Complex resonances of conducting spheres with lossy coatings

The electromagnetic scattering amplitude for metal objects coated with a lossless dielectric exhibits a large number of resonances versus real frequency. These resonances are a function of the object shape and size, coating thickness, and coating electrical properties. Previously, it was shown that for coated spheres and nonspherical bare objects, these resonances can be understood in terms of phase matched circumnavigating surface waves and the objects' complex eigenfrequencies. The effect of dielectric loss in the coating on the complex eigenfrequencies and phase velocities of these surface waves is presented for a metal sphere coated with a uniform homogeneous dielectric coating. It is seen that the positions of the complex resonance frequencies move away from the real frequency axis under the influence of dielectric loss in the coating. The effect of this is shown to correspond to the changes in the backscatter spectrum versus real frequency as computed using a Mie series expansion. The significant difference between the lossless and lossy dielectric-coated sphere cases is seen to be due to a modification of the phase velocities of surface waves, particularly the whispering gallery types, and the increased attenuation of the surface wave modes     

Mathematical analysis of lead field expansions

The solution to the bioelectromagnetic inverse problem is discussed in terms of a generalized lead field expansion, extended to weights depending polynomially on the current strength. The expansion coefficients are obtained from the resulting system of equations which relate the lead field expansion to the data. The framework supports a family of algorithms which include the class of minimum norm solutions and those of weighted minimum norm, including FOCUSS (suitably modified to conform to requirements of rotational invariance). The weighted-minimum-norm family is discussed in some detail, making explicit the dependence (or independence) of the weighting scheme on the modulus of the unknown current density vector. For all but the linear case, and with a single power in the weight, a highly nonlinear system of equations results. These are analyzed and their solution reduced to tractable problems for a finite number of degrees of freedom. In the simplest magnetic field tomography (MFT) case, this is shown to possess expected properties for localized distributed sources. A sensitivity analysis supports this conclusion.     

Asymptotic performance of single parity-check product codes

This article investigates the asymptotic performance of single parity-check (SPC) product codes (PCs) from a decoding point of view. Specifically, the probability of bit error is bounded before and after the decoding of each dimension, similar to the analysis of "iterated codes" by Elias (1954). It is shown that the asymptotic probability of bit error can be driven to zero as the number of dimensions, and hence the block length, increases at signal-to-noise ratios (SNRs) within 2 dB of capacity over the additive white Gaussian noise (AWGN) channel.     

High-yield flip-chip bonding and packaging of low-threshold VCSEL arrays on sapphire substrates

Oxide-confined top-emitting 850 nm and bottom-emitting 980 nm vertical-cavity surface-emitting laser (VCSEL) 8/spl times/8 arrays were designed and fabricated for applications of optical interconnects. The arrays were flip-chip bonded onto sapphire substrates that contain complimentary metal-oxide-semiconductor (CMOS) driver and fan-out circuitries. The off-sited bonding contacts and minimized bonding force produced very high yield of the hybridization process without causing damage to the VCSEL mesas. The hybridized devices were further mounted either on printed circuit board (PCB) or in 68-pin pin-grid-array (PGA) packages. The transparent sapphire substrate allowed optical outputs from the top-emitting VCSEL arrays to transmit directly through without additional substrate removal procedure. Lasing thresholds below 250 /spl mu/A for 850 nm VCSELs and 800 /spl mu/A for 980 nm VCSEL were found at room temperature. The oxide confinement apertures of VCSELs were measured to be around 6 /spl mu/m in diameter. High-speed data transmission demonstrated a bandwidth of up to 1 Gbits/s per channel for these hybridized VCSEL transmitters.     

Methyl bromide—Is there any future for this noteworthy fumigant?

