Explicit finite difference solution of the power flow equation in W-type optical fibers

Using the power flow equation, we have calculated spatial transients of power distribution and a steady-state distribution that are due to coupling of guided to leaky modes in W-type optical fibers (doubly clad fibers). A numerical solution has been obtained by the explicit finite difference method. Results show that power distribution in W-type optical fibers depends on both the intermediate layer width and the coupling strength. W-shaped index profile of optical fibers is effective in reducing modal dispersion and therefore in improving the fiber bandwidth. We have also shown that explicit finite difference method is effective and accurate for solving the power flow equation in W-type optical fibers.     

Insight into the CSA tensors of nucleobase carbons in RNA polynucleotides from solution measurements of residual CSA: towards new long-range orientational constraints

Using residual chemical shift anisotropies (RCSAs) measured in a weakly aligned stem-loop RNA, we examined the carbon chemical shift anisotropy (CSA) tensors of nucleobase adenine C2, pyrimidine C5 and C6, and purine C8. The differences between the measured RCSAs and the values back-calculated using three nucleobase carbon CSA sets [D. Stueber, D.M. Grant, 13C and 15N chemical shift tensors in adenosine, guanosine dihydrate, 2'-deoxythymidine, and cytidine, J. Am. Chem. Soc. 124 (2002) 10539-10551; D. Sitkoff, D.A. Case, Theories of chemical shift anisotropies in proteins and nucleic acids, Prog. NMR Spectrosc. 32 (1998) 165-190; R. Fiala, J. Czernek, V. Sklenar, Transverse relaxation optimized triple-resonance NMR experiments for nucleic acids, J. Biomol. NMR 16 (2000) 291-302] reported previously for mononucleotides (1.4 Hz) is significantly smaller than the predicted RCSA range (-10-10 Hz) but remains larger than the RCSA measurement uncertainty (0.8 Hz). Fitting of the traceless principal CSA values to the measured RCSAs using a grid search procedure yields a cytosine C5 CSA magnitude (CSAa=(3/2.(delta11(2)+delta22(2)+delta33(2)))1/2=173+/-21 ppm), which is significantly higher than the reported mononucleotide values (131-138 ppm) and a guanine C8 CSAa (148+/-13 ppm) that is in very good agreement with the mononucleotide value reported by solid-state NMR [134 ppm, D. Stueber, D.M. Grant, 13C and (15)N chemical shift tensors in adenosine, guanosine dihydrate, 2'-deoxythymidine, and cytidine, J. Am. Chem. Soc. 124 (2002) 10539-10551]. Owing to a unique sensitivity to directions normal to the base plane, the RCSAs can be translated into useful long-range orientational constraints for RNA structure determination even after allowing for substantial uncertainty in the nucleobase carbon CSA tensors.     

Estimation of Rayleigh scattering loss in a double-clad photonic crystal fiber

In this paper the effect of photonic crystal fiber’s structure parameters on Rayleigh scattering was investigated. Rayleigh scattering loss (RSL) has been numerically estimated by average Rayleigh scattering coefficient based on the empirical relations for \(V\) and \(W\) parameters of double-clad photonic crystal fibers (DC PCFs). The dependence of RSL on the two structural parameters—the air hole diameter and the hole pitch was demonstrated. We have shown that RSL depends on the index profiles because of the different optical power confinement factors in every layer of DC PCF. Using these results, the RSL can be optimized by adjusting the fiber parameters—air-hole diameter as well as the air-hole pitch.     

Thermal Diffusion Process Estimation for Fabrication of Graded Plastic Optical Fibre

Thermal diffusion of dopants is investigated in the process of generating the graded-index profile of plastic optical fibres. Because the diffusion coefficient in high polymers has been shown to depend strongly on dopant concentration, it is allowed in this work to vary with the radial coordinate of the multistep-core fibre. A novel multi-layer model is presented for solving the diffusion equation with the variable diffusion coefficient. It is solved by the finite difference method. The solution determines the dopant diffusion profile in the fibre. It is verified against a solution from the literature and two cases of fibres with diffused profiles. The application is demonstrated on two examples of graded-index plastic optical fibres, one originally with a two-step and the other with four-step core. The results indicate that closer to the core-cladding interface, the computed diffused profile with variable diffusion coefficient D is closer to target profile than the profile obtained with constant D for the same time of thermal process.     

