62,316 research outputs found
Fast tuneable InGaAsP DBR laser using quantum-confined stark-effect-induced refractive index change
We report a monolithically integrated InGaAsP DBR ridge waveguide laser that uses the quantum-confined Stark effect (QCSE) to achieve fast tuning response. The laser incorporates three sections: a forward-biased gain section, a reverse-biased phase section, and a reverse-biased DBR tuning section. The laser behavior is modeled using transmission matrix equations and tuning over similar to 8 nm is predicted. Devices were fabricated using post-growth shallow ion implantation to reduce the loss in the phase and DBR sections by quantum well intermixing. The lasing wavelength was measured while varying the reverse bias of the phase and DBR sections in the range 0 V to < - 2.5 V. Timing was noncontinuous over a similar to 7-nm-wavelength range, with a side-mode suppression ratio of similar to 20 dB. Coupled cavity effects due to the fabrication method used introduced discontinuities in tuning. The frequency modulation (FM) response was measured to be uniform within 2 dB over the frequency range 10 MHz to 10 GHz, indicating that tuning times of 100 ps are possible
A monolithic MQW InP-InGaAsP-Based optical comb generator
We report the first demonstration of a monolithic optical-frequency comb generator. The device is based on multi-section quaternary/quaternary eight-quantum-well InP-InGaAsP material in a frequency-modulated (FM) laser design. The modulation is generated using quantum-confined Stark-effect phase-induced refractive index modulation to achieve fast modulation up to 24.4 GHz. The laser was fabricated using a single epitaxial growth step and quantum-well intermixing to realize low-loss phase adjustment and modulation sections. The output was quasicontinuous wave with intensity modulation at less than 20% for a total output power of 2 mW. The linewidth of each line was limited by the linewidth of the free running laser at an optimum of 25 MHz full-width at half-maximum. The comb generator produces a number of lines with a spacing exactly equal to the modulation frequency (or a multiple of it), differential phase noise between adjacent lines of -82 dBc/Hz at 1-kHz offset (modulation source-limited), and a potential comb spectrum width of up to 2 THz (15 nm), though the comb spectrum was not continuous across the full span
A monolithic MQW InP/InGaAsP-based comb generator
We report a monolithic optical frequency comb generator using quaternary/quaternary multiple quantum well InV/InGaAsP material as phase modulator and gain medium in a Frequency Modulated (FM) laser design. The modulation was generated by quantum confined Stark effect to achieve a comb-line spacing of 24.4 GHz. The laser was fabricated using a single epitaxial growth step and quantum well intermixing to realize low loss phase and modulation sections. The resulting comb generator produces lines with a spacing exactly given by the modulation frequency, differential phase noise between adjacent lines of -82 dBc/Hz at 1 kHz offset and a comb spectrum width of up to 2 THz
Approximate Minimum Diameter
We study the minimum diameter problem for a set of inexact points. By
inexact, we mean that the precise location of the points is not known. Instead,
the location of each point is restricted to a contineus region (\impre model)
or a finite set of points (\indec model). Given a set of inexact points in
one of \impre or \indec models, we wish to provide a lower-bound on the
diameter of the real points.
In the first part of the paper, we focus on \indec model. We present an
time
approximation algorithm of factor for finding minimum diameter
of a set of points in dimensions. This improves the previously proposed
algorithms for this problem substantially.
Next, we consider the problem in \impre model. In -dimensional space, we
propose a polynomial time -approximation algorithm. In addition, for
, we define the notion of -separability and use our algorithm for
\indec model to obtain -approximation algorithm for a set of
-separable regions in time
Effect of Soft and Softer Handoffs on CDMA System Capacity
Abstract? The effect of soft and softer handoffs on code-division multiple-access (CDMA) system capacity is evaluated for unsectorized and sectorized hexagonal cells according to an average bit energy-to-interference power spectral density, which corresponds to a bit-error rate (BER) of 10-3. The effect of imperfect sectorization on sectorization efficiency is also considered. On the reverse link, there is no capacity loss as no extra channels are needed to perform soft handoff, while the macrodiversity provided by soft handoff can improve the reverse-link quality and extend the cell coverage. On the forward link, when soft handoff is employed in unsectorized cells, the capacity loss due to two traffic channels assigned to a user in the handoff zone is 0.2% or 1.1% for a voice activity factor of 3/8 or 1/2, respectively. As the forward-link capacity is higher than that of the reverse link, this small capacity loss does not affect the system capacity. For sectorized cells having three sectors per cell, there are overlapping coverage areas between sectors, where mobiles in these areas are subjected to an increase in cochannel iterference. For an overlapping angle of 5°, the sectorization efficiency is 0.96 and 0.7 for the reverse-link and forward-link systems, respectively. When soft and softer handoffs are employed, the forward-link sectorization efficiency is improved to 0.97. We find the application of soft and softer handoff improves not only the forward-link capacity, but also the signal-to-interference ratio (SIR) for mobiles near the cell and sector boundaries. Index Terms?CDMA, macrodiversity, sectorization efficiency, soft handoff, softer handoff
