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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Disorder-Induced Acceleration of Condensation in Multimode Fibers
摘要： Recent studies on wave turbulence revealed that a purely classical system of random waves can exhibit a process of condensation, whose thermodynamic properties are analogous to those of Bose-Einstein condensation [1-3]. Classical wave condensation finds its origin in the natural thermalization toward the Rayleigh-Jeans equilibrium distribution, whose divergence is responsible for the macroscopic occupation of the fundamental mode of the system. The experimental study of condensation in a conservative (cavity-less) configuration constitutes a major challenge, because of the prohibitive large propagation lengths required to achieve thermalization. In contrast with this commonly accepted opinion, a remarkable phenomenon of spatial beam self-cleaning has been recently discovered in graded-index multimode optical fibers (MMFs) [4-7]. This phenomenon is due to a purely conservative Kerr nonlinearity [7] and its underlying mechanism still remains debated. Light propagation in MMFs is known to be affected by a structural disorder of the material due to inherent imperfections and external perturbations. On the basis of the wave turbulence theory, we formulate a nonequilibrium kinetic description of the random waves that accounts for the impact of disorder. The theory reveals that a structural disorder is responsible for a dramatic acceleration of the process of condensation by several orders of magnitudes. This counterintuitive mechanism of condensation acceleration provides a natural explanation for the effect of spatial beam self-cleaning: As a consequence of the fast condensation process, the beam power rapidly flows toward the fundamental mode of the MMF, which becomes macroscopically populated to the detriment of the other modes that exhibit energy equipartition, as predicted by the Rayleigh-Jeans distribution [1]. The simulations of the nonlinear Schrodinger equation (NLSE) are in quantitative agreement with those of the derived kinetic equation, and thus confirm the validity of the theory and the effect of acceleration of condensation mediated by disorder (see Fig. 1). Furthermore, the derived kinetic equation also explains why spatial beam self-cleaning has not been observed in step-index MMFs. We performed experiments in a MMF to evidence the transition to light condensation by varying the coherence of the input beam (as described in Fig. 1c). When a large number of modes are excited, the output intensity distribution tends to relax toward the thermal Rayleigh-Jeans distribution, i.e., the 'temperature' is above the critical value for condensation (E > Ecrit in Fig. 1c) and spatial beam self-cleaning is not observed. By reducing the excitation of modes (E < Ecrit in Fig. 1c), the power gradually condenses into the fundamental mode of the MMF, leading to a cleaned beam with a measured condensate fraction as large as ～60%.
关键词： multimode fibers,spatial beam self-cleaning,condensation,wave turbulence,disorder
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - All-Fiber High-Average-Power 4.42-μm Raman Laser Based on Silica Fiber
摘要： Mid-infrared lasers operating at wavelengths above ~3 μm are required for many applications. The lasers of a particular interest are those that can provide high average power and high quality of the output beam. A promising approach to realize such lasers is the use of gas-filled hollow-core fibers (HCF), which combine mid-infrared light guidance with all the advantages of well-established silica fiber technology. Recent progress in silica HCFs for mid-infrared spectral range has resulted in demonstration of various gas fiber lasers at wavelengths from 2.8 to 4.6 μm [1-5]. However, those lasers use bulk elements to couple the pump into a HCF, thus losing such advantages as compactness and alignment-free operation, which are expected from fiber lasers. In this work we demonstrate an all-fiber gas Raman laser operating in a single transversal mode at 4.42 μm and generating average output power as high as 360 mW. The Raman laser was constructed using silica-based fibers only (Fig.1a). A pump source was a high-power erbium-doped fiber laser emitting 2-ns pulses with 75-kHz repetition rate at 1.56 μm. The output fiber of the pump laser was fusion spliced to a revolver-type HCF. In spite of a large difference in mode field diameters of the solid-core and hollow-core fibers (20 and 55 μm, respectively), a 3-dB splice losses were obtained. The splice point was mechanically stable and no splice loss degradation was observed when HCF was filled by molecular hydrogen up to pressure of 50 atm. By using a 3.2-m-long hydrogen-filled revolver-type HCF we realized an efficient 1.56 to 4.42 μm Raman conversion in a single-pass single-cascade scheme. When 2.3 W of the pump average power was coupled to the HCF, the Raman laser generated as high as 360 mW of average power at 4.42 μm (Fig.1b). The quantum conversion efficiency was as high as 46 %. A beam quality factor of the output radiation at λ=4.42 μm was evaluated to be M2=1.4. Our results demonstrate that gas fiber Raman lasers based on revolver hollow-core silica fibers are a viable solution for the development of all-fiber mid-infrared lasers that combine a compact alignment-free design with high power single mode operation. An application area of such laser sources includes, but is not limited to, a high-average-power mid-infrared supercontinuum generation, which is of high importance for numerous applications in science, biomedicine and technology.
