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Granular materials like cereals and sands are widely present in different industrial fields like food and agricultural industry. Capacitive planar sensors for granular materials suffer from an additional source of uncertainty related with the random distribution of the grains inside the sensitivity volume. In this letter, we propose a capacitive planar sensor with a new electrode geometry for increasing the sensitivity volume, thus improving the accuracy of the measurements. The geometry is optimized through a design based on numerical simulations in order to maximize the sensitivity volume without reducing the dynamics of the capacitance variation. Experimental results obtained by considering two different granular materials show a reduction of the standard deviation due to granularity of 20% with respect to a classical interdigital electrode structure. Finally, it is experimentally shown that the increased accuracy of the proposed geometry can be fruitfully used for contaminant detection in a mixture of granular materials.

This is the pre-review draft of the second of two papers submitted to Journal of Lightwave Technology, where the effect on confinement loss of thickness variations along the perimeter of the tubes composing the cladding of inhibited-coupling guiding Tubular Lattice hollow-core fibers is investigated by using the Azimuthal Fourier Decomposition technique (developed in Part I) for the description of the cladding modal dynamics and their interaction with fundamental core mode. The results show that the thickness inhomogeneity affects the confinement loss spectrum through confinement loss increase and frequency red- and blue-shift of the high loss spectral regions. The magnitudes of the confinement loss increase and the high-loss region frequency shift strongly depend on the spatial distribution of the thickness inhomogeneity. The study provides insight into the loss mechanism of non-ideal tube lattice fibers, it allows quantifying the impact of such kind of structural deformations, identifying the route to make fibers more resilient to such fabrication imperfections, and highlighting once again the importance played in inhibited-coupling fibers by the interaction between core modes and the intricate set of cladding modes.

This is the pre-review draft of the first of two papers submitted to Journal of Lightwave Technology, where we propose and apply a methodology for the analysis of the loss in tube lattice fibers (TLFs) based on the azimuthal Fourier decomposition (AFD) of the fiber’s modes along the perimeters of the tubes composing the fiber cladding. This technique combined with the coupled mode theory constitutes an effective approach for gaining insight in the waveguiding mechanism and for additional loss analysis due to fiber non-idealities. In this part I we describe the approach and apply it to loss analysis of ideal TLFs. The approach is then applied to the analysis of tube thickness variation effects in the part II.

While optical fibers display excellent performances in the infrared, visible and ultraviolet ranges remain poorly addressed by them. Obtaining better fibers for the short-wavelength range has been restricted, in all fiber optics, by scattering processes. In hollow-core fibers, the scattering loss arises from the core roughness and represents the limiting factor in reducing their loss regardless of the fiber cladding confinement power. To attain fibers performing at short wavelengths, it is paramount developing means to minimize the height variations on the fiber microstructure boundaries. Here, we report on the reduction of the core surface roughness of hollow-core fibers by modifying their fabrication technique. In the novel process proposed herein, counter directional gas fluxes are applied within the fiber holes during fabrication to attain an increased shear rate on its microstructure. The effect of the process on the surface roughness has been quantified by optical profilometry and the results showed that the root-mean-square surface roughness has been reduced from 0.40 nm to 0.15 nm. The improvement in the fiber core surface quality entailed fibers with ultralow loss in the short-wavelength range. We report on fibers with record loss values as low as 50 dB/km at 290 nm, 9.7 dB/km at 369 nm, 5.0 dB/km at 480 nm, and 1.8 dB/km at 719 nm. The results reveal this new approach as a promising path for the development of hollow-core fibers guiding at short wavelengths with loss that can potentially be orders of magnitude lower than the ones achievable with their silica-core counterparts.

International audience; We report on the development of new state-of-the-art ultralow-loss hollow-core fibers guiding in the short-wavelength range. The new record low attenuation values are as low as 50.0 dB/km at 290 nm, 9.7 dB/km at 369 nm, 5.0 dB/km at 480 nm, 0.9 dB/km at 558 nm, and 1.8 dB/km at 719 nm. Notably, the core roughness levels of the new state-of-the-art fibers are under the thermodynamic equilibrium limit set by the surface capillary waves scenario in amorphous silica.

