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.
Fabrication and characterization of submicron optical waveguides is one of the major challenges in modern photonics, as they find many applications from optical sensors to plasmonic devices. Here we report on a novel technique that allows for a complete and precise characterization of silica optical nanofibers. Our method relies on the Brillouin backscattering spectrum analysis that directly depends on the waveguide geometry. Our method was applied to several fiber tapers with diameter ranging from 500 nm to 3 μm. Results were compared to scanning electron microscopy (SEM) images and numerical simulations with very good agreement and similar sensitivity.
Light propagation in small-core photonic crystal fibers enables tight optical confinement over long propagation lengths to enhance light-matter interactions. Not only can photonic crystal fibers compress light spatially, they also provide a tunable means to control light-hypersound interactions. By exploring Brillouin light scattering in a small-core and high air-filling fraction microstructured fiber, we report the observation of Brillouin scattering from surface acoustic waves at lower frequencies than standard Brillouin scattering from bulk acoustic waves. This effect could find potential applications for optical sensing technologies that exploit surface acoustic waves.
We investigate Brillouin scattering in hybrid As<sub>2</sub>Se<sub>3</sub> PMMA tapered fiber and demonstrate that Brillouin
frequency shift can be widely tuned over a broad radio-frequency range by varying the core diameter of the
optical tapered fiber.
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
Sub-micron waveguides and cavities have been shown to produce the confinement of elastic and optical waves in the
same devices in order to benefit from their interaction. It has been shown that square and honeycomb lattices are the
most suitable to produce simultaneous photonic and phononic band gaps on suspended silicon slabs. The introduction of
line defects on such "phoxonic" (or optomechanical) crystals should lead to an enhanced interaction between confined
light and sound. In this work we report on the experimental measurements of light guiding through waveguides created
in these kinds of two-dimensional photonic crystal membranes. The dimensions of the fabricated structures are chosen to
provide a "phoxonic" bandgap with a photonic gap around 1550 nm. For both kinds of lattice, we observe a high-transmission
band when introducing a linear defect, although it is observed for TM polarization in the honeycomb lattice
and for TE in the square. Using the plane-wave expansion and the finite element methods we demonstrate that the guided
modes are below the light line and, therefore, without additional losses beside fabrication imperfections. Our results lead
us to conclude that waveguides implemented in honeycomb and square lattice "phoxonic" crystals are a very suitable
platform to observe an enhanced interaction between propagating photons and phonons.
We demonstrate that the acoustic phonons involved in stimulated Brillouin scattering (both forward and backward)
in phoxonic waveguide can be completely described by using electrostrictive forces. Numerical calculation
for bridge waveguide in silicon and silica illustrate the model.
This paper presents helpful expressions predicting the filling time of gaseous species inside photonic crystal fibres.
Based on the theory of diffusion, our gas-filling model can be applied to any given fibre geometry or length by
calculating diffusion coefficients. This was experimentally validated by monitoring the filling process of acetylene gas in
several fibre samples of various geometries and lengths. The measured filling times agree well within ±15% with the
predicted values for all fibre samples. In addition the pressure dependence of the diffusion coefficient was
experimentally verified by filling a given fibre sample with acetylene gas at various pressures. Finally ideal conditions
for gas light interaction are determined to ensure optimal efficiency of the sensor by considering the gas flow dynamics
in the design of microstructured fibres for gas detection and all-fibre gas cell applications.
Optical fibre sensors based on stimulated Brillouin scattering have now clearly demonstrated their excellent capability
for long-range distributed strain and temperature measurements. The fibre is used as sensing element and a value for
temperature and/or strain can be obtained from any point along the fibre. While classical configurations have practically
a spatial resolution limited by the phonon lifetime to 1 meter, novel approaches have been demonstrated these past years
that can overcome this limit. This can be achieved either by the prior activation of the acoustic wave by a long lasting
pre-pumping signal, leading to the optimized configuration using Brillouin echoes, or by probing a classically generated
steady acoustic wave using a ultra-short pulse propagating in the orthogonal polarization of a highly birefringent fibre.
These novel configurations can offer spatial resolutions in the centimetre range, while preserving the full accuracy on the
determination of temperature and strain.
Photonic Crystal Fibers (PCF) play a crucial role for fundamental investigations such as acousto-optical interactions
as well as for applications, such as distributed sensors. One limiting factor for these experiments is the
fiber inhomogeneity owing to the drawing process. In this paper we study the effect of structural irregularities on
both the backward and forward Brillouin scattering by comparing two PCFs drawn with different parameters, in
order to minimize diameter fluctuations. We fully characterize their Brillouin properties including the backward
Brillouin spectrum, the Brillouin threshold, a distributed measurement along the fibers and polarized Guided
Acoustic Wave Brillouin Scattering (GAWBS). In the Brillouin spectrum we observe a single peak as in a singlemode
fiber whereas former investigations have often shown a multiple peak spectrum in PCFs with small core.