关键词： gas-filled hollow-core fibers,silica fiber,mid-infrared lasers,Raman laser,high power
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Wavelength Independent Image Classification through a Multimode Fiber using Deep Neural Networks
摘要： Deep Neural Networks (DNNs) have been increasingly implemented in different research fields or industrial applications. Large amounts of data are processed daily in order to extract useful information using machine learning techniques. Many research groups have shown impressive results on improving resolution in microscopy and quantitative phase retrieval by training DNNs on real datasets. Recently, recovery and reconstruction of images after they have propagated through multimode optical fibers (MMFs) have also been achieved using DNNs. When images propagate through MMFs they suffer severe scrambling because the information gets distributed among the different spatial modes that the fiber supports. Furthermore, since the fiber modes propagate with different velocities, the local information of the input decorrelates after a few millimeters along the MMF, thus resulting in the formation of a speckle pattern at the output. Recovery of information from such speckle patterns is of practical interest for integrating the MMFs for endoscopic applications in medicine or for signal recovery in telecommunications.
关键词： Wavelength Independence,Image Classification,Speckle Patterns,Multimode Optical Fibers,Deep Neural Networks
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Detection and Tracking of Multiple Individual Nanoparticles in Antiresonant Hollow-Core Fibers
摘要： Directly studying the dynamics of nanoparticles with subwavelength dimensions, for example protein interactions or viral self-assembly, is difficult using conventional light microscopes due to Abbe’s resolution limit. Methods to overcome this limit such as fluorescence microscopy usually require to label particles and suffer from photobleaching in the case of long illumination times. A recent tracking method based on elastic light scattering from nano-objects inside a microstructured fiber which includes a nanometer sized channel managed to circumvent these limitations [1]. However, due to the small channel size, this approach imposes high spatial constraints on the particle motion, impedes the investigation of multi-particle dynamics and allows very little control over the liquid flow inside the fiber. In this work, we demonstrate that antiresonant hollow-core fibers (ARHCFs) open new perspectives for the detection and tracking of unlabeled, individual nanoparticles in statistically large numbers simultaneously. Gold nanospheres of diameters as small as 40nm were introduced into the hollow inner channels of an ARHCF with hexagonally shaped core section of around 30μm in diameter (see Fig. 1a,b). The fiber was integrated into an optofluidic chip system which allows controlling the liquid flow [2], and illuminated by a laser source (wavelength at 532nm). The light scattered off the particles was collected from the transverse direction by a low NA microscope objective (see Fig. 1a) and imaged onto a high speed camera. Applying an appropriate algorithm [3], the obtained data was analyzed and individual particle trajectories deduced (see Fig. 1c). Therefore a single video contains statistical data, e.g. for particle size estimations via the mean squared displacement method for a large number of freely diffusing particles (see Fig. 1d). Since particle tracking for tens of seconds at kHz image rates is possible, this novel method holds strong potential for the investigation of yet unexplored multi-particle dynamics at the nanoscale.
关键词： nanoparticles,antiresonant hollow-core fibers,tracking,detection,optofluidic chip
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Gain-Managed Nonlinear Fiber Amplifier
摘要： We demonstrate a novel pulse evolution in a fiber amplifier. We show that this pulse evolution is driven by a nonlinear attractor and paves the way toward simple, compact fiber systems producing high-energy, ~30-fs pulses.
关键词： Lasers, fiber,Nonlinear optics, fibers,Ultrafast lasers
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Broad-Band Optical Parametric Amplification in a Tapered Silicon Core Fiber Pumped in the Telecom Band
摘要： Silicon core ?bers (SCFs) are emerging as a robust and ?exible waveguide platform that can exploit the high nonlinear coef?cients of silicon to perform compact nonlinear processing within an integrated ?ber geometry. Optical parametric ampli?cation (OPA) based on four-wave mixing (FWM) has been studied intensively over the past decade using chip-based silicon waveguides for a variety of telecommunication applications such as all-optical signal sampling, time-demultiplexing, pulse generation, and wavelength conversion. However, due to the strong nonlinear absorption associated with two photon and free carrier effects in silicon at telecom wavelengths, net on/off parametric gain has not been observed using a continuous-wave (CW) signal, and even when using pulses the gain has been limited to be 5.2 dB. Here, by tapering the SCF to tailor the dimensions such that we can achieve high coupling ef?ciency into a core with a sub-micron-sized diameter, required to access the anomalous dispersion region, we demonstrate OPA with a positive on/off gain up to 6.6 dB and bandwidth of more than 220 nm. Eventually, when fully integrated with conventional ?bers and components to improve the robustness of the system, we expect these tapered SCFs will ?nd wide-ranging applications in areas that require ef?cient wavelength conversion across a broad spectral band.
