The properties of MOVPE grown 1.3 μm DFB MQW lasers infilled with semi-insulating InP fabricated on semi-insulating substrates

The use of optoelectronic integrated circuits (OEICs) is now emerging as a practical technology for a variety of applications, particularly in advanced telecommunications. OEICs consist of a range of devices such as lasers, waveguides, modulators, amplifiers, transistors, detectors, etc. fabricated on the same substrate. When a semi-insulating substrate is used, these devices can be electrically isolated by channel etching, resulting in a low capacitance structure with reduced electrical interference between the subcomponents. One of the devices which is particularly advantageous for this type of integration scheme is the distributed feedback (DFB) laser. The laser can be made to function more efficiently by minimizing the current flowing outside the active region. This can be achieved by surrounding the active region with semi-insulating iron doped InP. This work describes for the first time, the MOVPE growth, fabrication, and device characterization of 1.3 um buried heterostructure DFB MQW lasers, which combine the advantages of using both a semi-insulating substrate and a semi-insulating infill region in the same device structure. The potential advantage of this design scheme is improved OEIC performance as a result of, reduced capacitance and electrical crosstalk, enhanced laser output power, higher speed, increased efficiency, wider operating temperature and reduced threshold current. The laser active region consists of 8 x 140 Å quantum wells of GalnAsP (λ = 1.3 μm) and 110 Åbarriers of GalnAsP (λ= 1.07 μm). Single mode 1.3 urn devices of length 250 μm operating at room temperature produced threshold currents of 8 mA, efficiencies of up to 25%, output powers of 18 mW at 80 mA (pulsed), and a frequency response greater than 12GHz. The parasitic capacitance was estimated to be less than 3 pF.     

A new look at spotlight mode synthetic aperture radar astomography: imaging 3-D targets

A new 3D tomographic formulation of spotlight mode synthetic aperture radar (SAR) is developed. This extends the pioneering work of Munson et al. (1983), who first formally described SAR in terms of tomography but who made the simplifying assumption that the target scene was 2D. The present authors treat the more general and practical case in which the radar target reflectivities comprise a 3D function. The main goal is to demonstrate that the demodulated radar return data from a spotlight mode collection represent a certain set of samples of the 3D Fourier transform of the target reflectivity function and to do so using a tomographic paradigm instead of traditional range-Doppler analysis. They also show that the tomographic approach is useful in interpreting the reconstructed 2D SAR image corresponding to a 3D scene. Specifically, the well-known SAR phenomenon of layover is easily explained in terms of tomographic projections and is shown to be analogous to the projection effect in conventional optical imaging.     

1.4-/spl mu/m InGaAsP-InP strained multiple-quantum-well laser for broad-wavelength tunability

InGaAsP-InP strained multiple-quantum-well (MQW) lasers for extended wavelength tunability in external cavity operation were designed, fabricated, and tested. The active layer was a strain compensated structure consisting of three 3.2/spl plusmn/0.3 nm and three 6.4/spl plusmn/0.3 nm 1.0% compressive strained wells and five 10.3/spl plusmn/0.3 nm 0.45% tensile strained barrier layers. A 2-/spl mu/m-wide ridge waveguide laser of length 250 /spl mu/m, when used in a grating external cavity and with no coatings to alter the reflectivity of the facets, was observed to operate over a range >110 nm. The lasers were designed for applications in trace gas and liquid detection with the goal to maximize the tunable range when operated in external cavities and with no facet coatings.     

Information Processing With Pure Spin Currents in Silicon: Spin Injection, Extraction, Manipulation, and Detection

We demonstrate that information can be transmitted and processed with pure spin currents in silicon. Fe/Al₂O₃ tunnel barrier contacts are used to produce significant electron spin polarization in the silicon, generating a spin current which flows outside of the charge current path. The spin orientation of this pure spin current is controlled in one of three ways: 1) by switching the magnetization of the Fe contact; 2) by changing the polarity of the bias on the Fe/Al₂O₃ "injector" contact, which enables the generation of either majority or minority spin populations in the Si, providing a way to electrically manipulate the injected spin orientation without changing the magnetization of the contact itself; and 3) by inducing spin precession through the application of a small perpendicular magnetic field. Spin polarization by electrical extraction is as effective as that achieved by the more common electrical spin injection. The output characteristics of a planar silicon three-terminal device are very similar to those of nonvolatile giant magnetoresistance metal spin-valve structures.     

Temperature compensation of total power radiometers

This paper describes a new technique to compensate output variations of total power radiometers due to physical temperature changes of the instrument. This technique performs the correction without the addition of expensive microwave hardware required in Dicke switching or many other widely used methods. A characterization period, over which the input antenna temperature is known, indicates the appropriate output adjustment needed for a change in physical temperature of the radiometer. The method effectively corrects the output in an example radiometer system built with inexpensive commercially available parts. For a 30-K variation in physical temperature, the measured data shows an improvement from 60-K peak-to-peak error to 6.9 K with an average absolute error of 1.1 K.     

