La Le Gal

Salle

M Benamira
email: m_benamira@univ-jijel.dz

• M T Caldes

• O Joubert

• A Le Gal

La Salle

Preparation of Anode Supported Solid Oxide Fuel Cells

Keywords: Tape casting, SOFC, EIS, Pr2Nio4+δ

Introduction

Nowadays, solid oxide fuel cells (SOFCs) operate at 800-1000 °C. Reducing the operating temperature of solid oxide fuel cells requires the development of new electrolyte materials with good performance at intermediate temperatures (500-700 °C). BaIn0.3Ti0.7O2.85 (BIT07) has the required transport properties and redox stability to be considered as a promising electrolyte material with an ionic conductivity of 10 -2 S cm -1 at 700 °C and chemical stability under a wide range of partial pressures of oxygen [START_REF] Moser | Development of new anodes compatible with the solid oxide fuel cell electrolyte BaIn0.3Ti0.7O2.85[END_REF][START_REF] Benamira | Optimization of SOFC anode/electrolyte assembly based on BaIn0.3Ti0.7O2.85 (BIT07)/Ni-BIT07 using an interfacial anodic layer[END_REF][START_REF] Benamira | New anode materials for IT-SOFC derived from the electrolyte BaIn0.3Ti0.7O2.85 by lanthanum and manganese doping[END_REF][START_REF] Letilly | Validation of BaIn0.3Ti0.7O2.85 as SOFC electrolyte with Nd2NiO4, LSM and LSCF as cathodes[END_REF]. The praseodymium nickelate Pr2NiO4+δ (so-called PNO) is a mixed ionic electronic conducting (MIEC) oxide widely studied as oxygen electrode for solid oxide fuel cells and solid oxide electrolysis cells (SOEC) as well. MIEC oxide attracted particular attention since the oxygen reduction reaction (ORR) has been extended to the whole volume of the electrode, compared to a pure electronically conducting cathode where the ORR is restrained to the triple phase boundary (TPB) points located at the cathode/electrolyte interface [START_REF] Lay | Effect of Ce-doping on the electrical and electrocatalytical behavior of La/Sr chromo-manganite perovskite as new SOFC anode[END_REF][START_REF] Benamira | Enhancing oxygen reduction reaction of YSZ/La2NiO4+δ using an ultrathin La2NiO4+δ interfacial layer[END_REF][START_REF] Caldes | Metallic nanoparticles and proton conductivity: improving proton conductivity of BaCe0.9Y0.1O3-δ and La0.75Sr0.25Cr0.5Mn0.5O3-δ by Ni-doping[END_REF][START_REF] Bassat | Anisotropic oxygen diffusion properties in Pr2NiO4+δ and Nd2NiO4 + δ single crystals[END_REF][START_REF] Allançon | Influence of oxygen on structural transitions in Pr2NiO4+δ[END_REF].

This paper aims to study the influence of the presence of the material used as an interlayer between the PNO electrode and the BIT07 electrolyte on the electro-chemical performance of the SOFC. Ce0.8Gd0.2O1.9 (namely GDC) has been selected as interlayer material. It has also been demonstrated that BIT07 can be used with well-known cathode materials, such as La0.58Sr0.4Co0.2Fe0.8O3δ (LSCF) or Nd2NiO4+ δ, and that best results regarding electrochemical performance have been obtained with LSCF.

Our main objective in this work is to prepare a complete Ni-BIT07/BIT07/PNO and Ni-BIT07/BIT07/GDC//PNO cells by tape casting and screen printing. They have been tested under air on the cathode side and wet H2 (3% H2O) on the anode side. The performance has been discussed in terms of total resistance determined from current density/voltage curve (J/U) and Electrochemical Impedance Spectroscopy (EIS) measurements at the Open Circuit Voltage (OCV) to understand the origin of this resistance, and to determine the relevant parameters that can be modified in order to minimize it. The stability of the cell presenting the highest performance has been studied at the operating temperature of 700 °C.

Experimental

BaIn0.3Ti0.7O2.85 (BIT07) was synthesised by solid state reaction by using stoichiometric amounts of BaCO3 (Alfa Aesar), In2O3 (Alfa Aesar) and TiO2 (Merck). Reac-tants were thoroughly mixed using acetone in an agate mortar and calcined at 1200 °C for 24 h. The obtained powder was again ground well, mixed, pressed into disks and sintered at 1350 °C for 24 h, ground and passed through mesh 100. Pr2NiO4+δ powder was provided by Marion Technologie and NiO powder (grain size 0.5-1 μm) was provided by Pharmacie Centrale de France. X-ray diffraction (XRD) was used to characterize the structure of the as-prepared powders. The patterns were obtained using a Brüker "D8 Advance" powder diffractometer with a Cu Kα radiation (λ = 1.54056 Å) and 2θ varying from 20 to 80° by steps increments of 0.0146° and 0.2 s counting time per step. The Scanning Electron Microscopy (SEM) was performed on a FEG-SEM JEOL 7600. By Brunauer-Emmett-Teller (BET), the surface areas of the powders were measured and carried out on a Micromeritics 3Flex analyzer determined by N2 adsorption-desorption isotherm measurement at -196°C.

Before cell fabrication and in order to decrease the grain sizes, the as-prepared powders of BIT07 were ball-milled at 500 rpm: 60 h. The anode composition has been Current collectors, made of discs (5 mm in diameter) of gold grid (Goodfellow, wire diameter: 60 μm) are attached on both the electrodes using gold ink. The cell is placed at 120 °C for overnight to evaporate the solvents and to obtain good electrical contacts. The current-voltage characteristic is measured with the use of a laboratory-made testing system. The cell was sealed on the top of an alumina sample tube. Thus separating the atmospheres between the inside and outside of the tube. The fuel-gas supply tube is situated inside the sample tube. The system was kept vertically in a tubular furnace. The measurements of current density (J) and voltage (U) were done by digital multimeters Keithley 197 and Protek 506, respectively. Current drawn in the circuit was varied using a rheostat. Prior to the measurements, nickel oxide is reduced in situ at 700 °C for 2 h under wet H2 and measurements were made at 700 °C, under wet (3% H2O) H2 on the anode side, and air on the cathode side.

Electrochemical performances of single cells were studied by electrochemical impedance spectroscopy (EIS). The measurements were realized using a frequency response analyzer Solartron 1260. The impedance spectra were recorded over a frequency range 2 MHz to 0.01 Hz with a signal amplitude of 10 mV and with 10 points per decade under open circuit conditions from 500 to 700 °C. The EIS diagrams were normalized to the electrode area and fitted using equivalent circuits with the ZView ® software (Scribner Associates). Post mortem (after measurements) analyses of the cells have been carried out by SEM, using a JEOL 7600. In both cases, the polarisation curves (U/J) are clearly linear in the studied current density range. In both cases, the OCV is lower than the theoretical value expected, suggesting a small leakage between gas chambers. Table 3.1 gathers the cells characteristics obtained from the voltage and power density versus current density characteristics and also from the Nyquist diagrams of the single cells.

Results and Discussion

Characterization of Material

The extrapolated maximum power densities are of about 28.1 and 22.06 mW cm -2 for the two cells, respectively. The ASR value obtained from the EIS measurements, 10.41 for cells A at 700 °C, is in agreement with the ones obtained from the U/J Table 3.1 Characteristics of the cell Ni-BIT07/BIT07 and Pr2NiO4+δ sintered at 1050 °C recorded at OCV, ASR(U/J) calculated from U/J curves, ASR(EIS), Rs and Rp obtained from the EIS measurements

T (°C) OCV (V) Pmax (mW cm -2 ) ASR(U-J) (Q cm 2 ) ASR (Nyquist ) (Q cm 2 ) Re.S (Q cm 2 )
Rp.S (Q cm In order to understand the origin of the low performances of the two cells, the reac-tivity test between BIT07 and Pr2NiO4+δ powders for more than 168 h at 1100 °C was done as shown in Fig. 3.3. Indeed, the DRX analysis reveals the significant pres-ence of a new phase (Pr1,6Ba0,4NiO4) due to the reaction between the electrolyte and cathode. Considering the reactivity of the Pr2NiO4+δ with BIT07, the higher power densities would expect to be achieved by using a thin interfacial layer of GDC.

A thin interfacial layer (5 μm) of GDC is deposited to the electrolyte/anode half-cell by screen-printing and co-fired at 1350 °C for 8 h. Then a PNO cathode is deposited on the electrolyte by screen-printing. To demonstrate the benefit of incorporating a thin layer at the interface between the electrode and electrolyte, the I-V-P curves of the fuel cell are shown in Fig. 3.4. The power density obtained from the cell with GDC thin film was about 119.21 mW/cm 2 at 700 °C, compared with about 28.1 mW/cm 2 for the cell without thin film.

Conclusion

In this work, anode supported cells based on BIT07 as an electrolyte, BIT07-Ni as anode and Pr2NiO4+δ as cathode have been successfully realized by tape casting and screenprinting. The reactivity test between BIT07 and Pr2NiO4+δ powders reveals the presence of a new phase (Pr1,6Ba0,4NiO4) which explain the low performances of the single cell.

