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We report the development of a silica glass single-mode polarization-maintaining fiber with birefringence induced by artificial anisotropic glass in the circular core without any external stress zones or structured cladding. The fiber core is composed of silica and germanium-doped silica nanorods ordered in submicrometer interleaved layers. The fiber has a measured cut-off wavelength at 1113 nm, phase birefringence of 0.3×10−4, and an effective mode diameter of 10.5 µm at the wavelength of 1550 nm. The polarization extinction ratio in the fiber is 20 dB at 1550 nm. The fiber is compatible with the standard SMF-28 fiber and can be easily integrated using standard fusion splicing with losses of 0.1 dB.

We report high efficiency, miniaturized, ultra-fast coherent beam combining with 3D printed micro-optics directly on the tip of a multicore fiber bundle. The highly compact device foot-print (180 micron diameter) facilitates its incorporation into a minimally invasive ultra-thin nonlinear endoscope to perform two-photon imaging, Comment: Published in Optics Letters

The incorporation of Ce3+ ions in silicate glasses is a crucial issue for luminescence-based sensing applications. In this article, we report on silica glass preforms doped with cerium ions fabricated by modified chemical vapor deposition (MCVD) under different atmospheres in order to favor the Ce3+ oxidation state. Structural analysis and photophysical investigations are performed on the obtained glass rods. The preform fabricated under reducing atmosphere presents the highest photoluminescence (PL) quantum yield (QY). This preform drawn into a 125 µm-optical fiber, with a Ce-doped core diameter of about 40 µm, is characterized to confirm the presence of Ce3+ ions inside this optical fiber core. The fiber is then tested in an all-fibered X-ray dosimeter configuration. We demonstrate that this fiber allows the remote monitoring of the X-ray dose rate (flux) through a radioluminescence (RL) signal generated around 460 nm. The response dependence of RL versus dose rate exhibits a linear behavior over five decades, at least from 330 µGy(SiO2)/s up to 22.6 Gy(SiO2)/s. These results attest the potentialities of the MCVD-made Ce-doped material, obtained under reducing atmosphere, for real-time remote ionizing radiation dosimetry.

International audience; This work reports on the fabrication and the characterization of hybrid optical fibers with silicon core and silica cladding. Adopting a two-step manufacturing technique derived from the stack-and-draw method, silicon-core fibers with core dimensions ranging from about 0.8 mu m to 3.5 mu m have been successively drawn into hundreds of meter-long fibers. A 3.3 mu m diameter core fiber has been more extensively characterized and background losses for this as-drawn fiber are less than 0.2 dB/cm between 1250 nm and 1650 nm, with a minimum of 0.12 dB/cm around 1600 nm. The crystalline state of the core and the limited impact of oxygen contamination were confirmed by Raman scattering, x-ray diffraction, HR-STEM, and ToF-SIMS analysis. Transmission peaks associated with specific modal distributions are evidenced under certain injection conditions and their positions are shown to be in good accordance with mode cut-off wavelengths of a step-index silicon-core fiber.

We report on the experimental influence of filter shape on the dynamic of an all-fiber Mamyshev oscillator. Significant variation of output pulse energy and various regimes (harmonic mode-locking, multipulses) are observed.

Radiation-induced emission of doped sol-gel silica glass samples was investigated under a pulsed 20-MeV electron beam. The studied samples were drawn rods doped with cerium, copper, or gadolinium ions, which were connected to multimode pure-silica core fibers to transport the induced luminescence from the irradiation area to a signal readout system. The luminescence pulses in the samples induced by the electron bunches were studied as a function of deposited dose per electron bunch. All the investigated samples were found to have a linear response in terms of luminescence as a function of electron bunch sizes between 10−5 Gy/bunch and 1.5×10−2 Gy/bunch. The presented results show that these types of doped silica rods can be used for monitoring a pulsed electron beam, as well as to evaluate the dose deposited by the individual electron bunches. The electron accelerator used in the experiment was a medical type used for radiation therapy treatments, and these silica rod samples show high potential for dosimetry in radiotherapy contexts.

All-optical wavelength conversion of dual-polarization signals over 9.6 nm in C+L-band is demonstrated in a twin-core highly nonlinear fibre for the first time. Conversion efficiencies between -1 dB and -8.5 dB for both cores are obtained simultaneously.

We evaluate the temperature effect on the X-ray radiation-induced luminescence (RIL) of Cu- or Ce-single-doped and CuCe-codoped silica glass. Previous investigations showed that these optical materials exhibit interesting dosimetry properties, such as very good detection capabilities and linear response over a large range of dose rate. However, several radiation environments, such as space and particle accelerators, also require a careful assessment of the temperature effect. With this aim, we characterize their RIL efficiency and spectral dependence at several irradiation temperatures. We demonstrate that all the investigated materials present a nonnegligible temperature dependence of the RIL in the range −120 °C to 80 °C, whilst preserving linear dose rate dependence at each irradiation temperature. However, the temperature effect on the RIL signal is still compensable via calibration and temperature monitoring.

