On the Evolutionary Optimization of Many Conflicting Objectives

This study explores the utility of multiobjective evolutionary algorithms (using standard Pareto ranking and diversity-promoting selection mechanisms) for solving optimization tasks with many conflicting objectives. Optimizer behavior is assessed for a grid of mutation and recombination operator configurations. Performance maps are obtained for the dual aims of proximity to, and distribution across, the optimal tradeoff surface. Performance sweet-spots for both variation operators are observed to contract as the number of objectives is increased. Classical settings for recombination are shown to be suitable for small numbers of objectives but correspond to very poor performance for higher numbers of objectives, even when large population sizes are used. Explanations for this behavior are offered via the concepts of dominance resistance and active diversity promotion.     

Ventilatory management in the postanesthesia care unit

Postoperative ventilatory depression is common in patients who have received intravenous and inhalational anesthetic agents. Prompt assessment and treatment of ventilatory depression are essential to minimize morbidity and mortality.     

CTLA4 mediates antigen-specific apoptosis of human T cells

The regulation of T cell-mediated immune responses requires a balance between amplification and generation of effector function and subsequent selective termination by clonal deletion. Although apoptosis of previously activated T cells can be induced by signaling of the tumor necrosis factor receptor family, these molecules do not appear to regulate T-cell clonal deletion in an antigen-specific fashion. We demonstrate that cross-linking of the inducible T-cell surface molecule CTLA4 can mediate apoptosis of previously activated human T lymphocytes. This function appears to be antigen-restricted, since a concomitant signal T-cell receptor signal is required. Regulation of this pathway may provide a novel therapeutic strategy to delete antigen-specific activated T cells.     

Pitfalls in quantitative contrast echocardiography: the steps to quantitation of perfusion

Current methods used clinically to assess myocardial perfusion are invasive and expensive. As the technology of ultrasound imaging improves, CE may provide a relatively inexpensive, noninvasive means of quantitating myocardial perfusion. Issues regarding stability of microbubble contrast agents must be studied more closely under physiologic conditions. As such, encapsulated microbubbles may provide more stability under physiologic pressures than free gas microbubbles. Introducing high concentrations of contrast, either by hyperconcentrating the contrast agent or by increasing the injection rate, may provide greater stability under physiologic conditions. Further, before quantitative statement of tissue perfusion can be made, the relationship between tracer concentration and system response must be established. Further, a "linear" postprocessing ultrasound setting does not eliminate this requirement as data must still undergo nonlinear transformation during log compression and time-gain compensation. Additionally, issues regarding "electronic thresholding" must be explored more extensively in vivo. Commercial ultrasound scanners, in their present form, may not offer adequate sensitivity for absolute quantitative studies. Further development of modified ultrasound systems may provide sufficient sensitivity for quantitative perfusion imaging. CE offers a potentially powerful tool in the clinical management of patients with ischemic heart disease. Conventional coronary angiography provides information on the size of a lesion, but accompanying tissue perfusion distal to the lesion cannot be determined. Doppler ultrasonography determines velocity of blood flow in large vessels but does not offer the potential to quantitate tissue perfusion. Clearly, CE has a place in the future of diagnostic imaging. The recent work of Ito et al. demonstrated the qualitative potential of CE in the identification of "areas at risk" in patients who had undergone thrombolysis or percutaneous transluminal coronary angioplasty after an acute myocardial infarction. With further improvement in the ultrasound imaging techniques and microbubble stability, CE may offer an inexpensive, noninvasive means of assessing myocardial perfusion.     

Measurement and analysis of pump-dependent refractive index anddispersion effects in erbium-doped fiber amplifiers

In a doped fiber, at wavelengths close to an absorption, the refractive index, and hence the dispersion, is expected to be a strong function of wavelength, as described by the Kramers-Kronig relationship. Furthermore, this spectral variation itself will be a function of pumping. This paper describes an accurate and sensitive experimental determination of the pump dependence of the refractive index in an erbium-doped fiber amplifier. A Mach-Zehnder interferometric measurement is described where only one arm is comprised of doped fiber. Particular attention is paid to accounting for the significant difference in power levels in the two arms and the effects of polarization and incoherent light. The measured refractive-index change matches well with that theoretically predicted. The pumped and unpumped dispersion of the amplifier are calculated. While the extra dispersion is of similar magnitude to that of silica fiber, the impact on long telecommunications systems is likely to be negligible. However, this extra dispersion may be important in shorter systems such as soliton lasers. Given the good agreement between theory and experiment, the variation of dispersion with fractional inversion is calculated from absorption and gain measurements. The presence of codopants is known to alter the absorption and emission spectra; the effect on the dispersion is calculated, and a near linear dependence on germanium concentration is observed     

The corrosion behavior of BK-7 glasses for use in non-hermeticelectro-optic devices

Examines the stability and corrosion resistance, of four different commonly used optical glasses, including BK-7 made by three different manufacturers in hot humid ambients. We found that although all three BK-7's are considered industrial equivalents, they have slightly different chemical compositions, and exhibit significantly different corrosion behaviors. The corrosion rates varied by at least a factor of two. This improvement appears to be related to the ZnO content in the glass. We found one borosilicate optical glass with similar optical properties to BK-7 which has at least a factor of 3 times better corrosion resistance than the best BK-7. We propose a six stage corrosion model to explain the corrosion behavior of borosilicate glasses in hot humid environments: 1) adsorption of “bulk” like water at defective sites on the surface of the glass; 2) ion exchange between the alkali ions in the glass and protons in the “bulk” like water 3) growth of the “bulk” like water droplet followed by more ion exchange until the entire surface of the glass is covered with a “bulk” like water film; 4) dissolution of the silicate network by the basic “bulk” like water film and formation ionic SiO^$species in the “bulk” like water; 5) reaction between the ionic SiO^$ species and the hydrolized alkali in solution; 6) precipitation and growth of the alkali silicate reaction product. Finally, we determine the acceleration factor for device failure due to corrosion of one type of BK-7 glass. The temperature (T) dependence of failure is assumed to take an exponential form with an activation energy between 0.64-0.81 eV. The relative humidity (RH) dependence of failure is estimated to be of the form Rate ∝ exp {constant(RH²)}. The fitting constant is estimated to fall between 4.8×10^-4 /%² and 6.0×10^-4/%²     

Single transverse mode operation at 1345 nm wavelength of adiode-laser pumped neodymium:ZBLAN multimode fiber laser

A diode-laser pumped multitransverse-mode neodymium-doped fluoride fiber laser is shown to oscillate in the lowest-order transverse mode when the cavity conditions are optimized. The combination of good pump coupling efficiency, laser operating efficiency, and good overlap of the HE₁₁ mode to that of a single-mode fiber, allows up to 12 mW of continuous wave (CW) power to be coupled into a standard single-mode fiber at a 1345-nm wavelength. Improved lens design should significantly increase the power injected into the single-mode fiber from the multimode fiber laser     

Trapping second-order nonlinearity in erbium-doped germanosilicate ring in thermally poled optical fibre

An optical fibre with an erbium (Er)-doped germanosilicate ring structure around the anode hole has been fabricated for stabilising the thermal poling induced second-order nonlinearity (SON). The SON spatial distribution was visualised with second-harmonic microscopy. It was found that the SON layer was trapped in the Er-doped ring under normal poling conditions. Only at higher poling voltages and for much longer poling time could some SON be formed at the core-cladding interface, while most of the SON layer was still firmly confined within the Er-doped ring.