The power density of a single cell with GDC thin film at the interface electrolyte/cathode was significantly higher than that of a single cell without interfacial layer. This is attributed to increased interfacial area. The factors such as the better electrocatalytic activity of ceria and the intrinsic properties of nanostructured thin films.

Fig. 3 . 1

 31 Fig. 3.1 Preparation of Ni-BIT07/BIT07/PNO anode supported cell. a Anode of BIT07-NiO, b electrolyte BIT07, c deposition of PNO cathode, d current collector of gold grid

Fig. 3 . 2

 32 Fig. 3.2 Performance of a single cell consisting in: a Ni-BIT07/BIT07 and Pr2NiO4+δ sintered at 1050 °C, b Ni-BIT07/BIT07 and Pr2NiO4+δ sintered at 1100 °C as the cathode; I-V curves and resulting power densities

Figure 3 .

 3 Figure 3.2 presents the voltage (U) and power density (P) versus current density (J) characteristics obtained for two single cells at different temperatures 1050 and 1100 °C, respectively, Fig. 3.2a, b.In both cases, the polarisation curves (U/J) are clearly linear in the studied current density range. In both cases, the OCV is lower than the theoretical value expected, suggesting a small leakage between gas chambers. Table3.1 gathers the cells characteristics obtained from the voltage and power density versus current

Fig. 3 .

 3 Fig. 3.3 X-ray diffraction patterns of BIT07/Pr2NiO4+δ composite pellets that were calcined at 1100 °C for different times

Fig. 3 . 4 a

 34 Fig. 3.4 a Performance of a single cell consisting in Ni-BIT07/BIT07/GDC/Pr2NiO4+δ; I-V curves and resulting power densities, b scanning electron microscope image (cross-section) of single cell


Pierre Béjot
email: pierre.bejot@u-bourgogne.fr

Bertrand Kibler

Spatiotemporal Helicon Wavepackets

Keywords: space-time wave packets, nonlinear optics, orbital angular momentum, optical fibers

come

Introduction

Light structuring [START_REF] Forbes | Structured light[END_REF] has found far-reaching applications covering numerous fields such as fundamental physics [START_REF] Hansen | Singular atom optics with spinor Bose-Einstein condensates[END_REF], telecommunications [START_REF] Wang | Terabit free-space data transmission employing orbital angular momentum multiplexing[END_REF][START_REF] Xie | Ultra-broadband on-chip twisted light emitter for optical communications[END_REF], particle manipulation [START_REF] Dholakia | Shaping the future of manipulation[END_REF][START_REF] Woerdemann | Advanced optical trapping by complex beam shaping[END_REF], quantum information technology [START_REF] Sit | High-dimensional intracity quantum cryptography with structured photons[END_REF][START_REF] Erhard | Twisted photons: New quantum perspectives in high dimensions[END_REF], laser micromachining [START_REF] Mathis | Micromachining along a curve: Femtosecond laser micromachining of curved profiles in diamond and silicon using accelerating beams[END_REF][START_REF] Courvoisier | Ultrafast laser micro-and nano-processing with nondiffracting and curved beams: invited paper for the section: hot topics in ultrafast lasers[END_REF], and high-resolution imaging [START_REF] Fahrbach | Microscopy with self-reconstructing beams[END_REF][START_REF] Zhan | Cylindrical vector beams: from mathematical concepts to applications[END_REF][START_REF] Liu | Super-resolution radar imaging based on experimental OAM beams[END_REF][START_REF] Nylk | Light-sheet microscopy with attenuation-compensated propagation-invariant beams[END_REF]. Propagation-invariant or non-diffracting beams belong to the important class of structured optical fields that has received a great deal of attention [START_REF] Durnin | Diffraction-free beams[END_REF][START_REF]Nondiffracting optical beams: Physical properties, experiments, and applications[END_REF]. Propagationinvariant beams are particular solutions of Maxwell's equations that have the particularity to sustain their shape all along the propagation. The simplest well-known (theoretical) diffraction-free beams are monochromatic plane waves which are solutions of Maxwell's equation in free space in Cartesian coordinates. Apart this trivial solution, monochromatic invariant beams have been widely studied in free space, both theoretically and experimentally since several decades. For instance, one can cite Bessel [START_REF] Durnin | Diffraction-free beams[END_REF], Mathieu [START_REF] Gutiérrez-Vega | Alternative formulation for invariant optical fields: Mathieu beams[END_REF], Weber [START_REF] Bandres | Nondiffracting accelerating waves: Weber waves and parabolic momentum[END_REF], and self-accelerating Airy beams [START_REF] Siviloglou | Observation of accelerating Airy beams[END_REF], which are solutions of free space Maxwell equations in cylindrical, elliptic, or parabolic accelerating coordinates.

Orbital angular momentum (OAM)-carrying beams (i.e., vortex beams) are also solutions of Maxwell's equations and have drawn increasing interest in recent years [START_REF] Allen | IV. The Orbital Angular Momentum of Light[END_REF][START_REF] Shen | Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities[END_REF]. They are characterized by a phase singularity at the center, a helical wavefront, and their so-called topological charge, an integer number counting how many wavefront rotations take place over one propagation wavelength. Even if it is not a general law for such beams, some of them are also diffraction-free. This is in particular the case for higher-order Bessel beams. This is important to note that, even if OAM beams own helical wavefronts and that the direction of their Poynting vectors rotates around the propagation axis, their intensity patterns do not. Indeed, their donut-like intensity pattern being cylindrically symmetric, it cannot exhibit any rotation in space. However, particular superposition of diffraction-free Bessel beams that carry OAM can provide rotating complex beams with spiraling (or screw-shaped) trajectories of their intensity distribution during propagation like helicon beams [START_REF] Chávez-Cerda | Nondiffracting beams: travelling, standing, rotating and spiral waves[END_REF][START_REF] Paterson | Helicon waves: Propagation-invariant waves in a rotating coordinate system[END_REF][START_REF] Lee | Optical solenoid beams[END_REF][START_REF] Vetter | Generalized radially self-accelerating Helicon beams[END_REF]. More complex forms of OAM-carrying beams have been also investigated numerically such as rotating-revolving Laguerre-Gaussian beams using multiple optical-frequency-comb lines [START_REF] Zhao | Dynamic spatiotemporal beams that combine two independent and controllable orbital-angular-momenta using multiple optical-frequency-comb lines[END_REF]. More generally, by carefully controlling the interference of structured light that caries OAM, twisting and accelerating light over both the radial and the angular directions can be demonstrated [START_REF] Schulze | Accelerated rotation with orbital angular momentum modes[END_REF][START_REF] Brimis | Tornado waves[END_REF], as well as possibly providing an optical pulling force through tractor beams [START_REF] Ruffner | Optical Conveyors: A Class of Active Tractor Beams[END_REF][START_REF] Ding | Photonic tractor beams: a review[END_REF].

On the other hand, propagation-invariant pulsed beams (without carrying OAM) have also been intensively studied. Such solutions are not only diffraction-free as their monochromatic counterpart but are also insensitive to dispersion, i.e., they do not spread in time as they propagate in dispersive bulk media. Brittingham's focus-wave modes [START_REF] Brittingham | Focus waves modes in homogeneous Maxwell's equations: Transverse electric mode[END_REF], Mackinnon's wave packet [START_REF] Mackinnon | A nondispersive de Broglie wave packet[END_REF], Bessel-pulse X-pulses [START_REF] Sõnajalg | Demonstration of the Bessel-X pulse propagating with strong lateral and longitudinal localization in a dispersive medium[END_REF], and space-time light sheets [START_REF] Kondakci | Diffraction-free space-time light sheets[END_REF] belong to this particular class of propagation-invariant wavepackets. The non-dispersive and non-diffractive nature of such fully localized (in space and time) waves is due to their intrinsic coupled spatiotemporal properties by which the combined effects of spatial diffraction and temporal dispersion cancel each other. Moreover, it has been shown that X-waves can also spontaneously emerge when intense ultrashort pulses nonlinearly propagate in dispersive bulk media [START_REF] Conti | Nonlinear Electromagnetic X waves[END_REF].

The monochromatic diffraction-free beams are well-known in structured media, and more particularly in waveguides, they are called optical modes. A generalization of propagationinvariant conical waves in dispersive waveguides, called discretized conical waves, has been also derived [START_REF] Zamboni-Rached | Superluminal localized solutions to Maxwell equations propagating along a waveguide: The finite-energy case[END_REF][START_REF] Kibler | Discretized conical waves in multimode optical fibers[END_REF]. As their bulk counterpart, such wavepackets do not spread in space and time. Moreover, it has also been shown that such waves are naturally generated when a rather intense ultrashort pulse having a cylindrical symmetry around the propagation axis nonlinearly propagates in the waveguide. The intrinsic difference between discretized and bulk conical waves comes from the discrete number of modes that can propagate in a waveguide, so that their spectrum is also discrete. In this direction of research, propagationinvariant wavepackets in the form of light sheets have been recently used to demonstrate hybrid guiding in unpatterned films with controllable group index [START_REF] Shiri | Hybrid guided spacetime optical modes in unpatterned films[END_REF], and spatiotemporal evolution of pulses in a planar polariton waveguide has revealed spontaneous X-wave formation [START_REF] Walker | Spatiotemporal continuum generation in polariton waveguides[END_REF].

