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We are reporting on SiO2-Al2O3-La2O3 glasses—with and without Yb2O3—suitable for nonlinear and fiber laser applications. We will also be presenting successful supercontinuum generation and fiber laser operation around 1060 nm in step-index fibers. We optimized the glass compositions in terms of thermal and optical requirements for both a high La2O3 and Yb2O3 concentration and good compatibility with a silica cladding. We also produced bulk samples, unstructured fibers, and structured fibers with a SiO2-Al2O3-La2O3 core and silica cladding for this purpose.

International audience; We demonstrate a widely tunable Q-switched dualwavelength fiber laser emitting synchronized pulses in the 2 µm spectral range. Owing to the use of a thulium doped rod-type Fully-Aperiodic Large Pitch Fiber (FA-LPF), together with an acousto-optic modulator (AOM), and two Volume Bragg Gratings (VBGs), the wavelength separation was shown to be continuously tunable from 1 nm to 120 nm (~0.1-10 THz). A peak power of more than 8 kW was demonstrated over the whole tuning range for a repetition rate of 1 KHz and a 26 ns pulse duration. The repetition rate was modulated from 1 kHz to 30 kHz, and the laser pulse duration measured between 23 ns and 130 ns, depending of the repetition rate and the wavelength separation.

International audience; In this invited communication, we perform a review on Fully-Aperiodic Large Pitch Fibers. Major advantages and drawbacks of this design are presented first as well as the synthesis method, which enables the success of their fabrication. Some laser experiments are then detailed in order to prove the relevance of the aperiodic fiber concept. Very last experimental results and some prospects will be finally shown during the conference.

International audience

Recently, significant work has been conducted to reach high energy or peak power in fiber lasers. Microstructured fibers with large mode areas were developed to address this concern [1,2] and have allowed to access to the best state-of-the-art performances in terms of pulse energy, average power and peak power [3,4]. Although these fibers were designed for power scaling while keeping a single transverse mode propagation, the onset of transverse modal instabilities (TMI) degrades significantly the beam quality owing to the re-confinement of one or more higher order modes (HOMs) in the gain area. That effect suddenly appears when a certain average power threshold is exceeded. To push further the TMI power threshold, an original aperiodic pattern made of solid low-index inclusions embedded into the optical cladding was proposed to enhance the HOMs delocalization out of the gain region and thus ensure an effective single-mode emission. Such fibers are called Fully-Aperiodic Large-Pitch Fibers (FA-LPF). In this work, we realize for the first time a burn-in experiment with a 84 µm core Yb-doped FA-LPF in amplification regime. Using a 400 W pump diode at 976 nm and two different seeders, the power scaling as well as the spatial beam quality and its temporal behavior [5] were investigated in amplifier configuration in two different temporal regimes (nanosecond and picosecond pulses). After 800 hours, the maximum extracted average signal power decreases from 139W to 128W in picosecond regime and no TMI have been observed. Explanations on the power decrease will be given during the conference.

In this paper, we demonstrate a widely tunable dual-wavelength fiber laser, which is based on a thulium-doped fiber and volume Bragg gratings triggering the emission wavelengths in the 2-μm band. A continuously tunable wavelength separation range (Δλ), from 1 to 144 nm (0.08–11.47 THz) is demonstrated, with > 4.5 W of output power over the tuning range. The laser emission exhibits a linewidth (FWHM) lower than 0.1 nm independent from wavelengths and Δλ, with more than 45 dB of a signal-to-noise contrast. A maximum output power of 7 W is obtained for shorter Δλ (1–14 nm), corresponding to a slope efficiency of 33% with respect to the incident pump power at 790 nm.

