High average power and short pulse duration continuous wave mode-locked Nd : GdVO4 laser with a semiconductor absorber mirror

Passive mode locking of a solid-state Nd:GdVO4 laser is demonstrated. The laser is mode locked by use of a semiconductor absorber mirror (SAM). A low Nd3+ doped Nd:GdVO4 crystal is used to mitigate the thermal lens effect of the laser crystal at a high pump power. The maximum average output power is up to 6.5 W, and the pulse duration is as short as 6.2 ps. The optic-to-optic conversion efficiency is 32.5% and the repetition rate is about 110 MHz.

Photoluminescence properties of a GaN0.015As0.985/GaAs single quantum well under short pulse excitation

Under short pulse laser excitation, we have observed an extra high-energy photoluminescence (PL) emission from GaNAs/GaAs single quantum wells (QWs). It dominates the PL spectra under high excitation and/or at high temperature. By measuring the PL dependence on both temperature and excitation power and by analyzing the time-resolved PL results, we have attributed the PL peak to the recombination of delocalized excitons in QWs. Furthermore, a competition process between localized and delocalized excitons is observed in the temperature-dependent PL spectra under the short pulse excitation. This competition is believed to be responsible for the temperature-induced S-shaped PL shift often observed in the disordered alloy semiconductor system under continuous-wave excitation. (C) 2001 American Institute of Physics.

Tandem electroabsorption modulators integrated with DFB laser by ultra-low-pressure selective-area-growth MOCVD for 10 GHz optical short pulse generation - art. no. 60200O

A novel device of tandem multiple quantum wells (MQWs) electroabsorption modulators (EAMs) monolithically integrated with DFB laser is fabricated by ultra-low-pressure (22 mbar) selective area guowth (SAG) MOCVD technique. Experimental results exhibit superior device characteristics with low threshold of 19 mX output light power of 4.5 mW and over 20 dB extinction ratio when coupled into a single mode Fiber. Moreover, over 10 GHz modulation bandwidth is developed with a driving voltage of 2 V. Using I this sinusoidal voltage driven integrated device, 10GHz repetition rate pulse with a width of 13.7 ps without any compression elements is obtained.

Short range scattering mechanism of type-II GaSb/GaAs quantum dots on the transport properties of two-dimensional electron gas

We have studied the scattering process of AlGaAs/GaAs two-dimensional electron gas with the nearby embedded GaSb/GaAs type-II quantum dots (QDs) at low temperature. Quantum Hall effect and Shubnikov-de Haas oscillation were performed to measure the electron density n(2D), the transport lifetime tau(t) and the quantum lifetime tau(q) under various biased gate voltage. By comparing measured results of QDs sample with that of reference sample without embedded QDs, mobilities (transport mobility mu(t) and quantum mobility mu(q)) dominated by GaSb QDs scattering were extracted as functions of n(2D). It was found that the ratios of tau(t) to tau(q) were varying within the range of 1-4, implying the scattering mechanism belonging to the sort of short-range interaction. In the framework of Born approximation, a scattering model considering rectangular-shaped potential with constant barrier height was successfully applied to explain the transport experimental data. In addition, an oscillating ratio of tau(t)/tau(q) with the increasing n(2D) was predicted in the model.

Picosecond ultra-wideband short pulse radiated by antenna arrays

This report presents the experiments to study the characteristics of the picosecond ultra-wideband pulses coherent radiation. The testing involves bow-tie horn antennas for both the transninting and receiving antenna. Sixteen channels of electrical pulses with 290 ps duration and jitter < 30 ps have been used. The antenna arrays with various frames of 4 x 1, 4 x 2, 4 x 3, 4 x 4 are employed to radiate the pulses. The receiving antenna measures the electrical field in different distance front the transmitting antennas arrant The results show that if the pulses are in coherent condition, the peak power pulse of output by antennas array with N elements are N-2 of that of the single element antenna. (c) 2007 Wiley Periodicals, Inc.