When one now considers all the concepts above listed, namely the propagationinvariance, the pulsed nature, the OAM features of light waves and the structured property of optical media like waveguides, it appears essential to establish a global framework able to account for their simultaneous combination in both bulk media and waveguides for future developments and applications of light structuring. To this end, in this article, we present a novel class of invariant spatiotemporal wavepackets guided in multimode optical fibers, namely OAM-carrying helicon wavepackets. Note that our generalized approach is also valid for bulk media, since the latter can be considered as a waveguide of infinite dimension. The main feature of such dispersion-and diffraction-free electric fields is that they rotate in both time and space as they propagate, without any space-time distortion, at a chosen groupvelocity in the fiber core. Such an amazing property is a consequence of tight correlations between the spatial modes, the topological charges, and the frequencies embedded in the optical field. Beyond the possible linear shaping of such structured waves, we also reveal their spontaneous emergence triggered by the nonlinear propagation of a rather intense ultrashort pulse in fiber OAM modes. In particular, we will show that such wavepackets, spiraling in both time and space, are linked to the formation of helical optical shocks.

The article is organized as follows. We first recall the theoretical description of OAM modes and then provide the construction of linear spatiotemporal helicon wavepackets based on a strict multidimensional phase-matching condition. A simple analytic example is then presented and discussed in the case of a fused silica rod (i.e., at the frontier between bulk media and fibers). Next we numerically investigate the nonlinear generation of such wave structures through the use of a multimode fiber and its OAM modes pumped by an intense ultrashort pulse. In particular, we confirm that the formalism of helicon wavepackets introduced in the first section allows us to predict accurately the mode-resolved output spectrum of the laser pulse at the output of the multimode fiber.

Theoretical description: spatiotemporal helicon wavepackets

Basic considerations

This section is devoted to the theoretical description of propagation-invariant (dispersion-and diffraction-free) space-time wavepackets that evolve on spiraling trajectories around the propagation axis in structured/finite media. For simplicity, we will restrict our study to a scalar approach by considering the weak guidance approximation. Moreover, we will study only waveguides whose refractive index presents a cylindrical symmetry around the propagation axis (i.e., standard optical fibers). Using the above restriction, the partial differential equation driving the linear propagation of an electric field is given in cylindrical coordinates by: [START_REF] Forbes | Structured light[END_REF] where is the light velocity in vacuum, is the radial-dependent refractive index at the angular frequency , and is the Fourier transform of with respect to the time coordinate:

. ( 2
)
Before going ahead, let us find solutions of Eq. 1 that write

Such solutions are called optical modes of the fiber and is called the propagation constant of the mode. Note that modes are not necessarily propagating but can be exponentially decaying along provided that is complex. Moreover, note that, in the context of optical fibers, a mode can propagate in the fiber cladding, even if it is of common use to call "modes" only the ones confined within the core. Since the electric field is intrinsically -periodic (and assumed piecewise-continuous) with respect to , its angular dependence can be developed as a Fourier-series:

By injecting Eqs. 3 and 4 in Eq. 1, one obtains:

(

Since the refractive index is assumed here to be angularly invariant, in bra-ket notation, one has: [START_REF] Woerdemann | Advanced optical trapping by complex beam shaping[END_REF] where is the Kronecker delta. It then indicates that optical modes of such fibers are necessarily of the form:

In other words, functions describe the angular part of the optical modes, and this means that OAM modes form a basis set to represent spatial modes. This is not true anymore for a fiber whose refractive index depends on the angular coordinate , since, in this case, the operator will couple the different functions . Equation 7gives the functional form of the optical modes in an angularly-invariant waveguide. Moreover, one will impose that the electric field vanishes at a given radius , which is located sufficiently far from the waveguide:

Imposing such a Dirichlet boundary condition then discretizes the number of eligible solutions. Consequently, for a given couple , the solutions (i.e., modes) can be indexed and sorted by an integer number , the radial order related to the number of concentric intensity rings in the intensity profile. As a consequence, a fiber mode can be represented by a triplet . Moreover, after an appropriate normalization, the modes form an orthonormal basis so that any electric field can be expressed as [START_REF] Mathis | Micromachining along a curve: Femtosecond laser micromachining of curved profiles in diamond and silicon using accelerating beams[END_REF] where is the transverse envelope of the mode and are the electric field coordinates in the modal basis.

Using the unidirectional pulse propagation equation (UPPE) [START_REF] Andreasen | Nonlinear propagation of light in structured media: Generalized unidirectional pulse propagation equations[END_REF], the linear evolution of an electric field along the propagation axis is given in the modal basis by [START_REF] Courvoisier | Ultrafast laser micro-and nano-processing with nondiffracting and curved beams: invited paper for the section: hot topics in ultrafast lasers[END_REF] where is the propagation constant of the OAM mode at the frequency .

Recall that and refer here to azimuthal and radial indices, where is the topological charge, related to the phase front of the OAM mode. OAM modes are higherorder modes defined on a different basis as compared to more conventional modes in fiber, such as linearly polarized (LP) modes and vector modes, so that they can be also regarded as the linear combination of the latter [START_REF] Ramachandran | Optical vortices in fiber[END_REF][START_REF] Ma | Propagation stability in optical fibers: role of path memory and angular momentum[END_REF]. For example, we can here write , as well as corresponds to the fundamental mode of the fiber. The or terms represent a or phase shift in the linear combinations.

Definition of helicon wavepackets

One can now define families of modes in such a way that their propagation constants all verify the same relation: [START_REF] Fahrbach | Microscopy with self-reconstructing beams[END_REF] where , , and are arbitrarily chosen constants. Note that, depending on the chosen constants defining a family, the latter can contain a priori several frequencies in the same spatial mode and a given frequency can satisfy relation [START_REF] Courvoisier | Ultrafast laser micro-and nano-processing with nondiffracting and curved beams: invited paper for the section: hot topics in ultrafast lasers[END_REF] in different spatial modes. A corresponding electric field can be built from the linear superposition of the modes belonging to the same family:

Using Eqs. 10-11, one then obtains the evolution of the above electric field:

where and . Accordingly, any electric field built from a given family is a diffraction-and dispersion-free space-time wavepacket propagating at the group velocity whose intensity continuously rotates around the propagation axis with a spatial period . In other words, the built families are perfectly invariant fields in a frame propagating at the velocity and rotating around the axis with a period .

Wavepackets with spiraling trajectories then result from the linear superposition of waves oscillating at different frequencies and carrying different topological charges.

Several remarks have to be made concerning such wave structures. First, the fact that the spatiotemporal intensity rotates around the propagation axis does not necessarily imply that the fluence (i.e., the energy per surface unit) does. In fact, using the Parseval theorem, one can show that the fluence exhibits the same trajectory (with or without rotation around the propagation axis) only if the defined family embeds at least two modes and such that with . In addition, the instantaneous power of the wavepacket (i.e., the total energy per time unit) is constant except if the electric field embeds at least two distinct frequencies in the same spatial mode.

The above construction of wavepackets, composed of discrete frequencies, implies an infinite energy, which cannot correspond to a realistic physical situation, except if one uses a set of continuous wave lasers (or multiple frequency-comb lines) combined with spatial shaping and multiplexing. On the other hand, the carrier frequencies involved could have a finite narrow bandwidth (i.e., associated with a temporal envelope ). As a consequence, the electric field constructed from a given family would write in good approximation (i.e., taking into account only the group-velocity difference between spatial modes) as [START_REF] Nylk | Light-sheet microscopy with attenuation-compensated propagation-invariant beams[END_REF] so that its evolution along the propagation axis is [START_REF] Durnin | Diffraction-free beams[END_REF] where with the inverse of the group velocity of the mode taken at the frequency . Accordingly, a wavepacket embedding a finite amount of energy is necessarily dispersive, i.e., not strictly speaking dispersion-and diffraction-free. Note however, that the effect of dispersion, i.e., the length from which the dispersion of the group velocities in the different modes will impact the propagation, will strongly depend on the envelope duration. Then, the wavepacket can be considered as invariant but over a finite distance.

Last but not least, if now, one comes back to the essential issue of establishing a general framework able to link the concepts of propagation-invariance, pulsed nature, and OAM, it is important to notice that modes of a given family lye on a surface S in the three-dimensional (l,p,) modal space. This surface embeds both helicon (monochromatic) beams [START_REF] Paterson | Helicon waves: Propagation-invariant waves in a rotating coordinate system[END_REF][START_REF] Vetter | Generalized radially self-accelerating Helicon beams[END_REF] and discretized conical wavepackets [START_REF] Kibler | Discretized conical waves in multimode optical fibers[END_REF], which are located at the intersection between S and planes of constant  and constant topological charge l, respectively. Our theory then naturally encompasses both concepts which now appear as special cases of spatiotemporal helicon wavepackets.

