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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

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.

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.

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 (

The lensless endoscope represents the ultimate limit in miniaturization of imaging tools: an image can be transmitted through a (multi-mode or multi-core) fiber by numerical or physical inversion of the fiber's pre- measured transmission matrix. However, the transmission matrix changes completely with only minute conformational changes of the fiber, which has so far limited lensless endoscopes to fibers that must be kept static. In this work we report a lensless endoscope which is exempt from the requirement of static fiber by designing and employing a custom-designed conformationally invariant fiber. We give experimental and theoretical validations and determine the parameter space over which the invariance is maintained.

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.

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.

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.

A class of miniaturized imaged systems based on multicore fibers (MCFs) and wavefront shaping, also known as lensless endoscopes, have emerged as promising candidates for non-invasive imaging deep inside the tissue. At the current stage of their development, these systems already provide features like pixelation-free wide-field and point-scanning imaging and are compatible with the majority of nonlinear imaging techniques, offering diffraction-limited resolution over a field of view only limited by the single fiber numerical aperture. Widely spaced single-mode cores in such an MCF are designed for very weak inter-core coupling, which ensures an infinite memory effect and provides good ultrashort pulse delivery framework for nonlinear imaging. In a true endoscopic setting, however, where the fiber geometry is subject to continuous deformation (at a rate from several Hz to several tens of Hz), results in inter-core phase and group delay dispersion (GDD) that changes as the fiber bends. This consequently degrades the PSF in terms of size and power in the focal spot, eventually rendering impossible to produce non-linear imaging. We addressed this issue by implementing an active measurement and compensation loop at the proximal end of an MCF, allowing to correct in real time the GDD changes and to minimize the temporal dispersion of the delivered laser pulses. We evaluate this approach against a passive scheme where a static pre-compensation is used in a compliment with a specifically designed MCF, allowing to drastically increase the MCF bending robustness.

Lensless endoscopes have generated a great deal of interest in the development of minimally invasive probes for imaging in sensitive and hitherto inaccessible regions as found in deep brain imaging. Lifting the requirement for the opto-mechanical elements at the distal end of the fiber reduces the footprint of the endoscope down to the fundamental limit, the fiber itself. Our approach, using specially designed multicore fibers allows us to i) generate two-photon fluorescence contrast with femtosecond pulses, ii) image at high speeds with resonant scanners, iii) simple and non-interferometric calibration schemes and iii) exhibits a high resilience to spatio-temporal distortion of the focus due to fiber bending. In this contribution, we will discuss how novel designs of the MCFs can lift several of the instrumental complexity typically associated wavefront shaping and high speed imaging. We show that the use of sparse arrays of fiber cores can provide pixelation-free imaging with no artifacts when employed in the wavefront domain as opposed to conventional fiber bundles. Furthermore, we examine the unique properties of the MCFs which allow for fast and non-interferometric calibration schemes and can tolerate severe bending with an intact focus. The inclusion of a secondary cladding on the MCF allows us high sensitivity detection though the fiber (NA = 0.6) whilst preserving the advantages of a sparse MCF. The combination of these new developments brings us towards the application of these ultrathin probes in realistic imaging conditions.

We propose and fabricate a novel ring-core photonic crystal fiber made of a circular ring core surrounded by a cladding constituted of air holes organized in a first circular ring surrounded by hexagonal ones. The fiber efficiently supports four different groups of orbital angular momentum (OAM) modes. The effective indices of spin-orbit aligned and spin-orbit anti-aligned modes in the same OAM modes group are separated by at least 2.13×10−3 at 1550 nm. The realized fiber is expected to be a good platform for applications involving OAM modes.

International audience; We investigate optical parametric amplification in a fiber pumped by a shaped pulse. The temporal structure of the pump impacts directly the instantaneous spectral gain and its temporal measurement is achieved in the spectral domain thanks to a chirped signal with a broad spectrum.

We present the experimental investigation of timing jitter and relative intensity noise of a Mamyshev ring oscillator operating in the fundamental mode-lock regime. We find that both timing jitter and intensity noise spectra are correlated to the output optical power with noise increase close to the loss of the mode-locking. In addition, we have investigated the dependence of the spectral filters wavelength separation on both timing jitter and intensity noise showing a severe degradation with filters overlapping.

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.

International audience; We report two-photon lensless imaging through a novel golden spiral multicore fiber. This unique layout optimizes the sidelobe levels, field of view, cross-talk, group delay and mode density to achieve a sidelobe contrast of atleast 10.9 dB. We demonstrate experimentally the ability to generate and scan a focal point with a femtosecond pulse and perform two-photon imaging.

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.

Spatially and spectrally resolved imaging has become an important tool for multimode fiber characterization, both in the realms of group index dispersion measurement and modal geometry analysis. However, limited resolution in group delay space, distributed scattering and noise sometimes make the task of identifying meaningful modal interference figures difficult. In this paper, an application of multivariate statistical analysis to S $^2$ datasets that effectively isolates mode pair interferences, both spectrally and spatially, is presented. A method to use those components to retrieve the power in each mode is also shown.