In this paper a multiband automotive antenna for 5G and Vehicle to Everything (V2X) communications is presented. The antenna is designed to fit the commonly used sharkfin radome. To overcome the problem of the small volume available a 3D full metal radiating element is proposed. In order to validate the design a prototype is realized and measured. The proposed antenna achieves good matching conditions (Voltage Standing Wave Ratio (VSWR) < 2.5) trough the worldwide 5G frequency bands. Furthermore, the radiation characteristics fulfill the goal showing a Linear Average Gain (LAG) ranging between -5 dBi and 0 dBi.

In this paper an antenna module for automotive 5G communication is presented. The antenna is designed to cover all the 3GPP n258 frequency band i.e. from 24.25 GHz to 27.5 GHz. At these frequencies, the path loss and obstacles attenuation are high. For this reason, to reach an acceptable communication distance a relatively high gain is required. Furthermore, to achieve a more robust system to multipath interferences the antenna is designed to operate with dual linear polarization. The proposed antenna is composed by an array of 8 dual polarized stacked patches. Finally, in order to suppress the radiation pattern distortion due to surface waves (SWs) propagation, an Electronic Band Gap (EBG) structure is implemented. To validate the design a prototype has been realized and measured.

Hollow-Core Inhibited Coupling Fibers have been the target of great investigation efforts due to their peculiar properties [1] . These features, in addition to their capability to handle high power levels, make this kind of fibers very attractive for a wide range of applications such as high-power lasing, light-gas interaction, plasma photonics, quantum physics, terahertz systems, and gas- and bio-sensing. The guidance mechanism of Inhibited Coupling (IC) fibers relies on the spatial mismatch between core modes and cladding modes (CLMs). In this way, the coupling between core and CLMs, caused by their spatial overlap, is dramatically reduced [1] . An effective way to strengthen this effect is to deploy hypocycloid core-contour (i.e. negative curvature) fiber designs [2] . A negative curvature core can be simply obtained by fabricating a cladding structure made of a layer of thin isolated glass tubes arranged around the hollow core (Tube Lattice Fibers – TLFs) [3] , [4] . Although TLFs have reached an extremely low transmission loss and are intensely developed [5] , [6] a gap remains between measured transmission and the theoretical minimum given by confinement loss (CL). This difference can be ascribed to geometrical non-idealities of the fiber structure, introduced during the fabrication process, even though a theoretical analysis showing the real reason for this gap is not yet available.

International audience; We propose a hybrid Kagome-tubular lattice hollow-core fiber for ultra-low loss and single-mode operation. The fiber displays a minimum loss of 1.6dB/km at 1050nm and a higherorder modes extinction of 47dB for a 10m-long fiber.

International audience; This paper proposes a circularly-polarized continuous transverse stub(CTS) array antenna for space applications. The radiating structure consists of open-ended parallel plate waveguides(PPW), supporting the propagation of the transverse electromagnetic(TEM) and first transverse electric(TE1) modes. Such modes are orthogonally-polarized in the PPW cross-section and, if properly excited, can generate circular polarization(CP). The antenna radiating section consists of 32 long slots, parallel-fed by a quasi-optical system through a PPW-based corporate feed network(CFN) and an electric field rotator. The proposed antenna is matched in the 27.5-31GHz band while scanning up to +-20° in elevation, with an axial ratio(AR) below 3dB. © 2021 IEICE.