Simple example of helicon wavepackets

Before studying how the above wavepackets can emerge from nonlinear propagation in a standard multimode fiber, we first construct such space-time wave structures in a simple case at the frontier between bulk media and fibers. In particular, we consider a dispersive medium of finite transversal dimension (radius ), often used as a toy model for calculating the guided modes of hollow core capillaries. Its modes have the advantage to be perfectly analytic as follows:

(

where is the root of Bessel function of first kind , , and is the frequency-dependent refractive index of the medium. In the paraxial approximation, the propagation constants can be approximated by [START_REF] Gutiérrez-Vega | Alternative formulation for invariant optical fields: Mathieu beams[END_REF] Note that such an approximation does not change the underlying physics but it allows to deal with more tractable analytical formula. Moreover, we assume that the wavevector can be developed in frequency as a Taylor series around a central carrier frequency .

Accordingly, one has

, ( 18
)
where and .We can then define the family of modes that satisfies:

. ( 19
)
As a result, for each couple , one has to find the roots of a order polynomial in .

The purely real roots then correspond to frequencies that satisfy Eq. 19. As an example, Figure 1(a) depicts the family of modes given by the above condition with , when considering a fused silica rod ( 100 µm) and a central wavelength 800 nm. This arbitrary choice of the constant is fixed to the difference between propagation constants of the two first OAM modes of our silica rod at , thus defining the rotation period of the fluence, and the two first OAM modes at will be contained in the family presented in this particular example. Note that we added an arbitrary group delay of 3 ps m -1 , i.e., the group velocity of the helicon wavepacket is

. Then, we observe an evident combination of X-shaped patterns in the or planes, whose individual element for the latter is similar to the well-known X-shaped conical wave studied in previous works [START_REF] Conti | Nonlinear Electromagnetic X waves[END_REF][START_REF] Zamboni-Rached | Superluminal localized solutions to Maxwell equations propagating along a waveguide: The finite-energy case[END_REF][START_REF] Kibler | Discretized conical waves in multimode optical fibers[END_REF].

The linear construction of the helicon wavepacket then results from the superposition of frequencies with equal spectral amplitudes and phases (see Fig. 1(b-d)). Since the present example corresponds to the particular case, where two distinct angular modes oscillates at the same frequency (here, ), the fluence distribution is angularly asymmetric with zero-value shifted from the origin (i.e., distinct from a perfect donut-like pattern), as shown in Fig. 1(c). The spatial phase distribution at is shown in Fig. 1(d), the complex pattern results from the superposition of two OAM fiber modes and is characterized by a singularity shifted from the origin. The spatial phase distribution for other frequencies that contain a single mode is not shown here since featured by typical signatures related to the topological charge (i.e., a phase singularity at the center and times 2πazimuthal phase changes).

The full electric field at a given propagation distance is given by Eqs. 12-13. Figure 1(e) shows the evolution of the fluence along propagation which implies the spiraling trajectories of both fluence and phase profiles while maintaining their initial distribution. The rotation period is directly given by 16 cm, and the direction of rotation is counter clockwise as driven by the negative value of . As shown in Fig. 1(f) that displays the isosurface of the spatiotemporal intensity at half maximum, the wavepacket looks like a corkscrew in space-time coordinates, the points of high intensity being extremely localized in both space and time since all frequencies are in phase. Nevertheless, all modes used for the construction contain a single frequency so that the instantaneous power (integrated over spatial coordinates) is constant. Note that despite the time-localization of our helicon wavepacket, this ultrashort structure does not disperse and simply rotates during its propagation around the -and -axis because of the inherent invariant nature of the constructed wavepacket (see supplementary movie 1).

Note that more complex fluence and phase profiles could be easily generated by choosing a superposition of multiple higher-order modes at the phase-matched discrete frequencies.

This would provide simultaneous revolving and rotating of such profiles in the x-y plane during propagation [START_REF] Zhao | Dynamic spatiotemporal beams that combine two independent and controllable orbital-angular-momenta using multiple optical-frequency-comb lines[END_REF].

Numerical simulations: nonlinear generation in multimode fibers

Multimode fibers are known as versatile platforms, in particular, for sculpting supercontinuum as a function of the initial pump spatial profile [START_REF] Eftekhar | Versatile supercontinuum generation in parabolic multimode optical fibers[END_REF][START_REF] Tzang | Adaptive wavefront shaping for controlling nonlinear multimode interactions in optical fibres[END_REF][START_REF] Deliancourt | Wavefront shaping for optimized many-mode Kerr beam self-cleaning in graded-index multimode fiber[END_REF]. In the following, we

show how spatiotemporal helicon wavepackets can be nonlinearly generated by appropriate and realistic initial pumping conditions. Our numerical approach of nonlinear pulse propagation is based on the multimode unidirectional pulse propagation equation recently derived in [START_REF] Béjot | Multimodal unidirectional pulse propagation equation[END_REF], which describes the evolution of the complex electric field in the scalar approximation:

, ( 22
)
is the vacuum permittivity, and is the nonlinear polarization expressed in the modal basis. Moreover, for convenience, the equation is written in a local frame propagating at an arbitrarily chosen velocity . The latter is often chosen as the group velocity of the fundamental mode calculated at . In the context of fiber propagation, using a complex representation of the electric field (expressed now so that , being the pulse intensity), UPPE rewrites: [START_REF] Paterson | Helicon waves: Propagation-invariant waves in a rotating coordinate system[END_REF] where is the effective refractive index of the fundamental mode at , is the nonlinear refractive index of the medium (here for silica glass, we used 3.2 10 -20 m 2 /W).

The function is the Raman response with fraction 0.18 for fused silica glass. For simplicity, we have neglected here the nonlinear term responsible for third harmonic generation. We recall that the present simulations are full 3D+1 simulations. Note that the effect of usual fiber losses (less than 10 dB/km) is found here to be negligible over the considered propagation distance. We solve the propagation by a split-step algorithm as described in Ref. [START_REF] Béjot | Multimodal unidirectional pulse propagation equation[END_REF]. Counting both radial and angular modes, we considered more than 1500 modes for our calculations. Using such a number of modes would be completely intractable if an algorithm based on multimode generalized nonlinear Schrodinger equation (MM-GNLSE) were employed [START_REF] Tarnowski | Numerical modelings of ultrashort pulse propagation and conical emission in multimode optical fibers[END_REF].

As an example (see Fig. 2), we investigate the nonlinear propagation of 100-fs Gaussian pulse at 1300 nm, with 450-nJ energy, close to the zero dispersion of a standard stepindex multimode fiber (core radius 52.5 µm and numerical aperture 0.22). The associated peak power is around 0.8 times the critical power of silica glass. The energy is initially equally-distributed on the two first OAM modes of the fiber ( 0, 0 and +1).

Note that such an initial condition could be experimentally realized using a spatial beam shaper. The propagation constant difference between the OAM modes at is -220 rad m -1 . Accordingly, the laser pulse initially rotates around the propagation axis with a period 2.8 cm, as shown in Fig. 2(c). We observe about 3 rotations of the guided beam in the fiber core (denoted by the contour-plot of fluence at half-maximum) over the 8.5-cm-long propagation distance under study. The detailed nonlinear propagation for temporal power and power spectrum is shown in Fig. 2(a-b), respectively. We observe that the spectral dynamics reaches a stationary state after 7 cm (see Fig. (e) Corresponding iso-surface at 1/e 2 of the 3D-intensity profile (see also its evolution during nonlinear propagation in supplementary movie 2).

A clear structure emerges in the mode-resolved spectrum at the distance of occurrence of this sub-pulse, in the form of discretized conical fish-wave (also known as a combination of both X-wave and O-wave) over angular indices for low radial indices , as depicted in Fig. 2(d).

This particular shape corresponds to the formation of a spatiotemporal helicon wavepacket belonging to a particular family of modes as defined in the previous section by Eqs. 11 and 12. In other words, this simply corresponds to the linear superposition of phase-matched resonant radiations over the fiber OAM modes. More particularly, the emerging helicon wavepacket propagates and rotates according to the velocity features of the most intense subpulse.

Based on the three-wave mixing (TWM) picture [START_REF] Kolesik | Interpretation of the spectrally resolved far field of femtosecond pulses propagating in bulk nonlinear dispersive media[END_REF], previously developed for describing the generation of conical waves in bulk media, the underlying basic idea is here to consider that the OAM-carrying supercontinuum generated during the nonlinear propagation scatters on a "material wave" which represents the nonlinearly-induced change of the refractive index of the medium. While previous conical waves in bulk media and multimode fibers are solely characterized by their propagation velocity (see Refs. [START_REF] Kibler | Discretized conical waves in multimode optical fibers[END_REF][START_REF] Kolesik | Interpretation of the spectrally resolved far field of femtosecond pulses propagating in bulk nonlinear dispersive media[END_REF]), it is here defined by two parameters, namely its propagation velocity and rotation period due to their spiraling trajectory based on OAM fiber modes. Accordingly, the energy of the scattered waves will be concentrated in families of modes propagating and rotating at the same rates than the material wave (MW):

where , , and are the propagation constant, the inverse of group velocity, and the rotation rate of the material wave, respectively. Here, the material wave corresponds to the most intense sub-pulse that is triggered and that swirls around the propagation axis.

