74 research outputs found
Taming of Modulation Instability by Spatio-Temporal Modulation of the Potential
Spontaneous pattern formation in a variety of spatially extended nonlinear
system always occurs through a modulation instability: homogeneous state of the
system becomes unstable with respect to growing modulation modes. Therefore,
the manipulation of the modulation instability is of primary importance in
controlling and manipulating the character of spatial patterns initiated by
that instability. We show that the spatio-temporal periodic modulation of the
potential of the spatially extended system results in a modification of its
pattern forming instability. Depending on the modulation character the
instability can be partially suppressed, can change its spectrum (for instance
the long wave instability can transform into short wave instability), can split
into two, or can be completely eliminated. The latter result is of especial
practical interest, as can be used to stabilize the intrinsically unstable
system. The result bears general character, as it is shown here on a universal
model of Complex Ginzburg-Landau equations in one and two spatial dimension
(and time). The physical mechanism of instability suppression can be applied to
a variety of intrinsically unstable dissipative systems, like self-focusing
lasers, reaction-diffusion systems, as well as in unstable conservative
systems, like attractive Bose Einstein condensates.Comment: 5 pages, 4 figures, 1 supplementary video fil
Regularization of Vertical-Cavity Surface-Emitting Lasers emission by periodic non-Hermitian potentials
We propose a novel physical mechanism based on periodic non-Hermitian
potentials to efficiently control the complex spatial dynamics of broad-area
lasers, particularly in Vertical-Cavity Surface-Emitting Lasers (VCSELs),
achieving a stable emission of maximum brightness. Radially dephased periodic
refractive index and gain-loss modulations accumulate the generated light from
the entire active layer and concentrate it around the structure axis to emit
narrow, bright beams. The effect is due to asymmetric-inward radial coupling
between transverse modes, for particular phase differences of the refractive
index and gain-loss modulations. Light is confined into a central beam with
large intensity opening the path to design compact, bright and efficient
broad-area light sources. We perform a comprehensive analysis to explore the
maximum central intensity enhancement and concentration regimes. The study
reveals that the optimum schemes are those holding unidirectional inward
coupling but not fulfilling a perfect local PT-symmetry.Comment: 4 pages, 4 figure
Slow light enabled wavelength demultiplexing
Photonic crystal waveguides supporting band gap guided modes hold great potential to tailor the group
velocity of propagating light. We propose and explore different wavelength demultiplexer design approaches that utilize
slow light concept. By altering the dielectric filling factors of each waveguide segment, one can show that different
frequencies can be separated and extracted at different locations along the cascaded waveguide. Furthermore, to
eliminate the inherent reflection loss of such a design, a composite structure involving a tapered waveguide with a
side-coupled resonator is also presented. Such a structure features not only a forward propagating wave but also a
backward propagating wave acting as a feedback mechanism for the drop channels. We show that by careful design of
the waveguide and the resonator, the destructive and instructive interference of these waves can effectively eliminate
the reflection loss and increase the coupling efficiency, respectively. Numerical and experimental verification of the
proposed structures show that the targeted frequencies can be coupled out with low cross-talks and moderate quality
factors, while maintaining a compact size. © 2016 IEEE.Peer ReviewedPostprint (published version
Non-Hermitian arrangement for stable semiconductor laser arrays
We propose and explore a physical mechanism for the stabilization of the
complex spatiotemporal dynamics in arrays (bars) of broad area laser diodes
taking advantage of the symmetry breaking in non-Hermitian potentials. We show
that such stabilization can be achieved by specific pump and index profiles
leading to a PT-symmetric coupling between nearest neighboring lasers within
the semiconductor bar. A numerical analysis is performed using a complete
(2+1)-dimensional space-temporal model, including transverse and longitudinal
spatial degrees of freedom and temporal evolution of the electric field and
carriers. We show regimes of temporal stabilization and light emission spatial
redistribution and enhancement. We also consider a simplified (1+1)-dimensional
model for an array of lasers holding the proposed non-Hermitian coupling with a
global axisymmetric geometry. We numerically demonstrate a two-fold benefit:
the control over the temporal dynamics over the EELs bar and the field
concentration on the central lasers leading to a brighter output beam,
facilitating a direct coupling to an optical fiber
Diagnóstico diferencial de las enfermedades prostáticas
Este trabajo incluye una revisión de los métodos de diagnóstico de las enfermedades prostáticas; además describe cómo podemos distinguir entre cada una de ellas, dependiendo del método de diagnóstico utilizado.This paper reviews the prostatic pathologies diagnosis methods; it also describes the way we can distinguish between each one depending on the one we use
Non-Hermitian Mode Cleaning in Periodically Modulated Multimode Fibers
We show that the simultaneous modulation of the propagation constant and of
the gain/loss coefficient along the multimode fibers results in unidirectional
coupling among the modes, which, depending on the modulation parameters, leads
to the enhancement or reduction of the excitation of higher order transverse
modes. In the latter case, effective mode-cleaning is predicted, in ideal case
resulting in single-mode spatially coherent output. The effect is
semi-analytically predicted on a simplified Gaussian beam approximation and
numerically proven by solving the wave propagation equation introducing the
modulated potential
Restricted Hilbert transform for non-Hermitian management of fields
Non-Hermitian systems exploiting the synergy between gain and loss have
recently become the focus of interest to discover novel physical phenomena. The
spatial symmetry breaking in such systems allows tailoring the wave propagation
at will. Inspired by such property, we propose a feasible approach based on
local Hilbert transform to control the field flows in two- or higher
dimensional non-Hermitian systems, restricting the complex refractive index
within practical limits. We propose an iterative procedure to reduce the
dimensionality of complex refractive index parameter space to two, one or zero
dimensions. The proposed method provides a flexible way to systematically
design locally PT-symmetric systems realizable with a limited collection of
realistic materials.Comment: 16 pages, 10 figure
Stabilization of Broad Area Semiconductor Laser sources by simultaneous index and pump modulations
We show that the emission of broad area semiconductor amplifiers and lasers
can be efficiently stabilized by introducing, two-dimensional periodic
modulations simultaneously on both the refractive index and the pump
(gain-loss) profiles, in the transverse and longitudinal directions. The
interplay between such index and gain-loss modulations efficiently suppresses
the pattern forming instabilities, leading to highly stable and bright narrow
output beams from such sources. We also determine the stabilization performance
of the device as a function of pump current and linewidth enhancement factor.Comment: 4 pages, 4 figure
Nueva estación fenológica asociada al Observatorio Meteorológico de Guadalajara
Ponencia presentada en: XXXIII Jornadas Científicas de la AME y el XIV Encuentro Hispano Luso de Meteorología celebrado en Oviedo, del 7 al 9 de abril de 2014
High-Directional Wave Propagation in Periodic Gain/Loss Modulated Materials
Amplification/attenuation of light waves in artificial materials with a
gain/loss modulation on the wavelength scale can be sensitive to the
propagation direction. We give a numerical proof of the high anisotropy of the
gain/loss in two dimensional periodic structures with square and rhombic
lattice symmetry by solving the full set of Maxwell's equations using the
finite difference time domain method. Anisotropy of amplification/attenuation
leads to the narrowing of the angular spectrum of propagating radiation with
wavevectors close to the edges of the first Brillouin Zone. The effect provides
a novel and useful method to filter out high spatial harmonics from noisy
beams
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