Remarkable recent demonstrations of ultra-low-loss inhibited-coupling (IC) hollow-core photonic-crystal fibres (HCPCFs) established them as serious candidates for next-generation long-haul fibre optics systems. A hindrance to this prospect and also to short-haul applications such as micromachining, where stable and high-quality beam delivery is needed, is the difficulty in designing and fabricating an IC-guiding fibre that combines ultra-low loss, truly robust single-modeness, and polarisation-maintaining operation. The design solutions proposed to date require a trade-off between low loss and truly single-modeness. Here, we propose a novel IC-HCPCF for achieving low-loss and effective single-mode operation. The fibre is endowed with a hybrid cladding composed of a Kagome-tubular lattice (HKT). This new concept of a microstructured cladding allows us to significantly reduce the confinement loss and, at the same time, preserve truly robust single-mode operation. Experimental results show an HKT-IC-HCPCF with a minimum loss of 1.6 dB/km at 1050 nm and a higher-order mode extinction ratio as high as 47.0 dB for a 10 m long fibre. The robustness of the fibre single-modeness is tested by moving the fibre and varying the coupling conditions. The design proposed herein opens a new route for the development of HCPCFs that combine robust ultra-low-loss transmission and single-mode beam delivery and provides new insight into IC guidance., Hollow-core photonic crystal fibres: Low loss with hybrid-lattice cladding A hybrid microstructured cladding significantly reduces confinement loss and preserves single-mode operation in hollow-core photonic crystal fibres. The hybrid cladding was conceptualised and designed by Fetah Benabid of XLIM Institute, CNRS, and colleagues in France and Italy. It is made of an inner cladding of six tubes connected to an outer basket weave-like ‘Kagome’ lattice via a number of small, thin tubes. Hollow-core photonic crystal fibres surrounded by this hybrid cladding demonstrated a minimum loss figure of 1.6 decibels per kilometre at wavelengths of 1050 nanometres with robust single-mode operation in a ten-metre-long fibre. The design demonstrates the concept’s potential for single-mode waveguides and could contribute towards realising the next generation of long-haul optical fibres.

We numerically demonstrate that a hollow-core fiber with a cladding comprised with a first-ring with non-touching tubes surrounded by a photonic bandgap outer-cladding combines low confinement loss, low optical overlap with silica and single modedness.

International audience; We report on inhibited-coupling hollow-core optical fibers exhibiting specific modal operation and low loss in the short-wavelength range. We show that judicious choice of the fiber cladding structure allows obtaining fibers that favor the propagation of higher-order modes and fibers which combine effective single-mode operation and low loss. Moreover, we report on recent improvements in hollow-core fibers with ultralow loss in the visible and ultraviolet range.

A low cost and low-profile multiband 3D antenna for roof-top vehicular applications is presented. The radiating element is obtained through a folded metal sheet shaping a geometry that makes the antenna works in the LTE (Long-Term Evolution) bands. Thanks to its particular shape and its small size the antenna can be positioned under the automotive common used shark-fin case. Furthermore, the design allows an easy industrialization process, using low cost material. In fact, the designed shape needs only to be cut and folded without any welding process. In addition to the designed antenna, simulations and measurements take into account another radiating element in order to analyze its behavior in a MIMO (Multiple-Input-Multiple-Output) configuration. For the proposed solution good performances have been demonstrated from both numerical and measurements results.

Multi-core fibers (MCFs) are promising solutions for high power fiber based devices as they reduce nonlinearity and other unwanted detrimental effects, like transverse mode instability, by transporting, instead of a single high power beam, several low-powered ones to be coherently combined at the fiber output. This method relies on accurate evaluation of the phase differences between signals in different cores, which are significantly impacted by changes in the effective index of the propagating modes. For this to be effective, spatial heat generation must be accounted for. In particular, the heat flux from the doped cores to the external boundary causes a temperature gradient across the fiber, which affects the refractive index distribution, creating the chance for effective index change and thus dephasing of the output beams, which is harmful for beam combining. The results of in-depth numerical analysis on the performance of 9-core and 16-core MCFs under thermal effects are presented by studying the mode phase sensitivity to heat load and by introducing a coupled-mode theory model to study possible optical coupling effects. The effectively single-mode condition is also investigated by calculating the core modal overlap differences between fundamental and higher-order modes.

A new methodology to the analysis of the results of Finite-Element Modeling (FEM) simulations at electric field singularities is proposed. The method, that can be easily applied in the post-processing phase of the electromagnetic FEM analysis workflow, is based on the weighted averaging of the calculated electric field magnitude within small volumes including the singularity point under investigation. In the paper, the proposed approach is applied to the electrical stress analysis of a high-voltage device modeled by means of a commercial electromagnetic FEM tool. In comparison to the conventional metric of the maximum field evaluation usually adopted for the analysis of electrical stress in insulators, our approach features several advantages: (i) the outcome of the analysis is independent of the numerical grid refinement at the singularity, thus allowing direct comparison of calculated electric field with the material dielectric strength; (ii) the method is robust against slight modifications of the geometrical shape of the singularity; (iii) on the other hand, for a given shape, the analysis outcome responds to significant variations of the singularity size or, in other words, of its sharpness; (iv) in the analysis of highvoltage devices, the approach can be applied for the estimation of the discharge volumes corresponding to different singularity types of different device geometries. In the paper, the new methodology is explained in details and is applied to simple but significant case studies.