Since the nonlinear process takes place over a finite length , Eq. 24 is not a strict phasematching condition, it can be satisfied over a certain tolerance related to [START_REF] Kibler | Discretized conical waves in multimode optical fibers[END_REF][START_REF] Kolesik | Interpretation of the spectrally resolved far field of femtosecond pulses propagating in bulk nonlinear dispersive media[END_REF]. Equation 24, together with the precise knowledge of the frequency-dependent propagation constants, then allows to estimate the mode-resolved output spectrum shape of the pulse impacted by the conical wave emission. is equal to the propagation constant of the input pump pulse . From Fig. 2(a) and the white dashed arrow, the inverse of group velocity is found to be , with 800 fs m -1 .

Concerning the rotation rate , one can expect that it is also impacted by the nonlinear propagation in a similar way to the group velocity. This is corroborated by the 3Dintensity profile spiraling in time and obtained at the distance of conical wave emission, and depicted in Fig. 2(e). The iso-surface of the intensity clearly exhibits distinct sub-pulses spiraling, characterized by different positions and shapes in space, thus suggesting a variation of rotation feature along the pulse splitting. To better unveil this effect, we show the corresponding cross-sections of spatial intensity profiles at distinct times in Fig. 3(a-c), namely the center of our retarded time frame, and the temporal locations of the maximal intensity of the two first sub-pulses, respectively. As above suggested, we clearly observe that the intensity peaks do not rotate at the same velocity as the initial propagation constant difference between the OAM modes at . The angular delay between the most intense subpulse (Fig. 2(d)) and the linear rotation at is found to be ~1.46 rad after 6.1 cm. As a consequence, the rotation rate of the material wave can be determined as , with 24 rad m -1 . From the above determination of and , we can now provide a detailed analysis of the 3D-phase-matching pattern depicted in Fig. 2(e) by using our theoretical approach, in order to confirm the emission of the helicon wavepacket. Figure 4 shows the angularly-resolved spectra for the five first radial and angular indices obtained from numerical simulation at the output of the fiber (i.e., after 8.5 cm of propagation). The white iso-contour superimposed to the mode-resolved output spectrum delimits the inner angular-spectral region that satisfies Eq. 24 with parameters fixed above when 5 mm (roughly the distance corresponding to the lifetime of the most intense subpulse). All the mode-resolved output spectral shapes are well fitted by this contour, which confirms that a helicon wavepacket rotating at and propagating at group velocity is generated during the nonlinear propagation. A clear conical wave of fish-wave type is revealed over the lowest orders of OAM fiber modes, a typical signature of pumping close to the zero dispersion of the medium or waveguide [START_REF] Kibler | Discretized conical waves in multimode optical fibers[END_REF][START_REF] Kolesik | Interpretation of the spectrally resolved far field of femtosecond pulses propagating in bulk nonlinear dispersive media[END_REF]. We also notice that the bandwidth of phase-matching condition strongly reduces for increasing values of radial or angular indices (i.e., higher order modes). Note that, the tolerance of phase-matching condition in Eq. 24 (as the wavepacket is generated over a finite distance) makes that spectral components forming the wavepacket are not delta functions distributed over a given family of modes, in contrast with purely non-spreading wavepackets. Each mode composing the helicon wavepacket here supports a certain bandwidth, thus the latter will be dispersive, as described by Eq. 15.

Discussion and conclusion

Firstly, we have shown that a pump short pulse built from the superposition of two beams having different topological charges nonlinearly propagates along a helical trajectory in the waveguide. Close to the critical power, the nonlinearly-induced formation of an ultrabroadband localized structure (pulse or shock front according to the dispersion regime) then takes place, thus generating the main axial spectral broadening. Besides, it also emits resonant radiations through intra-and intermodal properties (conical spectral broadening) satisfying the phase-matching condition described by Eq. 25, and whose linear superposition forms a spatiotemporal helicon wavepacket with its group velocity and rotating rate (i.e., distinct from characteristics of initial pump pulse due to the nonlinear effects which implies corrections and ). It appears that our analysis also unveils the intensity dependence of both group velocity and rotation rate of the ultra-broadband localized structure at the origin of the helicon wavepacket. One well-known physical effect at the origin of such a dependence of the group velocity is the self-steepening effect. It usually induces a clear optical shock appearing on the trailing edge of an ultrashort pulse when studied in the normal dispersion regime, which can also seed a conical wave of X-type [START_REF] Kibler | Discretized conical waves in multimode optical fibers[END_REF][START_REF] Kolesik | Interpretation of the spectrally resolved far field of femtosecond pulses propagating in bulk nonlinear dispersive media[END_REF]. In the present case, this effect is less noticeable but still present at the pulse splitting stage. The formation of an optical shock results from the velocity difference between the pulse peak and its tails due to the nonlinear dependence of the refractive index. In the same way, we observe such equivalent dynamics in the spatial domain, as shown in Figs. Secondly, it is worth mentioning that, while the above example here considered a superposition of and as initial conditions, the dynamics still remains the same if one uses another superposition of two others OAM modes. For instance, our simulations showed (data not shown) that using an initial superposition of and also leads to the emergence of a helicon wavepacket. In that case, however, the generated wavepacket is only composed of modes with even topological charges due to the selection rules imposed by the symmetry. More generally, close to the critical power, we have found that preparing the initial pump pulse as a superposition of multiple OAMs of low topological charges always leads to the emergence of spatiotemporal helicon wavepackets. This is due to the low dispersion of the propagation constant with respect to the topological charge . Even prepared with multiple low-order OAMs, the pump pulse still rotates around the propagation axis without significant angular dispersion. Indeed, the nonlinear propagation tends to excite the modes that follow "the symmetry" of the field from which it is generated. It leads to the preferential generation of a given helicon wavepacket family, as shown in Fig. 4.

Accordingly, our theoretical framework can foresee the global shape of the output spectrum, but not the amplitude of the different components of this family, which strongly depends on the values of the overlap integral between modes (which also depends on the waveguide considered).

Finally, OAM fiber modes are known to provide an additional degree of freedom with which to control nonlinear wave propagation [START_REF] Rottwitt | Intermodal Raman amplification of OAMfiber modes[END_REF][START_REF] Liu | Nonlinear four-wave mixing with enhanced diversity and selectivity via spin and orbital angular momentum conservation[END_REF], but this also raises the usual challenge of mode coupling that OAM modes face when propagating in a conventional multimode fiber [START_REF] Ramachandran | Optical vortices in fiber[END_REF][START_REF] Ma | Propagation stability in optical fibers: role of path memory and angular momentum[END_REF]. A relatively small difference between the effective refractive indices of two modes favors power exchange (i.e., coupling) during propagation. Recently, specially designed fibers (i.e., vortex fibers) have been developed to reduce the mutual mode coupling and support stable propagation of OAM modes over long distances [START_REF] Ramachandran | Optical vortices in fiber[END_REF][START_REF] Ma | Propagation stability in optical fibers: role of path memory and angular momentum[END_REF]. We recall that our description and findings are valid in any waveguide with axisymmetric refractive index profile, so that we expect that future studies in specialty OAM waveguides will be successful.

Note that the spontaneous generation of helicon wavepackets in the case of nonlinear propagation can be also observed on very short distances as shown previously.

In conclusion, we have presented a novel kind of propagation-invariant space-time wavepackets that have the particularity to rotate around propagation and time axis. The invariance property of such electric fields is due to tight space-OAM-frequency correlations that compensate for dispersion and diffraction at the same time. The possibilities and interests of such helicon wavepackets are twofold. First, with the actual technologies of modal multiplexing and fiber fabrication, the experimental generation of such linear wavepackets in multimode fibers seems attainable. Beyond the fundamental interest of their observations, we strongly believe that potential applications in laser-matter manipulation could emerge from the generation of such wavepackets. Our approach could be also extended to more complex structured beams with time-varying OAM also known as self-torqued beams [START_REF] Rego | Generation of extreme-ultraviolet beams with time-varying orbital angular momentum[END_REF]. Second, we have shown that twirling wavepackets can naturally emerge from the nonlinear propagation of rather intense ultrashort pulses. As for conical waves emerging during filamentation in bulks, the formalism of helicon wavepackets brings a proper understanding of the phenomena taking place when intense pulses embedding different OAM beams propagate in bulks and waveguides. Given the current strong interest of both OAM-carrying beams and fiber modes, we expect that this work will stimulate further researches in the field. More specifically, we anticipate that space-time helicon wavepackets could emerge during the filamentation process in bulk media since Laguerre-Gauss beams are known to exhibit an OAM-dependent Gouy phase. Superimposed Laguerre-Gauss beams whose azimuthal mode index is correlated to their frequency are associated to helical structures for their phase and intensity profiles in space close to the waist [START_REF] Zhao | Dynamic spatiotemporal beams that combine two independent and controllable orbital-angular-momenta using multiple optical-frequency-comb lines[END_REF][START_REF] Pariente | Spatio-temporal light springs: extended encoding of orbital angular momentum in ultrashort pulses[END_REF]. As a consequence, such a field combined with our phasematching could then generate space-time helicon wavepacket if its power exceeds the critical power necessary to trigger the filamentation process. If so, it suggests important applications in terms of processing of transparent dielectrics.

