1.3-μm Quantum-well InGaAsP, AlGaInAs, and InGaAsN laser material gain: A theoretical study

Due to the keen interest in improving the high-speed and high-temperature performance of 1.3-μm wavelength lasers, we compare, for the first time, the material gain of three different competing active layer materials, namely InGaAsP-InGaAsP, AlGaInAs-AlGaInAs, and InGaAsN-GaAs. We present a theoretical study of the gain of each quantum-well material system and present the factors that influence the material gain performance of each system. We find that AIGaInAs and InGaAsN active layer materials have substantially better material gain performance than the commonly used InGaAsP, both at room temperature and at high temperature.

Characterization of the temperature sensitivity of gain and recombination mechanisms in 1.3-μm AlGaInAs MQW lasers

The potential of 1.3-μm AlGaInAs multiple quantum-well (MQW) laser diodes for uncooled operation in high-speed optical communication systems is experimentally evaluated by characterizing the temperature dependence of key parameters such as the threshold current, transparency current density, optical gain and carrier lifetime. Detailed measurements performed in the 20°C-100°C temperature range indicate a localized T0 value of 68 K at 98°C for a device with a 2.8μm ridge width and 700-μm cavity length. The transparency current density is measured for temperatures from 20°C to 60°C and found to increase at a rate of 7.7 A·cm -2 · °C-1. Optical gain characterizations show that the peak modal gain at threshold is independent of temperature, whereas the differential gain decreases linearly with temperature at a rate of 3 × 10-4 A-1·°C-1. The differential carrier lifetime is determined from electrical impedance measurements and found to decrease with temperature. From the measured carrier lifetime we derive the monomolecular (A), radiative (B), and nonradiative Auger (C) recombination coefficients and determine their temperature dependence in the 20 °C-80 °C range. Our study shows that A is temperature independent, B decreases with temperature, and C exhibits a less pronounced increase with temperature. The experimental observations are discussed and compared with theoretical predictions and measurements performed on other material systems. © 2005 IEEE.

Spray visualisation and acoustic analysis in a gas-turbine type burner

This paper presents the characterisation of self-excited oscillations in a kerosene burner. The combustion instability exhibits two different modes and frequencies depending on the air flow rate. Experimental results reveal the influence of the spray to shift between these two modes. Pressure and heat release fluctuations have been measured simultaneously and the flame transfer function has been calculated from these measurements. The Mie scattering technique has been used to record spray fluctuations in reacting conditions with a high speed camera. Innovative image processing has enabled us to obtain fluctuations of the Mie scattered light from the spray as a temporal signal acquired simultaneously with pressure fluctuations. This has been used to determine a transfer function relating the image intensity and hence the spray fluctuations to changes in air velocity. This function has identified the different role the spray plays in the two modes of instability. At low air flow rates, the spray responds to an unsteady air flow rate and the time varying spray characteristics lead to unsteady combustion. At higher air flow rates, effective evaporation means that the spray dynamics are less important, leading to a different flame transfer function and frequency of self-excited oscillation. In conclusion, the combustion instabilities observed are closely related with the fluctuations of the spray motion and evaporation.

Strong plasmonic enhancement of photovoltage in graphene.

From the wide spectrum of potential applications of graphene, ranging from transistors and chemical sensors to nanoelectromechanical devices and composites, the field of photonics and optoelectronics is believed to be one of the most promising. Indeed, graphene's suitability for high-speed photodetection was demonstrated in an optical communication link operating at 10 Gbit s(-1). However, the low responsivity of graphene-based photodetectors compared with traditional III-V-based ones is a potential drawback. Here we show that, by combining graphene with plasmonic nanostructures, the efficiency of graphene-based photodetectors can be increased by up to 20 times, because of efficient field concentration in the area of a p-n junction. Additionally, wavelength and polarization selectivity can be achieved by employing nanostructures of different geometries.

Understanding jet noise.

