Sylvie Lebrun

Brillouin light scattering from surface acoustic waves in a subwavelength-diameter optical fibre

Brillouin scattering in optical fibres is a fundamental interaction between light and sound with important implications ranging from optical sensors to slow and fast light. In usual optical fibres, light both excites and feels shear and longitudinal bulk elastic waves, giving rise to forward-guided acoustic wave Brillouin scattering and backward-stimulated Brillouin scattering. In a subwavelength-diameter optical fibre, the situation changes dramatically, as we here report with the first experimental observation of Brillouin light scattering from surface acoustic waves. These Rayleigh-type surface waves travel the wire surface at a specific velocity of 3,400 m s−1 and backscatter the light with a Doppler shift of about 6 GHz. As these acoustic resonances are sensitive to surface defects or features, surface acoustic wave Brillouin scattering opens new opportunities for various sensing applications, but also in other domains such as microwave photonics and nonlinear plasmonics.

Characterization of self-phase modulation in liquid filled hollow core photonic bandgap fibers

We experimentally and theoretically study self-phase modulation by Kerr effect in a liquid filled hollow core photonic crystal fiber. We perform a complete characterization of the linear optical properties of the hollow core photonic bandgap fiber filled with deuterated acetone to determine all the characteristics of the propagation mode. The nonlinear coefficient of the fiber is determined by fitting the output spectra broadened by self-phase modulation with a new analytical expression giving the spectra of a hyperbolic secant pulse transmitted through a Kerr medium. The experiment allows a precise determination of the nonlinear index change nI2 of acetone-d6 equal to (1.15±0.17)×10−19 m2 W−1.

Stimulated Raman scattering in the evanescent field of liquid immersed tapered nanofibers

We present the first experimental demonstrations of stimulated Raman scattering in a liquid probed by the evanescent field of a tapered silica nanofiber. Raman scattering by a pure liquid, ethanol, or mixture of liquids, toluene diluted in ethanol, is investigated. Given the large choice of available materials for the medium surrounding the nanofiber, these demonstrations pave the way to the exploration of a new class of experiments and devices.

OPTICAL CHARACTERIZATIONS OF A RAMAN GENERATOR BASED ON A HOLLOW CORE PHOTONIC CRYSTAL FIBER FILLED WITH A LIQUID

We present a Raman generator based on a 35 cm long hollow core photonic crystal fiber entirely filled with ethanol emitting on a nearly-Gaussian beam. The power conversion efficiency from the pump at 532 nm to the first Stokes order of ethanol at 630 nm is 67%. Temporal profiles, mode patterns and power stability are also analyzed.

Efficient stimulated Raman scattering in hybrid liquid-silica fibers for wavelength conversion

Wavelength Raman converters have been developed for years to provide an elegant solution to easily shift the wavelength of existing lasers. In the pulse regime, due to relatively low Raman gains, these converters are usually limited to high-energy pulses, typically a few J or a few mJ in the nanosecond or picosecond regime. In order to build efficient Raman converters with lower energy pulses, we have developed a new class of fiber wavelength shifters based on Stimulated Raman Scattering in the liquid filling the hollow core of photonic bandgap fibers or Kagome fibers. The liquid choice, the design of the photonic crystal microstructure, the fiber length and its diameter give us enough degrees of freedom to realize efficient and versatile shifters, each being optimized for a specific wavelength shift. Connecting such a fiber device to a fixed wavelength laser allows delivering a new wavelength. With the same laser, another wavelength can be obtained by connecting another shifter. Using microlasers delivering 532 nm sub-nanosecond pulses of about 1 μJ, we already built a full series of shifters to reach any wavelength among: 556 nm; 561 nm; 582 nm; 595 nm; 612 nm; 630 nm; 650 nm; 667 nm; 772 nm. Hereafter, we detail how we design and optimize these new devices.

Observation of surface acoustic wave Brillouin scattering in optical microfibers

We report the observation of surface-acoustic-wave Brillouin scattering (SAWBS) in a subwavelength-diameter fiber and show that this effect relies on the generation by the electrostrictive force of phonons confined at the surface of the microwire.

Diameter and tensile strain measurements of optical nanofibers using Brillouin reflectometry

We demonstrate a simple and efficient technique that allows for a complete characterization of silica-based tapered optical fibers with sub-wavelength diameters ranging from 0.5 μm to 1.2 μm. The technique is based on Brillouin reflectometry using a single-ended heterodyne detection. It has a high precision sensitivity down to 1% owing to the strong dependence of the Brillouin spectrum on the taper diameter. We further investigate the tensile strain dependence of the Brillouin spectrum for an optical microfiber up to 5% of elongation. The results show strong dependences of several Brillouin resonances with different strain coefficients ranging from 290 MHz/% to 410 MHz/% with a specific nonlinear deviation at high strain. Those results therefore show that optical micro and nanofibers could find potential application for sensitive strain optical sensing.