In this work, we propose analytical formulas for the estimation of dispersion properties and effective area of the fundamental mode of hollow-core inhibited coupling fibers with a microstructured cladding composed by a ring of dielectric tubes. The formulas are based on a model which has already been successfully applied to the estimation of confinement loss. The model takes into account the effects of the coupling of the fundamental core mode with the cladding modes in the context of the single-tube approximation. Effective index, group velocity dispersion, and effective area of the fundamental mode are estimated and compared with the results obtained from numerical simulations, by considering ten different fibers. The comparison shows a good accuracy of the proposed formulas, which do not require any tuning of fitting parameters. On the basis of the analysis carried out, a scaling law relating the effective area to the core radius is also given. Finally, the formulas give a good estimation of the same parameters of other Hollow-core inhibited coupling fibers, such as nested, ice-cream, and kagome fibers.

The results about the measurements of complex permittivity of hydraulic mineral oil is here presented. Two different oil samples were analyzed using this method: a non-used one and a sample with 1000 hours of usage. The obtained data were collected by using the cavity perturbation technique exploiting a coaxial cavity resonator. To overcame the approximations that are usually made in this technique a particular measurement procedure based on 3D-full wave electromagnetic simulations has been performed.

Multi-core fibers are a technology currently at the forefront of research due to the increasing need for efficient transport of high-power continuous and pulsed laser beams [1], Power scaling is gained by propagating independent beams through spatial multiplexing in different cores to avoid thermal instability issues [2], shifting the traditional trade-off involved in enlarging the mode area while preserving single-mode operation. Phase difference compensation between the various beams is achieved by a piezo mirror system, and experimental results show that onset of multi-modality in heated cores is one of the main causes of efficiency reduction [2].

In this work, we propose an analytical model for estimating confinement loss in Tube Lattice Fibers. It is based on the single-tube model and the inhibited coupling waveguiding mechanism. The comparison with numerical simulations of tube lattice fibers having different geometrical parameters and dielectric refractive indexes demonstrates the model validity and effectiveness. Being based only on analytical closed formulas, it constitutes a useful tool for rapid estimation of TLF CL. It also gives a more in-depth insight into the TLF guiding mechanisms, confirming the inhibited coupling is an appropriate and effective model for such kind of fibers.

Photonic Crystal Fibers represent a good platform for the development of sensitive and cheap sensors for the detection of protein and DNA sequence. The holes running along the fiber allows the infiltration of biological substances and for biological layers to attach on the air-dielectric interfaces. In addition, the presence of a hollow core can further increase the infiltration feasibility and the sensor sensitivity. Recently, the possibility in using hollow core tube lattice fibers (HC-TLFs) for label-free DNA detection have been numerically investigated. The sensing is based on the waveguiding mechanism, that is inhibited-coupling which makes the HC-TLF transmission properties particularly sensitive to the thickness of the glass struts composing the microstructured cladding of those fibers. The molecular interactions between the surface of the glass and the target to be detected results in a generation of a biological layer which modifies the strut thickness and thus the fiber transmission properties. The aim of the present work is to experimentally demonstrate that HC-TLF can be successfully used as bio sensor for proteins. After a functionalization of the silica surface through a solution of aptes, a layer of biotin was deposited on inner surfaces of the fiber. The fiber was than infiltrated with a solution containing streptavidin and an additional bio-layer is deposited on the surfaces. The experimental results demonstrated a red shift of the entire fiber transmission spectrum of about 7 nm corresponding to an additional bio-layer with thickness of 6.45 nm which is fully compatible with the size of streptavidin molecules of about 6nm.

