672 research outputs found
Extreme Events in Resonant Radiation from Three-dimensional Light Bullets
We report measurements that show extreme events in the statistics of resonant
radiation emitted from spatiotemporal light bullets. We trace the origin of
these extreme events back to instabilities leading to steep gradients in the
temporal profile of the intense light bullet that occur during the initial
collapse dynamics. Numerical simulations reproduce the extreme valued
statistics of the resonant radiation which are found to be intrinsically linked
to the simultaneous occurrence of both temporal and spatial self-focusing
dynamics. Small fluctuations in both the input energy and in the spatial phase
curvature explain the observed extreme behaviour.Comment: 5 pages, 5 figures, submitte
Evolution in prostheses for sprinters with lower-limb amputation
Depuis une quinzaine d'annĂ©es, les progrĂšs techniques en appareillage ont Ă©tĂ© le facteur dĂ©terminant de la progression des performances des sportifs amputĂ©s de membre infĂ©rieur. Pour l'amputĂ© tibial, la prothĂšse de course comprend un manchon gel et une emboĂźture solidarisĂ©s par un accrochage distal ou un vide virtuel. Par ses qualitĂ©s dynamiques, le pied en fibre de carbone, garni de pointes, assure des performances remarquables. Pour l'amputĂ© fĂ©moral, Ă©quipĂ© des mĂȘmes pieds prothĂ©tiques, le genou est Ă biellettes et Ă contrĂŽle des phases d'appui et pendulaire. Par rapport au coureur valide, le temps d'appui sur le membre appareillĂ© est raccourci tandis que celui sur le membre sain est allongĂ©. L'asymĂ©trie du sprint de l'amputĂ© tibial est discrĂšte. C'est le travail des extenseurs de hanche qui est la principale compensation au dĂ©ficit de propulsion dĂ» Ă l'amputation. Chez l'amputĂ© fĂ©moral, l'absence de genou aggrave l'asymĂ©trie. L'extension totale du genou prothĂ©tique, prĂ©coce en fin de phase oscillante et persistant pendant toute la phase d'appui, impose une compensation par une augmentation d'extension de la hanche controlatĂ©rale. Les transferts de charge de travail entre cĂŽtĂ© amputĂ© et sain, par l'intermĂ©diaire d'une hyperlordose lombaire, mettent en jeu le bassin, le tronc et les Ă©paules. Les amputĂ©s sportifs font progresser la recherche en appareillage. Leurs orthoprothĂ©sistes acquiĂšrent avec eux un savoir-faire dont bĂ©nĂ©ficient leurs patients non-sportifs.For about 15 years, technical advances in prosthetic treatment have been the main factor in the increased performance of athletes with lower-limb amputation. For trans-tibial amputation, the prosthesis for sprinting is composed of a gel liner and a socket joined by a locking or virtual vacuum liner. Because of these dynamic properties, the carbon prosthetic foot equipped with tacks ensures outstanding performance. For trans-femoral amputation, a hydraulic swing and a stance control unit are added to the same prosthesis. In comparison with the able-bodied runner, athletes with amputation have smaller loading times in the prosthetic limb and larger ones in the sound limb. The length of the energy-storing prosthetic foot is determined by the âup-on-the-toesâ running gait. The sprinting gait with trans-tibial amputation is almost symmetrical. The hip extensor effort is the main compensation of propulsion reduction with lower-limb amputation. With trans-femoral amputation, the lack of knee increases the asymmetry. The total prosthetic knee extension (early in late-swing phase and lasting during total stance phase) compensates with extension of both hips, especially the opposite one. The amputation and sound limb load transfer with lumbar hyperlordosis concern the pelvis, trunk and shoulders. Because of athletes with amputation, research in prosthetic treatment has progressed. The development of orthotics and prostheses for such athletes has benefited non-athletes with amputation
Real-time measurements of dissipative solitons in a mode-locked fiber laser
Dissipative solitons are remarkable localized states of a physical system
that arise from the dynamical balance between nonlinearity, dispersion and
environmental energy exchange. They are the most universal form of soliton that
can exist in nature, and are seen in far-from-equilibrium systems in many
fields including chemistry, biology, and physics. There has been particular
interest in studying their properties in mode-locked lasers producing
ultrashort light pulses, but experiments have been limited by the lack of
convenient measurement techniques able to track the soliton evolution in
real-time. Here, we use dispersive Fourier transform and time lens measurements
to simultaneously measure real-time spectral and temporal evolution of
dissipative solitons in a fiber laser as the turn-on dynamics pass through a
transient unstable regime with complex break-up and collision dynamics before
stabilizing to a regular mode-locked pulse train. Our measurements enable
reconstruction of the soliton amplitude and phase and calculation of the
corresponding complex-valued eigenvalue spectrum to provide further physical
insight. These findings are significant in showing how real-time measurements
can provide new perspectives into the ultrafast transient dynamics of complex
systems.Comment: See also M. Narhi, P. Ryczkowski, C. Billet, G. Genty, J. M. Dudley,
