Bidirectional optical coupler for plastic optical fibers

We have developed a low-loss bidirectional optical coupler for high-speed optical communication with plastic optical fibers (POFs). The coupler, which is fabricated by an injection molding method that uses poly (methyl methacrylate), has an antisymmetric tapered shape. We show that the coupler has low insertion and branching losses. The tapered shape of the receiving branch reduces beam diameter and increases detection efficiency coupling to a photodetector, whose area is smaller than that of the plastic optical fiber. The possibility of more than 15-m bidirectional transmission with a signaling bit rate up to 500 Mbits/s for simplex step-index POFs is demonstrated.     

A novel technique for the analysis of dielectric height variationsin microstrip circuits

This paper presents a technique based on the full-wave, two-dimensional spectral domain method and the distributed transmission line approach to analyze structures with a combination of conductor width and dielectric height variations. The technique is applied to a regular taper and the results are compared with existing models. A matched taper transition is then designed based on simultaneous variation of the conductor width and dielectric height such that their ratio is kept constant. The analysis of such tapers is carried out and compared to measured data. It is shown that significant reduction in the return loss can be achieved with such transitions over fairly wide, tunable frequency ranges     

Beam test performance of a scintillator-based detector for the charge identification of relativistic ions

We report on the measurements performed with relativistic ions from Be to Fe, at the Fragment Separator (FRS) of the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, to test the performance of charge-sensitive detectors that were designed to separate - via multiple dE/dx measurements - fully stripped nuclei of cosmic origin in the experiment CALET. The latter is a space mission by the Japanese Space Agency (JAXA) scheduled to be launched to the International Space Station (ISS) in 2013. The CALET instrument is managed by an international collaboration and it is scheduled to take data for 5 years on the Exposure Facility (JEM-EF) of the Japanese module KIBO on the ISS.The aim of the test was to accurately measure the response of the scintillator to different nuclear species and parametrize the saturation of the scintillation light in order to assess the impact of this effect on the charge resolution of the instrument.     

Correlation between energy level alignment and device performance in planar heterojunction organic photovoltaics

Information on the interfacial electronic structure in organic photovoltaics (OPVs) is essential for fully understanding features of device operation such as the photocurrent generation and relative energy band offsets at the donor/acceptor interface, which directly affect the open circuit voltage (V_oc). Kelvin probe (KP) measurements fully reveal the energy level alignment in a prototype OPV with a copper phthalocyanine (CuPc)/fullerene (C₆₀) planar heterojunction. Energy level pinning at the CuPc/C₆₀ junction fixes the energy band offsets of C₆₀. A downward energy shift of about 0.9 eV appears at the C₆₀/bathocuproine junction, which may act as a hole-blocking barrier. A combination of KP and current density–voltage measurements indicates that photocurrent generation depends strongly on the magnitude of the upward energy shift at the CuPc/C₆₀ junction. The dependence of V_oc on the substrate work function is also discussed in terms of the energy level alignment at indium tin oxide/CuPc/C₆₀ junctions.     

Asymptotic boundary conditions for finite element analysis ofthree-dimensional transmission line discontinuities

The finite element method is used to analyze open three-dimensional transmission line structures in the quasi-TEM regime. Starting from the general solution of the Laplace equation in spherical coordinates, a set of asymptotic boundary conditions are derived for three-dimensional quasi-static problems for a spherical outer boundary. The second-order boundary condition is generalized to a box-shaped outer boundary and implemented in the finite element method to solve the potential problem of a rectangular microstrip patch. Numerical results show that the asymptotic boundary conditions yield more accurate results than those obtainable with a perfectly conducting shield placed at the same location     

Systematic DC/small-signal/large-signal analysis of heterojunctionbipolar transistors using a new consistent nonlinear model

In this paper, a new systematic approach to heterojunction bipolar transistors (HBT's) characterization and modeling is presented. The proposed approach is based on a new compact HBT nonlinear circuit model which accounts for both self-heating and the temperature dependence effects. The model's parameters are extracted from measured dc-IV characteristics and S-parameters. The power characteristics of the device are then predicted using the extracted model without any further optimizations. The same model is also used for intermodulation distortion analysis. The model has been implemented in a number of commercial nonlinear simulators and in an in-house computer code. Results are presented for two different size devices showing good agreement with measurements     

Suppression of the nicotinic acetylcholine response in rat superior cervical ganglionic neurons by steroids

The effects of various types of steroids on the nicotinic acetylcholine (ACh) receptor (nAChR)-mediated responses were investigated in superior cervical ganglionic neurons acutely dissociated from rats using nystatin perforated patch recording. ACh induced a peak followed by a gradual decrease in the inward current at a holding potential of -40 mV. Nicotine, but not muscarine, mimicked ACh. Hydrocortisone at a concentration of >10(-6) M reversibly suppressed both the peak and steady-state nicotine-induced currents (Inic) in a noncompetitive manner. The inhibition of Inic by hydrocortisone did not show any voltage dependency and persisted in the presence of either cyclic AMP modulators, forskolin and 3-isobutyl-1-methylxanthine, or a protein kinase A inhibitor, N-[2-(methylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-89). Beta-estradiol, androsterone, aldosterone, and 17alpha-estradiol mimicked hydrocortisone in its inhibitory action on ACh-induced currents (I(ACh)). The potency for the inhibitory actions on I(ACh) was as follows: androsterone > beta-estradiol > hydrocortisone > or = aldosterone = 17alpha-estradiol. Cholesterol had no effect on the I(ACh). In conclusion, the structural characteristics of a steroid are thus considered to be necessary to block nicotinic I(ACh) in rat superior cervical ganglionic cells, whereas the cholesterol side chain might disturb the inhibitory action of the steroid skeleton on nAChRs.