Understanding cladding properties is crucial for designing microstructured optical fibers. This is particularly acute for Inhibited-Coupling guiding fibers because of the reliance of their core guidance on the core and cladding mode-field overlap integral. Consequently, careful planning of the fiber cladding parameters allows obtaining fibers with optimized characteristics such as low loss and broad transmission bandwidth. In this manuscript, we report on how one can tailor the modal properties of hollow-core photonic crystal fibers by adequately modifying the fiber cladding. We show that the alteration of the position of the tubular fibers cladding tubes can alter the loss hierarchy of the modes in these fibers, and exhibit salient polarization propriety. In this context, we present two fibers with different cladding structures which favor propagation of higher order core modes – namely LP11 and LP21 modes. Additionally, we provide discussions on mode transformations in these fibers and show that one can obtain uncommon intensity and polarization profiles at the fiber output. This allows the fiber to act as a mode intensity and polarization shaper. We envisage this novel concept can be useful for a variety of applications such as hollow core fiber based atom optics, atom-surface physics, sensing and nonlinear optics.

Multi-core fiber capability to deliver several independent beams in a single structure has been deeply investigated to obtain spatial multiplexing in optical communication. Recently, the coherent beam multiplexing idea has been extended to high power fiber laser field, where multi-core fiber amplifiers, combining low power beams, promise to overcome thermal mode instability, which characterizes single-core fiber amplifiers. Although coherent output beam combination is advantaged in multi-core fiber, the understanding of core phase shifts is necessary to implement efficient beam combination. In presence of thermal load, induced by pump-to-signal conversion quantum defect, a refractive index gradient is formed on the multi-core fiber amplifier cross-section, thus changing core propagation properties and possibly creating unwanted core couplings. In this work a 9-core double-cladding fiber amplifier is numerically investigated by varying the core thermal load, from 2 to 15 W/m, in order to understand the structure propagation mismatch. The 9 cores are organized in a 3×3 regular grid, each core has a diameter of 19 μm and a spacing of 55 μm. Cores numerical aperture is 0.06. The outer cladding has a diameter of 340 μm. A comparison between a rod-type fiber amplifier configuration and a flexible fiber amplifier has been performed. Results show that the cores can be divided in three groups according to their propagation properties: central core, side cores, and corner ones. The phase shift between these groups, or equivalently the effective index difference, becomes higher with the increase of thermal load. These observations are fundamental to implement a model for beam propagation in presence of thermal effect, to investigate the amplification dynamics along z-direction.

In this study, a low profile wideband 3D antenna for roof-top LTE vehicular applications is presented. The structure consists of folded metal sheets welded together building a geometry that allows covering two LTE bands: 700MHz–960MHz, 1700MHz–2700MHz. The shape and the size of the antenna allow to fit it under the shark-fin case, and thanks to the chosen material the production costs are low. Simulations and measurements have been performed taking into account two antennas working as a MIMO configuration. Both numerical and measurement results show good performances in terms of return loss, coupling and gain.

Unravelling superradiance, also known as superfluorescence, relies on an ensemble of phase-matched dipole oscillators and the suppression of inhomogeneous broadening. Here we report on a novel superradiance platform that combines an optical lattice free from the ac Stark shift and a hollow-core photonic crystal fibre, enabling an extended atom-light interaction over $2 \mathrm{mm}$ free from the Doppler effect. This system allows controlling the atom spatial distribution and spectral homogeneity whilst efficiently coupling the radiation field to an optical fibre. The experimentally-observed and theoretically-corroborated temporal, spectral and spatial dynamic behaviours of the superradiance, e.g., superradiance ringing and density-dependent frequency shift, demonstrate a unique interplay between the trapped atoms and the fibre-guided field with multiple transverse modes. Our theory indicates the resulting temporal evolution of the guided light shows a minimal beam radius of $3.1 \mathrm{\mu m}$ that is three times smaller than that of the lowest-loss fibre mode., Comment: 41 pages, 8 figures in total, the first 30 pages and 6 figures are main article and the remaining pages and figures are supplemental material