Jets are one of the most fascinating topics in fluid mechanics. For aeronautics, turbulent jet-noise modelling is particularly challenging, not only because of the poor understanding of high Reynolds number turbulence, but also because of the extremely low acoustic efficiency of high-speed jets. Turbulent jet-noise models starting from the classical Lighthill acoustic analogy to state-of-the art models were considered. No attempt was made to present any complete overview of jet-noise theories. Instead, the aim was to emphasize the importance of sound generation and mean-flow propagation effects, as well as their interference, for the understanding and prediction of jet noise.

Optimization of semiconductor optical amplifiers for all optical wavelength conversion

In recent years a variety of experimental and theoretical work has been reported on the use of semiconductor optical amplifiers for high speed wavelength conversion. However little work has addressed the dynamic limitations of this conversion process in detail with a view to device optimization. In this paper, a detailed study of the conversion process is carried out in order to optimize device parameters and drive conditions for increased conversion speed and improved modulation index.

Bearings and energy storage

Passive magnetic bearings are ideal components for energy storage flywheels which require small dynamic loads and low-maintenance bearings with minimal power requirements. High temperature superconductors such as YBCO can be used to fabricate these bearings and achieve the desired magnetic properties. Stiffness and gap decay due to high speed can be addressed by dynamically altering bearing geometry to provide active control with bulk materials.

Evaluation of two-phase induction motor drives for low-cost applications

An experimental evaluation of small two-phase induction motor drives operating with different inverter topologies is described. Results show that a PWM-based four-switch inverter, having only low-side switches is attractive for high-speed low-cost applications where speeds greater than those that can be obtained using single phase induction motors are required.

Uncooled, 10Gb/s operation of two-contact InGaAsP lasers with low drive current

Uncooled, high-speed modulation of two-contact lasers is presented with ultra-low drive currents. Practical operation at 10Gb/s up to temperatures of 85°C and extinction ratios of 6dB are found for current swings which are less than 40% of conventional lasers.

Superconducting micro-bearings

This paper presents research into superconducting Micro-Bearings for MEMS systems. Advanced silicon processing techniques developed for the Very Large Scale Integration (VLSI) industry have been exploited in recent years to enable the production of micro-engineered moving mechanical systems. These devices commonly known as Micro-ElectroMechanical Systems (MEMS) have many potential advantages. In many respects the effect of scaling a machine from macro-sized to micro-sized are either neutral or beneficial. However in one important respect the scaling produces a severely detrimental effect. That respect is in the tribology and the subsequent wear on the high speed rotating machines. This leads to very short device lifetimes. This paper presents results obtained from a MEMS motor supported on superconducting bearings. The bearings are self-positioning, relying on, the Meissner effect to provide a levitation force which moves the rotor into position and flux pinning to provide stability thereafter. The rotor is driven by a simple electrostatic type motor in which photo resist is used to pattern the motor poles directly onto the rotor. © 2005 IEEE.

An experimental study of unsteady separated shock wave boundary layer interactions

An experimental investigation of the unsteady interaction between a turbulent boundary layer and a normal shock wave of strength M∞ = 1.4 subject to periodic forcing in a parallel walled duct has been conducted. Emphasis has been placed on the mechanism by which changes in the global flow field influence the local interaction structure. Static pressure measurements and high speed Schlieren images of the unsteady interaction have been obtained. The pressure rise across the interaction and the appearance of the local SBLI structure have been observed to vary during the cycle of periodic shock wave motion. The magnitude of the pressure rise across the interaction is found to be related to the relative Mach number of the unsteady shock wave as it undergoes periodic motion. Variations in the upstream Influence of the interaction are sensitive to the magnitude and direction of shock wave velocity and acceleration and it is proposed that a viscous lag exists between the point of boundary layer separation and the shock wave position. Further work exploring the implications of these findings is proposed, including studies of the variation in position of the points of boundary layer separation and reattachment.