International audience; Silicon-core fibers were drawn into hundreds of meter-long spans. The regular crystalline structure of the core and the absence of oxygen contamination are demonstrated together with loss below 0.2 dB/cm from 1250 to 1650 nm.

Cerium doped silica glasses are attractive materials for radiation dosimetry. In this article, Ce-doped air-clad optical fiber has been fabricated for real-time X-ray dose rate measurements through the radioluminescence (RL) signal. The structural properties of the obtained material were studied using Raman spectroscopy. The presence of Ce3+ ions inside the sol-gel-derived silica core was confirmed using photoluminescence (PL) measurements. This optical fiber, with a high numerical aperture, was tested as active and guiding material in an all-fibered remote X-ray dosimeter configuration. We demonstrated that the response dependence of RL versus dose rate is linear from 52 mGy(SiO2)/s up to at least 1.5 Gy(SiO2)/s, allowing to monitor the dose rate evolution during an irradiation run. The presented results confirm the potentialities of this microstructured optical fiber to monitor ionizing radiations in harsh environments. They also pave the way toward an alternative to the widely used approach that uses scintillator material at the end of a transport’s optical fiber.

The luminescence properties of several doped materials are explored in many fields of application, including ionizing radiation dosimetry. These luminescence properties are essentially provided by the doping element. In this article, we report thermally stimulated luminescence (TSL) measurements of the trapping and recombination centers involved in the luminescence process of sol–gel-derived glasses. Three-doped species will be concerned: one transition metal (Cu), one rare-earth element (Ce), and a combination of both in the sol–gel silica matrix. By comparing the performances of these samples according to the criteria required for dosimetry by both TSL and optically stimulated luminescence (OSL) (sensitivity, dose response, and thermal fading), the results show that the simply doped samples really are potential candidates for fibered-OSL dosimetry.

The photonic nanojet (PNJ) was first reported by Chen et al.in 2004 through finite-difference time domain modeling of cylindrical structures under plan wave illumination. PNJ is a propagative beam with a full width at half maximum (FWHM) slightly smaller than a half wavelength. The power density can be more than 200 times higher than the one of the incident wave. This concentration can be achieved using a transparent dielectric microcylinder with a wavelength-scale radius. Gustav Mie obtained the exact solution of Maxwell’s equations for a dielectric microsphere in 1908. Applying this method, PNJ in the 3D case of a dielectric sphere was first reported by Li and Lecler. The photonic nanojet appears as a narrow and elongated spot with a high intensity electromagnetic field. The key parameters of PNJs, which are their FWHM, focal distance, decay length, and light intensity maximum, have been the subject of extensive theoretical and experimental studies. An in-depth understanding of photonic nanojet is nevertheless needed to fully exploit the potential of microspheres as optical super-lens. The original properties of PNJs make them ideal candidate in a wide range of applications, including nonlinear absorption enhancement, single-molecule fluorescence measurement, laser surgery, super-resolved microscopy, manipulation and detection of single sub-100 nm nanoparticles and biomolecules and laser sub-micro etching. Recently, the possibility to obtain a PNJ using an optical fiber with a shaped tip has opened new interests in the challenging field of direct laser subwavelength micromachining. To start with, the fiber is easier to move than the microsphere. Furthermore, the sphere is corrupted after the first irradiation due to their contact with the sample. On the opposite, the fiber tips have no contact with the processed surface and are not altered by the removed material. PNJs have already demonstrated the ability to reduce the laser etching size using shaped optical fiber tips. With a shaped optical fiber tips, the PNJ phenomenon is only due to the fundamental mode of the fiber. However, a standard single-mode fiber has three drawbacks for PNJ generation. (1) The small core diameter (

International audience; Hybrid optical fibers, i.e. optical fibers that combine, in the same structure, glass with crystal, metal, polymer or a second type of glass, open access to a wide range of optical properties or optical functions not accessible to common single-glass-made optical fibers. Silicon-core fibers are one type of hybrid fibers that have been intensively studied since 2006 with the aim to take benefit of the mid-infrared transparency of silicon or to implement opto-electrical functions in the optical fiber itself. Some of the unique optical properties of these semiconductor-core fibers have been demonstrated but it is admitted that optical losses are still today a drag on the rise of performances and hence devote specific attention. Post-processing based on laser or thermal annealing can be applied on the as-drawn fibers to improve core crystallinity and then reduce optical losses. However, such processing techniques have been demonstrated on centimeter-long fibers only. In the present paper, we demonstrate as-drawn silicon-core fiber with loss level below 0.2 dB/cm on the 1250-1650nm wavelength range, this fiber being continuously manufactured over length exceeding one hundred of meters. Several fibers have been fabricated from a rod-in-stack approach and different core dimensions ranging from about 0.8 to 3.4 mu m have been successively realized and extensive characterizations (XRD, micro-Raman spectroscopy, TEM and ToF-SIMS analysis) have been conducted on the 3.4 mu m core fiber. The crystalline state of the core, the absence of oxygen contamination and the optical transmission from 1.1 to 4 mu m will be presented.

