An FDTD near- to far-zone transformation for scatterers buried instratified grounds

The finite-difference time-domain (EDTD) technique is being used with increasing frequency for modeling the scattering characteristics of buried objects. The FDTD has, for some time, been able to model the near-zone scattered fields of buried objects due to near-zone sources. This is adequate for modeling the scattered returns of ground-based ground-penetrating radar, but not for airborne radar. This paper describes an FDTD-compatible technique whereby far-zone scattered fields of objects buried in a stratified ground can be calculated. This technique uses the equivalence principle to model a buried object in terms of equivalent electric and magnetic currents. The fields radiated by these currents in the presence of a stratified ground are then calculated using the reciprocity theorem and the well-known field equations for plane waves in a stratified media. Numerical results are presented that show excellent agreement between this technique and both analytical and numerical results     

Frequency response of cross-phase modulation in multispan WDMoptical fiber systems

The spectral characteristics of cross-phase modulation (XPM) in multispan intensity-modulation direct-detection optical systems were investigated both experimentally and theoretically. XPM crosstalk levels and its spectral features were found to be strongly dependent on fiber dispersion and optical signal channel spacing. Interference between XPM-induced crosstalk effects created in different amplified fiber spans is also found to be important to determine the overall frequency response of XPM crosstalk effects     

FDTD modeling of scatterers in stratified media

The FDTD technique is well suited for calculating the fields scattered by buried objects when the sources are close enough to the air/ground interface so that they can be incorporated into the solution space. Difficulties arise, however, when the sources are far from the interface since the total fields in the solution space are not all outgoing waves. Using well-known formulas for the fields transmitted and reflected by stratified media, this paper discusses a method whereby the fields scattered by a buried object can be easily calculated by the FDTD technique when the incident field is a plane wave     

An Optimum Combination of Power Levels and Combiner Weighting for Generating 64-QAM from Three 4-PSK Signals

64 state quadrature amplitude modulation (64-QAM) can be generated by combining three 4-PSK signals, thereby allowing nonlinear amplifiers with smaller power ratings to be used. An optimum combination of amplifier power levels and combiner weighting is described which results in no lost power when the peak power state of the 64-QAM signal is being keyed.     

Genetics of ethanol-induced locomotor activation: detection of QTLs in a C57BL/6J x DBA/2J F2 intercross

Moderate doses of ethanol (1-2 g/kg) markedly increase locomotor activity in some inbred mouse strains, for example, the DBA/2J (D2), but have relatively little effect in other strains, for example, the C57BL/6J (B6). In the present study, we conducted a genome-wide search in a B6D2 F2 intercross (N = 925) for quantitative trait loci (QTLs) associated with the locomotor response. A QTL with a LOD score of 8.4 was detected on Chromosome (Chr) 2; this QTL accounted for 11.4% of the phenotypic variance and approximately 30% of the genetic variance. The QTL on Chr 2 is in the same general region as QTLs previously described for ethanol preference/consumption (Rodriguez et al. Alcohol Clin Exp Res 19, 367, 1995; Melo et al. Nat Genet 13, 147, 1996; Phillips et al. Mamm Genome, in press), acute ethanol withdrawal (Buck et al. J. Neurosci 17, 3946, 1997) and nitrous oxide withdrawal severity (Belknap et al. Behav Genet 23, 213, 1993). A logical candidate gene in the region of interest is the enzyme which synthesizes GABA, glutamic acid decarboxylase 1 (GadI).     

Modeling baluns with the method of moments

When using the method of moments (MoM) to model large arrays with large numbers of balanced feeds, the feed baluns can always be incorporated into the analysis by combining the multiport admittance matrix for the array with the admittance matrices of the baluns. This technique is straightforward but requires that the MoM equations be solved for a large number of right-hand sides. The paper shows how the effects of perfect baluns can be incorporated directly into the MoM equations, requiring only one right-hand side. This technique yields the exact results with a significant savings in computing resources     

Using model-based parameter estimation to increase the efficiencyof computing electromagnetic transfer functions

Two main ideas are introduced: (1) the use of model-based parameter estimation based on rational function approximations, which reduces the number of frequencies at which solutions or samples are required; (2) a sampling approach that uses frequency derivatives of the response and a novel analytical technique based on differentiating the moment method impedance equation, which provides derivative information in a time proportional to N² in contrast with the N³ dependence in solving the original problem. Antenna input admittances are modeled using frequency samples and derivatives. The rational function model is shown to offer a large advantage over polynomial interpolation of a frequency response. Application of the frequency-derivative approach is demonstrated for problems having well-defined resonances such as a dipole antenna, and for more challenging problems having narrow resonances     

A DC to multigigabit/s polarization-independent modulator based ona Sagnac interferometer

A broadband, electrically controlled polarization-independent intensity modulator (PLM) based on a Sagnac interferometer is presented. Influences from asymmetric modulation and traveling wave effects are analyzed and methods to eliminate these effects are proposed. Compared to Mach-Zehnder interferometer-based modulators, this modulator is independent of the input polarization state and less sensitive to ambient perturbations due to its reciprocity property. A prototype modulator operating at 1 and 2.488 Gb/s modulation rates is demonstrated