关键词： optical parametric amplification,nonlinear absorption,Silicon core fibers,telecom wavelengths,four-wave mixing
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Kerr Beam Self-Cleaning in the Telecom Band
摘要： Multimode graded index (GRIN) fibers received a renewed interest in recent years, in particular for the development of new laser sources [1]. In many cases, the use of GRIN fibers is limited by multimodal propagation, leading to a spatially modulated intensity distribution (speckles) at the fiber output. Recent studies have found that quasi-single mode propagation can be recovered in GRIN fibers by the so-called Kerr self-cleaning effect [2]. It consists in the spontaneous recovery of the spatial beam quality, without any frequency shift [2] (as opposed to, e.g., Raman beam self-cleaning [3]). This nonlinear process was only observed so far at laser wavelengths around 1 μm, for peak power levels above a certain threshold value. In this communication, we show that Kerr beam self-cleaning also occurs in a GRIN fiber pumped with chirped pulses in the telecom band (1562 nm). At these wavelengths, the power threshold of the self-cleaning process is decreased by one order of magnitude with respect to pumping at 1 μm.
关键词： GRIN fiber,multimode fibers,nonlinear optics,Kerr beam self-cleaning,telecom band
更新于2025-09-11 14:15:04
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[IEEE 2019 21st International Conference on Transparent Optical Networks (ICTON) - Angers, France (2019.7.9-2019.7.13)] 2019 21st International Conference on Transparent Optical Networks (ICTON) - A Materials Approach Toward the Mitigation of Nonlinearities in Glass Optical Fibers
摘要： Power scaling in high energy fiber-based laser systems is limited by optical nonlinearities. As opposed to the complex micro-structured large mode area (LMA) fibers typically developed to mitigate these parasitic nonlinear effects, the present work instead advocates another approach, a material approach, in which nonlinearities are mitigated through a judicious choice of dopants in the fiber core. To demonstrate the effectiveness of this approach, multicomponent core – silica glass cladding optical fibers are fabricated using the molten core method and the role played by the glass dopants in the mitigation of nonlinearities discussed. More specifically, highlights are made on multicomponent alkaline-earth (Sr, Ca) doped-silica fibers exhibiting concomitant reduction of > 6 dB in the Brillouin gain coefficient, 1 – 2 dB in the Raman gain coefficient, and 2 – 3 dB in the thermo-optic coefficient relative to conventional silica fibers.
关键词： Brillouin scattering,Raman scattering,optical nonlinearities,optical fibers,multicomponent silicates,glass
更新于2025-09-11 14:15:04
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[IEEE 2019 IEEE International Symposium on Measurements & Networking (M&N) - Catania, Italy (2019.7.8-2019.7.10)] 2019 IEEE International Symposium on Measurements & Networking (M&N) - Water monitoring in smart cities exploiting plastic optical fibers and molecularly imprinted polymers. The case of PFBS detection
摘要： The detection of perfluorobutanesulfonic acid (PFBS) in water is presented for the first time exploiting a low-cost optical chemical sensing strategy, based on plastic optical fibers and molecularly imprinted polymers (MIPs), able to detect pollutants in water. The sensor's response measured in this work for the detection of PFBS is similar to the one obtained with Perfluorooctanesulfonate and investigated. The detection limit of PFBS is considered of interest when compared to those obtained by using different expensive traditional approaches, e.g. high performance liquid chromatography-mass spectrometry (HPLC-MS). Moreover, the use of this sensing approach is a very attractive perspective for fast, in situ, and remote low-cost detection of pollutants in water for smart cities applications.
关键词： molecularly imprinted polymers,pollutants,PFAs,Surface plasmon resonance,plastic optical fibers,optical fiber sensors,water quality,PFBS,Chemical sensors
更新于2025-09-11 14:15:04
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Detection and imaging of local ply angle in carbon fiber reinforced plastics using laser ultrasound and tilt filter processing
摘要： Wrinkles in carbon-fiber reinforced plastics (CFRP) are extremely dangerous, dramatically changing the performance of a composite structure. However, wrinkles are one of the most difficult defects to detect and characterize with ultrasound since they do not produce large echoes like other flaws. Recent developments in laser-ultrasound (LU) have enabled non-contact, high-resolution imaging of CFRPs, providing sub-ply resolution and detailed visualization of CFRP structures. In this paper, we demonstrate that the superior resolution of LU systems can be used to help track ply orientation variations in composites. To improve the signal-to-noise ratio of LU generated signals, we propose a local cross-correlation based tilt-filtration procedure for coherent spatial signal averaging that accounts for the shape of subsequent layers. In addition, local ply angle is a straightforward output of the proposed processing, which can be used directly as an indicator of wrinkle severity.
关键词： D. Non-destructive testing,A. Carbon fibers,B. Directional orientation,D. Ultrasonics
更新于2025-09-11 14:15:04