[1] Using the Global Positioning System radio occultation (GPSRO) technique, the observation of the global ionosphere becomes possible. The irregularity in the ionospheric sporadic-E (Es) layer, which is probably caused by wind shear, can be investigated by analyzing the signal-to-noise ratio (SNR) of RO signal. In this study, the relation between the amplitude of RO signals and the electron density profiles of the ionosphere is simulated, and RO data recorded in the time period from mid-2008 to mid-2011 are used for the analysis. Based on the simulation results, the multiple-layer-type (MLT) and the single-layer-type (SLT) Es layers which are defined by the shape of SNR, are used to analyze the global distribution of Es layer. The seasonal MLT Es layer is compared with the seasonal wind shear, which is obtained from the Horizontal Wind Model (HWM07). Furthermore, the seasonal MLT Es layer is compared with the SLT Es layer, and the global altitude distributions of MLT and SLT Es layers are similar while the magnitude distributions are different. Unlike the MLT Es layer, the global distribution of the SLT Es layer is similar to the distribution of E region peak electron density (N m E), which is related to the solar zenith angle.

We report on the propagation characteristics of a plasmonic waveguide structure based on two coupled rectangular wedges. Dispersion, propagation loss, and field distributions are investigated by three-dimensional finite-difference time-domain method. The considered structure supports only one supermode over 30THz bandwidth, and the calculated propagation loss at lambda=1.55mum is 0.0257dB/mum, which is lower than the existing report by 1.7 times while keeping comparable field localizations. The all-planar structure in conjunction with the linearly dispersive characteristic over a wide operational bandwidth signifies its great potential for optical signal transporting in nanophotonic circuits.

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We present the numerical investigation of the optical limiting behavior based on coupled nonlinear plasmonic waveguides. Exploiting the strong localization of the electromagnetic fields at metal-dielectric interfaces, significant enhancement of the nonlinear absorption was achieved. Two types of optical limiters (OLs), one based on the enhanced optical Kerr (OK) effect and the other based on the enhanced two-photon absorption (TPA), are proposed. Their transmission characteristics at off-resonant band of Au are investigated. The simulation results reveal that the linear transmittances in both cases are higher than 85%, and the limiting thresholds are 250 GW/cm(2) and 42.69 GW/cm(2) for the OK and TPA based OLs, respectively. As compared with the non-structured slab waveguides, the optical limiting thresholds are greatly reduced. Wideband operation over 200 nm was confirmed and TPA induced free carrier absorption (FCA) discussed.

A three-dimensional ray tracing model with aiming algorithms for global positioning system (GPS) signal is proposed to make simulations conform to the realistic radio occultation (RO) signal propagation. The two aiming algorithms used in this study ensure the initial and end point ray trajectories are located in the prescribed region. In past studies, the ray tracing techniques applied to the RO signal simulation usually assumed a spherically symmetrical atmosphere for simplicity. The exact GPS and low earth orbit (LEO) satellite locations are not considered in the simulation. These two assumptions make the simulation unrealistic for GPS signal propagation in the RO technique. In the proposed model, the shape of the earth is assumed as an ellipse. The information from European Centre for Medium-Range Weather Forecasts (ECMWF) analysis is used to setup the atmosphere in the simulation. Two aiming algorithms are developed to determine the initial signal propagating direction to make the simulated signal start from the prescribed GPS satellite position and end in the close vicinity of the LEO satellite position. An ideal spherical symmetric atmospheric structure is used to verify the ray tracing model. The fractional difference between real and simulated refractivity results is less than 0.1%. Otherwise, the GPS and LEO satellite position in the Formosat-3/COSMIC observation and the ECMWF analysis, considering the earth's flattening, is also used to verify the aiming algorithms. All of the simulated signals end in close vicinity to the LEO satellite position in the simulation results.