A compound high-order polarization-independent birefringence filter using Sagnac interferometers

Polarization-independent fiber-optic filters are presented that replace the polarizers in traditional birefringence filters with a Sagnac interferometer. The performance of these filters is discussed when Solc and Lyot type birefringence combinations are used. A high-order, compound filter combining these two filters is presented, along with experimental verification.     

Automated determination of silicon isotope natural abundance by the acid decomposition of cesium hexafluosilicate

A procedure for the automated determination of isotopic abundances of silicon from biogenic and lithogenic particulate matter and from dissolved silicon in fresh or saltwaters is reported. Samples are purified using proven procedures through the reaction of Si with acidified ammonium molybdate, followed by precipitation with triethylamine and combustion of the precipitate to yield silicon dioxide. The silicon dioxide is converted to cesium hexafluosilicate by dissolution in hydrogen fluoride and the addition of cesium chloride. Isotopic analysis is accomplished by decomposing the cesium hexafluosilicate with concentrated sulfuric acid to generate silicon tetrafluoride gas. Silicon tetrafluoride is purified cryogenically and analyzed on a gas source isotope ratio mass spectrometer. Yields of silicon tetrafluoride are >99.5%. The procedure can be automated by modifying commercial inlet systems designed for carbonate analysis. The procedure is free of memory effects and isotopic biases. Reproducibility is +/-0.03-0.10 per thousand for a variety of natural and synthetic materials.     

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     

Modeling of thin dielectric structures using the finite-differencetime-domain technique

The finite-difference time-domain (FDTD) technique is applied to scattering problems involving thin dielectric sheets, conductor-backed dielectric sheets, and conductor-backed dielectric sheets containing cracks in the dielectric material. A smart cell technique is developed that enables these geometries to be modeled with a spatial grid that is much larger than the dielectric slab and crack widths. This technique is computationally more efficient than the `brute force' (or ordinary) FDTD approach, which must use cells small enough to resolve the dielectric sheets. Numerical results are presented which show that this technique yields accurate scattering results at a large savings in computational resources     

An adaptive first-order polarization-mode dispersion compensationsystem aided by polarization scrambling: theory and demonstration

An adaptive polarization-mode dispersion (PMD) compensation system has been developed to cancel the effects of first-order PMD by producing a complementary PMD vector in the receiver. Control parameters for the PMD compensation system comprised of a polarization controller and a PMD emulator are derived from the nonreturn-to-zero (NRZ) signal in the channel to be compensated. Estimates of the link's differential group delay (DGD) and principal states of polarization (PSPs) based on this signal are reliable when the signal power is equally split between the link's two PSP's; however this condition cannot be assumed. To meet this requirement, we scramble the state of polarization (SOP) of the input signal at a rate much greater than the response time of the PMD monitor signal so that each sample represents many different SOP alignments. This approach allows the effective cancellation of the first-order PMD effects within an optical fiber channel     

An FDTD/MoM hybrid technique for modeling complex antennas in thepresence of heterogeneous grounds

Calculating the current distribution and radiation patterns for ground-penetrating radar antennas is a challenging problem because of the complex interaction between the antenna, the ground, and any buried scatterer. Typically, numerical techniques that are well suited for modeling the antennas themselves are not well suited for modeling the heterogeneous grounds, and visa versa. For example the finite-difference time-domain (FDTD) technique is well suited for modeling fields in heterogeneous media, whereas the method of moments (MoM) is well suited for modeling complex antennas in free space. This paper describes a hybrid technique, based upon the equivalence principle, for calculating an antenna's current distribution radiation pattern when the antenna is located near an air-ground interface. The original problem is decomposed into two coupled equivalent problems: one for the antenna geometry and the other for the ground geometry, with field information passing between them via a rapidly converging iterative procedure. The fields in each region may be modeled using numerical techniques best suited to them. Results for several test cases are presented, using FDTD to model the ground problem and MoM for the antenna problem, that demonstrate the accuracy of this hybrid technique     

Subcarrier multiplexing for high-speed optical transmission

The performance of high-speed digital fiber-optic transmission using subcarrier multiplexing (SCM) is investigated both analytically and numerically. In order to reduce the impact of fiber chromatic dispersion and increase bandwidth efficiency, optical single-sideband (OSSB) modulation was used. Because frequency spacing between adjacent subcarriers can be much narrower than in a conventional DWDM system, nonlinear crosstalk must be considered. Although chromatic dispersion is not a limiting factor in SCM systems because the data rate at each subcarrier is low, polarization mode dispersion (PMD) has a big impact on the system performance if radiofrequency (RE) phase detection is used in the receiver. In order to optimize the system performance, tradeoffs must be made between data rate per subcarrier, levels of modulation, channel spacing between subcarriers, optical power, and modulation indexes. A 10-Gb/s SCM test bed has been set up in which 4 × 2.5 Gb/s data streams are combined into one wavelength that occupies a 20-GHz optical bandwidth. OSSB modulation is used in the experiment. The measured results agree well with the analytical prediction