Mario Zitelli, Fabio Mangini, Mario Ferraro, Alioune Niang, Denis Kharenko, Stefan Wabnitz
The process of high-energy soliton fission is experimentally and numerically investigated in a graded-index multimode fiber. Fission dynamics is analyzed by comparing numerical observations and simulations. A novel regime is observed, where solitons produced by the fission have a nearly constant Raman wavelength shift and same pulse width over a wide range of soliton energies.
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We present a theoretical and numerical study of light propagation in graded-index (GRIN) multimode fibers where the core diameter has been periodically modulated along the propagation direction. The additional degree of freedom represented by the modulation permits to modify the intrinsic spatiotemporal dynamics which appears in multimode fibers. More precisely, we show that modulating the core diameter at a periodicity close to the self-imaging distance allows to induce a Moiré-like pattern, which modifies the geometric parametric instability gain observed in homogeneous GRIN fibers.
Experimental and numerical studies of spatiotemporal femtosecond soliton propagation over up to 1 km spans of parabolic graded-index (GRIN) fibers reveal that initial multimode soliton pulses naturally and irreversibly evolve into a singlemode soliton. This is carried by the fundamental mode of the fiber, which acts as a dynamical attractor of the multimode system for up to the record value (for multimode fibers) of 5600 chromatic dispersion distances. This experimental evidence invalidates the use of variational approaches, which intrinsically require that the initial multimode propagation of…
Beam self-imaging in nonlinear graded-index multimode optical fibers is of interest for many applications, such as implementing a fast saturable absorber mechanism in fiber lasers via multimode interference. We obtain an exact solution for the nonlinear evolution of first and second order moments of a laser beam carried by a graded-index multimode fiber, predicting that the spatial self-imaging period does not vary with power. Whereas the amplitude of the oscillation of the beam width is power-dependent. We have experimentally studied the longitudinal evolution of beam self-imaging by means of…
We experimentally study polarization dynamics of Kerr beam self-cleaning in a graded-index multimode optical fiber. We show that spatial beam cleaning is accompanied by nonlinear polarization rotation, and a substantial increase of the degree of linear polarization.
We experimentally demonstrate spatial beam self-cleaning in an Yb-doped graded-index multimode fiber taper, both in passive and active configurations. The input laser beam at 1064 nm was injected for propagation from the small to the large core side of the taper, with laser diode pumping in a counterdirectional configuration. The Kerr effect permits to obtain high-beam quality amplification with no accompanying frequency conversions. As a result, our nonlinear taper amplifier may provide an important building block for multimode fiber lasers and amplifiers.
Since its first demonstration in graded-index multimode fibers, spatial beam self-cleaning has attracted a growing research interest. It allows for the propagation of beams with a bell-shaped spatial profile, thus enabling the use of multimode fibers for several applications, from biomedical imaging to high-power beam delivery. So far, beam self-cleaning has been experimentally studied under several different experimental conditions. Whereas it has been theoretically described as the irreversible energy transfer from high-order modes towards the fundamental mode, in analogy with a beam condens…
We present a new spatial-spectral mapping technique permitting to measure the beam intensity at the output of a graded-index (GRIN) multimode fiber with sub-nanometric spectral resolution. We apply this method to visualize the fine structure of the beam shape of a sideband generated at 1870 nm by geometric parametric instability (GPI) in a GRIN fiber. After spatial-spectral characterization, we amplify the GPI sideband with a Tm-doped fiber amplifier to obtain a microjoule-scale picosecond pump whose spectrum is finally broadened in a segment of InF3 optical fiber to achieve supercontinuum ran…
Multimode fibers (MMFs) are attractive ultra-thin replacements for state-of-the-art endoscopes, but the phase randomization in propagation through MMFs poses a major hurdle for imaging and focusing of light. Recently, this challenge has been addressed by pre-measuring the compensation for the fiber's complex input-output modes relations. Unfortunately, the sensitivity of this approach to fiber bending and temperature variations renders it inappropriate for many applications. Here, we demonstrate a truly endoscopic robust method for controlled in-situ focusing and scanning through a flexible un…
