Improved Poisson intensity estimation: denoising application using Poisson data

Recently, Timmermann and Nowak developed algorithms for estimating the means of independent Poisson random variables. The algorithms are based on a multiscale model where certain random variables are assumed to obey a beta-mixture density function. Timmermann and Nowak simplify the density estimation problem by assuming the beta parameters are known and only one mixture parameter is unknown. They use the observed data and the method of moments to estimate the unknown mixture parameter. Taking a different approach, we generate training data from the observed data and compute maximum likelihood estimates of all of the beta-mixture parameters. To assess the improved performance obtained by the proposed modification, we consider a denoising application using Poisson data.     

Protocol design for scalable and reliable group rekeying

We present the design and specification of a protocol for scalable and reliable group rekeying together with performance evaluation results. The protocol is based upon the use of key trees for secure groups and periodic batch rekeying. At the beginning of each rekey interval, the key server sends a rekey message to all users consisting of encrypted new keys (encryptions, in short) carried in a sequence of packets. We present a scheme for identifying keys, encryptions, and users, and a key assignment algorithm that ensures that the encryptions needed by a user are in the same packet. Our protocol provides reliable delivery of new keys to all users eventually. It also attempts to deliver new keys to all users with a high probability by the end of the rekey interval. For each rekey message, the protocol runs in two steps: a multicast step followed by a unicast step. Proactive forward error correction (FEC) multicast is used to reduce delivery latency. Our experiments show that a small FEC block size can be used to reduce encoding time at the server without increasing server bandwidth overhead. Early transition to unicast, after at most two multicast rounds, further reduces the worst-case delivery latency as well as user bandwidth requirement. The key server adaptively adjusts the proactivity factor based upon past feedback information; our experiments show that the number of NACKs after a multicast round can be effectively controlled around a target number. Throughout the protocol design, we strive to minimize processing and bandwidth requirements for both the key server and users.     

One-, two-, and three-photon pumped lasing in a novel liquid dye salt system

We report the observation of one-, two-, and three-photon pumped lasing in the same medium, a novel liquid dye salt system when excited by pulsed 0.532-, 1.06-, and /spl sim/1.49-/spl mu/m coherent radiation pulses, respectively. Since the gain medium is a liquid and not a solution, it contains a significantly higher effective dye concentration and, therefore, is highly suitable for multiphoton pumped lasing and optical power limiting applications. The lasing spectra, temporal waveforms, near- and far-field intensity distributions, and output/input efficiency were measured under the conditions of one-, two-, and three-photon pump configurations.     

The structure and maximal gain of CW-pumped GaP-AlGaP semiconductor Raman amplifier with tapers on both sides

GaP-AlGaP waveguide semiconductor Raman amplifiers (SRAs) tapered on both sides were fabricated by high-quality GaP-AlGaP liquid phase epitaxial growth using the temperature difference method with controlled vapor pressure (TDM-CVP), photolithography patterning, and reactive ion etching with PCl/sub 3/ gas. Although the finesse of the both-sides-tapered waveguide SRA is lower than previous values for straight or one-side-tapered waveguides, the CW-pumped gain was maximized, and a maximal gain of 4.2 dB was obtained. This letter presents the effect of tapered structures in SRA with CW pumping amplification.     

Analysis of a MEMS transmission line

A microelectromechanical system (MEMS) sound waveguide is considered as a transmission line for RF signals. We analyze a device geometry of a straight one-dimensional microsize silicon rod, where a longitudinal acoustic wave is generated and detected using capacitive transducers. Linear, isotropic, and nondispersive acoustic-wave propagation is assumed. Based on the calculation of the electromechanical impedance, an electrical equivalent model is derived for the acoustic transmission line. A numerical example and a comparison to measured properties of a MEMS-transmission-line resonator shows that the characteristic impedance level of the waveguide is typically high, which causes challenges for matched termination. Solutions to overcome the matching problems are discussed.     

