Determination of (13)C/(12)C-Ratios in Rumen Produced Methane and CO(2) of Cows, Sheep and Camels

Abstract Naturally produced methane shows different δ(13)C-values with respect to its origin, e.g., geological or biological. Methane-production of ruminants is considered to be the dominant source from the animal kingdom. Isotopic values of rumen methane-given in literature-range between -80‰ and -50‰ and are related to feed composition and also sampling techniques. Keeping cows, camels and sheep under identical feed conditions and sampling rumen gases via implanted fistulae we compared δ(PDB) (13)C-values of methane and CO(2) between the species. Referring to mean values obtained from 4 or 5 samples at different times of 11 animals (n = 47) we calculated δ(PDB) (13)C-medians resulting in small but not significant differences within and significant differences between the species for CO(2) and methane. The δ(PDB) (13)C-differences between methane and CO(2) were statistically equal within and also between the species. Therefore a linear regression of methane values on CO(2) is appropriate and leads to: δ(PDB) (13)C(methane)‰ = 1,57 * δ(PDB) (13)C(CO(2))‰-47‰ with a correlation coefficient of r = 0,87.     

An improved design of an integrated optical isolator based on non-reciprocalMach–Zehnder interferometry

Non-reciprocal rib waveguide structures can be used to realize integrated optical isolators. In this paper, we propose a concrete design for a Mach–Zehnder interferometer type isolator for TM modes. Just one of the arms, which are of equal length, is a non-reciprocal magneto-optic waveguide. The rest of the interferometer is reciprocal. Required fabrication tolerances are estimated, and the entire isolator is simulated by applying a finite difference beam propagation method.     

Wave-matching method for mode analysis of dielectric waveguides

Frequently the cross-section of a longitudinally homogeneous dielectric waveguide may be decomposed into rectangles with constant permittivity. For points inside these rectangles the wave equation for modal fields is solved analytically by expanding into functions with harmonic or exponential dependence on the transverse coordinates. Minimization of a least-squares expression for the remaining misfit on the boundary lines allows us to determine propagation constants and fields for guided modes. Semivectorial calculations for two sets of rib waveguides and the centre sections of a directional coupler and an MMI device show very good agreement with results found in the literature. This revised version was published online in November 2006 with corrections to the Cover Date.     

Vectorial wave-matching mode analysis of integrated optical waveguides

For mode fields of integrated optical waveguides with piecewise constant and rectangular permittivity profile, Maxwell's equations reduce to the two-dimensional Helmholtz wave equation, supplemented by continuity requirements on the boundaries between different media. Two basic components of the mode field are expanded into factorizing harmonic or exponential functions, separately on each rectangular region. We determine approximations for guided modes and propagation constants by means of a minimization procedure based on a least squares expression for the mismatch in the continuity requirements. Results for the hybrid mode fields of typical sample waveguides classify this approach as competitive with and superior to alternative methods for vectorial mode analysis, although some open questions remain.