Characteristics of plasma induced by interaction of a free-oscillated laser pulse with a coal target in air and combustible gas

Plasma in the air is successfully induced by a free-oscillated Nd:YAG laser pulse with a peak power of 10(2-3) W. The initial free electrons for the cascade breakdown process are from the ablated particles from the surface of a heated coal target, likewise induced by the focused laser beam. The laser field compensates the energy loss of the plasma when the corresponding temperature and the images are investigated by fitting the experimental spectra of B-2 Sigma(+) -> X-2 Sigma(+) band of CN radicals in the plasma with the simulated spectra and a 4-frame CCD camera. The electron density is estimated using a simplified Kramer formula. As this interaction occurs in a gas mixture of hydrogen and oxygen, the formation and development of the plasma are weakened or restrained due to the chaining branch reaction in which the OH radicals are accumulated and the laser energy is consumed. Moreover, this laser ignition will initiate the combustion or explosion process of combustible gas and the minimum ignition energy is measured at different initial pressures. The differences in the experimental results compared to those induced by a nanosecond Q-switched laser pulse with a peak power of 10(6-8) W are also discussed. (C) 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Gas-phase isothermal chromatography with short-lived technetium isotopes

In order to study the gas-phase chemical behavior of transactinides, an on-line isothermal chromatography apparatus has been developed and applied to separate short-lived technetium isotopes in the form of TcO3 from fission products. The fission products from a Cf-252 source were continuously and rapidly transported through the capillary to the isothermal chromatography apparatus using the N-2/KBr gas-jet techniques. Volatile oxide molecules were formed at the reaction zone kept at 900 degrees C since a trace amount of oxygen existed in the N-2 carrier gas. With the new developed isothermal chromatography apparatus, a selective separation of Tc from fission products was achieved. After isothermal chromatographic separation, Tc-101,Tc-103,Tc-104,Tc-105,Tc-106,Tc-107,Tc-108 were dominantly observed together with their Ru daughters in the gamma-spectrum, The chemical yields of Tc-101, and Tc-104 and Tc-105 isotopes with longer half-lives are about 55-57%, and those of Tc-103, Tc-106 and Tc-108 isotopes with shorter half-lives dropped down to 25-28%. The adsorption enthalpy of the investigated compounds on quartz surfaces was determined to be -150 +/- 5 kJ/mol by fitting the measured retention curves with a Monte Carlo model. The observed species of technetium oxide is attributed to TcO3, which is in good agreement with previous experimental results. That means our system worked properly and it can be used to investigate the gas-phase chemical behavior of transactinides.

Molecular dynamics and control following excitation with an ultra-short intense laser pulse

Vibronic excitations of the tri-atomic molecule OClO (A(2)A(2)(nu(1), nu(2), nu(3)) <-- (XB1)-B-2 (0, 0, 0)) with weak and strong ultra-short laser fields are studied within full quantum wavepacket dynamics in hyperspherical coordinates. Different dynamics is observed following excitation with laser pulses of different intensities. With a strong laser pulse, many vibrational states are excited and a spatially more localised wavepacket arises. The numerical results show that the population of different vibrational states of the wavepacket on the excited potential energy surface is altered by the intensity of the laser pulse. The numerical results also suggest a related effect on the phase of the wavepacket. These interesting phenomena can be understood by an analysis of the corresponding results for two model diatomic molecules. The possible physical mechanisms of control of chemical processes using strong laser fields are discussed. (C) 2004 Elsevier B.V. All rights reserved.

