4,377 research outputs found
Extending AMCW lidar depth-of-field using a coded aperture
By augmenting a high resolution full-field Amplitude Modulated Continuous Wave lidar system with a coded aperture, we show that depth-of-field can be extended using explicit, albeit blurred, range data to determine PSF scale. Because complex domain range-images contain explicit range information, the aperture design is unconstrained by the necessity for range determination by depth-from-defocus. The coded aperture design is shown to improve restoration quality over a circular aperture. A proof-of-concept algorithm using dynamic PSF determination and spatially variant Landweber iterations is developed and using an empirically sampled point spread function is shown to work in cases without serious multipath interference or high phase complexity
Nonlinear limits to the information capacity of optical fiber communications
The exponential growth in the rate at which information can be communicated
through an optical fiber is a key element in the so called information
revolution. However, like all exponential growth laws, there are physical
limits to be considered. The nonlinear nature of the propagation of light in
optical fiber has made these limits difficult to elucidate. Here we obtain
basic insights into the limits to the information capacity of an optical fiber
arising from these nonlinearities. The key simplification lies in relating the
nonlinear channel to a linear channel with multiplicative noise, for which we
are able to obtain analytical results. In fundamental distinction to the linear
additive noise case, the capacity does not grow indefinitely with increasing
signal power, but has a maximal value. The ideas presented here have broader
implications for other nonlinear information channels, such as those involved
in sensory transduction in neurobiology. These have been often examined using
additive noise linear channel models, and as we show here, nonlinearities can
change the picture qualitatively.Comment: 1 figure, 7 pages, submitted to Natur
Pulses and Snakes in Ginzburg--Landau Equation
Using a variational formulation for partial differential equations (PDEs)
combined with numerical simulations on ordinary differential equations (ODEs),
we find two categories (pulses and snakes) of dissipative solitons, and analyze
the dependence of both their shape and stability on the physical parameters of
the cubic-quintic Ginzburg-Landau equation (CGLE). In contrast to the regular
solitary waves investigated in numerous integrable and non-integrable systems
over the last three decades, these dissipative solitons are not stationary in
time. Rather, they are spatially confined pulse-type structures whose envelopes
exhibit complicated temporal dynamics. Numerical simulations reveal very
interesting bifurcations sequences as the parameters of the CGLE are varied.
Our predictions on the variation of the soliton amplitude, width, position,
speed and phase of the solutions using the variational formulation agree with
simulation results.Comment: 30 pages, 14 figure
Laser Guide Star for Large Segmented-Aperture Space Telescopes, Part I: Implications for Terrestrial Exoplanet Detection and Observatory Stability
Precision wavefront control on future segmented-aperture space telescopes
presents significant challenges, particularly in the context of high-contrast
exoplanet direct imaging. We present a new wavefront control architecture that
translates the ground-based artificial guide star concept to space with a laser
source aboard a second spacecraft, formation flying within the telescope
field-of-view. We describe the motivating problem of mirror segment motion and
develop wavefront sensing requirements as a function of guide star magnitude
and segment motion power spectrum. Several sample cases with different values
for transmitter power, pointing jitter, and wavelength are presented to
illustrate the advantages and challenges of having a non-stellar-magnitude
noise limited wavefront sensor for space telescopes. These notional designs
allow increased control authority, potentially relaxing spacecraft stability
requirements by two orders of magnitude, and increasing terrestrial exoplanet
discovery space by allowing high-contrast observations of stars of arbitrary
brightness.Comment: Submitted to A
Topographical coloured plasmonic coins
The use of metal nanostructures for colourization has attracted a great deal
of interest with the recent developments in plasmonics. However, the current
top-down colourization methods based on plasmonic concepts are tedious and time
consuming, and thus unviable for large-scale industrial applications. Here we
show a bottom-up approach where, upon picosecond laser exposure, a full colour
palette independent of viewing angle can be created on noble metals. We show
that colours are related to a single laser processing parameter, the total
accumulated fluence, which makes this process suitable for high throughput
industrial applications. Statistical image analyses of the laser irradiated
surfaces reveal various distributions of nanoparticle sizes which control
colour. Quantitative comparisons between experiments and large-scale
finite-difference time-domain computations, demonstrate that colours are
produced by selective absorption phenomena in heterogeneous nanoclusters.
Plasmonic cluster resonances are thus found to play the key role in colour
formation.Comment: 9 pages, 5 figure
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