Near‐Infrared Light‐Emitting Diodes (LEDs) Based on Poly(phenylene)/Yb‐tris(β‐Diketonate) Complexes

Near‐infrared‐emitting electroluminescent (EL) devices using blue‐light‐emitting polymers blended with the Yb complexes Yb(DBM)₃phen (DBM = dibenzoylmethane), Yb(DNM)₃phen (DNM = dinaphthoylmethane), and Yb(TPP)L(OEt) (L(OEt) = [(C₅H₅)Co{P(O)Et₂}₃]^–) have been studied. EL devices composed of Yb(DNM)₃phen blended with PPP‐OR11 showed enhanced near‐IR output at 977 nm when compared to those fabricated with Yb(DBM)₃phen/PPP‐OR11 blends. The maximum near‐IR external efficiencies of the devices with Yb(DBM)₃phen and Yb(DNM)₃phen are, respectively, 7 × 10^–5 (at 6 V and at 0.81 mA mm^–2) and 4 × 10^–4 (at 7 V, and 0.74 mA mm^–2). The optimal blend composition for EL device performance consisted of PPP‐OR11 blended with 10–20 mol‐% Yb(DNM)₃phen. A device fabricated using Yb‐(TPP)L(OEt)/PPP‐OR11 showed significantly enhanced near‐IR output efficiency, and future efforts will focus on devices fabricated using porphyrin‐based materials.     

SiGe HMODFET "KAIST" micropower model and amplifier realization

The recently published small-signal KAIST model is used successfully to fit the measured RF characteristics of a novel SiGe n-HMODFET device operating at micropower levels and extracted small-signal model parameters for this device under micropower operation are presented here for the first time. This model is then used to predict the performance of a simple micropower amplifier (sub 300-/spl mu/W total power consumption), realized in SiGe technology, and a comparison of modeled versus measured data is included.     

Integrated-optic dispersion slope equalizer for N/spl times/ several tens of Gb/s WDM transmission

We report a dispersion slope equalizer on a planar lightwave circuit for wavelength division multiplexing (WDM) transmission. This device consists of an array of lattice-formed equalizers with different compensation values fabricated on one wafer and arrayed-waveguide gratings for wavelength multi/demultiplexing. We describe its configuration, operational principle, parameter design, fabrication, and measured characteristics in detail. N/spl times/20 and N/spl times/40 Gb/s slope equalizers were fabricated and their characteristics agreed well with designed values. We also report a reduction in the bias electrical power needed for thermooptic phase shifters in the equalizer array that we realized by employing a phase trimming technique normally used for optical switches.     

High‐Tg Carbazole Derivatives as Blue‐Emitting Hole‐Transporting Materials for Electroluminescent Devices

A series of dicarbazolyl derivatives bridged by various aromatic spacers and decorated with peripheral diarylamines were synthesized using Ullmann and Pd‐catalyzed C–N coupling procedures. These derivatives emit blue light in solution. In general, they possess high glass‐transition temperatures (T_g > 125 °C) which vary with the bridging segment and methyl substitution on the peripheral amine. Double‐layer organic light‐emitting devices were successfully fabricated using these molecules as hole‐transporting and emitting materials. Devices of the configuration ITO/HTL/TPBI/Mg:Ag (ITO: indium tin oxide; HTL: hole‐transporting layer; TPBI: 1,3,5‐tris(N‐phenylbenzimidazol‐2‐yl)benzene) display blue emission from the HTL layer. The EL spectra of these devices appear slightly distorted due to the exciplex formation at the interfaces. However, for the devices of the configuration ITO/HTL/Alq₃/Mg:Ag (Alq₃ = tris(8‐hydroxyquinoline)aluminum) a bright green light from the Alq₃ layer was observed. This clearly demonstrates the facile hole‐transporting property of the materials described here.     

Cramer-Rao bounds for parametric shape estimation in inverse problems

We address the problem of computing fundamental performance bounds for estimation of object boundaries from noisy measurements in inverse problems, when the boundaries are parameterized by a finite number of unknown variables. Our model applies to multiple unknown objects, each with its own unknown gray level, or color, and boundary parameterization, on an arbitrary known background. While such fundamental bounds on the performance of shape estimation algorithms can in principle be derived from the Cramer-Rao lower bounds, very few results have been reported due to the difficulty of computing the derivatives of a functional with respect to shape deformation. We provide a general formula for computing Cramer-Rao lower bounds in inverse problems where the observations are related to the object by a general linear transform, followed by a possibly nonlinear and noisy measurement system. As an illustration, we derive explicit formulas for computed tomography, Fourier imaging, and deconvolution problems. The bounds reveal that highly accurate parametric reconstructions are possible in these examples, using severely limited and noisy data.