Reconstructing the Star Formation History of the Galaxy
The evolution of the star formation rate in the Galaxy is one of the key
ingredients quantifying the formation and determining the chemical and
luminosity evolution of galaxies. Many complementary methods exist to infer the
star formation history of the components of the Galaxy, from indirect methods
for analysis of low-precision data, to new exact analytic methods for analysis
of sufficiently high quality data. We summarise available general constraints
on star formation histories, showing that derived star formation rates are in
general comparable to those seen today. We then show how colour-magnitude
diagrams of volume- and absolute magnitude-limited samples of the solar
neighbourhood observed by Hipparcos may be analysed, using variational calculus
techniques, to reconstruct the local star formation history. The remarkable
accuracy of the data coupled to our maximum-likelihood variational method
allows objective quantification of the local star formation history with a time
resolution of ~ 50 Myr. Over the past 3Gyr, the solar neighbourhood star
formation rate has varied by a factor of ~ 4, with characteristic timescale
about 0.5Gyr, possibly triggered by interactions with spiral arms.Comment: 12 pages, Proc. of the Sept. 20-24, 1999 Vulcano Workshop ``The
chemical evolution of the Milky Way: stars vs. clusters'', eds. F. Matteucci
& F. Giovanell
Fat area and lipid droplet morphology of porcine oocytes during in vitro maturation with trans-10, cis-12 conjugated linoleic acid and forskolin
Lipid droplets (LD) in porcine oocytes form a dark mass reaching almost all cytoplasm. Herein we investigated changes in fat areas, cytoplasmic tone and LD morphology during in vitro maturation (IVM) of porcine oocytes cultured with 100mM trans-10, cis-12 conjugated
linoleic acid (t10,c12 CLA) or 10mM forskolin at different time periods. Four groups were constituted: control, excipient, t10,c12 CLA and forskolin, with drugs being supplemented during 44 to 48h and the initial 22 to 24h in Experiments 1 and 2, respectively. In Experiment 3,
forskolin was supplemented for the first 2 h. Matured oocytes were inseminated with frozen-thawed boar semen and cleavage rate recorded. Before and during IVM, samples of oocytes were evaluated for LD, total and fat areas and fat gray value or for meiotic progression. Results showed that forskolin supplementation during 44 to 48 h or 22 to 24 h inhibits oocyte maturation (exp. 1: forskolin = 5.1±8.0%, control = 72.6±5.0%; exp. 2: forskolin =24.3±7.4%, control =71.6±5.6%) and cleavage (exp. 1: forskolin=0.0±0.0%, control=55.4±4.1%; exp. 2: forskolin=8.3±3.3%, control=54.5±3.0%). Forskolin also reduced oocyte and fat areas. In Experiment 3, forskolin negative effect on oocyte maturation and cleavage disappeared, although minor (P<0.03) LD and oocyte fat areas were identified at 22 to 24 h of IVM. Oocytes supplemented with t10,c12 CLA during 44 to 48h presented a lighter (P<0.04) colour tone cytoplasm than those of control and forskolin. In conclusion, t10,c12 CLA and forskolin were capable of modifying the distribution and morphology of cytoplasmic LD during porcine oocyte maturation, thus reducing its lipid content in a time-dependent manner
Moody's Correlated Binomial Default Distributions for Inhomogeneous Portfolios
This paper generalizes Moody's correlated binomial default distribution for
homogeneous (exchangeable) credit portfolio, which is introduced by Witt, to
the case of inhomogeneous portfolios. As inhomogeneous portfolios, we consider
two cases. In the first case, we treat a portfolio whose assets have uniform
default correlation and non-uniform default probabilities. We obtain the
default probability distribution and study the effect of the inhomogeneity on
it. The second case corresponds to a portfolio with inhomogeneous default
correlation. Assets are categorized in several different sectors and the
inter-sector and intra-sector correlations are not the same. We construct the
joint default probabilities and obtain the default probability distribution. We
show that as the number of assets in each sector decreases, inter-sector
correlation becomes more important than intra-sector correlation. We study the
maximum values of the inter-sector default correlation. Our generalization
method can be applied to any correlated binomial default distribution model
which has explicit relations to the conditional default probabilities or
conditional default correlations, e.g. Credit Risk, implied default
distributions. We also compare some popular CDO pricing models from the
viewpoint of the range of the implied tranche correlation.Comment: 29 pages, 17 figures and 1 tabl
The Early Bird Catches The Term: Combining Twitter and News Data For Event Detection and Situational Awareness
Twitter updates now represent an enormous stream of information originating
from a wide variety of formal and informal sources, much of which is relevant
to real-world events. In this paper we adapt existing bio-surveillance
algorithms to detect localised spikes in Twitter activity corresponding to real
events with a high level of confidence. We then develop a methodology to
automatically summarise these events, both by providing the tweets which fully
describe the event and by linking to highly relevant news articles. We apply
our methods to outbreaks of illness and events strongly affecting sentiment. In
both case studies we are able to detect events verifiable by third party
sources and produce high quality summaries
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