Characterization of surface-plasmon modes in metal-clad optical waveguides

Finite-element analysis, based on the vector H-field formulation and incorporating the perturbation technique, is used to calculate the complex propagation characteristics of metal-coated dielectric waveguides. The propagation and attenuation characteristics of the surface-plasmon modes at the metal/dielectric interfaces are presented. The effects on the optical properties of metal-clad optical fibers with infinite and finite cladding thickness and the formation of the supermodes due to the coupling between the surface-plasmon modes in the presence of different surrounding materials are also investigated.     

A rigorous comparison of the performance of directional couplerswith multimode interference devices

The authors report, for the first time, a direct comparison between a directional coupler and a multimode interference-based device, in relation to their performance characteristics such as crosstalk, polarization dependence and the effect of fabrication tolerances. The coupling efficiency for a nonidentical coupler, produced inadvertently due to problems with fabrication tolerances, is also demonstrated. The vector finite element (FE) and the least squares boundary residual (LSBR) methods are employed as the numerical tools used in this simulation study     

A new rapidly absorbed paracetamol tablet containing sodium bicarbonate. II. Dissolution studies and in vitro/in vivo correlation

The objective of this study was to compare the in vitro dissolution profile of a new rapidly absorbed paracetamol tablet containing sodium bicarbonate (PS) with that of a conventional paracetamol tablet (P), and to relate these by deconvolution and mapping to in vivo release. The dissolution methods used include the standard procedure described in the USP monograph for paracetamol tablets, employing buffer at pH 5.8 or 0.05 M HCl at stirrer speeds between 10 and 50 rpm. The mapping process was developed and implemented in Microsoft Excel worksheets that iteratively calculated the optimal values of scale and shape factors which linked in vivo time to in vitro time. The in vitro-in vivo correlation (IVIVC) was carried out simultaneously for both formulations to produce common mapping factors. The USP method, using buffer at pH 5.8, demonstrated no difference between the two products. However, using an acidic medium the rate of dissolution of P but not of PS decreased with decreasing stirrer speed. A significant correlation (r = 0.773; p < .00001) was established between in vivo release and in vitro dissolution using the profiles obtained with 0.05 M HCl and a stirrer speed of 30 rpm. The scale factor for optimal simultaneous IVIVC in the fasting state was 2.54 and the shape factor was 0.16; corresponding values for mapping in the fed state were 3.37 and 0.13 (implying a larger in vitro-in vivo time difference but reduced shape difference in the fed state). The current IVIVC explains, in part, the observed in vivo variability of the two products. The approach to mapping may also be extended to different batches of these products, to predict the impact of any changes of in vitro dissolution on in vivo release and plasma drug concentration-time profiles.     

Learning and memory difficulties after environmental exposure to waterways containing toxin-producing Pfiesteria or Pfiesteria-like dinoflagellates

BACKGROUND: At the beginning of autumn, 1996, fish with "punched-out" skin lesions and erratic behaviour associated with exposure to toxins produced by Pfiesteria piscicida or Pfiesteria-like dinoflagellate species were seen in the Pocomoke River and adjacent waterways on the eastern shore of the Chesapeake Bay in Maryland, USA. In August, 1997, fish kills associated with Pfiesteria occurred in these same areas. People who had had contact with affected waterways reported symptoms, including memory difficulties, which raises questions about the human-health impact of environmental exposure to Pfiesteria toxins. METHODS: We assessed 24 people who had been exposed. We collected data on exposure history and symptoms, did a complete medical and laboratory assessment (13 people), and carried out a neuropsychological screening battery. Performance on neuropsychological measures was compared with a matched control group. RESULTS: People with high exposure were significantly more likely than occupationally matched controls to complain of neuropsychological symptoms (including new or increased forgetfulness); headache; and skin lesions or a burning sensation of skin on contact with water. No consistent physical or laboratory abnormalities were found. However, exposed people had significantly reduced scores on the Rey Auditory Verbal Learning and Stroop Color-Word tests (indicative of difficulties with learning and higher cognitive function), and the Grooved Pegboard task. There was a dose-response effect with the lowest scores among people with the highest exposure. By 3-6 months after cessation of exposure, all those assessed had test scores that had returned to within normal ranges. INTERPRETATION: People with environmental exposure to waterways in which Pfiesteria toxins are present are at risk of developing a reversible clinical syndrome characterised by difficulties with learning and higher cognitive functions. Risk of illness is directly related to degree of exposure, with the most prominent symptoms and signs occurring among people with chronic daily exposure to affected waterways.     