Finally, from a general point of view, our work provides a general approach to explore the dynamics of three-dimensional spiraling wavepackets with topological properties, the outlook of this topic going well beyond optics since being studied nowadays in various branches of physics such as acoustic spanners [START_REF] Brunet | Experimental observation of azimuthal shock waves on nonlinear acoustical vortices[END_REF], polaritonics [START_REF] Dominici | Full-Bloch beams and ultrafast Rabi-rotating vortices[END_REF], plasma waves [START_REF] Vieira | Optical control of the topology of laser-plasma accelerators[END_REF], and electron or particle beams [START_REF] Pierce | Coiling free electron matter waves[END_REF]. For instance, strong efforts will focus on investigating optical schemes to create and manipulate three-dimensional free electron wavepackets or to transfer the symmetry properties of the driving field to molecular dynamics. Other perspectives could rely on the possible emergence of coherent structures in the form of such space-time wavepackets from turbulence in three-dimensional fluid flows.

Fig. 1 .

 1 Fig. 1. Linear construction of a spatiotemporal helicon wavepacket in a fused silica rod. (a) Family of modes satisfying the phase-matching condition (19) for the following parameters: 800 nm, and 3 ps m -1 . (b) Power spectrum of the few selected modes involved in our linear superposition. (c) Corresponding initial fluence distribution in space. (d) Corresponding phase distribution at . (e) Evolution of iso-contour of the fluence with propagation distance (at 1/e 2 and half-maximum) showing the counterclockwise rotation around the propagation axis. (f) Corresponding iso-surface of the spatiotemporal intensity profile at half-maximum that features the localized helicon wavepacket (see also its propagation in supplementary movie 1).

  Fig.2(a), and characterized by a relative group delay of 800 fs m -1 . The simultaneous strong broadening associated to the most intense sub-pulse seeds linear waves, which are resonantly amplified in higher order modes.

Fig. 2 .

 2 Fig. 2. Nonlinear propagation of 100-fs pulses (450-nJ energy at 1300 nm) in a 8.5-cm-long segment of step-index multimode fiber. (a)-(b) Evolution of the instantaneous power and normalized full power spectrum with propagation distance, respectively. The white dashed arrow in panel (a) indicates the group-velocity of the most intense sub-pulse. (c) Evolution of iso-contour at half-maximum of the fluence with propagation distance. (d) Output moderesolved power spectrum as a function of and indices observed after 6.1 cm of propagation.

Fig. 3 .

 3 Fig. 3. Cross-sections of 2D-spatial intensity profiles studied in Fig. 2(e) at distinct times of our retarded frame, namely (a) 0 fs, (b) 30 fs, and (c) 47 fs. The gray circle corresponds to the core/cladding interface. The white dotted line is the angular position from (~13.4 rad from the input, i.e. 2.13 rotations) whereas the white dashed arrow indicates the angular position of the maximal intensity. (d) Intensity profiles along the angular position for the different times under study (note that the radial positions of maximal intensity were chosen).

Fig. 4 .

 4 Fig. 4. Angular-mode-resolved output spectra for the first radial (top panels) and angular (bottom panels) indices. The white contour corresponds to the phase-matching of theoretical radiations forming the spatiotemporal helicon wavepacket propagating with a group delay and rotating around the propagation axis with angular delay compared to the initial pump pulse centered at .

Acknowledgments. This work was supported by the Agence Nationale de la Recherche (EIPHI Graduate School, Contract No. ANR-17-EURE-0002, The Smile project; PIA2/ ISITE-BFC, contract ANR-15-IDEX-03, Breathing project), the European Regional Development Fund, and the Bourgogne Franche-Comté Region. The authors also thank DNUM-CCUB (Université de Bourgogne) for HPC resources used for numerical simulations.

Supplemental document. See Supplementary movies for supporting content.

Notes. The authors declare no competing financial interest.


Dragoş Dobre

Eddy Bajic
email: eddy.bajic@cran.uhp-nancy.fr

ACTIVE PRODUCT MODELING FOR CHEMICAL SECURITY MANAGEMENT BASED ON SMART OBJECT CONCEPT

Keywords: Smart Object, Active Security, Wireless Sensor Network, Chemical Product, Ambient Intelligence

Nowadays the management of the security of goods and persons is a stake mattering in industries. In general, product's security level is the matter of each individual product and it often depends on products' interactions within vicinity. An efficient security management needs to be based on distributed security model supported by each individual products. Ambient and communication technologies bring new visions in creating reliable security systems, where products are transformed into smart products. Thus, each product acts as an active brick of the whole security system by means of embedded security functions as monitoring, controlling, decision making and alarming actions according to security and dependency rules. We propose the Active Product concept that provides product reactive behaviors to changes and inappropriate storage conditions, based on the security rules it is compelled. In our approach a product as for example a chemical container can be aware of its handling and storage conditions, and of changes in its environment and can be sensitive to products which surround it. Using wireless sensors attached to products, we create a network of interactive and communicating products to manage an Active Security System in an Ambient Intelligent Environment where human operators and hazardous chemical products are interacting securily in warehoursing and handling area. This article describes the concept Active Product by mean of smart object functionalities applied to hazardous industrial product and it proposes an internal structure model and associated capabilities of Active Product entity dedicated to security management.

INTRODUCTION

Sometimes, without our concern, we trust our lives to others as we depend on security measures that others take for us in environments that are out of our control. This is a fact in industrial application cases as storage and transport management of chemical substances. Without a high security level, industrial companies and workers are exposed to big risks on both economical and human aspects. That is a good reason to develop a security management policy of hazardous products based on rules and constraints associated to individual products and to products interactions.

A security system must be implemented each time we work with barrels containing chemical substances. This system considers all factors that can increase the intrinsic risk level of the substance, potentially explosive, toxic or harmful for its environment and the personal manipulating it. One main European Union laws concerning chemical safety is the Council Directive 67/548/EEC, describing regulations for classification, packaging and labeling of dangerous substances.

In a warehouse application scenario, we must consider safety data sheets for each substance in according to European Directives 67/548/EEC (European-Community 1967) and 91/155/EEC (European-Community 1991). Chemical data sheets can be found in Merck1 catalog. One can use these to implement a set of rules to monitor and control the product security level. This article describes the concept and realization of an Active Security System (ASS) for security management for warehousing and transportation of hazardous chemical products. It is divided into five sections: the second section introduces the Active Security concept relying on a Smart Object approach. In the third section we describe the Particle Computer technology used for the smart product implementation. The next two sections describe the methodology, design and implementation of the ASS. The last chapter will conclude and give a vision for future developments.

AMBIENT INTELLIGENCE AND SMART OBJECTS

As technology becomes more and more interweaved with everyday activities, a new era arises, one that has been assigned to Ubiquitous Computing [START_REF] Weiser | The computer for the 21st century[END_REF]. This newborn concept relies heavily on the use of Smart Objects in order to manage the correct handling of sensitive information.

The Smart Object concept was introduced for the first time in [START_REF] Gershenfeld | When Things Start to Think[END_REF]. Also referred as intelligent product by [START_REF] Mcfarlane | The intelligent product in manufacturing control and management[END_REF] it is commonly accepted in the research community on automated manufacturing systems as the merging of physical and informational objects characterized by the following elementary features enriched several times : possesses a unique identification [START_REF] Wong | The intelligent product driven supply chain[END_REF]; is able to communicate effectively with its environment [START_REF] Kintzig | Objets Communicants[END_REF]; can retain or store data about itself; is capable of taking part or making decisions relevant to its destiny; develops interactions between products and their environment; offers objects advanced services along its life cycle [START_REF] Bajic | Ambient services modelling framework for intelligent products, Smart Object Systems[END_REF].

The Ambient Intelligence (AmI) is the domain of research for many companies and research centers. According to [START_REF] Remagnino | Ambient intelligence: A new multidisciplinary paradigm[END_REF], AmI is based on a distributed layered architecture enabling ubiquitous communication and advanced human -machine communication protocol, allowing persons to be assisted by the surroundings' objects which are sensitive to people's presence and respond to their needs. The key technologies used to deploy AmI [START_REF] Alcañiz | New technologies for ambient intelligence[END_REF] are Ubiquitous Computing, Ubiquitous Communication and Intelligent User Interfaces. Ubiquitous Computing refers to integration of microprocessors in everyday objects. Ubiquitous Communication enables objects to communicate with each other, with the environment and with persons that are present in their proximity.