Beam self-cleaning (BSC) in graded-index (GRIN) multimode fibres (MMFs) has been recently reported by different research groups. Driven by the interplay between Kerr effect and beam self-imaging, BSC counteracts random mode coupling, and forces laser beams to recover a quasi-single mode profile at the output of GRIN fibres. Here we show that the associated self-induced spatiotemporal reshaping allows for improving the performances of nonlinear fluorescence microscopy and endoscopy using multimode optical fibres. We experimentally demonstrate that the beam brightness increase, induced by self-c…
We study the spatio-temporal instability generated by a universal unstable attractor in normal dispersion graded-index multimode fiber (GRIN MMF) for femtosecond pulses. Our results present the generation of geometric parametric instability (GPI) sidebands with ultrashort input pulse for the first time. Observed GPI sidebands are 91 THz detuned from the pump wavelength, 800 nm. Detailed analysis carried out numerically by employing coupled-mode pulse propagation model including optical shock and Raman nonlinearity terms. A simplified theoretical model and numerically calculated spectra are wel…
Optical solitons in multimode fibers: recent advances
本文综述了多模光纤中光孤子的近期实验、数值与理论研究进展，重点介绍其形成条件、向基模拉曼孤子的演化、模式色散主导的裂变机制及其在空分复用系统中的应用潜力。
Massively Degenerate Coherent Perfect Absorption in Gradient-Index Fibers
本文提出基于梯度指数光纤的多模相干完美吸收器，利用自成像特性实现高效多模吸收，适用于光能收集、光学控制和成像等应用。
Observation of stability of Gaussian beams and off-axis beam-cleaning in graded-index media
针对模式数量趋近无穷、色散可忽略的大梯度折射率（GRIN）波导非线性效应研究空白，观测到GRIN棒中高斯光束的稳定性及空间复杂光束的轴上/离轴自净化现象，数值模拟验证实验并建立与GRIN光纤光束净化的关联。
We overview recent advances in the research on spatiotemporal beam shaping in nonlinear multimode optical fibers. An intense light beam coupled to a graded index (GRIN) highly multimode fiber undergoes a series of complex nonlinear processes when its power grows larger. Among them, the lowest threshold effect is the Kerr-induced beam self-cleaning, that redistributes most of the beam energy into a robust bell-shaped beam close to the fundamental mode. At higher powers a series of spectral sidebands is generated, thanks to the phase matching induced by the long period grating due to the periodi…
In this work, we unveil the unique complex dynamics of multimode soliton interactions in graded-index optical fibers through simulations and experiments. By generating two multimode solitons from the fission of an input femtosecond pulse, we examine the evolution of their Raman-induced red-shift when the input pulse energy grows larger. Remarkably, we find that the output red-shift of the trailing multimode soliton may be reduced, so that it accelerates until it collides with the leading multimode soliton. As a result of the inelastic collision, a significant energy transfer occurs between the…
A laser is based on the electromagnetic modes of its resonator, which provides the feedback required for oscillation. Enormous progress has been made in controlling the interactions of longitudinal modes in lasers with a single transverse mode. For example, the field of ultrafast science has been built on lasers that lock many longitudinal modes together to form ultrashort light pulses. However, coherent superposition of many longitudinal and transverse modes in a laser has received little attention. The multitude of disparate frequency spacings, strong dispersions, and complex nonlinear inter…
Spatiotemporal Nonlinear Pulse Dynamics in Multimode Silicon Nitride Waveguides
该研究提出基于开源多模非线性薛定谔方程的模拟方法，分析6μm宽氮化硅波导中飞秒脉冲演化，发现输入模式工程可调控非线性动力学，为集成光子平台中超连续谱生成等应用提供策略。
Probing intensity noise in ultrafast pulses using the dispersive Fourier transform augmented by quantum sensitivity analysis
本文结合 DFT 单发脉冲测量与 QSA 单步反向微分噪声分析，揭示孤子裂变和拉曼散射的功率相关平均场及量子噪声特性。
Simulating multimode nonlinearities at very high mode count via parallel factor decomposition
本文利用并行因子分解压缩非线性重叠张量，将多模非线性光学模拟的模式数从几十个提升至数千个，并验证了超千模孤子传播与噪声放大模拟，为多模非线性动力学研究提供了高效工具。
Progress in neuroscience constantly relies on the development of new techniques to investigate the complex dynamics of neuronal networks. An ongoing challenge is to achieve minimally-invasive and high-resolution observations of neuronal activity in vivo inside deep brain areas. A perspective strategy is to utilise holographic control of light propagation in complex media, which allows converting a hair-thin multimode optical fibre into an ultra-narrow imaging tool. Compared to current endoscopes based on GRIN lenses or fibre bundles, this concept offers a footprint reduction exceeding an order…
Wave turbulence, thermalization and multimode locking in optical fibers
该研究基于波湍流理论推导多模光纤光学热化的动力学方程，解释梯度折射率光纤的光束自清洁效应，分析折射率无序的作用，经模拟和实验验证，还发现光束自清洁伴随模式锁模，为相关研究提供新方向。
Tunable supercontinuum in multimode fiber via bending-induced dispersion modification
本文提出通过弯曲诱导局部色散修改高阶模以调控超连续谱，保持空间模不变，实现宽频带可调超连续光源，同时保持光束质量。
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