Absolute Minimal Sampling in High‐Dimensional NMR Spectroscopy

Standard three‐dimensional Fourier transform (FT) NMR experiments of molecular systems often involve prolonged measurement times due to extensive sampling required along the indirect time domains to obtain adequate spectral resolution. In recent years, a wealth of alternative sampling methods has been proposed to ease this bottleneck. However, due to their algorithmic complexity, for a given sample and experiment it is often hard to determine the minimal sampling requirement, and hence the maximal achievable experimental speed up. Herein we introduce an absolute minimal sampling (AMS) method that can be applied to common 3D NMR experiments. We show for the proteins ubiquitin and arginine kinase that for widely used experiments, such as 3D HNCO, accurate carbon frequencies can be obtained with a single time increment, while for others, such as 3D HN(CA)CO, all relevant information is obtained with as few as 6 increments amounting to a speed up of a factor 7–50.     

Quantifying millisecond exchange dynamics in proteins by CPMG relaxation dispersion NMR using side-chain 1H probes

A Carr-Purcell-Meiboom-Gill relaxation dispersion experiment is presented for quantifying millisecond time-scale chemical exchange at side-chain (1)H positions in proteins. Such experiments are not possible in a fully protonated molecule because of magnetization evolution from homonuclear scalar couplings that interferes with the extraction of accurate transverse relaxation rates. It is shown, however, that by using a labeling strategy whereby proteins are produced using {(13)C,(1)H}-glucose and D(2)O a significant number of 'isolated' side-chain (1)H spins are generated, eliminating such effects. It thus becomes possible to record (1)H dispersion profiles at the β positions of Asx, Cys, Ser, His, Phe, Tyr, and Trp as well as the γ positions of Glx, in addition to the methyl side-chain moieties. This brings the total of amino acid side-chain positions that can be simultaneously probed using a single (1)H dispersion experiment to 16. The utility of the approach is demonstrated with an application to the four-helix bundle colicin E7 immunity protein, Im7, which folds via a partially structured low populated intermediate that interconverts with the folded, ground state on the millisecond time-scale. The extracted (1)H chemical shift differences at side-chain positions provide valuable restraints in structural studies of invisible, excited states, complementing backbone chemical shifts that are available from existing relaxation dispersion experiments.     

Frequency response in step-index plastic optical fibers obtained by numerical solution of the time-dependent power flow equation

The time-dependent power flow equation is employed to calculate frequency response and bandwidth in addition to mode coupling and mode-dependent attenuation in step-index plastic optical fibers. Frequency response is specified in the paper as a function of fiber length. Results are found to match reported measurements better than the existing analytical solution does. Mode-dependent attenuation and mode dispersion and coupling are known to be strong in plastic optical fibers, leading to major implications for their frequency response.     

Influence of the medium electrolyte concentration on the electric polarizability of bacteria Escherichia coli in presence of ethanol

The electric polarizability is an important parameter of bacteria, giving information about the electric properties of the cells. In our previous works [A.M. Zhivkov, A.Y. Gyurova, Colloids Surf. B: Biointerfaces 66 (2008) 201; A.Y. Gyurova, A.M. Zhivkov, Biophys. Chem., 139 (2009) 8; A.M. Zhivkov, A.Y. Gyurova, J. Phys. Chem. B, 113 (2009) 8375] we have applied an experimental approach to distinguish the contribution of the components of the two types of interface electric polarizability—surface charge dependent (ChD) and Maxwell–Wagner (MW) polarizability. It is based on electro-optical study of the separate influence of the outer and inner medium electrolyte concentration, which changes the external ChD and internal MW components of polarizability; the last effect is reached by the membrane permeability increase in low ethanol concentration. In the present work we investigate the behavior of electric polarizability of Escherichia coli K12 at increasing the outer KCl concentration in presence of 10 vol.% ethanol in order to check if the polarizability components change independently from one another. The conclusion is that the outer electrolyte concentration influence indirectly the internal MW component by the trans-membrane concentration gradient, but the polarizability components themselves change independently.     

Investigation of influence of tilt angle on mode dispersion in step-index plastic optical fibers

Effect of numerical aperture on mode dispersion and bandwidth is reported for commercially available step-index plastic optical fibers. For the first time, the functional relationship is given between dispersion and the “tilt-angle” describing the slant of the input/output face for a fiber terminated by a plane not perpendicular to the fiber axis. This tilt of a non-squarely terminated fiber may be intentional as in some biomedical spectroscopic sensors, or otherwise when exploiting the quick-interconnectivity potential of plastic fibers.