Adding so smart capabilities to real-world objects is a common goal in Ubiquitous Computing community. Then two important decisions, needed to be made when designing smart objects, are whether they will be able to function without any infrastructure support (this means as fully autonomous objects) and whether they advertise and propagate their individual functionalities and properties.

Research activities in Smart Object community are mainly focused on service infrastructures specification and prototyping of Smart Objects which are functionally autonomous, in the fact that they can operate with and without any infrastructure. A survey can be found in [START_REF] Bajic | Service modeling for smart objects in the supply chain using rfid and upnp technologies[END_REF]. In the manufacturing system community, we can identify the holonic approach, promoted by IMS/HMS research project. The holon-product concept is similar to Smart Object definition, but it is often physically distributed and dependent on hierarchical system architecture to fulfill its objectives. Multi Agent Systems programming techniques is an implementation technology mainly used to test and evaluate distributed models of holons' collaborative interactions [START_REF] Fisher | Agent based design of holonic manufacturing systems[END_REF]. Our approach is closer to the Smart Object community to assume a fully autonomous product behavior by itself.

"Smart-its" projects community [START_REF] Holmquist | Building intelligent environments with smart-its[END_REF]) creates autonomous smart objects by attaching small computational devices (smart-it devices) to physical object, thus, they argue in favor of Cooperative Artifacts as communicating standalone devices that do not rely on exterior supervision.

In the aim of managing the security level of goods and people in the activities of storing and handling of hazardous chemicals products, we can identify a number of scenarios leading to dangerous situations that can occur in a variety of working environments: at a chemical plant, in an external warehouse or during transport, handling or moving between companies. Moreover, these environments are not under uniform control, but involve diverse ownerships and somehow diverse system infrastructures or organizations.

So, smart products can be a way to achieve interoperability among companies or infrastructures as they can rely very weakly on the hosting system without denying their intrinsic capabilities for their selfmonitoring or cross-regulation.

Thus, autonomous active products, implemented through containers equipped with Smart-its Particles, are able to collect data from the outside, process it and transform it into knowledge that is used in order to generate an action according to the situation [START_REF] Strohbach | Cooperative artefacts -a framework for embedding knowledge in real world objects, Smart Object Systems[END_REF].

The key point of our approach is that the knowledge associated within the active product is stored and processed within the product itself. Therefore a container is able to know its own status and can perform communication with other containers to make decision on a changing environment or product intrusion. The distributed knowledge base is a good way to solve the problem of variable surroundings appearing in the lifecycle of the chemical products. However, this might lead to a difficult decision when it comes to delimiting the information that should be stored in each object and to over flooding with messages from one product to another.

The presented paradigms and research objectives are obviously close to industrial practical needs mainly within the scope of security management covered by the present paper and which is the center topic of the European project FP6 Cobis [START_REF] Cobis | eec on the approximation of laws, regulations and administrative provisions relating to the classification, packaging and labelling of dangerous substances[END_REF]. This pinpoints the need to spend research efforts on both conceptual research activities on concepts and methodologies enhancements and also on development, implementation and testing of artifact solutions, feasibility demonstration and large scale evaluation.

We need a better understanding of the smart objects' capabilities and opportunities to face the research and industrial challenges and we urgently need to define and assess an internal smart object's model and associated implementation structures for evaluation and validation aspects.

The following sections detail our conceptual contribution and practical implementation and evaluation of Active Product concept.

SMART OBJECTS PLATFORM

Smart Object development needs some characteristics to be fulfilled. It can memorize its state; it has the ability to sense its environment, to communicate with the surrounding and to react, after taking a decision on its own.

Figure 1: Technologies used to implement a Smart Object

There exist several technologies that present capabilities requested by a Smart Object. In fig. 1 we illustrate our view in the ubiquitous technologies evolution [START_REF] Bajic | Service modeling for smart objects in the supply chain using rfid and upnp technologies[END_REF]. As the Barcode is used to identify the type of a product, using an optical lecture in direct view, the technology has been improved, offering the possibility to save proprietary information in non-volatile memory inside one RFID 2 tag. At 2 Radio Frequency Identification present, we dispose of computational and communication capabilities to deliver automatic treatment of the exchanged information by SmartCards and WSN3 .

Our first architecture of Smart Object concept implementation [START_REF] Cea | Ambient services interactions for smart objects in the supply chain[END_REF]) used RFID technologies, to save proprietary information, and UPnP services architecture4 , to attach a bunch of customized services to each physical product that can be requested from any place in the networked architecture as a service-based virtual image of the physical object. The need of a Middleware computer to interact continuously with the RFID tagged object is one inconvenience of the proposed architecture. Also, the computer needs network connection to allow communication between objects. So, the Smart Object represents a virtual representation of the physical object, an informational entity (UPnP Device) able to communicate, interact and to take decisions regarding its state. Using a specialized UPnP Control Point, the user can interact and exchange information to survey and control the Smart Object.

From the five technologies, the one which gives us all desired functionalities is the WSN. One node can survey its near environment, communicate with other nodes and have computational algorithmic behaviors according to perceived information. Examples of WSN nodes are Mica2 produced by Crossbow The project uses the pParticle ver. 2/32 wireless sensor with the full sensor circuit SSimp ver. 2/02. This WSN has been chosen for its capabilities (sensors, computational power, communication speed), which help us to detect all factors influencing the security level. One barrel contains a chemical product and has attached one pParticle device. Within the WSN, the Smart Objects can interact freely, exchanging objectto-object information.

The user-to-object interaction is implemented with Teco's wireless bridge, WBride. The user is exchanging information using a simply PDA with appropriate application.

Next section presents Particle platform by means of hardware devices and communication capabilities. We use these sensors to create an image of the ambient environment. This way, the product can actively verify its intrinsic security state, reacting to changes affecting its normal storage or handling conditions. Moreover, the voltage sensor surveys the battery life, one of the major problems in WSN. In addition, the device includes a RSSI 8 sensor to be used for distance measurement between two devices. The Smart Objects can automatically detect incompatibilities between them. RSSI is discussed in section 5.1.

Particle communication

The pParticle devices are equipped with a 125 kBit TR 1001 radio frequency module functioning at 868.35 MHz. The RF communication protocol is based on the wireless ad hoc customized protocol AwareCon [START_REF] Beigl | Awarecon: Situation aware context communication[END_REF]. Its design follows the fundamentals of the established OSI/ISO layered approach. Aware-Con (fig. 3) is composed of three layers: the physical radio frequency layer (RF), the Link Layer (LL) and the Application Convergence Layer (ACL). One of the most important aspects in designing a communication protocol for a distributed networked sensor systems is mobility. As a result, AwareCon is able to handle a high mobility of particle nodes, the main delay for the synchronization with another partner being around 40 ms.

The design of AwareCon also reflects the concept of a fully distributed system. All nodes have equal responsibilities to establish time slots, exchange synchronization signals and keep an established timing scheme alive. There are no access points or master devices like in WLAN, Bluetooth or many other known protocols. The channel access uses a nondestructive bit wise arbitration known from wired networks such as the CAN field bus. This access method achieves low collision rates especially for a high number of concurrent nodes and can also handle priorities. A common communication language is required to en-able the information exchange between nodes. In the Particle System, the proposed approach is ConCom [START_REF] Krohn | Concom -a language and protocol for communication of context[END_REF], an approach to represent transmitted data as tuples. A tuple is a byte sequence that starts with a type identifier (2 bytes), followed by a length statement (1 byte) and then the number of data bytes specified by the length. The first tuple is referred to as the subject and it is used to enable application specific data processing within the system. An application subscribes itself to a subject and filters thereafter all received information, while the procedure for sending from the application uses the subject as the prescript of the outgoing message.

AN ACTIVE SECURITY ENVIRON-MENT

As a standalone device, the hazardous product, acting as a Smart Object, collects data from the environment and process it. Its natural reaction to an environmental threat is an audible and/or visual alarm, in order to alert and announce of an immediate threat the afferent personnel manipulating it. Other reaction can be alarm messages sent to notify the security personnel, with detailed information concerning the cause.

When more products are close, Smart Objects can react to each other by requesting information or responding to another. In both cases the reactive behavior of the hazardous products creates an active security environment (fig. 4).

Figure 4: Active products in an Ambient intelligence environment

Object comportment is given by a series of information and rules. At a configuration stage, static information is stored on each product by central monitoring software: substance name, substance ID (from Merck international catalog for chemical products numbering), product's electronic product code (EPC), security phrases and security symbols according to European directives 67/548/EEC (fig. 5). This represents the core information, to uniquely identify the product as a hazardous and commercial item. Other useful information can be saved in product memory according to use case. Supply chain traceability and manufacturer's information are supported by EPC numbering schema and Internet information services support.