Since the proof of concept of Photonic Crystal Fibers (PCF) by Knight et al., their development over the last two decades has led to progressive enlargement of core sizes while maintaining a transverse single-mode operation enabling power scaling in fiber lasers and amplifiers by pushing further the nonlinear effects and damage thresholds. Numerous fiber designs and laser/amplifier architectures have been investigated in order to make the most of the PCF technology and mainly to mitigate a new deleterious phenomenon responsible of beam quality degradation, the Transverse Mode Instability (TMI), which arose in parallel of the high average powers reached with those fibers. In this context, our research group has developed a PCF, so called Fully-Aperiodic Large-Pitch Fibers (FA-LPF) which proved its relevance with passive as well as active fibers, manufactured with the powder sintering technology known as REPUSIL. In this work, the refractive index of the FA-LPF core is slightly lower than that of the background cladding material (Δn ~ -5x10-5). This depressed-index core feature enables a thermal resilience ensuring an effective single-mode propagation above a certain average power for core size as high as 110µm. Experimental results in amplifier set-up with a 110 µm Yb-doped depressed core FA-LPF led to 110W of amplified signal for 300W of pump with a M² < 1.3. No TMI phenomenon was observed even at maximum pump power despite the average power and the very large mode area involved.

In binary aluminosilicate liquids and glasses, heterogeneity on intermediate length scale is a crucial factor for optical fiber performance, determining the lower limit of optical attenuation and Rayleigh scattering, but also clustering and precipitation of optically active dopants, for example, in the fabrication of high-power laser gain media. Here, we consider the low-frequency vibrational modes of such materials for assessing structural heterogeneity on molecular scale. We determine the vibrational density of states VDoS g(ω) using low-temperature heat capacity data. From correlation with low-frequency Raman spectroscopy, we obtain the Raman coupling coefficient. Both experiments allow for the extraction of the average dynamic correlation length as a function of alumina content. We find that this value decreases from about 3.9 nm to 3.3 nm when mildly increasing the alumina content from zero (vitreous silica) to 7 mol%. At the same time, the average inter-particle distance increases slightly due to the presence of oxygen tricluster species. In accordance with Loewensteinian dynamics, this proves that mild alumina doping increases structural homogeneity on molecular scale.

International audience; We investigated a range of different mesoporous NiO electrodes prepared by different research groups and private firms in Europe to determine the parameters which influence good quality photoelectrochemical devices. This benchmarking study aims to solve some of the discrepancies in the literature regarding the performance of p-DSCs due to differences in the quality of the device fabrication. The information obtained will lay the foundation for future photocatalytic systems based on sensitized NiO so that new dyes and catalysts can be tested with a standardized material. The textural and electrochemical properties of the semiconducting material are key to the performance of photocathodes. We found that both commercial and non-commercial NiO gave promising solar cell and water-splitting devices. The NiO samples which had the two highest solar cell efficiency (0.145% and 0.089%) also gave the best overall theoretical H2 conversion.

The power scaling of fiber lasers and amplifiers has triggered an extensive development of large-mode area fibers among which the most promising are the distributed mode filtering fibers and the large-pitch fibers. These structures enable for an effective higher-order modes delocalization and subsequently a singlemode emission. An interesting alternative consists in using the fully-aperiodic large-pitch fibers, into which the standard air-silica photonic crystal cladding is replaced by an aperiodic pattern made of solid low-index inclusions cladding. However, in such a structure, the core and the background cladding material surrounding it must have rigorously the same refractive index. Current synthesis processes and measurement techniques offer respectively a maximum resolution of 5×10-4 and 1×10-4 while the indexmatching must be as precise as 1×10-5 . Lately a gain material with a refractive index 1.5×10-4 higher than that of the background cladding material was fabricated, thus re-confining the first higher-order modes in the core. A numerical study is carried out on the benefit of bending such fully-aperiodic fiber to counteract this phenomenon. Optimized bending axis and radius have been determined. Experiments are done in a laser cavity operating at 1030 nm using an 88cm-long 51μm core diameter ytterbium-doped fiber. Results demonstrate an improvement of the M2 from 1.7 when the fiber is kept straight to 1.2 when it is bent with a 100 to 60 cm bend radius. These primary results are promising for future power scaling.