The theoretical and experimental values for the Brillouin threshold are in good agreement, which results from
the single peak spectrum. By using a Brillouin echoes distributed sensing system (BEDS), we also investigate
the Brillouin spectrum along the fiber with a high spatial resolution of 30 cm. Our results reveal a clear-cut
difference between the distributed measurements in the two fibers and confirm the previous experiments. In the
same way the GAWBS allows us to estimate the uniformity of the fibers. The spectra show a main peak at about
750 MHz, in accordance with theoretical simulations of the acoustic mode and of the elasto-optical coefficient.
The fiber inhomogeneity impacts on the stability and the quality factor of the measured GAWBS spectra. We
finally show that the peak frequency of the trapped acoustic mode is more related to the optical effective area
rather than the core diameter of the PCF. Thus measuring the main GAWBS peak can be applied for the precise
measurement of the effective area of PCFs.
The absorption of light by a gas molecule has been measured comparatively using light propagating in normal conditions
and in a slow light regime. The experiment is designed to make the 2 measurements possible without modifying the
interaction conditions, so that the sole effect of slow light is unambiguously observed. A 26% group velocity reduction
induced by stimulated Brillouin scattering in a gas-filled microstructured fiber caused no observable change in the
measured absorption, so that it is proved that material slow light does not enhance Beer-Lambert absorption and has a
null impact on gas sensing or spectroscopic applications.
A general analytic solution for Brillouin distributed fibre sensors with sub-meter spatial resolution has been obtained by
solving the coupled wave equation by a perturbation method. The effect of the interaction of a square pump pulse with a
continuous signal is described in full generality for all possible pumping schemes and for any detuning with the resonance
condition of the Brillouin interaction. The model predicts how the acoustic wave, the signal amplitude and the gain spectral
profile depend upon the pumping scheme. The analytical solution is an unprecedented tool to optimize the sensor
configuration by determining the optimum pumping scheme.
A technique developed to acquire fast optical signals using low frequency detection and acquisition is presented here. It
is based on optical sampling that creates a replica of the fast signal on a much slower time scale by a simple strobe effect.
High bandwidth detection and acquisition is totally suppressed leading to a better response and a substantial cost
reduction. The performance is illustrated by comparative measurements using a Brillouin high resolution distributed
In Geotechnical Engineering, progressive failure in soil-structure interaction is one of the least understood problems. It is
difficult to study this phenomenon at laboratory scale, because of the large amount of strain gages required per unit
length/area of the structure, which would interfere with the mechanical properties of both the structure and the soil. The
recently developed Brillouin Echo Distributed Sensor (BEDS) technology overcomes this dilemma by distributed
readings and 5cm spatial resolution. A laboratory pullout testing program has been carried out to verify applicability of
BEDS for the study of progressive failure in the soil-structure interaction.
A novel configuration has been developed to optimize the response of Brillouin echoes for distributed fibre sensing.
Fully resolved measurements of the Brillouin frequency shift of a 5cm spot perturbation have been performed using a
500 ps (5cm) pulse width. The linewidth of the measured Brillouin gain spectrum remains comparable to the intrinsic
linewidth for any pulse width. The high accuracy and inherent stability of the technique have been successfully verified.
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.
We study guided acoustic wave Brillouin scattering (GAWBS) in several photonic crystal fibers (PCF) with different
kind of air-hole microstructure and we show this effect is enhanced only for a few acoustic phonons. The results of our
numerical simulations based on a finite element method reveal that these acosuti waves emitted in the GHz range are
indeed trapped within the air-hole microstructure, in good agreement with experimental observations. The periodic
wavelength-scale air-hole microstructure of solid-core PCFs can indeed drastically alter the transverse elastic waves
distribution and therefore forward Brillouin scattering compared to what is commonly observed in conventional all-silica
fibers. We show additionnally that the elasto-optic diffraction coefficient and the transverse acousto-optic field overlap
are maximum for these acoustic waves. For the most intense GAWBS modes, we investigate the scattering efficiency
and temperature dependence of the fundamental phonon frequency for sensing applications.
We describe the fabrication of wafer-scale alkali vapor cells based on silicon micromachining and anodic bonding. The principle of the proposed micromachined alkali cell is based on an extremely compact sealed vacuum cavity of a few cubic millimeters containing caesium vapors, illuminated by a high-frequency modulated laser beam. The alkali cells are formed by sealing an etched silicon wafer between two glass wafers. The technique of cell filling involves the use of an alkali dispenser. The activation of cesium vapors is made by local heating of the dispenser below temperature range causing degradations of cesium vapor purity. Thus, the procedure avoids negative effects of cesium chemistry on the quality of cell surfaces and sealing procedure. To demonstrate the clock operation, cesium absorption as well as coherent population trapping resonance was measured in the cells.