International audience; We report the fabrication and characterization of the first double clad tubular anti-resonant hollow core fiber. It allows to deliver ultrashort pulses without temporal nor spectral distortions in the 700-1000 nm wavelength range and to efficiently collect scattered light in a high numerical aperture double clad. The output fiber mode is shaped with a silica microsphere generating a photonic nanojet, making it well suitable for nonlinear microendoscopy application. Additionally, we provide an open access software allowing to find optimal drawing parameters for the fabrication of tubular hollow core fibers.

Optically activated glasses are essential to the development of new radiation detection systems. In this study, a bulk glassy rod co-doped with Cu and Ce ions, was prepared via the sol-gel technique and was drawn at about 2000 &deg, C into a cylindrical capillary rod to evaluate its optical and radioluminescence properties. The sample showed optical absorption and photoluminescence (PL) bands attributed to Cu+ and Ce3+ ions. The presence of these two ions inside the host silica glass matrix was also confirmed using PL kinetics measurements. The X-ray dose rate was remotely monitored via the radioluminescence (RL) signal emitted by the Cu/Ce scintillating sensor. In order to transport the optical signal from the irradiation zone to the detection located in the instrumentation zone, an optical transport fiber was spliced to the sample under test. This RL signal exhibited a linear behavior regarding the dose rate in the range at least between 1.1 mGy(SiO2)/s and 34 Gy(SiO2)/s. In addition, a spectroscopic analysis of this RL signal at different dose rates revealed that the same energy levels attributed to Cu+ and Ce3+ ions are involved in both the RL mechanism and the PL phenomenon. Moreover, integrated intensities of the RL sub-bands related to both Cu+ and Ce3+ ions depend linearly on the dose rate at least in the investigated range from 102 mGy(SiO2)/s up to 4725 mGy(SiO2)/s. The presence of Ce3+ ions also reduces the formation of HC1 color centers after X-ray irradiation.

This Letter reports on a large mode area pixelated Bragg fiber in which some high refractive index rods were replaced by boron-doped rods that allows polarization maintaining behavior while keeping single-mode behavior. The realized all-solid fiber has a core diameter of 35 µm. The fundamental mode is circular with a 25 µm mode field diameter around 1 µm wavelength, and the polarization extinction ratio reaches 30 dB. Finally, this fiber is single-mode and bendable up to a 20 cm radius with fundamental mode losses lower than 0.3 dB/m.

Optical fibers are close to ideal detectors to measure radiation beams for cancer therapy, either as beam monitors or for dosimetiy. Several organic and inorganic fibers were tested with proton and neutron beams. One material stood out in its almost complete lack of quenching for low proton energies.

International audience; We report an experimental investigation of the photoluminescence (PL) activities detected in as-drawn and γ-rayirradiated samples of a microstructured optical fiber elaborated with a silica-based core doped with Bismuthions. The presence of several visible emission bands is revealed and characterized under 325 nm laser excitation.In both samples, four narrow emission bands are studied and analyzed. They are similarly bleached by a long UVlaser exposure times. In addition, time-resolved luminescence data show that all these bands possess a similarlifetime of ∼4 μs Data analysis indicated that the energy levels scheme of the point defects responsible for thesebands consists of: one above 5 eV, one at ∼3.6 eV, and at least one in the range 2.6–2.8 eV and of the groundone. As a consequence, even if the exact structural model cannot yet be fully identified, we suggest that thebands originate from color centers related to the Bismuth ions.

Ce-doped sol-gel silica glasses were prepared for real-time X-ray dose or dose rate monitoring using radioluminescence ( RL ) and optically stimulated luminescence (OSL). A small piece of this glass scintillator was connected to two optical fibers, allowing a remote measurement of the ionizing radiation dose through RL and OSL signals. By using this Ce-doped material, we particularly demonstrated that the RL signal shows a linear behavior at least up to dose rates of 1.2 Gy/s. Furthermore, the linear dose response of the OSL signal can be extended up to 500 Gy, independently of the used dose rate.

We report an experimental investigation focused on the green emission detected in γ-ray irradiated Bismuth-doped photonic crystal fibers. Our photoluminescence spectra, recorded at room temperature, provide evidence for the presence of two emission bands both located at ~ 530 nm (2.34 eV). One emission is detected only in the Bi-doped core while the other, is detected in the cladding. These two emissions feature different excitation spectra and a fast and a slow decay lifetime. The origin of the fast emission decay, about ten nanoseconds, is tentatively attributed to a silica intrinsic defect, whereas the slow component, having lifetime of about 2 μs and featuring anti-stokes emission, is certainly caused by Bi related defects.

