conditions, the resonances are broadened and the dispersion coefficient is decreased to 1000 ps/(nm km). However, to the best of our knowledge, this is the largest dispersion coefficient reported using a large mode area fiber.

Effect of polymer coating on leakage losses in Bragg fibers

It is found that the reflection of leaky radiation from the interface between the outer silica cladding and the coating polymer greatly modifies the loss spectrum of Bragg fibers. A simple model that describes this effect is proposed and confirmed by measurement and computation.

Highly birefringent photonic bandgap Bragg fiber loop mirror for simultaneous measurement of strain and temperature

A highly birefringent photonic bandgap Bragg fiber loop mirror configuration for simultaneous measurement of strain and temperature is proposed. The group birefringence and the sharp loss peaks are observable in the spectral response. Because the sensing head presents different sensitivities for strain and temperature measurands, these physical parameters can be discriminated by using the matrix method. It should be noted that this Bragg fiber presents high sensitivity to temperature, of ∼5.75 nm/°C, due to the group birefringence variation. The rms deviations obtained are ±19.32 με and ±0.5 °C, for strain and temperature measurements, respectively.

Analysis of the Modal Content Into Large-Mode-Area Photonic Crystal Fibers Under Heat Load

Yb-doped double-cladding photonic crystal fibers are one of the key enabling factors for the development of high-power fiber lasers, due to their capability to provide a very large-mode-area together with the effective suppression of high-order modes, while allowing strong pump absorption and efficient conversion. Thermal effects are currently considered as the main bottleneck for future power scaling; since beyond a certain average power, they allow guidance of high order modes and energy transfer to them, causing a sudden degradation of the beam quality. In this paper, the effects of the heat load on the modes of double cladding fibers are thoroughly analyzed with a full-vector modal solver based on the finite-element method with integrated steady-state heat equation solver. Fibers with different inner cladding designs are compared to provide a deeper understanding of the mechanisms beyond the mode reconfinement and coupling. The influence of the fiber design on the robustness of the single-mode regime with respect to fiber heating has been demonstrated, providing a clear picture of the complex interaction between modes. On the basis of simulation results it has been possible to group fiber modes into three families characterized by peculiar reaction to heating.

Experimental and Numerical Characterization of a Hybrid Fabry-Pérot Cavity for Temperature Sensing

A hybrid Fabry-Pérot cavity sensing head based on a four-bridge microstructured fiber is characterized for temperature sensing. The characterization of this cavity is performed numerically and experimentally in the L-band. The sensing head output signal presents a linear variation with temperature changes, showing a sensitivity of 12.5 pm/°C. Moreover, this Fabry-Pérot cavity exhibits good sensitivity to polarization changes and high stability over time.

Reduction of bend loss in large-mode-area bragg fibres

The delivery or generation of high power in optical fibre requires the increase of the core size to increase the threshold of nonlinear effects and the damage threshold. However the bend loss strongly limits the increase of the effective area (Aeff). All-solid photonic bandgap fibres are attractive for the delivery of power since they can be made singlemode whatever the core diameter is. Moreover the silica core can be doped with rare-earth ions. A Bragg fibre is a bandgap fibre composed of a low index core surrounded by N concentric layers of high and low index. We have fabricated Large Mode Area Bragg fibres by the MCVD process. These Bragg fibres present a ratio Aeff/λ2 close to 500. A first Bragg fibre, defined by N = 3 and an index contrast between the cladding layers Δn = 0.01, exhibits a measured critical bend radius Rc close to 16 cm (bend loss equal to 3 dB/m). Increasing the index contrast Δn leads to a tighter field confinement. The field distribution of the guided mode strongly decays in the periodic cladding and is thus less sensitive to bending. We propose here the design of an improved Bragg fibre with a very large index contrast Δn = 0.035 which leads to a dramatic reduction of the bend loss. The critical bend radius was measured to be lower than 3 cm. This fibre is less bend sensitive than an equivalent solid core fibre, either a step-index fibre or a photonic crystal fibre.