Over the last decade, significant work has been carried out in order to increase the energy/peak power provided by fiber lasers. Indeed, new microstructured fibers with large (or very large) mode area cores (LMA) such as Distributed Mode Filtering (DMF) fibers and Large-Pitch Fibers (LPF) have been developed to address this concern. These technologies have allowed diffraction-limited emission with core diameters higher than 80 μm, and have state-of-the-art performances in terms of pulse energy or peak power while keeping an excellent spatial beam quality. Although these fibers were designed to reach high power levels while maintaining a single transverse mode propagation, power scaling becomes quickly limited by the onset of transverse modal instabilities (TMI). This effect suddenly arises when a certain average power threshold is exceeded, drastically degrading the emitted beam quality. In this work, we investigate the influence of the core dimensions and the refractive index mismatch between the active core and the background cladding material, on the TMI power threshold in rod-type Fully-Aperiodic-LPF. This fiber structure was specifically designed to enhance the higher-order modes (HOMs) delocalization out of the gain region and thus push further the onset of modal instabilities. Using a 400W pump diode at 976 nm, the power scaling, as well as the spatial beam quality and its temporal behavior were investigated in laser configuration, which theoretically provides a lower TMI power threshold than the amplifier one due to the lack of selective excitation of the fundamental mode.

International audience

We present the development of a tunable dual-wavelength fiber laser, based on a thulium-doped fiber and Volume Bragg Gratings (VBG). Dual-wavelength difference is continuously tunable from 1 nm to 144 nm (0.08-11.47 THz).

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International audience; The objective of this paper is to demonstrate the interest of a consolidation process associated with the powder-in-tube technique in order to fabricate a long length of specialty optical fibers. This so-called Modified Powder-in-Tube (MPIT) process is very flexible and paves the way to multimaterial optical fiber fabrications with different core and cladding glassy materials. Another feature of this technique lies in the sintering of the preform under reducing or oxidizing atmosphere. The fabrication of such optical fibers implies different constraints that we have to deal with, namely chemical species diffusion or mechanical stress due to the mismatches between thermal expansion coefficients and working temperatures of the fiber materials. This paper focuses on preliminary results obtained with a lanthano-aluminosilicate glass used as the core material for the fabrication of all-glass fibers or specialty Photonic Crystal Fibers (PCFs). To complete the panel of original microstructures now available by the MPIT technique, we also present several optical fibers in which metallic particles or microwires are included into a silica-based matrix.

International audience

We present an independently tunable pulsed dual-wavelength MOPA at 2µm based on a fiber-integrated theta ring oscillator. With a tailored tuning range for THz generation of 50nm, an output power of 12W has been achieved.

Two evolutions of fully aperiodic large-pitch fiber designs employing few stress-applying parts are presented. The induced elasto-optic stress discriminates the two orthogonal polarization modes (LPsub01x/suband LPsub01y/sub) of the fundamental mode, selectively delocalizing one of them into the cladding via a suitable coupling to one or several cladding modes. This ensures the propagation of a single linear polarization mode. For the largest core dimensions, however, the applied stress can strongly influence the intensity distributions of core modes, and a tailored design process must thwart this. The polarization properties are investigated experimentally with core scalability over a large spectral bandwidth into passive structures, leading to the evidencing of a single-mode single polarization over a large span from 1 to 1.6 μm with a core dimension of 80 μm and, notably, at 1400 nm for a core dimension of 140 μm. The polarization extinction ratio is also determined.

In this paper, a strategy consisting of precompensating the thermal-induced transverse refractive index changes is undertaken to push further the appearance threshold of a multimode regime. First, a standard air-silica large pitch fiber (LPF) and a fully aperiodic large pitch fiber are confronted in regard to their heat load resilience and capabilities for single-mode emission. Thereafter, slight refractive index depressions are judiciously introduced into the active core area. This approach enhances the delocalization of the high-order modes even under severe heat load levels. This combination of aperiodic cladding microstructuration and index-precompensation theoretically increases the multimode regime threshold while preserving large mode field areas. This investigation is performed at 1.03 and 2 μm operating wavelengths.

The first demonstration of a 40 μm core homogeneously ytterbium-doped fully aperiodic large-pitch fiber laser, to the best of our knowledge, is reported here. In this concept, the amplification of unwanted high-order modes is prevented by means of an aperiodic inner-cladding structure, while the core and inner-cladding material has a higher refractive index than pure silica. In a laser configuration, up to 252 W of extracted power, together with an optical-to-optical efficiency of 63% with respect to the incident pump power, have been achieved. While an average Msup2/supof 1.4 was measured, the emitted power becomes temporally unstable when exceeding 95 W, owing to the occurrence of modal instabilities.