Bottom simulating reflector and gas seepage in Okinawa Trough: Evidence of gas hydrate in an active back-arc basin

To look for gas hydrate, 22 multi-channel and 3 single-channel seismic lines on the East China Sea (ECS) shelf slope and at the bottom of the Okinawa Trough were examined. It was found that there was indeed bottom simulating reflector (BSR) occurrence, but it is very rare. Besides several BSRs, a gas seepage was also found. As shown by the data, both the BSR and gas seepage are all related with local geological structures, such as mud diapir, anticline, and fault-controlled graben-like structure. However, similar structural "anomalies" are quite common in the tectonically very active Okinawa Trough region, but very few of them have developed BSR or gas seepage. The article points out that the main reason is probably the low concentration of organic carbon of the sediment in this area. It was speculated that the rare occurrence of gas hydrates in this region is governed by structure-controlled fluid flow. Numerous faults and fractures form a network of high-permeability channels in the sediment and highly fractured igneous basement to allow fluid circulation and ventilation. Fluid flow in this tectonic environment is driven primarily by thermal buoyancy and takes place on a wide range of spatial scales. The fluid flow may play two roles to facilitate hydrate formation: to help gather enough methane into a small area and to modulate the thermal regime.

Multiphase technique improves mud-pulse velocity calculations

Measurement while drilling (MWD) has become a popular survey technology to monitor directional data, drilling data, formation evaluation data and safety data in the world. And closed loop drilling shows promise in recent years. Obviously, the method of tr

Detection and Characterization of Long-Pulse Low-Velocity Impact Damage in Plastic Bonded Explosives

Damage not only degrades the mechanical properties of explosives, but also influences the shock sensitivity, combustion and even detonation behavior of explosives. The study of impact damage is crucial in the vulnerability evaluation of explosives. A long-pulse low-velocity gas gun with a gas buffer was developed and used to induce impact damage in a hot pressed plastic bonded explosive. Various methods were used to detect and characterize the impact damage of the explosive. The microstructure was examined by use of polarized light microscopy. Fractal analysis of the micrographs was conducted by use of box counting method. The correlation between the fractal dimensions and microstructures was analyzed. Ultrasonic testing was conducted using a pulse through-transmission method to obtain the ultrasonic velocity and ultrasonic attenuation. Spectra analyses were carried out for recorded ultrasonic signals using fast Fourier transform. The correlations between the impact damage and ultrasonic parameters including ultrasonic velocities and attenuation coefficients were also analyzed. To quantitatively assess the impact induced explosive crystal fractures, particle size distribution analyses of explosive crystals were conducted by using a thorough etching technique, in which the explosives samples were soaked in a solution for enough time that the binder was totally removed. Impact induces a large extent of explosive crystal fractures and a large number of microcracks. The ultrasonic velocity decreases and attenuation coefficients increase with the presence of impact damage. Both ultrasonic parameters and fractal dimension can be used to quantitatively assess the impact damage of plastic bonded explosives.

K418与42CrMo异种金属的激光穿透焊接

实验研究了连续波Nd：YAG激光焊接速度、侧吹保护气流量和离焦量等参量对激光穿透焊接K418和42CrMo焊缝成形的影响。结果表明，K418与42CrMo激光穿透焊接有X形和T形两种典型的焊缝形貌，且焊缝形貌是不对称的。随着焊接速度的提高，焊接线能量降低，焊缝尺寸变小，且焊缝上部尺寸变化比下部尺寸变化慢，焊缝形貌由X形过渡到T形。当离焦量在瑞利长度范围内时，焊缝正面宽度变化很小；当离焦量超出瑞利长度范围时，在足够高的激光功率密度下，焊缝正面宽度快速增加。在激光功率为3kW，侧吹保护气角度为35°条件下，通过优化焊接速度、侧吹保护气流量和离焦量等参量可以得到最佳焊缝质量。

Arc Root Attachment on the Anode Surface of Arc Plasma Torch Observed with a Novel Method

The arc-root attachment on the anode surface of a dc non-transferred arc plasma torch has been successfully observed using a novel approach. A specially designed copper mirror with a boron nitride film coated on its surface central-region is employed to avoid the effect of intensive light emitted from the arc column upon the observation of weakly luminous arc root. It is found that the arc-root attachment is diffusive on the anode surface of the argon plasma torch, while constricted arc roots often occur when hydrogen or nitrogen is added into argon as the plasma-forming gas.