Ultrafast Simultaneous Real Time Spectral and Temporal Measurements of Fibre
Laser Modelocking Dynamics, 2017 Conference on Lasers and Electro-Optics
Europe & European Quantum Electronics Conference, paper EE-3.5 (2017
Hydrodynamic supercontinuum
We report the experimental observation of multi-bound-soliton solutions of the nonlinear Schrödinger equation (NLS) in the context of hydrodynamic surface gravity waves. Higher-order N-soliton solutions with N=2, 3 are studied in detail and shown to be
Robust design of Si/Si3N4 high contrast grating mirror for mid-infrared VCSEL application
A Si/Si3N4 high contrast grating mirror has been designed for a VCSEL
integration in mid-infrared ({\lambda} = 2.65 m). The use of an
optimization algorithm which maximizes a VCSEL mirror quality factor allowed
the adjustment of the grating parameters while keeping large and shallow
grating pattern. The robustness with respect to fabrication error has been
enhanced thanks to a precise study of the grating dimension tolerances. The
final mirror exhibits large high reflectivity bandwidth with a polarization
selectivity and several percent of tolerance on the grating dimensions.Comment: The final publication is available at http://www.springerlink.com,
Optical and Quantum Electronics (2012) Online Firs
Real-time full bandwidth measurement of spectral noise in supercontinuum generation
The ability to measure real-time fluctuations of ultrashort pulses
propagating in optical fiber has provided significant insights into fundamental
dynamical effects such as modulation instability and the formation of
frequency-shifting rogue wave solitons. We report here a detailed study of
real-time fluctuations across the full bandwidth of a fiber supercontinuum
which directly reveals the significant variation in measured noise statistics
across the spectrum, and which allows us to study correlations between widely
separated spectral components. For two different propagation distances
corresponding to the onset phase of spectral broadening and the fully-developed
supercontinuum, we measure real time noise across the supercontinuum bandwidth,
and we quantify the supercontinuum noise using statistical higher-order moments
and a frequency-dependent intensity correlation map. We identify correlated
spectral regions within the supercontinuum associated with simultaneous
sideband generation, as well as signatures of pump depletion and soliton-like
pump dynamics. Experimental results are in excellent agreement with
simulations
A Cell of Oneâs Own? Incarceration and Other Turning Points in Womenâs Journeys to Desistance
Research has shown the importance of turning points in desistance from criminal behavior. Using qualitative data from a sample of 100 formerly incarcerated mothers interviewed about their criminal behavior, this article explores their descriptions of transition moments and whether and how those moments affected their criminal behavior. The findings indicate that whereas parenting emerges as a turning point, the practical difficulties of reentry may reduce the impact of mothering on womenâs desistance. More self-focused turning points, such as those due to incarceration, arrest, and sobriety appeared to be particularly important to the womenâs desistance. This article emphasizes the need for research into the subjective and environmental factors that affect womenâs desistance behaviors
Revealing the last 13,500 years of environmental history from the multiproxy record of a mountain lake (Lago Enol, northern Iberian Peninsula)
This is the author's accepted manuscript. The final publication is available at Springer via http://dx.doi.org/10.1007/s10933-009-9387-7.We present the Holocene sequence from Lago Enol (43°16âČN, 4°59âČW, 1,070 m a.s.l.), Cantabrian Mountains, northern Spain. A multiproxy analysis provided comprehensive information about regional humidity and temperature changes. The analysis included sedimentological descriptions, physical properties, organic carbon and carbonate content, mineralogy and geochemical composition together with biological proxies including diatom and ostracod assemblages. A detailed pollen study enabled reconstruction of variations in vegetation cover, which were interpreted in the context of climate changes and human impact. Four distinct stages were recognized for the last 13,500 years: (1) a cold and dry episode that includes the Younger Dryas event (13,500â11,600 cal. year BP); (2) a humid and warmer period characterizing the onset of the Holocene (11,600â8,700 cal. year BP); (3) a tendency toward a drier climate during the middle Holocene (8,700â4,650 cal. year BP); and (4) a return to humid conditions following landscape modification by human activity (pastoral activities, deforestation) in the late Holocene (4,650â2,200 cal. year BP). Superimposed on relatively stable landscape conditions (e.g. maintenance of well established forests), the typical environmental variability of the southern European region is observed at this site.The Spanish Inter-Ministry Commission of Science and
Technology (CICYT), the
Spanish National Parks agency, the European Commission, the
Spanish Ministry of Science, and the European
Social Fund
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