This paper describes the first synthesis of YbPO 4 nano-cylinders and nanoparticles within optical fiber preforms. To obtain YbPO 4 -doped silica monoliths, phosphosilicate powders, previously prepared by a sol–gel method, were impregnated with a solution containing an ytterbium salt. Then, they were compressed into cylinders and sintered at high temperature. These doped monoliths were densified under controlled atmosphere and then drawn at high temperature into 1.6 mm diameter-rods. The composition, structural and optical properties of these monoliths and rods were investigated using Electron Probe Micro-Analysis, Transmission Electron Microscopy (TEM), Raman spectroscopy and IR absorption measurements. The size of the YbPO 4 nanoparticles within the monoliths were found to be in the range of 8–15 nm and 13–25 nm, depending on the Yb-doping amount. Upon drawing the most concentrated Yb-doped monoliths into rods, the nanoparticles evolve to form nano-cylinders with a mean diameter around 40 nm and a length of about 400 nm. Similar behavior was observed for the less concentrated Yb-doped monoliths, once drawn into rods but a lower number of nano-cylinders were found in this case. Structural analysis, based on Raman spectroscopy and TEM experiments, shows that all these nano-objects have a tetragonal xenotime structure (I4 1 /amd ). Moreover, it was found that the nano-cylinders were all aligned parallel to the drawing direction. These SiO 2 -P 2 O 5 :YbPO 4 glass ceramics exhibit a structured absorption spectra around 980 nm, which corroborates the Yb 3+ crystalline environment.

Optical fibers hold promise for accurate dosimetry in small field proton therapy due to their superior spatial resolution and the lack of significant Cerenkov contamination in proton beams. One known drawback for most scintillation detectors is signal quenching in areas of high linear energy transfer, as is the case in the Bragg peak region of a proton beam. In this study, we investigated the potential of innovative optical fiber bulk materials using the sol-gel technique for dosimetry in proton therapy. This type of glass is made of amorphous silica (SiO$${}_{2}$$ 2 ) and is doped with Gd$${}^{3+}$$ 3 + ions and possesses very interesting light emission properties with a luminescence band around 314 nm when exposed to protons. The fibers were manufactured at the University of Lille and tested at the TRIUMF Proton Therapy facility with 8.2–62.9 MeV protons and 2–6 nA of extracted beam current. Dose-rate dependence and quenching were measured and compared to other silica-based fibers also made by sol-gel techniques and doped with Ce$${}^{3+}$$ 3 + and Cu$${}^{+}$$ + . The three fibers present strong luminescence in the UV (Gd) or visible (Cu,Ce) under irradiation, with the emission intensities related directly to the proton flux. In addition, the 0.5 mm diameter Gd$${}^{3+}$$ 3 + -doped fiber shows superior resolution of the Bragg peak, indicating significantly reduced quenching in comparison to the Ce$${}^{3+}$$ 3 + and Cu$${}^{+}$$ + fibers with a Birks’ constant, k$${}_{B}$$ B , of (0.0162 $$\pm $$ ± 0.0003) cm/MeV in comparison to (0.0333 $$\pm $$ ± 0.0006) cm/MeV and (0.0352 $$\pm $$ ± 0.0003) cm/MeV, respectively. To our knowledge, this is the first report of such an interesting k$${}_{B}$$ B for a silica-based optical fiber material, showing clearly that this fiber presents lower quenching than common plastic scintillators. This result demonstrates the high potential of this inorganic fiber material for proton therapy dosimetry.

Recently, there has been a surge in reports of so-called ‘lensless endoscopes’, a term which denotes the active control of the fiber injected wavefront from the proximal side only to enable imaging without any distal optics, thus allowing the miniaturization of the endoscope to the fundamental limit, the size of the optical fiber itself. Our approach sets itself apart in several ways: (i) the capability to perform nonlinear image contrast using two-photon fluorescence; and (ii) high image acquisition rates and (iii) the active control of the distal focused spot while bending the fiber. We have developed a multicore fiber (MCF) specifically for our approach. It consists of 169 homogeneous single-mode cores, each of which functions as one ‘pixel’ of the wave front. These fiber cores are spatially arranged in an aperiodic pattern to eliminate the satellite grating induced repliqua and to extend the field of view. The composite waveform at the distal end is then generated by pre-shaping the beam before it is coupled into the fiber cores. Hence, the generation of a focal spot and its scanning can be performed without any opto-mechanical components on the fiber tip. The MCF is designed with a multi-mode inner cladding with a very high numerical aperture (NA) of 0.65. This permits the collection of the resulting fluorescence signal with a high efficiency. A dedicated twist fiber design makes the developed imaging endoscope insensitive to fiber bending.

Fiber Optical Parametric Chirped Pulse Amplification (FOPCPA) consists of amplifying temporally stretched short pulses in an optical fiber by a high intensity pump beam through a four-wave mixing process [1,2]. Such amplification scheme presents high gain, large bandwidth, repetition rate and wavelength broad flexibility and tunability but provides up to now only very weak signal energies (below the nJ level).