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 (

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; 10 11 Multicore fiber bundles are widely used in endoscopy due 12 to their miniature size and their direct imaging capabilities. 13 They have recently been used, in combination with spatial 14 light modulators, in various realizations of endoscopy with 15 little or no optics at the distal end. These schemes require 16 characterization of the relative phase offsets between the 17 different cores, typically done using off-axis holography, 18 thus requiring both an interferometric setup and, typically, 19 access to the distal tip. Here we explore the possibility of 20 employing phase retrieval to extract the necessary phase in-21 formation. We show that in the noise-free case, disordered 22 fiber bundles are superior for phase retrieval over their peri-23 odic counterparts, and demonstrate experimentally accurate 24 phase information for up to 10 simultaneously illuminated 25 cores. Thus, the phase retrieval is presented as a viable al-26 ternative for real-time monitoring of phase distortions in 27 multicore fiber bundles. Endoscopy is a powerful tool used to image hollow organs with-32 out the need for surgery [1]. Miniaturization of the endoscope 33 probe will allow for a wider range of accessible organs to be 34 examined and, thus, is crucially important for applications. 35 Some miniature endoscopes are based on a single-mode optical 36 fiber which is mechanically scanned [2,3] or spectrally spread at 37 the distal end [4,5]. The use of a single-mode fiber circumvents 38 distortions due to propagation within the fiber. Yet, for minia-39 turization purposes, it is generally desirable to minimize the 40 number of elements at the distal end of the endoscope [6]. 41 Indeed, various techniques have been suggested for endoscopic 42 imaging where all the active elements are in the proximal end of 43 the fiber. Recent implementations are typically based on multi-44 mode optical fibers [7-9], GRIN fibers, or bundles of single-45 mode fibers [10-13]. In all these, light propagation through the 46 device, reflected in relative phase accumulated by the various 47 modes propagating through the optical system (which is also 48 dependent on bending and temperature) has to be taken into 49 account. Confocal scanning endoscopy, circumventing the 50 need to account for the accumulated phase, was performed ei-51 ther by using multi-core fiber (MCF) bundles or by using 52 GRIN fibers while scanning a confocal pinhole (or an input 53 single-mode fiber) at the proximal end [14]. The use of speckle 54 correlations and the "memory effect" in multicore fiber bundles 55 have also been recently suggested as a means to perform either 56 scanning endoscopy [15] or wide-field imaging [16] without 57 the need to compensate for the differences in the phase intro-58 duced by the different cores. Active phase compensation is, 59 however, by far the most direct and efficient approach to ad-60 vanced endoscopy applications. 61 Indeed, following the dramatic advances in spatial light 62 modulation technology, active compensation of the phase dis-63 tortion due to propagation either in a multimode fiber [7-9] or 64 in a multicore fiber bundle [10-13] has been pursued in recent 65 years. This requires accurate characterization of the phase dis-66 tortion induced by propagation in the endoscope. In multi-67 mode fibers, the measurement of the fiber transmission 68 matrix is sufficient to perform widefield imaging [9,17]. In 69 multicore bundles, this reduces to the problem of characteriz-70 ing the phase delay within each core, which can then be com-71 pensated for via the use of a spatial light modulator (SLM) [18]. 72 Alternatively, using an SLM, it is possible to perform confocal 73 scanning by adding a combination of a linear and a parabolic 74 phase profile on top of the phase distortion correction [11]. 75 The latter approach is especially useful for multiphoton imag-76 ing [12]. In most of the above cases, off-axis holography or self-77 referenced interferometry, was used to characterize the phase 78 distortion. This requires a stable interferometric setup and usu-79 ally requires access to the distal end of the fiber, and is difficult 80 to implement for correction of time-dependent distortions. For 81 multicore bundles, it is also possible to separately characterize 82 the phase shift between individual pairs of cores [11] (which 83 can, in principle, even be done in backscattering mode, taking 84 advantage of the light reflected from the distal fiber end). This 85 is, however, a painstakingly slow process. Therefore, a rapid 86 characterization method for the time-varying phase distortion 1 Letter Vol. 42, No. 4 / /Optics Letters 1 0146-9592/17/040001-01 Journal

International audience; We examine the impact of fiber bends on ultrashort pulse propagation in a 169-core multicore fiber (MCF) by numerical simulations and experimental measurements. We show that an L-shaped bend (where only one end of the MCF is fixed) induces significant changes in group delays that are a function of core position but linear along the bending axis with a slope directly proportional to the bending angle. For U-and S-shaped bends (where both ends of the MCF are fixed) the induced refractive index and group delay changes are much smaller than the residual, intrinsic inter-core group delay differences of the unbent MCF. We further show that when used for point-scanning lensless endoscopy with ultrashort pulse excitation, bend-induced group delays in the MCF degrade the point-spread function due to spatiotemporal coupling. Our results show that bend-induced effects in MCFs can be parametrized with only two parameters: the angle of the bend axis and the amplitude of the bend. This remains valid for bend amplitudes up to at least 200 degrees.

We investigate basic nonlinear effects in optical fibers which opto-geometrical parameters oscillate along the propagation axis. These “topographic” fibers provide an additional degree of freedom leading to multiple quasi-phase matched modulation instability side lobes in single pass configuration or in passive cavities. Experimental results are confirmed by analytical and numerical ones.

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.

We demonstrate pixelation-free real-time widefield endoscopic imaging through an aperiodic multicore fiber (MCF) without any distal opto-mechanical elements or proximal scanners. Exploiting the memory effect in MCFs the images in our system are directly obtained without any post-processing using a static wavefront correction obtained from a single calibration procedure. Our approach allows for video-rate 3D widefield imaging of incoherently illuminated objects with imaging speed not limited by the wavefront shaping device refresh rate., Comment: Published in Optics Letters

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.

Characterization of spatial mode content and dispersion properties in fiber laser systems and space-division multiplexing (SDM) applications is key to understanding fiber properties and system performance. Several techniques exist for modal characterization but often present limitations in the context in which they can be used efficiently. In this paper, we present a powerful analysis scheme that removes several of those limitations and pushes modal content analysis to a new level.

International audience

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.

Coherent beam combining of 7 erbium-doped cores of a multicore fiber amplifier is demonstrated. Diffractive optical element is used to spatially combine output beams. 45% combination efficiency is achieved using stochastic parallel gradient descent algorithm.