Optical properties of pseudomorphic Si1-xGexfor Si-based waveguides at the λ=1300-nm and 1550-nmtelecommunications wavelength bands

The index of refraction for pseudomorphic Si_1-xGe_x layers grown on Si has been measured at wavelengths λ=1310 mn and λ=1550 nm. The refractive index values were obtained from waveguide mode profile measurements on a series of Si-Si_1-xGe _x-Si waveguides with Ge_x concentrations between x=0.01 and x=0.1. The index of refraction, n, is significantly larger for light polarized parallel to the growth direction than for light polarized in the plane of the epilayer. This birefringence is consistent with the anisotropic index change predicted using photoelastic theory, given the biaxial strain present in the pseudomorphic Si_1-xGe _x layers. At all wavelengths and polarizations, n varies linearly with the Ge concentration. The pseudomorphic Si_1-xGe _x waveguides layer are stable against lattice relaxation during short anneals at 950°C, but exhibit partial relaxation after annealing at 1200°C     

Jitter reduction through feedback for picosecond pulsed InGaAsPlasers

Following a comparison of picosecond-pulse generation techniques, feedback schemes are reported for the generation of picosecond pulse trains with improved jitter for both multicontact and conventional single-contact InGaAsP-InP lasers. Subpicosecond jitter is achieved for Q-switched laser sources using a novel optoelectronic feedback scheme. The use of resonant electrical feedback is shown to improve the timing jitter of gain-switched pulses by up to six times. Pulse-to-pulse timing jitter as low as 250 fs is demonstrated for a hybrid of optical and electrical feedback schemes. Limits for timing jitter in diode lasers are established for optoelectronic, electrical, and optical feedback schemes, and the key picosecond pulse generation schemes are compared in terms of timing jitter for the first time     

Error estimation for numerical differential equations

Given a capable human being and a computer, it is possible to make an approximation to the solution of a nonlinear differential equation. However, under the (usually correct) assumption that the equation is analytically intractable, the result of the computation is not the exact solution; indeed it may be so far from the exact solution as to be completely useless. We are interested in the relationship between the effort expended by the human and the computer, and the duality of the computed approximation to a partial or ordinary differential equation. To be specific, we would like to think in terms of a cost-benefit analysis. The cost of the computation is a combination of the human effort and computer resources used to obtain the approximation. The benefit includes, of course, the computed approximation, but it also includes an estimate of the quality of the approximation, that is, an error estimate. It is our opinion that in computational science, as with the experimental sciences, results should always be presented with some estimate of their accuracy. In addition, however, there is another facet to error estimation: one cannot even attempt a cost-benefit analysis or efficiency comparison of methods without an error estimate to evaluate the results     

Vector-valued time-frequency representations

Cohen's (1989) class of time frequency distributions (TFDs), which includes the spectrogram (SP), Wigner distribution (WD), and reduced interference distributions (RIDs) has become widely known as a useful signal analysis tool. It has been shown that every real-valued TFD can be written as a weighted sum of SPs. The “SP decomposition” has been used to construct fast approximations to desirable TFDs using the SP building block, for which there exist accessible and efficient hardware and software implementations. We introduce a class of linear, vector-valued time-frequency representations (TFRs) that are easily related to associated bilinear TFDs through the SP decomposition. We solve a least-squares signal synthesis problem on modified vector-valued TFRs that are associated with nonnegative TFDs as a weighted sum of least-squares short-time Fourier transform (STFT) signal synthesis schemes. We extend the solution to vector-valued TFRs associated with high-resolution TFDs in order to define a high-resolution alternative to STFT signal synthesis, as demonstrated by desirable properties and examples. The resulting signal synthesis methods can be realized as a weighted sum of STFT synthesis schemes, for which there exist accessible and efficient hardware and software implementations     

A NOVEL PASSIVE DUAL ENERGY RECOVERY SNUBBER FOR HIGH POWER GTO CONVERTERS

A novel passive dual energy recovery snubber circuit is presented where energy trapped in the snubber inductor and capacitor is recovered into the both DC rail and load, without any active devices or resistors. The maximum over-shoot voltage on the switch is fixed, peak switch current is low, circuit reset is fast and the operational range of load current is wide. This circuit is suitable for use in high frequency, single ended power gate turn-off (GTO) thyrister choppers. Main design equations, some simulations and practical results are included.     

