787 research outputs found
Highly sensitive refractometer with photonic crystal fiber long-period grating
We present highly sensitive refractometers based on a long-period grating in
a large mode area PCF. The maximum sensitivity is 1500 nm/RIU at a refractive
index of 1.33, the highest reported for any fiber grating. The minimal
detectable index change is . The high sensitivity is obtained
by infiltrating the sample into the holes of the photonic crystal fiber to give
a strong interaction between the sample and the probing field.Comment: 4 pages, 3 figures, journal paper, submitte
Solitons in quadratic nonlinear photonic crystals
We study solitons in one-dimensional quadratic nonlinear photonic crystals
with modulation of both the linear and nonlinear susceptibilities. We derive
averaged equations that include induced cubic nonlinearities and numerically
find previously unknown soliton families. The inclusion of the induced cubic
terms enables us to show that solitons still exist even when the effective
quadratic nonlinearity vanishes and conventional theory predicts that there can
be no soliton. We demonstrate that both bright and dark forms of these solitons
are stable under propagation.Comment: 4 pages with 6 figure
Nonlocal incoherent solitons
We investigate the propagation of partially coherent beams in spatially
nonlocal nonlinear media with a logarithmic type of nonlinearity. We derive
analytical formulas for the evolution of the beam parameters and conditions for
the formation of nonlocal incoherent solitons.Comment: 5 pages, 3 figure
Fusion, collapse, and stationary bound states of incoherently coupled waves in bulk cubic media
We study the interaction between two localized waves that propagate in a bulk (two transverse dimensions) Kerr medium, while being incoherently coupled through cross-phase modulation. The different types of stationary solitary wave solutions are found and their stability is discussed. The results of numerical simulations suggest that the solitary waves are unstable. We derive sufficient conditions for when the wave function is bound to collapse or spread out, and we develop a theory to describe the regions of different dynamical behavior. For localized waves with the same center we confirm these sufficient conditions numerically and show that only when the equations and the initial conditions are symmetric are they also close to being necessary conditions. Using Gaussian initial conditions we predict and confirm numerically the power-dependent characteristic initial separations that divide the phase space into collapsing and diffracting solutions, and further divide each of these regions into subregions of coupled (fusion) and uncoupled dynamics. Finally we illustrate how, close to the threshold of collapse, the waves can cross several times before eventually collapsing or diffracting
Escape angles in bulk chi(2) soliton interactions
We develop a theory for non-planar interaction between two identical type I
spatial solitons propagating at opposite, but arbitrary transverse angles in
quadratic nonlinear (or so-called chi(2)) bulk media. We predict quantitatively
the outwards escape angle, below which the solitons turn around and collide,
and above which they continue to move away from each other. For in-plane
interaction the theory allows prediction of the outcome of a collision through
the inwards escape angle, i.e. whether the solitons fuse or cross. We find an
analytical expression determining the inwards escape angle using Gaussian
approximations for the solitons. The theory is verified numerically.Comment: V1: 4 pages, 4 figures. V2: Accepted for publication in Physical
Review E. 5 pages, 4 figures. Fig. 2 changed to be for fixed soliton width
and to show soliton power. New simple relations in terms of power and pahse
mismatch are include
Generic features of modulational instability in nonlocal Kerr media
The modulational instability (MI) of plane waves in nonlocal Kerr media is
studied for a general, localized, response function. It is shown that there
always exists a finite number of well-separated MI gain bands, with each of
them characterised by a unique maximal growth rate. This is a general property
and is demonstrated here for the Gaussian, exponential, and rectangular
response functions. In case of a focusing nonlinearity it is shown that
although the nonlocality tends to suppress MI, it can never remove it
completely, irrespectively of the particular shape of the response function.
For a defocusing nonlinearity the stability properties depend sensitively on
the profile of the response function. It is shown that plane waves are always
stable for response functions with a positive-definite spectrum, such as
Gaussians and exponentials. On the other hand, response functions whose spectra
change sign (e.g., rectangular) will lead to MI in the high wavenumber regime,
provided the typical length scale of the response function exceeds a certain
threshold. Finally, we address the case of generalized multi-component response
functions consisting of a weighted sum of N response functions with known
properties.Comment: 9 pages, 5 figure
Accurate switching intensities and length scales in quasi-phase-matched materials
We consider unseeded Type I second-harmonic generation in quasi-phase-matched
(QPM) quadratic nonlinear materials and derive an accurate analytical
expression for the evolution of the average intensity. The intensity-dependent
nonlinear phase mismatch due to the QPM induced cubic nonlinearity is found.
The equivalent formula for the intensity for maximum conversion, the crossing
of which changes the nonlinear phase-shift of the fundamental over a period
abruptly by , corrects earlier estimates by more than a factor of 5. We
find the crystal lengths necessary to obtain an optimal flat phase versus
intensity response on either side of this separatrix intensity.Comment: 3 pages with 3 figure
Directional supercontinuum generation: the role of the soliton
In this paper we numerically study supercontinuum generation by pumping a
silicon nitride waveguide, with two zero-dispersion wavelengths, with
femtosecond pulses. The waveguide dispersion is designed so that the pump pulse
is in the normal-dispersion regime. We show that because of self-phase
modulation, the initial pulse broadens into the anomalous-dispersion regime,
which is sandwiched between the two normal-dispersion regimes, and here a
soliton is formed. The interaction of the soliton and the broadened pulse in
the normal-dispersion regime causes additional spectral broadening through
formation of dispersive waves by non-degenerate four-wave mixing and
cross-phase modulation. This broadening occurs mainly towards the second
normal-dispersion regime. We show that pumping in either normal-dispersion
regime allows broadening towards the other normal-dispersion regime. This
ability to steer the continuum extension towards the direction of the other
normal-dispersion regime beyond the sandwiched anomalous-dispersion regime
underlies the directional supercontinuum notation. We numerically confirm the
approach in a standard silica microstructured fiber geometry with two
zero-dispersion wavelengths
The complete modulational instability gain spectrum of nonlinear QPM gratings
We consider plane waves propagating in quadratic nonlinear slab waveguides
with nonlinear quasi-phase-matching gratings. We predict analytically and
verify numerically the complete gain spectrum for transverse modulational
instability, including hitherto undescribed higher order gain bands.Comment: 4 pages, 3 figures expanded with more explanation and mathematical
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