International audience; Evolutions of fully-aperiodic large-pitch-fibers employing stress-applying parts are presented to ensure the single propagation of one polarization state of the fundamental mode. The polarization properties are investigated with core scalability over a large spectral bandwidth. OCIS codes: (060.4005) Microstructured fibers, (060.2420) Fibers, polarization-maintaining Controlling the polarization state of coherent light is generally addressed by polarization-maintaining design like the well-known PANDA fibers [1], or a single-polarization propagation [2] owing to stress applying parts (SAP). The current study has been focusing on judiciously introducing stress into high power fibers to control the polarization state of the fundamental mode (FM). Fully-aperiodic large-pitch-fibers (FA-LPF) which have proved their potential to delocalize efficiently the HOM out of the gain region [3] are used here as basic concept. The numerical approach consists in adding SAP (low-index boron-doped silica inclusions) in the fiber structure in order to obtain enough birefringence while maintaining the effective single-mode operation. Single-polarization is therefore obtained due to a selective modal coupling of one of the FM polarization state with a HOM. Two designs are shown here, with two (resp. 6) boron inclusions (yellow point in Fig.1) implemented in the FA-LPF called 2SAP (resp. 6SAP). Fibers with different core diameters have been drawn from 40µm to 140µm. Fig 1: Observed near field intensity distributions of the transmitted fundamental mode for several core sizes versus the working wavelength for the two designs (a) FA-LPF 2SAP design and (b) FA-LPF 6SAP design. Each black point represents a measured configuration, and the green area arbitrarily represents the spectral range where the transmission shows a single-mode behavior. On the left of each graph, the fiber numerical design with SAP in yellow, low-index inclusions in light blue and pure silica in dark blue. To characterize these fibers, a supercontinuum source emitting a non-polarized radiation spanning from 400nm to 1700nm was used. The experimental setup consists of launching light into the fiber core and observing the output near field intensity distributions with a CCD camera. Narrow pass-band filters have been introduced to perform a spectral study. The FM suffers deformation due to the vicinity of SAP with the core when the ratio core diameter/wavelength increases. The 6SAP structure allows effective single-mode propagation over a wider area. Results are summed up in Fig.1. Then, a polarizer was added to force the polarization state at some point. Single-polarization propagation behavior was systematically observed for the two designs. A polarization extinction ratio equal to 11.2dB has been measured at 1050nm at the output end of a 50cm long, 60µm core diameter fiber. To conclude two evolutions of the FA-LPF design are reported and single-polarization propagation is obtained over a large spectral band for core diameter up to 140µm especially for the 6SAP structure.

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Structures of MgB2-based materials manufactured under pressure (up to 2 GPa) by different methods having high superconducting performance and connectivity are multiphase and contain different Mg-B-O compounds. Some oxygen can be incorporated into MgB2 and boron into MgO structures, MgBx (X=4-20) inclusions contain practically no oxygen. Regulating manufacturing temperature, pressure, introducing additions one can influence oxygen and boron distribution in the materials and thus, affect the formation, amount and sizes of Mg-B-O and MgBx inclusions and changing type of pinning, pinning force and so affect critical current density j c. The boron concentration increase in initial Mg and B mixture allows obtaining sample containing 88.5 wt% of MgB12 with TC of 37.4 K (estimated magnetically).

The critical current density, jc, of high-pressure synthesized MgB2-based balk materials correlates with the amount and distribution of higher borides (MgB12) and Mg-B-O inclusions, which in tern correlates with the synthesis temperature and presence of additions (Ti, Ta, SiC). High-pressure-synthesized materials with near MgB12 composition of matrix exhibited superconducting transition temperature, Tc, of about 37 K, rather high jc (5∙105 and 103 A/cm2 in 0 T and 3.5 T, respectively, at 20 K) and doubled matrix microhardness: 25±1.1 GPa at 4.9 N –load as compared to materials with MgB2).

