ALBERT

Notes: Irradiation dose affected production of volatiles in vacuum-and aerobic-packaged cooked pork sausage, but its effect on TBARS was minor. Storage increased production of volatiles and changed their composition only in aerobic-packaged sausage. Among volatile components, 1-heptene and 1-nonene were influenced most by irradiation dose, and aldehydes by packaging type. TBARS and volatiles of vacuum-packaged irradiated cooked sausage did not correlate well. However, TBARS had very high correlation with amount of aldehydes, total volatiles, ketones and alcohols with long retention times in aerobic-packaged pork sausage. Heptene and 1-nonene could be indicators for irradiation; and propanal, pentanal, and hexanal for oxygen-dependent changes of cooked meat.

Notes: A numerical optimization approach is introduced to the subject of dynamo theory. Conventional kinematic dynamo studies treat the induction equation as an eigenvalue problem by choosing a candidate velocity field and solving for a marginally stable solution of magnetic field and critical magnetic Reynolds number. The conventional approach has told us something about dynamo action and magnetic field morphology for specific velocities, but the arbitrary choice of fluid flow is a hit-or-miss affair; not all velocities sustain dynamo action, and of those that do, few yield mathematically tractable solutions. As a result, progress has been slow. Here we adopt a new approach, a non-linear numerical variational approach, which allows us to solve the induction equation simultaneously for both the magnetic field and the velocity field. The induction equation is discretized following the Bullard-Gellman formalism and the resulting algebraic equations solved by an iterative, globally convergent, Newton-Raphson method. The particular choice of optimization constraints allows one to design a dynamo which satisfies certain conditions; in this paper we minimize a linear combination of the kinetic energy (magnetic Reynolds number) and a smoothness norm on the magnetic field to produce efficient (low magnetic Reynolds number) well-converged (smooth magnetic field) solutions. We illustrate the optimization method by designing two dynamos based on a Kumar-Roberts velocity parametrization; a specific choice of the velocity parameters, KR, sustains a 3-D kinematic model of the geodynamo. Compared with KR, one of our new models, LG1, is designed to have a higher magnetic Reynolds number but smoother magnetic field, and the other, LG2, a lower magnetic Reynolds number and somewhat rougher magnetic field. We suggest that dynamo efficiency, defined by the magnetic Reynolds number, is achieved through reduced differential rotation and a favourable spatial distribution of the helicity. These examples demonstrate the value of the optimization method as a tool for exploring dynamo action with geophysically realistic flows. It can be extended to the dynamic dynamo problem and, by changing the constraints, be used to design dynamos with good numerical convergence, which match the observed geomagnetic surface field morphology and which place useful quantitative constraints on the physical nature of the geodynamo.

Notes: Abstract A numerical technique, based on transforming the scalar wave equation to a finite interval before discretization is applied, is proposed for the solution of the scalar wave equation defining the modal fields and propagation constants of optical fibres. It is used to compute the propagation constants and modal fields of weakly guiding fibres of circular cross-section when the refractive index profile has a Gaussian or smoothed-out form. The smoothed-out profiles are studied because they vary continuously from the Gaussian to the step profile. Results are compared with and shown to be accurately approximated by simply explicit functions, dependent only on the fibre parameter.

Notes: Abstract By transforming the infinitex-y plane onto the unit square and using two-dimensional Fourier series expansions, the modal fields and propagation constants of dielectric waveguides can be accurately determined within the scalar (weak-guidance) regime. In particular, the method gives cutoffV-values directly. By applying the technique to symmetric and asymmetric rectangular-core couplers, the second-mode cutoffV-value is determined numerically as a function of both the centre-to-centre core separation and the core aspect ratios. Approximate analytical expressions are derived from the exact numerical data which give the cutoff values to within 2.5%.

Notes: Abstract It is shown that by taking advantage of the maximum in the spectral dependence of the coupling coefficient, very broad-band symmetric and asymmetric single-mode planar couplers can be designed. The symmetric coupler allows at least 95.6% coupling between the two cores over the wavelength range 1.3–1.58 μm, while the asymmetric coupler can act as a 3 dB splitter with only ±1.6% variation in the splitting ratio over the wavelength range 1.3–1.67 μm. In both cases, the design includes the curved input-output arms, and the estimated bend loss is approximately 0.1 dB.

Notes: Abstract Experimental transmission spectra of ultraviolet (u.v.)-written Bragg gratings in depressed- and matched-cladding fibre are characterized and compared. In particular, we discuss how the location and strength of the spectral features vary with the degree of blazing, or angular tilt of the grating. The fine-structure detail on the short-wavelength side of the fundamental Bragg line is attributed to power coupling between the forwardpropagating fundamental (LP01) mode and discrete, backward-propagating cladding modes. Resonances corresponding to backward-propagating LP0n and LP1n modes are identified, and their relative strengths are compared with theoretical overlap calculations. Physical arguments are presented that explain the pronounced ‘ghost’-grating notch that appears in the transmission spectra of blazed, fibre Bragg gratings in depressed-cladding fibre.

Notes: Abstract The transverse shift in the field distribution and the correction to the propagation constant of the fundamental and symmetric cladding modes on bent finite-clad single-mode fibres and slab waveguides are evaluated from perturbation theory for effective index values extending below the cladding index. Analytical results are derived in both geometries for the step-profile that are valid within the overall limitations of the theory. However, it is found that, for the fibre geometry only, the method breaks down at certain discrete wavelengths because of degeneracies that occur between the HE12 (LP02) and TE01 (LP11) mode propagation constants.

Notes: Abstract The wavelength-independent, non-reciprocal splitting properties of the asymmetric, multimode Y-junction make it an ideal device for use as a low-loss, low-power tap in a concatenated linear bus. The properties of a two-dimensional slab model of the junction are investigated and the results of ray-tracing, modal and numerical beam propagation method (BPM) techniques are compared. Minimization of splicing loss between the ports of a square-core, three-dimensional buried channel waveguide version of the device and a multimode fibre is also addressed.

Notes: Abstract Simple approximations for the fields and propagation constants of low-order modes on circular weakly guiding fibres are derived using variational techniques. The high accuracy of these approximations for quasi-parabolic profiles is demonstrated in the case of the Gaussian profile, for which there are no analytical solutions of the scalar wave equation.