Development of Surface Grating Distributed Feedback Quantum Cascade Laser for High Output Power and Low Threshold Current Density

We report on the room-temperature cascade laser （QCL） at λ -4.7μm. cw operation of a surface grating Both grating design and material distributed feedback （DFB） quantum optimization are used to decrease the threshold current density and to increase the output power. For a high-reflectivity-coated 13-μm-wide and 4- mm-long laser, high wall-plug efficiency of 6% is obtained at 20℃ from a single facet producing over I W of ew output power. The threshold current density of DFB QCL is as low as 1.13kA/cm^2 at 10℃ and 1.34kA/cm2 at 30℃ in cw mode. Stable single-mode emission with a side-mode suppression ratio of about 30 dB is observed in tile working temperature range of 20-50℃.

Very low threshold operation of quantum cascade lasers

A strain-compensated InP-based quantum cascade laser（QCL） structure emitting at 4.6 μm is demonstrated,based on a two-phonon resonant design and grown by solid-source molecular beam epitaxy（MBE）.By optimizing the growth parameters,a very high quality heterostructure with the lowest threshold current densities ever reported for QCLs was fabricated.Threshold current densities as low as 0.47 kA/cm^2 in pulsed operation and 0.56 kA/cm^2 in continuous-wave（cw） operation at 293 K were achieved for this state-of-the-art QCL.A minimum power consumption of 3.65 W was measured for the QCL,uncooled,with a high-reflectivity（HR） coating on its rear facet.

Material Growth and Device Fabrication of GaN-Based Blue-Violet Laser Diodes

Studies on first GaN-based blue-violet laser diodes(LDs) in China mainland are reported.High quality GaN materials as well as GaN-based quantum wells laser structures are grown by metal-organic chemical vapor deposition method.The X-ray double-crystal diffraction rocking curve measurements show the full-width half maximum of 180″ and 185″ for (0002) symmetric reflection and (10 12) skew reflection,respectively.A room temperature mobility of 850cm2/(V·s) is obtained for a 3μm thick GaN film.Gain guided and ridge geometry waveguide laser diodes are fabricated with cleaved facet mirrors at room temperature under pulse current injection.The lasing wavelength is 405 9nm.A threshold current density of 5kA/cm2 and an output light power over 100mW are obtained for ridge geometry waveguide laser diodes.

High power 2-μm room-temperature continuous-wave operation of GaSb-based strained quantum-well lasers

A high power GaSb-based laser diode with lasing wavelength at 2 μm was fabricated and optimized. With the optimized epitaxial laser structure, the internal loss and the threshold current density decreased and the internal quantum efficiency increased. For uncoated broad-area lasers, the threshold current density was as low as 144 A/cm2 （72 A/cm^2 per quantum well）, and the slope efficiency was 0.2 W/A. The internal loss was 11 cm^-1 and the internal quantum efficiency was 27.1%. The maximum output power of 357 mW under continuous-wave operation at room temperature was achieved. The electrical and optical properties of the laser diode were improved.

Divertor detachment operation in helium plasmas with ITER-like tungsten divertor in EAST

Detachment in helium(He)discharges has been achieved in the EAST superconducting tokamak equipped with an ITER-like tungsten divertor.This paper presents the experimental observations of divertor detachment achieved by increasing the plasma density in He discharges.During density ramp-up,the particle flux shows a clear rollover,while the electron temperature around the outer strike point is decreasing simultaneously.The divertor detachment also exhibits a significant difference from that observed in comparable deuterium(D)discharges.The density threshold of detachment in the He plasma is higher than that in the D plasma for the same heating power,and increases with the heating power.Moreover,detachment assisted with neon(Ne)seeding was also performed in L-and H-mode plasmas,pointing to the direction for reducing the density threshold of detachment in He operation.However,excessive Ne seeding causes confinement degradation during the divertor detachment phase.The precise feedback control of impurity seeding will be performed in EAST to improve the compatibility of core plasma performance with divertor detachment for future high heating power operations.

Tunnel effects on ring road traffic flow based on an urgent-gentle class traffic model

To explore tunnel effects on ring road traffic flow,a macroscopic urgent-gentle class traffic model is put forward.The model identifies vehicles with urgent and gentle classes,chooses the tunnel speed limit as free flow speed to express the fundamental diagram in the tunnel,and adopts algebraic expressions to describe traffic pressure and sound speed.With two speed trajectories at the Kobotoke tunnel in Japan,the model is validated,with good agreement with observed data.Numerical results indicate that in the case of having no ramp effects,tunnel mean travel time is almost constant dependent on tunnel length.When initial density normalized by jam density is above a threshold of about 0.21,a traffic shock wave originates at the tunnel entrance and propagates backward.Such a threshold drops slightly as a result of on-ramp merging effect,the mean travel time drops as off-ramp diversion effect intensifies gradually.These findings deepen the understanding of tunnel effects on traffic flow in reality.