International audience; The influence of the size and the air-filling fraction of the inner microstructure on the first HOM confinement in Yb-doped LMA PCFs under different heat load values has been investigated with a full-vector modal solver based on the finite element method, used also to solve the steady-state heat equation. In particular, the air-cladding inner dimension and the air-hole diameter in Symmetry-Free PCFs and Large Pitch Fibers have been modified in order to study which conditions facilitate the coupling between HOM and cladding modes, thus improving the delocalization of the former and making the fiber single-mode behavior more robust.

International audience; Since the double-clad fiber architectures development, fiber-based laser have witnessed an impressive power scaling [1]. The extracted power rising has been accompanied by the development of Very Large Mode Area (VLMA) fiber designs allowing overcome some key hurdles like the non-linear process or photo-darkening [2]. However, due to the very large core size of fiber architectures, a new phenomenon, referring to modal instabilities, has been evidenced recently like the current limitation which hampers any further power increase in the field of fiber laser sources without a dramatic degradation of the emitted beam quality [3]. In order to push away the appearance power threshold of this limitation, new aperiodic cladding microstructurations have been proposed to improve the higher-order modes (HOM) rejection out of the gain region and then to optimize the amplification of the sole fundamental mode [4]. These aperiodic microstructures have proved recently their potential to enhance an efficient HOM delocalization enabling singlemode confinement in the core region with passive VLMA fibers [5].In this communication we report on the first high power emission demonstration obtained using a solid non-filamented core fully-aperiodic large pitch fiber manufactured by the REPUSIL method based on the sintering and vitrification of micrometric doped silica powders. Using a simple laser cavity, an average output power of 233 W was achieved with an available pump power of 400 W for the first time in such a fiber. The preliminary M2 measurements have shown an excellent beam quality with values less than 1.4.

International audience; We report on a triple clad large-mode-area Tm-doped fiber laser with 18 μm core diameter manufactured for the first time by an alternative manufacturing process named REPUSIL. This reactive powder sinter material enables similar properties compared to conventional CVD-made fiber lasers, while offering the potential of producing larger and more uniform material. The fiber characterization in a laser configuration provides a slope efficiency of 47.7% at 20°C, and 50.4% at 0°C with 8 W output power, with a laser peak emission at 1970 nm. Finally, a beam quality near the diffraction-limit (M 2 x;y < 1.1) is proved. Ytterbium-doped fiber lasers have transformed the world of laser sources in recent years [1], in particular, due to impressive works done to circumvent intrinsic limitations imposed by the fiber architecture itself. Recently, new challenges are turned toward the power scaling of fiber lasers based on thulium-doped fibers. The latter enable the generation of radiations in the 1.8– 2.1 μm [2,3] spectral region, which is of great interest for several application fields such as medicine (cornea [4] and prostate [5] surgery), and civil and defense security (remote sensing, LIDAR [6]). Most of the works done in the 2 μm region have been achieved with step-index thulium-doped fibers, the core size being theoretically limited to 30 μm to avoid a significant degradation of the beam quality. The kW-level in average powers in CW regime has then been reached in 2009 [7]. To delve further into the power scaling of such fiber lasers, first demonstrations of thulium-doped large-mode-area (LMA) photonic crystal fibers have been recently reported allowing, for instance, the ability to reach up to 200 mW in peak power in a pulsed regime [8]. Unfortunately, one of the main technological barriers faced for the fabrication of thulium-doped fibers remains obtaining high quantum efficiency. Indeed, thulium-doped fiber lasers are typically pumped at 790 nm where high-power laser diodes are commercially available. Therefore, the large quantum defect between the pump wavelength and the signal wavelength is larger than that obtained in the case of the ytterbium-doped fiber lasers. The Stokes limit in this case is of about 40%, meaning that a large amount of the absorbed power is released as heat, compromising even more the expected power scaling by the appearance of modal instabilities as has already been well described for ytterbium-doped fiber lasers. Nevertheless, the well-known cross-relaxation mechanism that can be observed in thulium-doped gain material allows us to overcome this poor quantum efficiency. In fact, an electron promoted to the excited state through the absorption of one pump photon has the energetic potential to release two laser photons owing to an energy transfer toward neighboring thulium ions (

International audience

Fully aperiodic large-pitch-fiber designs are updated to ensure the single propagation of one polarization state of the fundamental mode (employing stress-applying parts) over an extended range of thermal load to prevent modal instabilities.

