Prism coupling technique: a method for measurement of propagation constant and beat length in single-mode fibres

We report preliminary investigations on the application of the prism coupling technique to the measurement of the mode spectra of conventional and birefringent single-mode fibres (SMFs). Experimental results on the measurement of the propagation constant in normal SMF, and the application of this technique to the measurement of beat length on an elliptic-core SMF, are presented.     

The Weyl correspondence and time-frequency analysis

Describes the Weyl correspondence and its properties, showing how it gives a “window-independent” definition of time-frequency concentration for use in models in signal detection. The definition of concentration is justified by showing that it gives reasonable answers in certain intuitive cases. The Weyl correspondence expresses a linear transformation as a weighted superposition of time-frequency shifts of the signal, and then authors explain why this is not the same as “transforming” a signal into the time-frequency domain, multiplying by a weight in the transform domain and taking the inverse. The investigation into time-frequency concentration and the Weyl correspondence is justified by a new result. The authors show that convolving the Wigner distribution with a general smoothing function is equivalent to evaluating a weighted sum of spectrograms. This is a new interpretation of the process of smoothing the Wigner distribution to reduce cross-terms. It relates smoothing of the Wigner distribution to the “multiple window” technique pioneered by Thomson (1982)     

SYNTHESIS AND SPECTROSCOPIC DIFFERENTIATION OF 2- AND 4-ALKOXYTHIOTETRONIC ACIDS

O-Alkylation of thiotetronic acids gives a mixture of 2- and 4-position enol ether products. Comparison of the physical data revealed that UV spectroscopy was the most reliable method of distinguishing between these related ethers. We have determined that 4-position ethers have a distinct absorption between 235-240 nm, while 2-position ethers have two absorbance peaks, one between 205-220 nm and the other between 305-310 nm. This report describes the synthesis and unambiguous characterization of 2- and 4-methoxy-3,5-dimethylthiotetronic acids. The UV absorption properties of several other pairs of thiotetronic acid ethers confirm that these differences are general features that provide a simple method for distinguishing between 2- and 4-substituted isomers.     

The effect of replacing a benzene ring with a saturated six-membered ring on the mesomorphic properties of 4,4'-disubstituted diphenyldiacetylenes

A limited selection of ring modified diphenyldiacetylenes of the type where A=, Y=C_nH_2n+1, CF₃, F, COMe, NH₂, and NMe₂, and A=, trans and cis with Y=F and trans with Y=C₃H₇, were synthesized. Mesomorphic properties were determined by hot stage polarizing microscopy and DSC. These properties were generally poorer than those found in the parent benzene compounds. This was also true of some pyrimidine analogues reported earlier. Birefringence values also decreased as expected.     

An Optimal Control Problem for Flows with Discontinuities

In this paper, we study a design problem for a duct flow with a shock. The presence of the shock causes numerical difficulties. Good shock-capturing schemes with low continuity properties often cannot be combined successfully with efficient optimization methods requiring smooth functions. A remedy studied in this paper is to introduce the shock location as an explicit variable. This allows one to fit the shock and yields a problem with sufficiently smooth functions. We prove the existence of optimal solutions, Fréchet differentiability, and the existence of Lagrange multipliers. In the second part, we introduce and investigate the discrete problem and study the relations between the optimality conditions for the infinite-dimensional problem and the discretized one. This reveals important information for the numerical solution of the problem. Numerical examples are given to demonstrate the theoretical findings.     

Magnetic and151Eu mössbauer studies on EuTGa (T=Pd AND Pt) compounds

The compounds EuPdGa and EuPtGa show lattice volume anomaly indicating the abnormal valence state of Eu. in these compounds. Magnetization studies reveal that these compounds are magnetically ordered with a moment of 7 _B/f.u. at 5K. The Curie temperatures obtained from the low field ac susceptibility measurements are 38K for EuPdGa and 36K for EuPtGa.¹⁵¹Eu Mössbauer studies at 300K gave large negative isomer shifts (relative to SmF₃) and show a hyperfine split pattern at 4.2K in both the compounds. These results suggest that Eu is in a divalent state in EuPdGa and EuPtGa.     

An Electronic Circuit System for Time-Reversal of Ultra-Wideband Short Impulses Based on Frequency-Domain Approach

In this paper, a compact and low-cost electronic circuit system is designed for time-reversal of ultra-wideband short impulses (with nanosecond and sub-nanosecond temporal durations). A frequency-domain approach is adopted to avoid high sampling rate in time. Specifically, the proposed system obtains the discrete spectra of input impulses first; then realizes time-reversal in frequency domain; and finally synthesizes the time-reversed impulses using discrete continuous wave elements. This system is composed of common and commercially available circuits, and hence, can embody a system-on-chip implementation. Its performance is verified by circuit-electromagnetic co-simulations using impulses with 3–10-GHz frequency band coverage. Advanced Design System and two full-wave Maxwell's equations solvers are used for circuit and electromagnetic simulations, respectively, and their results are coupled and integrated. In the circuit part, most of nonidealities of realistic circuits are taken into account. It is shown by the simulation results that, although realistic circuits unavoidably introduce errors to time-reversal, such errors do not affect the “focusing” phenomena in the context of electromagnetic wave propagation. As a conclusion, the proposed system can be deployed in practical time-reversal communication and radar applications.      

Rheograms for asphalt from single viscosity measurement

Asphalt materials are used in a variety of applications such as road paving, waterproofing, roofing membranes, adhesive binders, rust proofing and water resistant coatings. There are available in a number of grades distinguished in terms of their softening point and flow resistance. The selection of the proper grade of asphalt for a particular application is governed by the desired flow behaviour. A knowledge of the complete flow curve depicting the variation of melt viscosity with shear rate at the relevant temperatures is necessary not only for proper grade selection, but also for specifying processing conditions for aggregate mixing and spraying. The rheological data are also useful in assessing end use performance. The scientific techniques for generating the rheological data involve the use of expensive, sophisticated instruments. Generation of the necessary flow data using these instruments is beyond the financial and technical means of most processors of asphalt materials. The engineering techniques involving the use of inexpensive vacuum viscometers are relatively easy, but provide a single point viscosity measurement at low shear rate. In the present work, a method is proposed for unifying the viscosity versus shear rate a data at various temperatures for a number of asphalt grades. A master curve has been generated that is independent of the grade of asphalt and the temperature of viscosity measurement. The master curve can be used to generate rheograms at desired temperatures for the asphalt grade of interest, knowing its zero-shear viscosity at that temperature.NCL Communication Number 2914.