Integratable, high speed buried ridge DFB lasers fabricated on semi-insulating substrates

The fabrication of buried ridge DFB lasers on semi-insulating substrates is described. A novel contacting mechanism was employed to give a series resistance of less than 4 Omega . Devices were fabricated at both 1.3 and 1.53 mu m with lasing thresholds as low as 16 mA. Single longitudinal mode operation was achieved with SMSR greater than 30 dB at both wavelengths. The structure gives an inherently low capacitance, which together with low threshold currents, low series resistance and fabrication on SI substrates makes these devices suitable for integration and high speed applications.<>     

Millimeter-wave components and subsystems built using microstriptechnology

The author describes the evaluation of microstrip as a transmission medium for components and subsystems operating in the millimeter wavebands. The manufacturing process for thin-film MICs on single-crystal quartz is described. The microwave integrated circuit (MIC) manufacturing process is suitable for high-value manufacturing of highly integrated subsystems since manufacturing costs for the circuits are almost independent of the number of circuit functions. Designs for various circuit elements are discussed, and an integrated subsystem is described in which MIC techniques are used to produce a miniature millimeter-wave distance-measuring sensor. Range measurement results for the miniature sensor are presented     

Investigation of small-scale unintended releases of hydrogen: Buoyancy effects

Measurements were performed in small-scale hydrogen leaks to characterize the dimensional properties and flow characteristics of the resulting ignitable hydrogen cloud. The data are intended to provide a technological basis for determining hazardous length scales associated with the formation of ignitable mixtures due to unintended releases. In contrast to a previous study where momentum-dominated releases were considered, the present study focuses on smaller-scale releases at lower flow rates where buoyancy becomes important. A turbulent jet flow is selected as representative of releases in which the leak geometry is circular. Laser-based Rayleigh scattering is used to characterize the hydrogen concentration field downstream of the leak. Particle Image Velocimetry (PIV) is also used to characterize the flow velocity. Time-averaged mean and fluctuating hydrogen concentration statistics are presented and compared with results in momentum-dominated flows to elucidate the effects of buoyancy on the H₂ dispersion process. Over the range of Froude numbers investigated (Fr = 268, 152 and 99), increasing effects of buoyancy are seen as the Fr is reduced and at downstream locations where the influence of buoyancy increases relative to jet momentum. The primary effect of buoyancy is to increase the centerline decay rate of the time-averaged H₂ mass fraction relative to momentum-dominated flows. Acceleration due to buoyancy also results in a slower decay of the time-averaged axial velocity component along the centerline. Radial profiles of the time-averaged H₂ mass fraction also collapse onto the same curves as results in momentum-dominated flows when plotted against the same similarity/scaling variables. While buoyancy is found to have a negligible effect on centerline velocity fluctuations, the maximum H₂ mass fraction fluctuation intensity increases by 70 percent in the buoyant regime and the peak value shifts from the mixing region to the jet centerline. The database presented should provide a good test for the validation of CFD models being developed to predict unintended hydrogen releases under conditions where buoyancy is important.     

正确测量转换速率

转换速率(Slew rate)限定一个放大器输出偏移的最大比率.它确定不失真带宽(ADC驱动电路的一个重要参数)的极限.转换速率还影响DAC输出级、滤波器、视频放大器以及数据采集器可能达到的性能.如果你的电路性能依赖这一参数,你就必须通过测量来验证放大器的转换速率.     

Improved Ni Schottky Contacts on <Emphasis Type="Italic">n</Emphasis>-Type 4H-SiC Using Thermal Processing

High-temperature processing was used to improve the barrier properties of three sets of n-type 4H-SiC Schottky diodes fabricated with Ni Schottky contacts. We obtained an optimum average barrier height of 1.78 eV and an ideality factor of 1.09 using current–voltage measurements on diodes annealed in vacuum at 500°C for 24 h. Nonannealed contacts had an average barrier height of 1.48 eV and an ideality factor of 1.85. The Rutherford backscattering spectra of the Ni/SiC contacts revealed the formation of a nickel silicide at the interface, accompanied by a substantial reduction in oxygen following annealing.