Reduction of Brillouin scattering for the optimization of liquid-core Raman wavelength converters

Raman wavelength converters based on hollow core microstructured fibres filled with gases or liquids have been widely studied in the last few decades [1, 2]. In these converters the wavelength of a laser pump beam is red-shifted by stimulated Raman scattering in the material filling the core of the fibre, giving rise to new discrete wavelengths that can be used in many domains of applications (biophotonics, environment …). The possibility to strongly confine the pump light over long distances is an asset to get a high conversion efficiency. However if no particular care is taken unwanted non linear effects can occur at the expense of the stimulated forward Raman scattering. These effects are the Raman and Brillouin back-scatterings. In this work we are interested in the optimization of liquid-core Raman converters operating with pulses of a few nanoseconds or less. In these temporal regimes competition between the abovementioned effects are known to be very strong. We validate here a solution to minimize Raman and Brillouin back-scatterings by an appropriate choice of the fibre length. The effective length of interaction Leff in the backward direction is estimated to be cΔt / 2n, where At is the pulse width duration and n the refractive index. In the forward direction, as the dispersion is negligible, this effective length is the length of the fibre L. In liquids, as backward and forward Raman gains are nearly the same, L has just to be slightly higher than Leff to avoid Raman back-scattering. In most liquids and for long pulses Brillouin gain is much higher than Raman gain (typically one order of magnitude). To decrease Brillouin scattering L has to be subsequently higher than Leff. We demonstrate experimentally these trends by building two Raman converters differing only by their lengths. Our initial aim was to realize a Raman converter emitting in the near IR range. The pump source is a microlaser at 532 nm delivering 900 ps pulses at a repetition rate of 4.5 kHz. We choose ethanol as the Raman liquid. When pumped at 532 nm, the first Stokes order of ethanol is at 630 nm and the second Stokes order is at 772 nm, i.e the near IR wavelength we want to generate. A Kagome fibre was used to realize the two converters. When empty this fibre guides in the near IR range over a 1 μm wide transmission band. When filled with ethanol this transmission band is blue-shifted and then covers the pump and the Stokes wavelengths. In our configuration Leff=0.1 m. The lengths of the two converters are 0.5 m and 1.5 m. The results are shown in figure 1. As expected we observed no Raman backscattering in both cases and Brillouin effect is strongly attenuated for the longer fibre. For instance at the critical point (when the output Raman Stokes energy equals the transmitted pump energy), the Brillouin energy is 66% of the forward Raman energy for the 0.5 m fibre and is only 9% for the 1.5 m fibre. As a consequence, the output energy at 772 nm is highly improved and we obtain an output energy of 0.9 μJ at 772 nm for an injected energy of 5.8 μJ.

All-optical generation of surface acoustic waves in a silica optical microwire

We demonstrate experimentally and numerically the generation of a new class of surface acoustic waves in a subwavelength-diameter silica microwire and term this new effect as surface acoustic wave Brillouin scattering (SAWBS).

Experimental demonstration of stimulated Raman scattering in the evanescent field of a tapered nanofiber immersed in a liquid

Summary form only given. Tapered nanofibers are fibers that are stretched until their diameter becomes comparable to the optical wavelength. As the length of such nanofibers can reach tens of centimeters, the guided mode that presents a strong evanescent field can be used to efficiently excite “evanescent nonlinearities” in the medium surrounding the nanofiber. This evanescent field was already used in many applications, for instance to probe the atomic fluorescence of gases, and also for spectroscopic measurements. However, to our knowledge, this field has never been used to perform optical nonlinearities, the so-called “evanescent nonlinearities”, except in two very recent communications . In this work, we present the demonstration of stimulated Raman scattering in the evanescent field of a nanofiber immersed in ethanol. The nanofiber was drawn from a telecom fiber with a home-made pulling platform. The targeted diameter and length are respectively 440 nm and 6 cm. The pump source was a frequency doubled Nd:YAG laser emitting pulses at 532 nm whose full width at half maximum is 510 ps. We observed the first Stokes of ethanol at 630 nm (see fig. below). The conversion efficiency is about 40 % for an input pump energy of 0.24 μJ. This first experimental demonstration of stimulated Raman scattering in the evanescent field of a nanofiber immersed in a liquid opens the way to the study of a new kind of experiments and components, thanks to the various materials that can form or surround the nanofiber and the feasibility of nanosystems such as nanoloops or nanoresonators.