A 15-mW fully integrated I/Q synthesizer for Bluetooth in 0.18-/spl mu/m CMOS

A low-power fully integrated synthesizer for Bluetooth applications is presented. The circuit with quadrature output signals at 2.45 GHz and 15-mW power dissipation has been designed in a digital 0.18-/spl mu/m CMOS process with 1.8-V supply voltage. The only external component is a 64-MHz crystal. Measurements have been performed on packaged samples mounted on an FR-4 board and show that the Bluetooth requirements are met. The measured phase noise is below -120 dBc/Hz at 3-MHz offset, and the resulting residual frequency modulation is 7.4-kHz rms. The tuning range consists of an analog and digital tuning mechanism, resulting in more than 15% overall tuning range.     

Evaluation of Corn/Soybeans Separability Using Thematic Mapper And Thematic Mapper Simulator Data

Multitemporal Thematic Mapper, Thematic Mapper Simulator, and detailed ground truth data were collected for a 9-by 1-km sample segment in Webster County, Iowa, in the summer of 1982. Three dates were acquired each with Thematic Mapper Simulator (June 7, June 23, and July 31) and Thematic Mapper (August 2, September 3, and October 21). The Thematic Mapper Simulator data were converted to equivalent TM count values using TM and TMS calibration data and model based estimates of atmospheric effects. The July 31, TMS image was compared to the August 2, TM image to verify the conversion process. A quantitative measure of proportion estimation variance (Fisher information) was used to evaluate the corn/soybeans separability for each TM band as a function of time during the growing season. The additional bands in the middle infrared allowed corn and soybeans to be separated much earlier than was possible with the visible and near-infrared bands alone. Using the TM and TMS data, temporal profiles of the TM principal components were developed. The greenness and brightness exhibited behavior similar to MSS greenness and brightness for corn and soybeans.     

Surface-constrained volumetric brain registration using harmonic mappings

In order to compare anatomical and functional brain imaging data across subjects, the images must first be registered to a common coordinate system in which anatomical features are aligned. Intensity-based volume registration methods can align subcortical structures well, but the variability in sulcal folding patterns typically results in misalignment of the cortical surface. Conversely, surface-based registration using sulcal features can produce excellent cortical alignment but the mapping between brains is restricted to the cortical surface. Here we describe a method for volumetric registration that also produces an accurate one-to-one point correspondence between cortical surfaces. This is achieved by first parameterizing and aligning the cortical surfaces using sulcal landmarks. We then use a constrained harmonic mapping to extend this surface correspondence to the entire cortical volume. Finally, this mapping is refined using an intensity-based warp. We demonstrate the utility of the method by applying it to T1-weighted magnetic resonance images (MRIs). We evaluate the performance of our proposed method relative to existing methods that use only intensity information; for this comparison we compute the intersubject alignment of expert-labeled subcortical structures after registration.     

Simulation Models for Investigation of Multiuser Scheduling in MIMO Broadcast Channels

Spatial correlation is a result of insufficient antenna spacing among multiple antenna elements, while temporal correlation is caused by Doppler spread. This paper compares the effect of spatial and temporal correlation in order to investigate the performance of multiuser scheduling algorithms in multiple‐input multiple‐output (MIMO) broadcast channels. This comparison includes the effect on the ergodic capacity, on fairness among users, and on the sum‐rate capacity of a multiuser scheduling algorithm utilizing statistical channel state information in spatio‐temporally correlated MIMO broadcast channels. Numerical results demonstrate that temporal correlation is more meaningful than spatial correlation in view of the multiuser scheduling algorithm in MIMO broadcast channels. Indeed, the multiuser scheduling algorithm can reduce the effect of the Doppler spread if it exploits the information of temporal correlation appropriately. However, the effect of spatial correlation can be minimized if the antenna spacing is sufficient in rich scattering MIMO channels regardless of the multiuser scheduling algorithm used.     

Hamilton-Jacobi skeleton on cortical surfaces

In this paper, we propose a new method to construct graphical representations of cortical folding patterns by computing skeletons on triangulated cortical surfaces. In our approach, a cortical surface is first partitioned into sulcal and gyral regions via the solution of a variational problem using graph cuts, which can guarantee global optimality. After that, we extend the method of Hamilton-Jacobi skeleton to subsets of triangulated surfaces, together with a geometrically intuitive pruning process that can trade off between skeleton complexity and the completeness of representing folding patterns. Compared with previous work that uses skeletons of 3-D volumes to represent sulcal patterns, the skeletons on cortical surfaces can be easily decomposed into branches and provide a simpler way to construct graphical representations of cortical morphometry. In our experiments, we demonstrate our method on two different cortical surface models, its ability of capturing major sulcal patterns and its application to compute skeletons of gyral regions.