33,055 research outputs found
Analog, Digital And Short Pulse Modulation Of Ultrafast Gallium Aluminum Arsenide Semiconductor Lasers
Semiconductor lasers are potentially devices of great importance for optical transmission as well as short pulse generation for various sampling, characteration and dispersion measurements. Since semiconductor lasers are currently driven devices, it is relatively easy to modulate the optical output and to generate short pulses, on the order of 10 ps long, by directly modulating the injection current into the laser. This paper will present some recent developments in injection lasers which are capable of being analog or digitally modulated at rates up to 10 GHz, as well as generating short optical pulses at repetition rates from several hundred megahertz to tens of gigahertz
A point process framework for modeling electrical stimulation of the auditory nerve
Model-based studies of auditory nerve responses to electrical stimulation can
provide insight into the functioning of cochlear implants. Ideally, these
studies can identify limitations in sound processing strategies and lead to
improved methods for providing sound information to cochlear implant users. To
accomplish this, models must accurately describe auditory nerve spiking while
avoiding excessive complexity that would preclude large-scale simulations of
populations of auditory nerve fibers and obscure insight into the mechanisms
that influence neural encoding of sound information. In this spirit, we develop
a point process model of the auditory nerve that provides a compact and
accurate description of neural responses to electric stimulation. Inspired by
the framework of generalized linear models, the proposed model consists of a
cascade of linear and nonlinear stages. We show how each of these stages can be
associated with biophysical mechanisms and related to models of neuronal
dynamics. Moreover, we derive a semi-analytical procedure that uniquely
determines each parameter in the model on the basis of fundamental statistics
from recordings of single fiber responses to electric stimulation, including
threshold, relative spread, jitter, and chronaxie. The model also accounts for
refractory and summation effects that influence the responses of auditory nerve
fibers to high pulse rate stimulation. Throughout, we compare model predictions
to published physiological data and explain differences in auditory nerve
responses to high and low pulse rate stimulation. We close by performing an
ideal observer analysis of simulated spike trains in response to sinusoidally
amplitude modulated stimuli and find that carrier pulse rate does not affect
modulation detection thresholds.Comment: 1 title page, 27 manuscript pages, 14 figures, 1 table, 1 appendi
Communication in a Poisson Field of Interferers -- Part II: Channel Capacity and Interference Spectrum
In Part I of this paper, we presented a mathematical model for communication
subject to both network interference and noise, where the interferers are
scattered according to a spatial Poisson process, and are operating
asynchronously in a wireless environment subject to path loss, shadowing, and
multipath fading. We determined the distribution of the aggregate interference
and the error performance of the link. In this second part, we characterize the
capacity of the link subject to both network interference and noise. Then, we
put forth the concept of spectral outage probability (SOP), a new
characterization of the aggregate radio-frequency emission generated by
communicating nodes in a wireless network. We present some applications of the
SOP, namely the establishment of spectral regulations and the design of covert
military networks. The proposed framework captures all the essential physical
parameters that affect the aggregate network emission, yet is simple enough to
provide insights that may be of value in the design and deployment of wireless
networks.Comment: To appear in IEEE Transactions on Wireless Communication
Image intensifier characterization
An image intensifier forms an integral part of a full-field image range finder under development at the University of Waikato. Operating as a high speed shutter with repetition rates up to 100 MHz, a method is described to characterise the response, both temporally and spatially, of the intensifier in order to correct for variations in the field of view and to optimise the operating conditions. A short pulse of visible light is emitted by a laser diode, uniformly illuminating the image intensifier, while a CCD camera captures the output from the intensifier. The phase of the laser pulse is continuously varied using a heterodyne configuration, automatically producing a set of samples covering the modulation cycle. The results show some anomalies in the response of our system and some simple solutions are proposed to correct for these
Physics and Applications of Laser Diode Chaos
An overview of chaos in laser diodes is provided which surveys experimental
achievements in the area and explains the theory behind the phenomenon. The
fundamental physics underpinning this behaviour and also the opportunities for
harnessing laser diode chaos for potential applications are discussed. The
availability and ease of operation of laser diodes, in a wide range of
configurations, make them a convenient test-bed for exploring basic aspects of
nonlinear and chaotic dynamics. It also makes them attractive for practical
tasks, such as chaos-based secure communications and random number generation.
Avenues for future research and development of chaotic laser diodes are also
identified.Comment: Published in Nature Photonic
Ultrafast Insulator-Metal Phase Transition in VO2 Studied by Multiterahertz Spectroscopy
The ultrafast photoinduced insulator-metal transition in VO2 is studied at
different temperatures and excitation fluences using multi-THz probe pulses.
The spectrally resolved mid-infrared response allows us to trace separately the
dynamics of lattice and electronic degrees of freedom with a time resolution of
40 fs. The critical fluence of the optical pump pulse which drives the system
into a long-lived metallic state is found to increase with decreasing
temperature. Under all measurement conditions we observe a modulation of the
eigenfrequencies of the optical phonon modes induced by their anharmonic
coupling to the coherent wave packet motion of V-V dimers at 6.1 THz.
Furthermore, we find a weak quadratic coupling of the electronic response to
the coherent dimer oscillation resulting in a modulation of the electronic
conductivity at twice the frequency of the wave packet motion. The findings are
discussed in the framework of a qualitative model based on an approximation of
local photoexcitation of the vanadium dimers from the insulating state.Comment: 10 pages, 8 figures submitted to Physical Review
Optimum frequency modulation receivers
Optimization of receivers for demodulating FM SIGNAL corrupted by Gaussian noise, and solution to maximum likelihood equatio
Nonlinear evolution of the plasma beatwave: Compressing the laser beatnotes via electromagnetic cascading
The near-resonant beatwave excitation of an electron plasma wave (EPW) can be
employed for generating the trains of few-femtosecond electromagnetic (EM)
pulses in rarefied plasmas. The EPW produces a co-moving index grating that
induces a laser phase modulation at the difference frequency. The bandwidth of
the phase-modulated laser is proportional to the product of the plasma length,
laser wavelength, and amplitude of the electron density perturbation. The laser
spectrum is composed of a cascade of red and blue sidebands shifted by integer
multiples of the beat frequency. When the beat frequency is lower than the
electron plasma frequency, the red-shifted spectral components are advanced in
time with respect to the blue-shifted ones near the center of each laser
beatnote. The group velocity dispersion of plasma compresses so chirped
beatnotes to a few-laser-cycle duration thus creating a train of sharp EM
spikes with the beat periodicity. Depending on the plasma and laser parameters,
chirping and compression can be implemented either concurrently in the same, or
sequentially in different plasmas. Evolution of the laser beatwave end electron
density perturbations is described in time and one spatial dimension in a
weakly relativistic approximation. Using the compression effect, we demonstrate
that the relativistic bi-stability regime of the EPW excitation [G. Shvets,
Phys. Rev. Lett. 93, 195004 (2004)] can be achieved with the initially
sub-threshold beatwave pulse.Comment: 13 pages, 11 figures, submitted to Physical Review
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