Educação matemática pela arte
Gusmão, Lucimar Donizete
2013-08-28
Search results
31 records were found.
The thermal diffusion of Nd^3+ and Gd^3+ ions in YVO_4 is characterized, yielding diffusion rates of 7X10^-19 and 44X10^-19 m^2s^-1, respectively, at 1400°C and activation energies of 5.3X10^-19 and 2.9X10^-19 J, respectively, for diffusion along the c axis. The fluorescence properties of the Nd^3+ -diffused YVO_4 agree well with those of bulk doped materials. The formation of a planar optical waveguide was also observed for one of the Nd^3+ -diffused samples. This characterization is a significant first step toward fabrication of waveguide lasers and amplifiers in this important laser material.
In this paper we utilize extrinsic information transfer
(EXIT) chart analysis in a coded direct-sequence codedivision
multiple-access (DS-CDMA) multiuser receiver to dynamically
derive the optimal decoding schedule in the unequal
user power level case. Conventional receivers generally follow a
predetermined decoding schedule. However, decoding delay and
complexity can be significantly reduced while maintaining BER
performance through optimisation of the decoding schedule. We
show dynamic scheduling, that is deriving the optimal schedule
on-line after each iteration of the receiver, is a more flexible
approach which improves the BER performance in comparison
to the static (off-line) optimised schedule for similar complexity
savings over the conventional receiver.
An iterative multiuser detection (IMUD) receiver is considered for a code-division multiple-access (CDMA) system
with turbo codes. Fidelity is considered as a metric for convergence analysis and an approximation of fidelity
as a function of variance of the log-likelihood ratios is introduced. Fidelity charts are compared with extrinsic
information transfer (EXIT) charts as tools for convergence analysis and a close approximation of the relationship
between fidelity and mutual information is proposed. We then use this function to approximate a closed form
expression for the EXIT function of an interference canceller. Further, we propose a new stopping criterion for the
turbo decoder and the IMUD receiver which achieves complexity saving of up to 50% over conventional designs.
In this paper we utilize extrinsic information transfer (EXIT)
chart analysis with unequal user power levels in a coded directsequence code-division multiple-access (DS-CDMA) multiuser
receiver to derive a dynamic decoding schedule. Conventional receivers generally follow a static predetermined decoding schedule. However, decoding delay and complexity
can be significantly reduced while maintaining BER performance through a dynamic decoding schedule optimized for a
given load and transmit power configuration. Verified through
simulations, we show that complexity savings of up to 5000
can be achieved. We also derive the effective EXIT charts
which can be used for the convergence analysis and performance predictions of unequal power CDMA systems.
An efficient, longitudinally diode-pumped, diffraction-limited, Nd:YAG double-clad planar waveguide laser was operated on four transitions of the Nd3+ ion. Optimised output powers of 4.3W, 3.5W, and 2.7W, were obtained for absorbed pump powers of ~7W, for the transitions at the lasing wavelengths of 1.064μm, 946nm, and 1.3 μm, respectively. Operation of the weak 4F3/2 → 4I15/2 transition, lasing at 1.833 μm, was demonstrated at an absorbed pump power threshold of 300mW and an output power of 400mW, with a non-optimised output coupling. Diffraction-limited performance was obtained in both the guided and non-guided axes.
We theoretically investigated the effect of the spatial distribution of the active-ion concentration in multimode step-index waveguides on transverse-mode selection for continuous-wave laser operation. We found that uniform doping of a central portion of as much as 60% of the full waveguide core width is highly effective for the selection of fundamental-mode operation, even under highly saturated, high-power conditions. Profiling the dopant distribution to match that of the particular mode desired was also found to be effective, especially if it is the saturated inversion profile that is matched to the shape of the mode.
We present a synchronously pumped optical parametric oscillator whose output characteristics have been controlled using adaptive pulse-shaping techniques. The principles for applying adaptive-control are explored, including the use of simulated-annealing algorithms and various feedback parameters.
We report the first neodymium-doped amorphous channel waveguide laser through extrusion. Single-mode operation was observed at 1058nm for a 10mm long channel waveguide. An output power of 13.3mW with 30% slope efficiency was measured.
We theoretically and numerically investigate indirect mid-infrared pulse shaping via parametric transfer, specifically difference-frequency generation. We define a quantitative measure for the fidelity of parametric transfer, and investigate the effect of material dispersion and process nonlinearity on the parametric transfer. We show that a good fidelity transfer of a broadbandwidth pulse can be efficiently achieved with a reasonable wavelength tunability, by careful design of the experimental configuration.
PbSe quantum dots (QDs) were grown in high-refractive-index low-melting-temperature lead-phosphate glass. The lowest energy exciton transition of the QDs was tuned over a wide range within the infrared spectral region (0.93-2.75μm) by a controlled heat treatment. The measured QD radius ranged between 2 and 5.3nm, with a time (t) dependence of t^0.29 for long dwelling times during the heat treatment, indicating that the QD growth mechanism tends to follow Lifshitz-Slyozov-Wagner theory. The QD saturable absorber behavior at 1.2μm had a measured saturation fluence of ~2.1μJ/cm2.