Transporting ensembles of electrons over long distances without losing their spin polarization is an important benchmark for spintronic devices. It requires usually to inject and to probe spin polarized electrons in conduction channels using ferromagnetic contacts or optical excitation. Parallel to this development, an important effort has been dedicated to the control of nanocircuits at the single electron level. The detection and the coherent manipulation of the spin of a single electron trapped in a quantum dot are now well established. Combined with the recent control of the displacement of individual electrons between two distant quantum dots, these achievements permit to envision the realization of spintronic protocols at the single electron level. Here, we demonstrate that spin information carried by one or two electrons can be transferred between two quantum dots separated by a distance of 4 micrometers with a classical fidelity of 65 %. We show that it is presently limited by spin flips occurring during the transfer procedure prior to and after the electron displacement. Being able to encode and control information in the spin degree of freedom of a single electron while being transferred over distances of a few microns on nanosecond timescales paves the way towards "quantum spintronics" devices where large scale spin-based quantum information processing could be implemented., Comment: 8 pages, 3 figures, supplementary material section

National audience; - Principales limites gênant la montée en puissance des sources laser à fibres optiques unimodales- Concepts avancés de structures de fibres optiques- Méthodes de fabrications, limitations et prospectives- Projets en cours, impact potentiel et perspectives

International audience; Thanks to their capability to provide very large mode area together with effective suppression of high-order modes, while allowing strong pump absorption and efficient conversion, Yb-doped double-cladding photonic crystal fibers are one of the key enabling factors for the development of high power fiber lasers. Thermal effects are currently appointed as the main bottleneck for future power scaling since, beyond a certain average power, they allow guidance of high order modes and energy transfer to them, causing a sudden degradation of the beam quality. In this paper the effects of heat load on the modes of double cladding fibers are thoroughly analyzed with a full-vector modal solver based on the finite-element method with integrated steady-state heat equation solver. Fibers with different inner cladding designs are compared to provide a deeper understanding of the mechanisms beyond the mode reconfinement and coupling. The influence of the fiber design on the robustness of the single-mode regime with respect to fiber heating has been demonstrated, providing a clear picture of the complex interaction between modes. On the basis of simulation results it has been possible to group fiber modes into three families characterized by peculiar reaction to heating. Index Terms—Photonic crystal fibers, thermo-optic effect, fiber lasers and amplifiers.

International audience; We are reporting on the characterization of a thulium-doped fiber laser applying new powder technology in the mode-locking regime. A high average output power of 185 mW at a repetition rate of 9 MHz was achieved directly from the oscillator, which resulted in 21 nJ of pulse energy. The single-pulse operation regime was confirmed by careful numerical modeling of the laser cavity.

National audience; Nous rapportons ici une fibre à large aire modale de 18 µm de diamètre de coeur dopée thulium fabriquée par un procédé alternatif nommé Repusil et présentant un profil à triple gaine. Cette fibre, caractérisée en configuration laser, nous a permis d'obtenir un faisceau monomode transverse proche de la limite de diffraction (M x,y 2 < 1,1) avec une efficacité laser de 50,4%.

International audience; This communication intends to summarize the recent strides carried out by the study of original Very Large Mode Area fibers (VLMA) that offer remarkable beam qualities. This work also aims in pushing farther the threshold of appearance for non-linear processes as well as thermal induced beam degradation hindering the power scaling in optical fibers. So as to fulfil this objective, thorough investigations of fibers modal content has been performed, leading to the evidencing of primordial statements for exacerbation of the beam quality and its robustness. Theoretical principles driven toward the conception of aperiodic Large Pitch Fibers (LPFs) will be reported together with experimental validation into passive fiber designs. The relevance of these original structures will then been discussed in regard to the power scaling.

International audience; We experimentally demonstrate a Type I grating effect in a lanthanum-aluminum silicate fiber for the first time, to the best of our knowledge. This grating shows a temperature sensitivity ~8.71 pm/oC, which is lower than that of FBG in standard single-mode fiber.

