Stella Foaleng Mafang

École Polytechnique Fédérale de Lausanne, Lausanne, VD, Switzerland

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Publications (26)17.71 Total impact

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    Luc Thévenaz · Stella Foaleng Mafang · Jie Lin
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    ABSTRACT: Energy transfer between the interacting waves in a distributed Brillouin sensor can result in a distorted measurement of the local Brillouin gain spectrum, leading to systematic errors. It is demonstrated that this depletion effect can be precisely modelled. This has been validated by experimental tests in an excellent quantitative agreement. Strict guidelines can be enunciated from the model to make the impact of depletion negligible, for any type and any length of fiber. (C) 2013 Optical Society of America
    Full-text · Article · Jun 2013 · Optics Express
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    Full-text · Dataset · Dec 2012
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    ABSTRACT: Like photonic crystals have revolutionized the way of manipulating optical waves at the sub-micron scale, phononic crystals have more recently played similar decisive role for sound waves, or more generally elastic waves. Then, the idea of coupling light and sound in purposely designed microstructures is now emerging. In this respect, the periodic, wavelength-scale (for both optic and high-frequency acoustic waves) transverse air-hole microstructure of photonic crystal fibers (PCFs) provides additional degrees of freedom for light-sound interactions. PCFs can indeed exhibit photonic and phononic bandgap effects, allowing for tight confinement and joint waveguiding of both types of waves [1]. Electrostriction-driven Brillouin phenomena, namely backward Stimulated Brillouin Scattering (SBS) and forward Guided Acoustic Wave Brillouin Scattering (GAWBS), constitute an important category of such opto-acoustic coupling. The geometry of PCFs can dramatically modify the Brillouin spectrum, the gain and the stimulated Brillouin threshold, globally yielding much richer opto-acoustic dynamics and spectral features than in conventional fibers [2-11]. Specific transverse or longitudinal guided acoustics modes in the 100 MHz-10 GHz range can thus be selectively excited, resonantly enhanced and tightly confined within the microstructure, with an intimate dependence on its μm or sub-μm geometry. All these specific features have great potential for developing novel PCF-based distributed Brillouin sensors [6,7,12-14], for high-resolution longitudinal mapping of the intrinsic fluctuations of the fiber microstructure [15], and more generally for developing original tools of optical signal processing [1-4,9,16,17]. This talk will give a comprehensive overview of these original behaviors in a range of PCFs.
    Full-text · Conference Paper · Jan 2012
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    Luc Thevenaz · Stella Foaleng Mafang · Jie Lin
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    ABSTRACT: The energy transfer between the two interacting optical waves in a distributed sensor based on stimulated Brillouin scattering can lead to a non-uniform spectral distribution of the pumping power after a long propagation. This results in a spectrally distorted gain that biases the determination of the maximum gain frequency. A quantitative analytical model gives an expression for the tolerable pump power change keeping the maximum bias within a given accuracy.
    Full-text · Article · May 2011 · Proceedings of SPIE - The International Society for Optical Engineering
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    Luc Thévenaz · Stella Foaleng Mafang · Jie Lin
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    ABSTRACT: Energy transfer between the interacting waves in a distributed Brillouin sensor can result in a distorted measurement of the local Brillouin gain spectrum, leading to systematic error. We demonstrate here that this behavior can be fully and precisely modeled, and an excellent quantitative agreement is found with experimental tests. Strict guidelines can be enunciated from this description to make the impact of depletion negligible, for any type and any length of fiber.
    Full-text · Article · May 2011 · Proceedings of SPIE - The International Society for Optical Engineering
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    Jean-Charles Beugnot · Moshe Tur · Stella Foaleng Mafang · Luc Thévenaz
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    ABSTRACT: A general analytic solution for Brillouin distributed sensing in optical fibers with sub-meter spatial resolution is obtained by solving the acoustical-optical coupled wave equations by a perturbation method. The Brillouin interaction of a triad of square pump pulses with a continuous signal is described, covering a wide range of pumping schemes. The model predicts how the acoustic wave, the signal amplitude and the optical gain spectral profile depend upon the pumping scheme. Sub-meter spatial resolution is demonstrated for bright-, dark- and π-shifted interrogating pump pulses, together with disturbing echo effects, and the results compare favorably with experimental data. This analytic solution is an excellent tool not only for optimizing the pumping scheme but also for post-processing the measured data to remove resolution degrading features.