A profound knowledge of the cladding mode properties and their interaction with the core modes is decisive for successfully designing inhibited-coupling (IC) guiding hollow-core photonic-crystal fibers (HC-PCFs). In IC fibers, the core and cladding modes coupling is robustly minimized due to a strong mismatch between their transverse phases and a small overlap between their fields. In the latest years, great attention has been given to IC fibers, especially since the introduction of the hypocycloid core contour concept [3], which entailed a dramatic loss reduction and preceded the development of single-ring tubular-lattice (SR-TL) HC-PCFs [4]. The interest on SR-TL HC-PCFs is due to their noteworthy properties such as geometric simplicity and the absence of connecting nodes in the fiber cladding, which favors IC guidance. Efforts on SR-TL HC-PCFs development entailed the accomplishment of ultra-low loss and broad transmission bandwidth [5]. Here, we show that the alteration of the lattice-tubes azimuthal position in SR-TL HC-PCFs causes modifications in the mode loss hierarchy and can be used to select the propagation of specific higher order modes (HOMs). It can be done by conveniently enlarging the spacing between the cladding tubes and by providing additional leakage to the fundamental mode while keeping low impact in the confinement loss (CL) of specific HOMs. It allows obtaining uncommon intensity profiles and polarization properties at the fiber output allowing it to act as a mode shaper.

In this paper, we report how tube lattice hollow-core fibers can be successfully used to build sensors for molecule detection. The inner silica surface of the fiber is functionalized and coated with a probe layer, which permits to bond only with a particular molecule (the target). When the fiber is infiltrated with a solution containing the target, an additional layer is created on the silica surface, causing a redshift of the fiber transmission spectrum. The technique does not require any additional transducer component, such as Bragg gratings, amplifying techniques, such as nano-particles, nor coherent sources. It simply consists of the measurement of the transmission spectrum of a piece of fiber some tens of centimeters long. The principle is validated with experimental results showing the detection of the streptavidin protein. A solution containing streptavidin was flowed through the hollow core of the fiber coated with biotin. The measurement of the transmitted spectrum before and after the infiltration showed the presence of a few nanometer thick bio-layer.

International audience; Emission at 4.6 μm was observed from an N 2 O filled hollow core fiber laser. 8-ns pump pulses at 1.517 μm excited a vibrational overtone resulting in lasing on an R and P branch fundamental transition from the upper pump state. At optimum gas pressure of 80 Torr, photon conversion efficiency of 9% and slope efficiency of 3% were observed from a mirrorless laser. The laser threshold occurred at absorbed pump energy of 150 nJ in a 45-cm long fiber with 85 μm core diameter. The observed dependence of the laser output on gas pressure was shown to be a result of line broadening and relaxation rates.

We report on the development of hypocycloid core-contour inhibited-coupling (IC) Kagome hollow-core photonic crystal fibers (HC-PCFs) with record transmission loss and spectral coverage that include the common industrial laser wavelengths. Using the scaling of the confinement loss with the core-contour negative curvature and the silica strut thickness, we fabricated an IC Kagome HC-PCF for Yb and Nd:Yag laser guidance with record loss level of 8.5 dB/km associated with a 225-nm-wide 3-dB bandwidth. A second HC-PCF is fabricated with reduced silica strut thickness while keeping the hypocycloid core contour. It exhibits a fundamental transmission window spanning down to the Ti:Sa spectral range and a loss figure of 30 dB/km at 750 nm. The fibers’ modal properties and bending sensitivity show these HC-PCFs to be ideal for ultralow-loss, flexible, and robust laser beam delivery.

We present results on multi-core fibers (MCFs) for optical amplification/lasing obtained by combined thermal-optical finite-element method (FEM) simulations. Refractive index variation due to quantum defect caused heating is simulated to evaluate thermal dissipation-induced index gradients.

In this work, we propose an analytical formula for estimating confinement loss in Tube Lattice Fibers. The formula is based on a single-tube model and the comparison with numerical simulations of three TLFs designed to work in the UV, NIR, and THz spectral ranges shows good agreement.

In this work we report how tubular lattice hollow core fibers can be successfully used as sensors for proteins such as streptavidin. After a functionalization of the silica surface, a layer of biotin was deposited on the fiber core inner surface. The fiber was than infiltrated with a solution containing streptavidin and a red shift of the fiber transmission spectrum was experimentally observed.