The photonic nanojet (PNJ) generated by a shaped optical fiber tip is an attractive technology for laser micro-machining. The working distance has the same order of size as the fiber core diameter; therefore, multimode (MM) fibers are generally preferred. However, the PNJ is due to the fundamental mode and, therefore, the energy coupled on the high-order modes does not contribute to the process. We demonstrate the benefit of a large-mode-area (LMA) optical fiber in the generation of the PNJ. A homemade 40 μm mode field diameter LMA fiber is compared with a 100/140 MM-shaped fiber tip. Similar micro-peaks are obtained, and an energy gain is demonstrated. The coupled energy required was eight times less intense with the LMA fiber, which may open new possibilities for laser micro- and nano-processing.

Gadolinium-doped silica glass was prepared, using the sol-gel route, for ionizing radiation dosimetry applications. Such a glassy rod was drawn to a cane at a temperature of 2000 °C. The structural and optical properties of the obtained material were studied using Raman, optical absorption, and photoluminescence spectroscopies. Thereafter, a small piece of this Gd-doped scintillating cane was spliced to a transport passive optical fiber, allowing the remote monitoring of the X-ray dose rate through a radioluminescence (RL) signal. The sample exhibited a linear RL intensity response versus the dose rate from 125 µGy(SiO2)/s up to 12.25 Gy/s. These results confirm the potentialities of this material for real-time remote ionizing radiation dosimetry. peerReviewed

We present a narrow-bandwidth (167 pm) few-nanosecond fiber laser based on tapered fiber amplification and phase modulation to mitigate stimulated Brillouin scattering (SBS). We compare the use of chirped diode seeding and standard sinusoidal phase modulation. Below SBS appearance threshold, output temporal profiles can be accurately tailored. Typical results for flat-top profiles include 170 kW peak-power (for 3 ns duration) and 1.25 mJ pulse energy (for 10 ns duration).

We propose and fabricate a novel ring-core photonic crystal fiber that supports 4 groups of OAM modes with $2.13\times 10^{-3}$ minimum difference of effective indices at 1550 nm. The realized fiber is expected to be a good platform for OAM mode transmission.

A simple technique for far-field single-shot non-interferometric determination of the phase transmission matrix of a multicore fiber with over 100 cores is presented. This phase retrieval technique relies on the aperiodic arrangement of the cores., Submitted to Optics Letters

International audience; Bismuth-doped glasses are attractive materials for the development of new optical amplifiers and tunable laser sources. In this paper, Bi-dopedphosphosilicate glasses are prepared using the sol-doping technique in combination with shaping using cold isostatic pressing. After a first drawingat high temperature (2000 C), the obtained glass cane presents a wide near infrared (NIR) photoluminescence band under visible excitation. The authors show that this NIR optical activity, connected to phosphorus center, is preserved after a second drawing into a conventional optical fiber. These results confirm the potentialities of this synthesis approach for the fabrication of Bi-activated optical fibers.

International audience; Distributed temperature sensing is performed up to 800°C by Optical Frequency Domain Reflectometry on a fiber with enhanced Rayleigh scattering profile. This fiber was drawn from a preform core-doped by zirconia-coated gold nanoparticles.

A new few-mode fiber design taking advantage of a micro-structured core consisting of 19 secondary cores embedded in a pedestal geometry is presented. This design offers the possibility of precisely tailoring the rare earth ion distribution in the core in order to manage the differential modal gain. An optimized configuration of an erbium-doped few-mode fiber supporting 10 modes in the C-band with a theoretical low gain excursion is designed and realized. Preliminary optical characterizations of this fiber are presented.

We present our recent progress in developing a two-photon lensless endoscope based on ultrathin multicore fiber and wavefront shaping. We discuss the field-of-view extension, intrinsic group delay compensation and bending compensation in such devices with the aim of imaging the deep brain with minimal invasion.

International audience; This paper reports the design and the fabrication of an all-solid photonic bandgap fiber with core diameter larger than 100 µm, a record effective mode area of about 3700 µm 2 at 1035 nm and robust single-mode behavior on propagation length as short as 90 cm. These properties are obtained by using a pixelated Bragg fiber geometry together with an heterostructuration of the cladding and the appropriated generalized half wave stack condition applied to the first three higher order modes. We detail the numerical study that permitted to select the most efficient cladding geometry and present the experimental results that validate our approach.

We investigate the potential of innovative optical fiber bulk materials made by the sol-gel technique for proton therapy dosimetry. These types of glass are made of amorphous silica (a-SiO 2 doped with either Copper (Cu) or Cerium (Ce) ions. All these optimized materials possess very interesting light emission properties when exposed to protons. Online measurement of the strong radiation-induced luminescence allows the monitoring of the time evolution of the proton flux with millisecond resolution and the cumulated proton fluence can be precisely deduced by integrating this radiation-induced luminescence signal. Preliminary tests presented here have been performed at the TRIUMF Proton Therapy facility in Vancouver, Canada, where ocular cancer is treated clinically with 74 MeV protons and a proton current of 6 nA.