International audience; In this paper, we investigate the potential of various large mode area bers under thermal load, that is the state-of-the-art air-silica large pitch bers, as well as the recently devised symmetry-reduced photonic crystal ber and aperiodic all-solid by carefully considering the degrees of freedom oered all along the ber fabrication. This work aims to discuss the mode ltering ability of these structures in regard to the power scaling and to conrm their potential for robust singlemode operation at high power level. Structural principles contributing to improve their performances such as the impact of air holes / solid inclusions size will be presented. We also intend to establish that the range of average absorbed/output power for which a robust singlemode operation is available can be shifted to full user requests in term of power range.

We report on simultaneous observations of the magnetar SGR J1745-2900 at frequencies $\nu = 2.54$ to $225\,\rm{GHz}$ using the Nancay 94-m equivalent, Effelsberg 100-m, and IRAM 30-m radio telescopes. We detect SGR J1745-2900 up to 225 GHz, the highest radio frequency detection of pulsed emission from a neutron star to date. Strong single pulses are also observed from 4.85 up to 154 GHz. At the millimetre band we see significant flux density and spectral index variabilities on time scales of tens of minutes, plus variability between days at all frequencies. Additionally, SGR J1745-2900 was observed at a different epoch at frequencies 296 to 472 GHz using the APEX 12-m radio telescope, with no detections. Over the period MJD 56859.83-56862.93 the fitted spectrum yields a spectral index of $\left = -0.4 \pm 0.1$ for a reference flux density $\left< S_{154} \right> = 1.1 \pm 0.2\rm{\,mJy}$ (with $S_{\nu} \propto {\nu}^{\alpha})$, a flat spectrum alike those of the other radio-loud magnetars. These results show that strongly magnetized neutron stars can be effective radio emitters at frequencies notably higher to what was previously known and that pulsar searches in the Galactic Centre are possible in the millimetre band., Comment: 5 pages, 3 figures. Published in MNRAS Letters; v2: minor typos corrected, one reference updated

International audience; We report on the first high power laser emission of a solid triple-clad fully-aperiodic large mode area fiber with non-filamented core based on Repusil process. The average power is 184 W with a singlemode fashion.

We report here on an experimental investigation of the temporal behavior of transverse mode instabilities into “fully aperiodic large-pitch fibers” (FA-LPFs) operated in high-power continuous-wave laser configuration. To ensure an effective transverse single-mode emission into FA-LPFs, a perfect index matching between the active core and the background cladding materials (Δn=0) is required. The original design of such fibers enables an effective transverse single-mode emission by strengthening the higher-order mode delocalization out of the gain region, even for high heat load levels, consequently leading to the improvement of the beam spatial quality. The study was conducted over fibers of various gain region diameters, from 58 to 100 μm, for a refractive index mismatch Δn of about +8×10−5. The emitted beam is characterized using both M2 measurements and time traces to study the changeover of a stable temporal behavior to an unstable one.

National audience; Nous rapportons ici la première démonstration expérimentale d'une nouvelle architecture de fibres optiques à très grande surface de mode destinée à renforcer la propagation d'un mode transverse unique. L'originalité de ce travail repose sur une rupture contrôlée de la périodicité de la gaine optique microstructurée afin de favoriser la délocalisation des modes d'ordre supérieur au bénéfice du confinement du seul mode fondamental. Le caractère monomode des structures fabriquées a été conforté en utilisant un interféromètre à faible cohérence. Comme prédit lors d'une approche numérique préliminaire, un tel comportement est mis en évidence sur une large bande spectrale allant de 1 à 2 µm et ce pour des diamètres de coeur allant de 30 à 140 µm.

International audience

International audience; We report here on the first experimental demonstration, to the best of our knowledge, of a new generation of very large mode area (VLMA) fibers intended to strengthen single-mode propagation. The originality of this work relies on an aperiodicity of the inner cladding microstructuration exacerbating the spatial rejection of higher-order-modes (HOMs) while preserving a significant confinement of the fundamental mode. The single-mode behavior was demonstrated using an optical low-coherence interferometry measurement based on the group-velocity dispersion. As suggested through a preliminary numerical approach, this outstanding characteristic/behavior is evidenced over a large spectral range spanning from 1 to 2 μm for a core diameter of 60 μm. Core scalability was also investigated.