Modelling of waterfront cantilever sheet pile walls subjected to earthquake loading

The seismic performance of waterfront cantilever sheet pile retaining walls is of continuing interest to geotechnical engineers as these structures suffer severe damage and even complete failure during earthquakes. This is often precipitated by liquefaction of the surrounding soil, either in the backfill or in front of the wall. This paper presents results from a series of small-scale plane strain models that were tested on a 1-g shaking table and recorded using a high-speed, high-resolution digital camera. The technique of Particle Image Velocimetry (PIV) was applied in order to allow the failure mechanisms to be visualised. It is shown that using PIV analyses it is possible to obtain failure mechanisms for a cantilever wall in liquefiable soil. These failure mechanisms are compared with those obtained for a cantilever wall in dry soil, previously carried out at a similar scale. It was observed that seismic liquefaction causes significant displacement in much larger zones of soil near the retaining wall compared to an equivalent dry case. The failure mechanism for a cantilever wall with liquefiable backfill, but with a remediated zone designed not to liquefy, is also presented and compared to the unremediated case.

Analysis of in-cylinder hydrocarbons in a multi-cylinder gasoline HCCI engine using gas chromatography

Gasoline Homogeneous Charge Compression Ignition (HCCI) combustion has been studied widely in the past decade. However, in HCCI engines using negative valve overlap (NVO), there is still uncertainty as to whether the effect of pilot injection during NVO on the start of combustion is primarily due to heat release of the pilot fuel during NVO or whether it is due to pilot fuel reformation. This paper presents data taken on a 4-cylinder gasoline direct injection, spark ignition/HCCI engine with a dual cam system, capable of recompressing residual gas. Engine in-cylinder samples are extracted at various points during the engine cycle through a high-speed sampling system and directly analysed with a gas chromatograph and flame ionisation detector. Engine parameter sweeps are performed for different pilot injection timings and quantities at a medium load point. Results show that for lean engine running conditions, earlier pilot injection timing leads to partial oxidation of the injected pilot fuel during NVO, while the fraction of light hydrocarbons remains constant for all parameter variations investigated. The same applies for a variation in pilot fuel amount. Thus there is evidence that in lean conditions, pilot injection-related NVO effects are dominated by heat release rather than fuel reformation. © 2009 SAE International.

Flame imaging of gas-turbine relight

High-altitude relight inside a lean-direct-injection gas-turbine combustor is investigated experimentally by highspeed imaging. Realistic operating conditions are simulated in a ground-based test facility, with two conditions being studied: one inside and one outside the combustor ignition loop. The motion of hot gases during the early stages of relight is recorded using a high-speed camera. An algorithm is developed to track the flame movement and breakup, revealing important characteristics of the flame development process, including stabilization timescales, spatial trajectories, and typical velocities of hot gas motion. Although the observed patterns of ignition failure are in broad agreement with results from laboratory-scale studies, other aspects of relight behavior are not reproduced in laboratory experiments employing simplified flow geometries and operating conditions. For example, when the spark discharge occurs, the air velocity below the igniter in a real combustor is much less strongly correlated to ignition outcome than laboratory studies would suggest. Nevertheless, later flame development and stabilization are largely controlled by the cold flowfield, implying that the location of the igniter may, in the first instance, be selected based on the combustor cold flow. Copyright © 2010.

VGs for a normal SBLI with a downstream diffuser

New types of vortex generators for boundary layer control were investigated experimentally in a flow field which contains a Mach 1.4 normal Shockwave followed by a subsonic diffuser. A parametric study of device height and distance upstream of the normal shock was undertaken with two novel devices: ramped-vanes and split-ramps. Flowfield diagnostics included high-speed Schlieren, oil flow visualization, and pitot-static pressure measurements. A number of flowfield parameters including flow separation, pressure recovery, centerline incompressible boundary layer shape factor, and shock stability were analyzed and compared to the baseline. All configurations tested yielded an elimination of centerline flow separation with the presence of the vortex generators. However, the devices also tended to increase the three-dimensionality of the flow with increased side-wall interaction. When located 25δo upstream of the normal shock, the largest ramped-vane device (whose height was about 0.75 the incoming uncontrolled boundary layer thickness, δo) yielded the smallest centerline incompressible shape factor and the least streamwise oscillations of the normal shock. However, additional studies are needed to better understand the corner interaction effects, which are substantial. © 2010 by the American Institute of Aeronautics and Astronautics, Inc.