The Laser Megajoule (LMJ) is a French large scale laser facility dedicated to inertial fusion and plasma physics research. LMJ front-ends are based on fiber laser technology at nanojoule range [1]. Scaling the energy of those fiber seeders to the millijoule range is a way to upgrade LMJ’s front ends architecture and could also be used as seeder for lasers for ELI project for example. However, required performances are so restrictive (optical-signal-to-noise ratio higher than 50 dB, temporally-shaped nanosecond pulses and spatial single-mode top-hat beam output) that such fiber systems are very tricky to build. High-energy fiber amplifiers In 2015, we have demonstrated, an all-fiber MOPA prototype able to produce a millijoule seeder, but unfortunately not 100% conform for all LMJ’s performances. A major difficulty was to manage the frequency modulation used to avoid stimulated Brillouin scattering, to amplitude modulation (FM-AM) conversion, this limits the energy at 170µJ. For upgrading the energy to the millijoule range, it’s necessary to use an amplifier with a larger core fiber. However, this fiber must still be flexible; polarization maintaining and exhibit a strictly single-mode behaviour. We are thus developing a new amplifier architecture based on an Yb-doped tapered fiber: its core diameter is from a narrow input to a wide output (MFD 8 to 26 µm). A S² measurement on a 2,5m long tapered fiber rolled-up on 22 cm diameter confirmed that this original geometry allows obtaining strictly single-mode behaviour. In a 1 kHz repetition rate regime, we already obtain 750 µJ pulses, and we are on the way to mJ, respecting LMJ performances. Beam delivery In LMJ architecture the distance between the nanojoule fiber seeder and the amplifier stages is about 16 m. Beam delivery is achieved with a standard PM fiber, such a solution is no longer achievable with hundreds of kilowatt peak powers. An efficient way to minimize nonlinear effects is to use hollow-core (HC) fibers. The comparison between the different fibers will be presented in the conference. Fiber spatial beam shaping Spatial beam shaping (top-hat profile) is mandatory to optimize the energy extraction in free-space amplifier. It would be very interesting to obtain a flat-top beam in an all-fiber way. Accordingly, we have design and realize a large mode area single-mode top-hat fiber able to deliver a coherent top-hat beam. This fiber, with larger MFD adapted to mJ pulse, will be implemented to perform the spatial beam shaping from coherent Gaussian profile to coherent top-hat intensity profile in the mJ range. In conclusion, we will present an all-fiber MOPA built to fulfil stringent requirements for large scale laser facility seeding. We have already achieved 750 µJ with 10 ns square pulses. Transport of high peak power pulses over 17 m in a hollow-core fiber has been achieved and points out FM to AM conversion management issues. Moreover, spatial beam shaping is obtained by using specifically designed single-mode fibers. Various optimizations are currently under progress and will be presented.

The Laser megajoule (LMJ) is a French large scale laser facility dedicated to inertial fusion research. Its front-ends are based on fiber laser technology and generate highly controlled beams in the nanojoule range. Scaling the energy of those fiber seeders to the millijoule range is a way explored to upgrade LMJ’s architecture. We report on a fully integrated narrow line-width all-fiber MOPA prototype at 1053 nm designed to meet stringent requirements of large-scale laser facilities seeding. We achieve 750 µJ temporally-shaped pulses of few nanoseconds at 1 kHz. Thanks to its original longitudinal geometry and its wide output core (26µm MFD), the Yb-doped tapered fiber used in the power amplifier stage ensures a single-mode operation and negligible spectro-temporal distortions. The transport of 30 kW peak power pulses (from tapered fiber) in a 17 m long large mode area (39µm) hollow-core (HC) fiber is presented and points out frequency modulation to amplitude modulation conversion management issues. A S² measurement of this fiber allows to attribute this conversion to a slightly multimode behavior (< 13dB of extinction between the fundamental mode and higher order modes). Other HC fibers exhibiting a really single-mode behavior (

International audience; Hybrid optical fibers consisting of glass clad and semiconductor core materials are extensively investigated dueto their interesting properties that make them attractive for several technological applications. Here, we reporton a new approach for the fabrication of such multi-material optical fiber preform using the sol–gel technique. Itconsists in mechanically drilling a central hole in a silica xerogel rod, then in filling this hole with a centimeterlengthsilicon rod, and finally in densifying the silica glass jacket around the silicon core. By drawing thispreform, a homogenous hybrid rod-type fiber was obtained. The composition and the structural properties of thepristine silicon rod, of the hybrid preform and of the drawn fiber have been investigated using electron probemicroanalysis, Raman spectroscopy and transmission electron microscopy. We have shown that the silicon corematerial preserves its composition and polycrystalline structure all along the fabrication process.