Measurement of up- and down-lead fiber sensitivity caused by the lead in a multimode fiber in an interferometric system

The results of a comparative experimental study on the effect of the modal noise induced by lead-in fibers in an extrinsic interferometric system, illuminated by high- and low-coherence light sources, respectively, are reported. When the up-lead fiber was subject to a perturbation, the sensitivity of the system was reduced by 20.9 dB through the use of a high-coherence source, and by 1.8 dB through the use of a low-coherence source. When the down-lead fiber was perturbed, the sensitivity dropped by 30.3 dB and 4.9 dB for high- and low-coherence sources, respectively. The results from the experimental analysis supported qualitatively by simple theory show that the use of a low-coherence light source can greatly suppress the modal noise induced in both the up- and down-lead fibers, if the coherence length of the light source used is less than the optical path difference between two adjacent fiber modes. This shows the practicality of the use of niultimode fibers in an interferometric system with a suitable lig t source.     

A field deployable, multiplexed Bragg grating sensor system used in an extensive highway bridge monitoring evaluation tests

A multiplexed optical fiber Bragg grating sensor system with a measurement bandwidth of up to 200 Hz enabling dynamic loading events, e.g., road traffic, to be observed has been designed, installed, and tested over an 18-month period on a 346-m road bridge in Norway, for design verification and structural integrity monitoring purposes. A network of 32 fiber Bragg sensors was surface bonded along with a corresponding set of resistive strain gauges for comparative tests to be made. The wavelength data were calibrated against two thermally stabilized (/spl sim/0.15 pm) reference gratings, which rejected common mode noise and provided absolute wavelength scaling. These data provides independent strain and temperature information. Long-term test results showed good linearity and repeatability of <10 /spl mu//spl epsiv/ over the test period with a precision of /spl plusmn/5 /spl mu//spl epsiv/ and a resolution of /spl plusmn/1 /spl mu//spl epsiv/. The readings from the FBG sensors were comparable to those from the foil gauge sensors to within /spl plusmn/4 /spl mu//spl epsiv/.     

Velocity matching of a GaAs electro-optic modulator

New designs for the velocity matching of a deep-etched semiconductor electro-optic modulator are presented. A tantalum pentoxide (Ta2O5) coating is considered here for achieving velocity matching between the microwave and the optical signals. The effects of the velocity mismatch, the conductor loss, the dielectric loss, and the impedance mismatch are studied in relation to the optical bandwidth of a high-speed semiconductor modulator. It is shown that both the dielectric loss and the impedance matching play key roles for velocity-matched high-speed modulators with low conductor loss. The effects of Ta2O5 thickness on the overall bandwidth and on the half-wave voltage-length product VpiL are also reported.     

The effect of selected water-soluble excipients on the dissolution of paracetamol and Ibuprofen

The purpose of this investigation was to study the dissolution behavior of paracetamol and ibuprofen in the presence of a range of selected potential excipients. First, a pH-solubility profile was generated for both drugs, and the effect of changing hydrodynamic conditions on the intrinsic dissolution rate was investigated. It was established that both drugs dissolved according to the diffusion-layer model. Paracetamol solubility (approximately 20.3 mg mL^- 1) did not vary from pH 1.2-8.0, corresponding to the in vivo range in the gastrointestinal tract. Ibuprofen had an intrinsic solubility of approximately 0.06 mg mL^- 1, and pK_a was calculated as 4.4. Second, the effects of selected potential excipients (lactose, potassium bicarbonate, sodium bicarbonate, sodium chloride, and tartaric acid) were evaluated by measuring the effect of the inclusion of each additive in the dissolution medium on drug solubility, drug intrinsic dissolution rate, and solution viscosity. The results were evaluated using the diffusion-layer model, and it was determined that for paracetamol, the collected data fitted the model for all the excipients studied. For ibuprofen, it was found that there were differences between the excipients that raised the solution pH above the pK_a to those that did not. For the excipients raising the pH above the pK_a, the effect on intrinsic dissolution rate was not as high as that expected from the change in drug solubility. It was postulated that this might be due to lack of penetration of the excipient into the drug boundary layer microenvironment. Formulators may calculate the effect of adding an excipient based on solubility increases but may not find the dissolution rate improvement expected.     

Fluorescence decay characteristic of Tm-doped YAG crystal fiber for sensor applications, investigated from room temperature to 1400/spl deg/C

An yttrium aluminum garnet (YAG) crystal fiber with a thulium-doped end tip was specially grown by means of the laser heated pedestal growth approach and designed to be incorporated in a fiber-optic temperature probe. The fluorescence decay characteristics of the crystal fiber, including the temperature dependence of both the fluorescence lifetime and intensity, were comprehensively investigated. Experimental results indicated that the crystal fiber showed a monotonic relationship between the fluorescence lifetime and temperature with an average lifetime sensitivity of 3 /spl mu/s /spl deg/C over a wide temperature range, taking measurement from room temperature to 1200/spl deg/C. Good stability (up to 1400/spl deg/C) was observed with high repeatability of the fluorescence lifetime during the annealing process carried out on the fiber over this temperature range. The fiber was found to be an excellent candidate material to be used as a fluorescence decay-based fiber thermometer probe and the results are presented on its performance.