Electrostriction and guidance of sound by light in optical fibers

We investigate the generation of phonon wavepackets in optical fibers via electrostriction from coherent optical waves. Solving the elastodynamic equation subject to the electrostrictive force, we are able to reproduce experimental spectra found in standard and photonic crystal fibers. We discuss the two important practical cases of forward interaction, dominated by elastic resonances of the fiber, and backward interaction, for which an efficient mechanism of phonon guidance by light is found. The latter result describes the formation of the coherent phonon wavepacket involved in stimulated Brillouin scattering

Generation of phonons from electrostriction in small-core optical waveguides

International audienceWe investigate the generation of acoustic phonons from electrostriction of optical waves in small core waveguides. We specifically consider simple step-index strip waveguides composed of silica or silicon in air, with sub-micron lateral dimensions. Such waveguides support one or a few optical modes, but a rich spectrum of acoustic phonons that becomes densely populated as the phonon frequency increases. We evaluate rigorously the phonon energy density that results from the electrostriction of two frequency detuned guided optical waves, that are either co- or contra-propagating, including phonon loss. Plotting this energy density as a function of frequency detuning reveals the phonon wave packets that are electrostrictively active and gives a quantitative estimation of the energy transfer from optical waves to particular phonons. Furthermore, in the backward interaction geometry, the dispersion relation of such phonons can be accessed directly by varying the optical wavelength

Subwavelength-diameter optical fibers for Brillouin scattering

International audienceBy confining light into subwavelength-diameter tapered optical fibers, we demonstrated a new type of Brillouin light scattering from surface and hybrid acoustic waves. This could be used to develop new optical sensors that are innovative and highly sensitive

Multimode Brillouin spectrum in a long tapered birefringent photonic crystal fiber

International audienceWe investigate the stimulated Brillouin scattering (SBS) in a long tapered birefringent solid-core photonic crystal fiber (PCF) and compare our results with a similar but untapered PCF. It is shown that the taper generates a broadband and multi-peaked Brillouin spectrum, while significantly increasing the threshold power. Furthermore, we observe that the strong fiber birefringence gives rise to a frequency shift of the Brillouin spectrum which increases along the fiber. Numerical simulations are also presented to account for the taper effect and on the birefringence. Our findings open a new mean to control or inhibit the SBS by tapering photonic crystal fibers

Analytical modeling of the gas-filling dynamics in photonic crystal fibers

We present useful expressions predicting the filling time of gaseous species inside photonic crystal fibers. Based on the theory of diffusion, this gas-filling model can be applied to any given fiber geometry or length by calculating diffusion coefficients. This was experimentally validated by monitoring the filling process of acetylene gas in several fiber samples of various geometries and lengths. The measured filling times agree well, within ±15%, with the predicted values for all fiber samples. In addition, the pressure dependence of the diffusion coefficient was experimentally verified by filling a given fiber sample with acetylene gas at various pressures. Finally, optimized conditions for gas–light interaction are determined by considering the gas flow dynamics in the design of microstructured fibers for gas detection and all-fiber gas cell applications

Suspended-core fibres as optical gas sensing cells: study and implementation

We have thoroughly studied and modelled many important aspects for the realization of gas-light interactions in suspended-core fibres. The fraction of the optical field propagating in holes could be calculated from the fibre geometry to predict the total absorption for a given molecular absorption line and fibre length. In addition, the gas diffusion into the fibre holes could be modelled to precisely anticipate the filling time for a given fibre geometry and length. This was experimentally validated by preparing several samples of suspended-core fibres showing various lengths. These samples were filled with acetylene at low pressure (< 50 mbar) and were hermetically and permanently sealed by fusion splicing each fibre end to a plain single-mode silica fibre. The adequacy between the modelling and the experimental results turned out to be excellent. Several physical parameters essential for the fibre characterization could be extracted from a set of measurements, sketching a specific metrological approach dedicated to this type of fibre. Finally, applications and advanced experiments that can be specifically carried out using these fibres are discussed