The periodic arrangement of core positions in multi-core fiber bundles introduces ‘ghost’ artifacts to endoscopic images obtained through them, whether in wide-field imaging (based on either direct imaging or speckle correlations) or in confocal scanning microscopy using wavefront shaping. Here we introduce partially disordered multi-core bundles as a means to overcome these artifacts. The benefits of their use will be discussed in the context of multiphoton scanning microscopy utilizing a spatial light modulator in the proximal end, and in the more general case of widefield imaging. We also show that both numerically and experimentally that the presence of disorder also enables to apply phase retrieval methods to characterize the phase distortion introduced due to propagation in the bundle without the need of an interferometrically stabilized reference. Thus, in addition to overcoming the challenge of ghost artifacts, disordered multi-core fibers have the potential to overcome another challenge, movement-induced phase distortions, by enabling real-time characterization of this phase distortion in reflection mode only via the proximal end.

Accurate control of both the doping distribution inside the fiber core and the low refractive index contrast between the fiber core and cladding materials is essential for the development of high-power fiber lasers based on the use of single-mode large-mode-area (LMA) optical fibers. Herein, sol-gel monolithic F/Yb3+-codoped silica glasses were prepared from porous large silica xerogels doped with ytterbium salt solution, which had been subjected to fluorination with hexafluoroethane gas, before subsequent sintering. The fluorine content inside the doped glass has been varied by adjusting the fluorination duration. The space homogeneity of fluorine and ytterbium concentrations in the cylindrical preforms has been checked by chemical analysis and Raman spectroscopy. Moreover, the glass with the lowest fluorine content has been successfully integrated as a core material in a microstructured optical fiber made using the stack-and-draw method. This fiber was tested in an all-fiber cavity laser architecture to evaluate potential lasing performances of the F/Yb3+-codoped silica glass. It presents a maximum efficiency of 70.4%, achieved at 1031 nm from a 1.16 m length fiber. These results confirm the potentialities of the obtained F/Yb3+-codoped glasses for the fabrication of LMA optical fiber lasers.

International audience; Active phase locking of a multicore erbium-doped fiber amplifier is demonstrated for 180 ns narrow-linewidth pulses at 1545 nm. A spatial light modulator is used at the input of the amplifier to control the optical phase of 7 beams injected in the hexagonally-arranged cores, ensuring efficient combining through a SPGD algorithm. At the output, combining is performed using a diffractive optical element. This experiment establishes multicore amplifiers as a promising way to scale the energy of Brillouin-limited pulsed amplifiers for LIDAR applications. We also present a simple lensless technique to measure phase shifts between pairs of adjacent channels that could be implemented in future active coherent combining systems.

Ionic copper-doped silica glasses are attractive materials for radiation detection and dosimetry based on radioluminescence (RL) and optically stimulated luminescence (OSL). We characterized the optical responses, under X-rays, of a Cu+-doped silica glass rod prepared via the sol-gel technique and spliced to transport pure silica optical fibers. Both the low-dose and the low-dose rate regimes were specially investigated at room temperature. RL and photoluminescence (PL) measurements confirmed that the Cu+ ions are the luminescent centers of interest for the targeted applications. RL dose rate response dependence shows a linear behavior from 260 μGy(SiO2)/s up to 23 mGy(SiO2)/s, allowing the monitoring of the dose rate evolutions during an irradiation run. The OSL response also linearly depends on the dose from 42 mGy(SiO2) to 200 Gy(SiO2), enabling a precise dose measurement shortly after the irradiation. The presented results confirm the potentialities of this material to monitor ionizing radiations in harsh environments.

We report on the first polarization maintaining single-mode fiber that delivers a flat-top intensity profile at 1050 nm. A high quality fundamental flat mode was obtained. We showed that our fiber can be consider ed as single-mode in practice with low confinement losses. Its birefringence was measured to be 0.6x10 -4 , and the PER was measured at more than 20 dB even for a 20 m fiber long. Strategies to enhance this birefringence preserving the flat top profile and the single-mode behaviour as well are also discussed. Keywords: birefringence, microstructured fiber, single-mode , flat-top 1. INTRODUCTION Due to their advantages over bulk lasers, as compactness, lo ng term stability and reliability, good heat dissipation and no free-space alignment to name a few, fiber lasers are widespread amongst industrial and academic communities 1 . For most applications, a Gaussian-like beam is required or adequate. However for targ eted application as industrial processes (marking, drilling, cutting, heat treatment ), biological tissue in teractions or seeding large-scale laser facilities like the Laser MegaJoule (LMJ)