    Full-text · Article · Apr 2011 · Optics Express
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    Stella Foaleng Mafang
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    ABSTRACT: Brillouin scattering is particularly efficient and attractive for the implementation of strain and temperature distributed sensing in optical fibres. Recently a trend has been observed that modern advanced applications require a substantial step towards better spatial resolution, while preserving temperature/strain precision over a long range. For this purpose the state of the art does not satisfy all these requirements. In this thesis we present a radically new approach named Brillouin Echoes distributed sensing (BEDS) that allows covering these requirements. In the first part, we propose an updated configuration of the classical existing Brillouin sensor for time domain analysis allowing drastic noise reduction. Then we investigate the limitations (due to non-linear effects) of the classical Brillouin sensor in terms of long distance range measurements. The identified nonlinear effects are pump depletion due to SBS itself, self-phase modulation (SPM), modulation instability (MI), which occurs only in fibres presenting an anomalous dispersion at the pump wavelength and Raman scattering (RS). We propose the modeling of the pump depletion effect to obtain analytical expressions that are useful for the proper design of a BOTDA sensor and for the determination of a very small depletion. The model confirmed by experimental measurements is informative on the conditions maximizing the depletion effect; therefore a standard configuration can be defined to test the value of the depletion in the set-up. Furthermore, we demonstrate that SPM-induced spectral broadening can have a significant effect on the measured effective gain linewidth. Modeling and experiments have undoubtedly demonstrated that the effective gain linewidth can easily experience a two-fold increase in standard conditions when the pulse intensity profile is Gaussian. We showed that the problem can be practically circumvented by using a clean rectangular pulse with very sharp rising and falling edges. The theoretical and experimental analysis of the undesirable effects of MI and forward RS in distributed BOTDA sensors systems gives a simplified expression to predict the critical power for a given distance range. MI turns out to be the dominant nonlinear limitation since it shows the lowest critical power, but it is less critical since it can be avoided to a wide extent by using the fibre in the normal dispersion spectral region such as a DSF in the C-band. On the other hand Raman scattering can be avoided only by limiting the optical pump power and therefore is the ultimate nonlinear limitation in a distributed sensing system. Under similar conditions RS shows a critical power ∼5 times larger than MI. In the second part, we present the new approach Brillouin echo distributed sensing (BEDS) which has proved to be a powerful solution to realize sub-metric spatial resolutions in Brillouin distributed measurements. We have demonstrated both theoretically and experimentally that an optimized configuration is reached when the optical wave is π-phase shifted. The experimental tests have shown a spatial resolution down to 5 cm, with a clear margin for further improvement down to a real centimetric spatial resolution. This optimized configuration produce the best contrast independently of the pulse intensity, with a factor 2 of improvement compared to other techniques based on the same approach (dark pulse, bright pulse). This extends the dynamic range by 3 dB, which corresponds in standard loss conditions to a 5 km extension of the sensing range. An analytical developed model has proved to be an excellent tool not only for optimizing the pumping scheme but also in post-processing the measured data. Finally the potentialities of BEDS technology provide solutions in real contexts. Using the BEDS technology in landslide monitoring at laboratory scale, for the first time it became possible to observe the failure propagation in laboratory scale with an accurate precision. Furthermore, using BEDS we have proposed and demonstrated the possibility of mapping geometrical structure fluctuations along a photonic crystal fibre (PCF). Both long- and short-scale longitudinal fluctuations in the Brillouin frequency shift have been identify and quantify. Observation of Brillouin linewidth broadening in PCF fibre through distributed measurement of the Brillouin gain spectrum using BEDS has allowed fundamental understanding of SBS in PCF fibre and in their design in view of applications to optical-strain/temperature sensing.