Mechanism and threshold fluence of nanosecond pulsed laser paint removal

In this paper, the nanosecond pulse laser surface treatment of the waterborne anti-rust paint on HT250 gray cast iron was carried out. The area and depth of the per-pulse laser ablation paint layer were measured. The threshold of laser energy density was determined through the relations with ablation area and depth. The paint removal mechanism was discussed by analyzing the ablation features of the paint layer on laser cleaning. The features of the paint removal under various laser energy densities were characterized, and the process parameters in the experiments were investigated.The results showed that there were four thresholds and three kinds of mechanisms in the paint removal process with nanosecond pulsed laser. The ablation threshold of substrate was deepened on the laser parameters. The ablation processes were included thermal ablation, thermal vibration and paint ionization concurrently, respectively. The surface cracks and paint debris were observed at the edge of the cleaning path, which were ascribed to the vibration effect by laser. In addition, the vibration effect could significantly increase the width of paint removal. Paint ionization has also a significant influence on the substrate morphology. Paint ionization would have an obvious impact on the formation of the substrate morphology. It is desirable to fabricate approach to remove the paint layer without damaging the substrate under optimized laser parameters by nanosecond pulse laser.

Impacts of Down-Up Hill Segment on the Threshold of Shock Formation of Ring Road Vehicular Flow

The study of impacts of down-up hill road segment on the density threshold of traffic shock formation in ring road vehicular flow is helpful to the deep understanding of sags’bottleneck effect.Sags are freeway segments along which the gradient increases gradually in the traffic direction.The main aim of this paper is to seek the density threshold of shock formation of vehicular flow in ring road with down-up hill segment,because down-up hill roadway segment is a source to cause capacity reduction that is an attractive topic in vehicular traffic science.To seek the density threshold numerically,a viscoelastic continuum model[1]is extended and used.To solve the model equations,a fifth-order weighted essentially non-oscillatory scheme for spatial discretization,and a 3rd order Runge-Kutta scheme for time partial derivative term are used.Validation by existing observation data and the Navier-Stokes like model[2]extended as EZM is done before conducting extensive numerical simulations.For ring road vehicular flow with three separated down-up hill segments,it is found that the density threshold of shock formation decreases monotonically with the relative difference of free flow speed,this variation can be simply fitted by a third order polynomial.

A study of the strong pulses detected from PSR B0656+14 using the Urumqi 25-m radio telescope at 1540 MHz

We report on the properties of strong pulses from PSR B0656＋14 by analyzing the data obtained using the Urumqi 25-m radio telescope at 1540 MHz from August 2007 to September 2010.In 44 h of observational data,a total of 67 pulses with signal-to-noise ratios above a 5σthreshold were detected.The peak flux densities of these pulses are 58 to 194 times that of the average profile,and their pulse energies are 3 to 68 times that of the average pulse.These pulses are clustered around phases about 5-ahead of the peak of the average profile.Compared with the width of the average profile,they are relatively narrow,with the full widths at half-maximum ranging from 0.28 ° to 1.78 °.The distribution of pulse-energies follows a lognormal distribution.These sporadic strong pulses detected from PSR B0656＋14 have different characteristics from both typical giant pulses and its regular pulses.

Managing crop damage caused by house mice(Mus domesticus)in Australia

A large-scale outbreak of the house mouse populations occurs in grain growing in Australia on average once every four years.High densities of mice cause major yield losses to cereal crops,and low to moderate densities of mice also cause some losses.Several predictive models based on rainfall patterns have been developed to forecast mouse density.These models carry some uncertainty and the economic value of basing management actions on these models is not clear.Baiting is the most commonly used method and zinc phosphide and other rodenticide bait are effective in reducing up to 90%of mouse populations.Ecologically-based best farming practice for controlling mice has recently been developed on the basis of long-term field studies of mouse populations.No effective biological control method has been developed for mice.However,grain growers still cannot make economically rational decisions to implement control because they do not know the pest threshold density(D_(T))above which the economic benefits of control exceed the economic costs of control.Applied predator-prey theory suggests that understanding the relationship between mouse density and damage is the basis for determining D_(T).Understanding this relationship is the first research priority for managing mouse damage.The other research priority is to develop a reliable method to estimate unbiased mouse density.

Enhanced spontaneous emission factor for microcavity lasers

The microcavity and the influence of nonradiative recombination can control spontaneous emission. An analytic resolution of rate equation is studied for microcavity lasers. The relationship between output properties and structural parameters of multi-quantum wells （MQWs） is obtained. One of the most important consequences of the increased spontaneous emission factor is the reduction of laser threshold. It is found that the characteristic curve of a ＂thresholdless＂ laser is strongly nonradiative depopulation-dependent. The light output is increased by the enhanced well number and the reduced width. In particular, there is an optimal well number corresponding to the lowest threshold current density for MQW structure in the microcavity lasers.