For many years methyl bromide has played a significant role in controlling insect pests in durable and perishable agricultural commodities, pests in the soil and in structures. Although about 10% of the chemical produced is used globally for disinfesting stored products its importance for that purpose cannot be understressed, since it is the only remaining fumigant in worldwide use other than phosphine. Methyl bromide is the only fumigant available for quarantine treatment of commodities where rapid disinfestation techniques are essential. Although methyl bromide arises from natural sources, principally the oceans, most of the chemical found in the stratosphere is now thought to originate from man-made sources. Methyl bromide has been formally listed as an ozone-depleting substance by the Parties to the Montreal Protocol, with agreement from 1995 to limit future production. Reviews of both the current uses of methyl bromide, and of potential substitutes should permit a decision on a general control scheme for the chemical by 1995. Separately, in the U.S.A., under the Clean Air Act, methyl bromide will be phased out by the year 2001. Other countries or political unions are also currently examining the feasibility of reducing the quantities of methyl bromide used. Future restrictions on methyl bromide are expected to have a particular impact on developing countries, particularly those exporting agricultural products. This has been recognized in exempting these countries from the controls to be introduced in 1995. Despite many uncertainties regarding its role in ozone depletion it seems certain that methyl bromide will, eventually, be removed from the list of the few remaining products capable of preventing pest damage to food and other valuable commodities.     

Compression of Q-switched and mode-locked CW Nd:YAG laser pulses

Using the optical compression technique with a single-mode optical fiber and a grating pair, the 85 ps pulses from aQ-switched, mode-locked CW Nd:YAG laser have been compressed by a factor of 30. On average, an overall enhancement of 7 × in the peak power was achieved.     

Maximum Likelihood Sequence Detection for QPSK on Nonlinear, Band-Limited Channels

A receiver structure based on maximum likelihood sequence estimation (MLSE) for receiving quaternary phase shift keyed signals over band-limited, nonlinear satellite channels, in the presence of additive downlink Gaussian noise, is derived. This work is an extension of work done by Mesiya et al. [2] for binary PSK transmission. Error performance of the receiver at low to moderate signal-to-noise ratios is estimated using computer simulation. For moderate to high signal-to-noise ratios, an upper bound on the probability of symbol error is obtained. A suboptimum receiver structure is then developed and its performance evaluated using computer simulation. It is shown that the performance of the suboptimum receiver is very close to that of the MLSE receiver.     

The variation of bistatic rough surface scattering cross section for a physical optics model

The general behavior of a rough surface scattering cross section is examined as a function of incident and scattering angles, surface roughness, dielectric constant, and polarization for physical optics (PO) conditions. Quite distinct and complicated variations are observed. For some conditions, deep nulls occur in the normalized bistatic cross sectionsigma deg; as a function of seatlering angle, while other parameter sets yield no such pattern. These results are analyzed and interpreted. The differences in the angular variation insigma degfor different polarizations suggested that, for a given set of conditions, it would be possible to minimize the scattering from a rough surface. This topic is addressed in the second part of the paper. For the case of a given incident polarization, a technique is presented to optimize the bistatic observation of terrain scattering by using combinations of receiver polarization which vary as the scattering angles are changed.     

Hot deboning beef with and without electrical stimulation

Two hot deboning procedures for producing vacuum packed primal beef joints were compared with conventional side chilling followed by cold deboning. Cold shortening toughness was avoided in the hot procedures either by a delay before chilling or by electrical stimulation of the carcass. Overall evaporative losses were 0·6% with hot deboning and 1·9% with cold deboning. Hot deboning reduced drip loss but the effect was smaller after electrical stimulation. The colour of large muscles which cool unevenly on the side was more uniform after hot deboning, but again the improvement was smaller after electrical stimulation. There was no difference in bacterial contamination or growth on hot and cold deboned meat, and instrumental and sensory assessments confirmed that eating quality was similar for all three treatments.     

Optimization of Chemiluminescent Nitrogen Detection for Packed-Column Supercritical Fluid Chromatography with Methanol-Modified CO(2)

A chemiluminescent nitrogen detector was optimized for packed-column supercritical fluid chromatography. Methanol modifier concentrations of 15% or less, an ozone flow of 5.8 mL/min, and a decompressed CO(2) flow between 240 and 310 mL/min were found to exhibit a maximum sensitivity of 5 ng for sulfamethazine (1 ng of N). The addition of a membrane drier to the "optimized" system further decreased the minimum detectable quantity (MDQ) to 0.5 ng (0.1 ng of N). In addition, by using a microbore column (2 mm i.d.) instead of an analytical scale column, the postcolumn decompressed flow split could be eliminated, further reducing the MDQ to 0.125 ng of sulfamethazine (0.025 ng of N).