Recent advances in wavefront shaping techniques have enabled so-called lensless endoscopes using fiber probes. Unlocking the full potential of such endoscopes call for the capability of optically sectioned and/or label free imaging. Or in other words, imaging through fibers must retain the functionality of a nonlinear microscope. This is a key challenge due to the temporal broadening of ultrashort pulses in fibers owing to modal dispersion. Here, we detail the first ever demonstration of two photon fluorescence imaging at the distal tip of a conventional graded index (GRIN) multimode fiber. GRIN fibers possess a high mode density, excellent throughput and limited temporal broadening. These features, in addition to its ready availability, make them attractive candidates for ultrathin endoscopes. In our approach, we apply the transmission matrix formalism and treat these fibers akin to highly scattering media. This lets us retrieve combinations of input modes that would generate intense focal spots throughout the field of view. Furthermore, we identify a regime where the modal dispersion in the fiber is minimal and two-photon excitation with femtosecond light pulses is possible. This allows us to perform two-photon imaging with ultrashort pulses in an epi-detection configuration analogous to conventional nonlinear microscopes. Finally, these concepts are validated by acquiring optically sectioned two photon fluorescence images of 3D samples with cellular resolution. We believe this first report of an ultrathin rigid endoscope of only 125 µm thickness would further accelerate the development of novel tools for demanding applications in biological imaging and opto-genetics.

Compactness, long term stability and no free-space alignment are important advantages of fiber lasers over bulky systems. These fiber lasers have also demonstrated their capability to deliver high-power pulses and are thus suitable for numerous applications. Nevertheless the intensity profile delivered usually has a Gaussian-like shape, which most of the time is sufficient, but it could be interesting, for many applications (laser-biological tissues interactions, heat treatment, industrial laser processing or for seeding large-scale laser facilities like Laser MegaJoule) to obtain a homogeneous intensity profile at the fiber laser output. Moreover several of these applications required a linearly polarized output beam. In order to achieve all these requirements we have developed and realized a new fiber design. This fiber is the first polarization maintaining single-mode fiber delivering a flat top intensity. A high quality flat mode was obtained at 1.05μm through the use of a well-tailored index profile and single-mode behavior was verified by shifting the injection and using the S² imaging. Moreover, boron Stress Applying Parts (SAPs) including in the cladding led to a birefringence of 0.6x10-4 and a measured PER better than 20dB even for a long fiber length (~20 m). Alongside the fabrication, we developed a simulation code, using Comsol Multiphysics®, to take into account the stress dependency induced by the SAPs. Further modeling allows us to present an effectively single-mode fiber design, delivering a top-hat mode profile and exhibiting a polarizing behavior. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We report on the design and the fabrication of a new design of an all-solid Bragg fiber based on the pixelization and heterostructuration of a cladding made of only two high index rings. The thickness of the low index ring as well as the geometry of the heterostructuration (its symmetry and the number of removed pixels) have been chosen to maximize the confinement losses of the Higher Order Modes (HOM) (above 10 dB/m) while keeping the Fundamental Mode (FM) losses low (below 0.1 dB/m). The proposed geometry allows having access to different Mode Field Diameter (MFD) from 54 μm to 60 μm at 1 μm wavelength by drawing the same stack to different fiber (and hence, core) diameters. As a result, a record MFD of 60 μm is reported for a Solid Core Photonic Bandgap Fiber (SC-PBGF) and single-mode behavior is obtained experimentally even for a short fiber length (few tens centimeters) maintained straight.

International audience; To extend the use of erbium- (Er-)/aluminum- (Al-) codoped optical fibers in hostile environments, the reduction of the Al amount has been identified as a serious way to harden them against harsh radiation. In this work, sol–gel monolithic Er3+-doped and Er3+/Al3+-codoped silica glasses were prepared from nanoporous silica xerogels soaked in a solution containing an Er salt together or not with an Al salt. After sintering, these glasses were used as the core material of microstructured optical fibers made by the stack-and-draw method. The influence of Al incorporation on the optical properties of Er3+-doped silica glasses and fibers is investigated. This approach enabled the preparation of silica glasses containing dispersed Er3+ ions with low Al content. The obtained fibers have been tested in an all-fibered cavity laser architecture. The Er3+/Al3+-codoped fiber laser presents a maximum efficiency of 27% at 1530 nm. We show that without Al doping, the laser exhibits lower performances that depend on Er content inside the doped fiber core. The effect of Er pair-induced quenching also has been investigated through nonsaturable absorption experiments, which clearly indicate that the fraction of Er ion pairs is significantly reduced in the Al-codoped fiber.

International audience; We investigated the capability of micro-structured optical fibers to develop multi-functional, remotely-controlled, Optical Frequency Domain Reflectometry (OFDR) distributed fiber based sensors to monitor temperature in nuclear power plants or high energy physics facilities. As pure-silica-core fibers are amongst the most radiation resistant waveguides, we characterized the response of two fibers with the same microstructure, one possessing a core elaborated with F300 Heraeus rod representing the state-of-the art for such fiber technology and one innovative sample based on pure sol-gel silica. Our measurements reveal that the Xray radiations do not affect the capacity of the OFDR sensing using these fibers to monitor the temperature up to 1 MGy dose whereas the sensing distance remains affected by RIA phenomena.