609 research outputs found
Interplay of Kerr and Raman beam cleaning with a multimode microstructure fiber
We experimentally study the competition between Kerr beam self-cleaning and
Raman beam cleanup in a multimode air-silica microstructure optical fiber. Kerr
beam self-cleaning of the pump is observed for a certain range of input powers
only. Raman Stokes beam generation and cleanup lead to both depletion and
degradation of beam quality for the pump. The interplay of modal four-wave
mixing and Raman scattering in the infrared domain lead to the generation of a
multimode supercontinuum ranging from 500 nm up to 1800 nm
Inner speech sustains predictable task switching: direct evidence in adults
It has been proposed that inner speech supports task selection in task-switching studies, especially when the need for endogenous control is increased. This has been established through the suppression of inner speech in cognitive-flexibility tasks that leads to poorer performance. The aim of this study is to quantify the role of inner speech in a flexibility task by using surface laryngeal electromyography, which, contrary to previous studies, enables participants to freely verbalise the tasks. We manipulated endogenous and exogenous flexibility in a mathematical switching task paradigm. Experiment 1 shows that inner speech acts as a support for switching and is recruited more often when the tasks are of an endogenous type. The main result of Experiment 2 that language is recruited more for the mixing cost than for the switch cost (regardless of the endogenous factor) extends past findings obtained through articulatory suppression
Full vectorial analysis of polarization effects in optical nanowires
We develop a full theoretical analysis of the nonlinear interactions of the
two polarizations of a waveguide by means of a vectorial model of pulse
propagation which applies to high index subwavelength waveguides. In such
waveguides there is an anisotropy in the nonlinear behavior of the two
polarizations that originates entirely from the waveguide structure, and leads
to switching properties. We determine the stability properties of the steady
state solutions by means of a Lagrangian formulation. We find all static
solutions of the nonlinear system, including those that are periodic with
respect to the optical fiber length as well as nonperiodic soliton solutions,
and analyze these solutions by means of a Hamiltonian formulation. We discuss
in particular the switching solutions which lie near the unstable steady
states, since they lead to self-polarization flipping which can in principle be
employed to construct fast optical switches and optical logic gates
Spatio-temporal beam dynamics in multimode nonlinear optical fibers
ABSTRACT
We overview recent advances in the spatio-temporal nonlinear dynamics of optical pulses propagating in multimode optical fibers. The Kerr effect leads to spatial beam self-cleaning in a graded-index multimode optical fiber, followed by sideband series generation spanning multiple octaves. Effectively single mode supercontinuum spanning from the visible to the mid-infrared was also demonstrated. Enhancement of Kerr beam self-cleaning was observed in active fiber with quasi-step index profile. Moreover, mutual self-cleaning was recently reported for both the fundamental and the second harmonic beams in optically poled multimode fibers with cubic and quadratic nonlinearity
Spatiotemporal Characterization of Supercontinuum Extending from the Visible to the Mid-Infrared in Multimode Graded-Index Optical Fiber
We experimentally demonstrate that pumping a graded-index multimode fiber
with sub-ns pulses from a microchip Nd:YAG laser leads to spectrally flat
supercontinuum generation with a uniform bell-shaped spatial beam profile
extending from the visible to the mid-infrared at 2500\,nm. We study the
development of the supercontinuum along the multimode fiber by the cut-back
method, which permits us to analyze the competition between the Kerr-induced
geometric parametric instability and stimulated Raman scattering. We also
performed a spectrally resolved temporal analysis of the supercontinuum
emission.Comment: 5 pages 7 figure
Spatial beam self-cleaning and supercontinuum generation with Yb-doped multimode graded-index fiber taper based on accelerating self-imaging and dissipative landscape
We experimentally demonstrate spatial beam self-cleaning and supercontinuum generation in a tapered Ytterbium-doped multimode optical fiber with parabolic core refractive index profile when 1064 nm pulsed beams propagate from wider (122 µm) into smaller (37 µm) diameter. In the passive mode, increasing the input beam peak power above 20 kW leads to a bell-shaped output beam profile. In the active configuration, gain from the pump laser diode permits to combine beam self-cleaning with supercontinuum generation between 520-2600 nm. By taper cut-back, we observed that the dissipative landscape, i.e., a non-monotonic variation of the average beam power along the MMF, leads to modal transitions of self-cleaned beams along the taper length
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