    Preview · Article · Jan 2011
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    ABSTRACT: We present a novel distributed sensing technique that makes possible the observation of Brillouin gain spectral distribution and acoustic decay time in photonic crystal fiber as well as in standard single-mode fiber.
    Full-text · Conference Paper · Oct 2010
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    ABSTRACT: The spectral broadening of the pump pulse through self phase modulation in a time domain distributed Brillouin sensor is demonstrated to have a non-negligible detrimental effect, leading to a doubling of the effective gain linewidth after some 20 km in standard conditions. The theoretical modeling is fully confirmed by experimental results.
    Full-text · Article · Sep 2010 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: 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.
    Full-text · Article · Sep 2010 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: We present a novel distributed sensing technique that allows for the observation of Brillouin gain spectral distribution and acoustic decay time in photonic crystal fiber as well as in standard single-mode fiber.
    Full-text · Conference Paper · Aug 2010
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    ABSTRACT: The absorption of light by a gas molecule has been measured comparatively using light propagating in normal and slow light regime, clearly resulting in no observable change in the measured absorption. The experiment is designed to make the 2 measurements possible without modifying the interaction conditions, so that the sole effect of slow light is observed.
    Full-text · Conference Paper · Jan 2010
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    ABSTRACT: 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.
    Full-text · Article · Oct 2009 · Proceedings of SPIE - The International Society for Optical Engineering
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    Stella Foaleng Mafang · Jean-Charles Beugnot · Luc Thevenaz · Julian D. C. Jones
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    ABSTRACT: 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.
    Full-text · Article · Oct 2009 · Proceedings of SPIE - The International Society for Optical Engineering
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    Stella Foaleng Mafang · Jean-Charles Beugnot · Luc Thevenaz · Julian D. C. Jones
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    ABSTRACT: 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 fibre sensor.
    Full-text · Article · Oct 2009 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: 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.
    Full-text · Article · Oct 2009 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: We experimentally demonstrate that material slow light does not enhance Beer-Lambert absorption. A 26% group velocity reduction induced by stimulated Brillouin scattering in a gas-filled microstructured fiber caused no observable change in the measured absorption.
    Full-text · Article · Jul 2009
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    Silvia Diaz · Stella Foaleng Mafang · Manuel Lopez-Amo · Luc Thevenaz
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    ABSTRACT: A novel configuration for a Brillouin distributed fiber sensor based on Brillouin optical time-domain analysis is proposed. This configuration eliminates many intensity noise issues found in previous schemes. Resolution of 7 m all over a 47 km single-mode fiber was achieved and resolution down to 30 cm in a few kilometer fiber. Noise reduction makes possible measurements with a 16 times averaging.
    Full-text · Article · Aug 2008 · IEEE Sensors Journal
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    Luc Thevenaz · Stella Foaleng Mafang
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    ABSTRACT: A simple physical description of the nonlinear optical interaction based on Brillouin echoes is presented. This technique makes potentially possible distributed Brillouin sensing down to centimeter spatial resolution while preserving the narrowband feature of the natural Brillouin gain spectrum. Experimental conditions for the generation of Brillouin echoes are described and demonstrations of distributed measurements using a 1 ns (10 cm) pulse are presented.
    Full-text · Article · Jan 2008 · Proceedings of SPIE - The International Society for Optical Engineering
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    ABSTRACT: We provide a complete experimental characterization of stimulated Brillouin scattering in a 160 m long solid-core photonic crystal fiber, including threshold and spectrum measurements as well as position-resolved mapping of the Brillouin frequency shift. In particular, a three-fold increase of the Brillouin threshold power is observed, in excellent agreement with the spectrally-broadened Brillouin gain spectrum. Distributed measurements additionally reveal that the rise of the Brillouin threshold results from the broadband nature of the gain spectrum all along the fiber and is strongly influenced by strain. Our experiments confirm that these unique fibers can be exploited for the passive control or the suppression of Brillouin scattering.
    Full-text · Article · Dec 2007 · Optics Express