Figure 5: Chemical product security symbols: toxic, irritant, inflammable, explosive, corrosive, oxidant, dangerous for the environment, radioactive Each product has two security levels: the intrinsic and the extrinsic. The intrinsic security level is dependent of environmental factors (temperature, shocks, etc.) that influence in a direct mode the product. The extrinsic security level is given by the compatibility between chemical substances.

Working or surrounded with hazardous products creates dangerous space where certain ambient and environmental parameters influence the intrinsic security level of each product. Variation of the ambient factors can disturb the normal and balanced state of the chemical substance inside a barrel. We consider the following environmental factors are capital for intrinsic security monitoring of each hazardous product (a radioactive sensor should be also of great interest) (fig. 6): temperature : product react violently, changing its structure and stability, ending with an explosion; acceleration : product reacts to shocks; light and infrared light : product reacts and alters with light; audio level : product reacts to frequencies stimulation or audio levels.

The intrinsic security level is processed using sensors data, considering a set of boundaries limits for pa-Figure 6: Product sensitivity to Ambient characteristics rameters values. These boundaries represent the security phrases registered in memory. For the temperature value we considered the upper and lower limit. For light, infrared light, audio and acceleration values we used only upper limits. Using the safety data sheet, we can establish the specific value for each limit. Therefore, the smart product determines its internal security level with reasoning, by inference on the sensor's values and boundaries limits.

The extrinsic security level is deducted using the nature and the compatibility agreements given by other products located in the vicinity. A compatibility relationship can be derived from the security symbols, coded in the product static information. Using a compatibility matrix that uses security symbols, we succeeded to determine if two products are compatible or not. Based on the compatibility matrix, some security symbols are incompatible with others, so every substance has compatible and incompatible symbols. The extrinsic security level of a hazardous product is Good, when all products in the vicinity are compatible, and Dangerous, in all other cases. Figure 7: Chemical product security level: Good, Bad or Dangerous

In the active security environment, a Smart Object sends product information: the substance Merck ID and its security symbols. Other Smart Object, receiving the information, calculates the message RSSI value, verifies the compatibility with the first prod-uct, using its symbols, and determines its extrinsic security level.

The global security level for the hazardous product is a logical combination of intrinsic and extrinsic security levels with alphabet G (Good), B (Bad), D (Dangerous) where B and D are absorbent states.

Manipulating the Smart Object, the worker receives security information from all barrels around him, allowing to know, in real-time, what is happening in the warehouse and to intervene in case of a problem.

SMART OBJECT'S BEHAVIOR FOR SE-CURITY MANAGEMENT

Each hazardous product, equipped with pParticle device, is transformed into a Smart Object. Its main activity is the security management, based on product's characteristics, limitations rules, security symbols and compatibility matrix, coded as static information. Hazardous product has the ability to monitor its near environment and communicates with other hazardous products or systems. In an AmI application, the main point is human-tosystems interaction. This is why one model must begin with use cases and with scenarios study. Within this study we propose messages that need to be exchanged between a Smart Object and another Smart Object or user application. In fig. 8 we present a sequence diagram containing messages exchanged in our environment. Other messages, sent automatically by the object, announcing its presence (greeting), to calculate the distance between two objects (RSSI) and to alert in case of a security problem (alert).

Using these messages, we created an internal algorithmic behavior of a Smart Object, illustrated in fig. 9. It is built on three main loops: not configured loop, new message received loop and internal check loop.

In our scenario, a new barrel enters the warehouse. Not being configured, it sends periodically not-configured messages to the supervisor system. This will stop as it receives a configuration. Qualified personnel remotely configure the smart product with suitable information related to the substance inside the barrel. The configuration is composed by product proprietary information and the security rules. This can be done automatically at manufacturing stage in a M2M 9 approach. 9 Machine to Machine After receiving the configuration, the Smart Object enters IDLE state and sends greeting messages. In IDLE state the Smart Object reads all sensors values, calculates the RSSI values for each Smart Object in the proximity and calculates, using these values, a product global security level. The global security level can be good, warning or dangerous. The normal state is the good security level. A product is in this state if all sensors are in a limit of 90% of recorded security limits.

The greeting message is sent in broadcast. As information it contains the substance ID and the product global security level. As no condition monitoring is changing, the Smart Object remains in this state.

When a sensor records more than 90% of security limit, the Smart Object records the value and enters the warning state or security level 2. If it succeeds 100% of the limit, it enters in security level 3 or alarm. In both cases, the smart product sounds an audible alarm and sends alarm messages. Alarm message indicates the sensors out of range. In case of product incompatibility, the message contains the ID of the concerning barrel. As soon as the threat has disappeared, it returns to IDLE state.

All messages are broadcasted (greeting, command and RSSI). They are used for object-to-object or object-to-supervisor communication. The command messages are mainly used by the supervision application to write the Smart Objects configuration, to read the configuration and to monitor the sensors values. The monitoring is the only message sent to a unique object.

RSSI for extrinsic security level determination

For extrinsic security level calculation, RSSI method is used. We estimate the distance with other hazardous products that could worsen the global security level by way of their incompatibility. To easily attach the pParticles devices on industrial chemical containers, we integrated them in conductive ABS plastic boxes. Particle's antenna is left on the outside, so it can be freely tilted to any position. After conducting experiments, with and without packaging, we concluded that it has no influence on the message transmission quality.

Many other factors, mostly controllable, affect the values indicated by the RSSI. One of them is the particle's antenna orientation. During our experiments we noticed the variation of indications as follows: when the antenna is pointed upwards the RSSI value is smaller than when it is positioned horizontally.

Yet another factor that influences the measurements is the place where the particles are positioned. When they are situated on the room floor, the signal strength is quite inferior to that when particles are positioned on top of the barrel. This can limit the detection of one barrel fallen on the ground.

Figure 10: RSSI measurements distributions RSSI level is obviously conditioned by the batteries charge level. Therefore, transmission signal strength must be accorded consequently. Moreover, the signal strength becomes a real problem when obstacles are positioned between the smart products. For example, paperboard boxes extend the interval between consecutive RSSI measurements, but the electronic devices act like a screen and disturb completely the communication.

An important issue is that not all particles receive the same RSSI value for a given distance. Even though the circuit is the same, an experiment with two particles has shown that the average RSSI measurements over a period of time differ substantially for the two devices. Fig. 10 shows the distribution functions of RSSI measurements for two Smart products A and B, at a distance of 1m from each other. The measurement is asymmetric, as one particle indicates an average RSSI of 43.70 and the other 47.41. During our experiments, we have found relevant indications of the RSSI for distances up to 3 meters. This distance is appropriate to define a security bubble around a product. For longer distances, RSSI messages were becoming either too scarce or completely absent. Fig. 11 reveals the RSSI indications at various distances and the standard deviation tube.

CONCLUSION

Our contribution in the presented work aims at responding to both conceptual and practical objectives of Active Security Management supported by interactive Smart Products. Conceptual aspect is covered with the proposal of the Active Product concept along with smart objects interaction architecture. An internal behavior model of an Active Product, supported by a generic algorithm, is proposed to perform security management over a surrounding community of products. Practical contribution is given with the implementation and evaluation of Smart-its platform for validation of Active Product capabilities, which show the limits and current uncertainties of the Ambient Intelligence technology.

Providing capabilities for ambient sensing and monitoring, vicinity communication, and a rule-based decision making according to security compatibility regulation specifications, we grant to the hazardous products an autonomous and adaptive security-oriented behavior. Proactive alarm and preventive actions can be embodied in each hazardous product to prevent a catastrophic situation in chemical areas.

Our model for a smart product's internal behavior, dedicated to security management, defines the concept of an Active Security Distributed Management System. Both theoretical and technological gaps are still outstanding: product on-board energy saving, robustness of product's behavior in large scale community, refinement of internal product behavior model to allow adaptability and reconfigurability in different use case scenarios, outside security domain. We will deeply investigate the monitoring of the product's extrinsic security by the cooperation with distant products.

  5 and pParticle produces by Teco 6 and commercialized by Particle Computer 7 .

  Figure 2: The pParticle 2/32 with SSimp 2/02

Figure 3 :

 3 Figure 3: The AwareCon frame structure

Figure 8 :

 8 Figure 8: Messages exchanged between Smart Objects

Figure 9 :

 9 Figure 9: The algorithmic behavior for security management on a Smart Product

Figure 11 :

 11 Figure 11: RSSI measurement for products distance evaluation

Table 1 :

 1

	pParticle base characteristics
	Feature	Description
	Microcontroller	PIC 18F6720 at 20 MHz
	Internal Memory	128kbyte program Flash
	RAM	4kbyte
	EEPROM	1kbyte
	Additional Memory 512 kbyte FLASH
	Communication	RF through RFM TR1001
	Bandwidth	125kbit
	RF Power	<1mV
	Interface	21 pin connector
	Power regulation	0.9 to 3.3 V
	Board core voltage	3.3V
	Size	45x18 mm (no battery)
	Code	in C, OTAP

http://www.merck.de

Wireless Sensor Network

Universal Plug and Play

http://www.xbow.com

http://particle.teco.edu

http://www.particle-computer.de