Transient inversion soft X-ray lasers

The collisionally excited transient inversion scheme is shown to produce exceptionally high gain coefficients and gain-length products. Data are presented for the Ne-like titanium and germanium and Ni-like silver X-ray lasers (XRL's) pumped using a combination of nanosecond and picosecond duration laser pulses. This method leads to a dramatic reduction of the required pump energy and makes down-sizing of XRL's possible, an important prerequisite if they are to become commonly used tools in the long-term     

Studies on collisional pumping of soft X-ray lasers at ILE

X-ray laser research performed at the Institute of Laser Engineering, Osaka University, are reported. The performance of X-ray lasing pumped by a train of 100-ps pulses at the wavelength range between 14 and 4 nm was investigated. The irradiance on the target was up to ~3×10¹⁴ W·cm. The double target amplification was successfully demonstrated with two beam irradiation for Ag, Nd, Yb, Hf, and Ta lasing. Moreover double-pass amplification at 7.9 nm has been demonstrated. The higher atomic number X-ray lasing strongly depends upon the transverse distance of the two targets     

Near-keV coherent X-ray generation with sub-10-fs lasers

Recent advances in solid-state laser technology and ultrafast optics have led to the generation of optical pulses as short as 5 fs with peak powers up to 0.1 TW from a compact kilohertz-repetition-rate all-solid-state laser. This source significantly pushes the frontiers of nonlinear optics. Intriguing possibilities include the development of a compact laser-driven coherent soft-X-ray source at photon energies near 1 keV, and the generation of autosecond XUV pulses. This paper analyzes strong-field ionization and high-harmonic generation in the few-cycle regime, and reviews the first experimental realization of high-harmonic generation with 5-fs pulses. These experiments have resulted in the emission of coherent X-rays at photon energies greater than 0.5 keV (λ<2.5 nm), representing the highest photon energies achieved with a laser-driven coherent source so far. The generated XUV beam is well collimated and predicted to be delivered, after suitable spectral filtering, in a single burst of autosecond duration     

The development of X-ray free-electron lasers

We review and discuss the theoretical and experimental work that has led to the development of short wavelength free-electron lasers operating as single pass amplifiers, starting from the spontaneous undulator radiation, in the self amplified spontaneous emission mode. This work has led to several projects to build this type of free-electron lasers operating at a wavelength of about 0.1 nm, producing coherent X-ray pulses with an unprecedented brilliance and peak power, and pulse length in the femtosecond range. One such project, the LCLS, is presently under construction and is expected to be operational in 2008.     

Single-pulse low-energy-driven transient inversion X-ray lasers

The paper deals with some aspects of a new pump method applied to collisional transient X-ray lasers working with ions of the Ni-like isoelectronic sequence. Specifically, Ni-like silver is the element of main interest. It is shown that a single profiled picosecond pulse with energy of about 1 J is able to excite efficiently the active medium. A small amount of the produced plasma facilitates its heating and following excitation. Application of other available pump laser systems and the resulting medium kinetics are analyzed. Pump pulse profiling and control over the pulse shape in a laser driver are highlighted on the background of the recent development in laser driver technology. Prospects and possible development trends are discussed as well.     

Transient stabilization of structure preserving power systems with excitation control via energy-shaping

In this paper, the transient stability of multimachine power systems based on structure preserving model (SPM) is considered. The interconnection and damping assignment passivity-based control (IDA-PBC) methodology is extended to solve the excitation regulation problem of SPM represented by a set of differential-algebraic equations. By shaping the total energy function via the introduction of a virtual coupling between the electrical and the mechanical dynamics of the power system, a decentralized excitation control law is proposed to ensure the asymptotic stability of the closed-loop system. The controller is proved to be effective in damping the oscillations and enhancing the system stability by the results of simulation research.     

Progress on collisionally pumped optical-field-ionization soft X-ray lasers

We present the status of optical field ionization soft X-ray lasers. The amplifying medium is generated by focusing a high-energy circularly polarized 30-fs 10-Hz Ti: sapphire laser system in a gaseous medium. Using xenon or krypton, strong laser emission at 41.8 and 32.8 nm, respectively, has been observed. After presenting the basis of the physics, we present recent characterization of the sources as well as dramatic improvement of their performances using the waveguiding technique.     

Usefulness of X-ray lasers for science and technology

Recent realization of saturated X-ray lasers (XRL's) has considerably extended the range of optical properties of soft X-ray sources toward high brightness and large coherence length. Consequently, new results may be expected from studies previously experienced with traditional sources such as synchrotron radiation. On the other hand, XRL's open new fields of research owing to their high brightness. In this paper, we present some of the first experiments utilizing XRL sources     

计及风电随机激励的电力系统暂态稳定分析

为了探讨风电随机激励对电力系统暂态稳定的影响,提出一种基于随机微分理论对含风电电力系统进行建模和稳定分析的方法。首先将异步风机机械功率作为随机激励,在暂态过程中利用伊藤型随机微分方程对异步风机的转子运动方程进行建模,将传统的微分代数方程模型扩展成随机微分代数方程模型;然后针对新的模型,通过时域仿真进行求解,分析风电功率随机波动对电力系统暂态过程的影响。算例结果表明:相比于确定性和概率性暂态稳定分析方法,所提方法能够更好地揭示风电不确定性对电力系统暂态稳定性的影响。     

Photon-timing detector module for single-molecule spectroscopy with 60-ps resolution

Advanced fluorescence measurements on single molecules demand single-photon detectors with high-quantum detection efficiency, low noise, and high time resolution. We have developed a compact (82/spl times/60/spl times/30 mm) and versatile single-photon timing module (SPTM), based on a planar epitaxial single photon avalanche diodes (SPAD) working with a monolithic integrated active quenching and active reset circuit (i-AQC) and cooled by a Peltier element. The main operating parameters are computer controlled via RS-232 interface and the photon counting rate can be continuously monitored. The photon detection efficiency is 45% at 500 nm with cooling at -15/spl deg/C, the dark counting rate is 5 c/s with SPAD operating at 5 V excess bias voltage, 10c/s operating at 10 V. The time resolution obtained with tightly focused illumination has 60-ps full-width at half-maximum. Comparative tests with the SPTM prototype and with an advanced commercially available photon counting module confirmed that the time resolution and sensitivity of the SPTM make it possible to resolve and measure even short lifetime components of a single molecule. The SPTM thus made possible experiments leading to a deeper insight into angstrom-scale structural changes of single-protein molecules.     

Random lasers with coherent feedback

We review our recent work on lasing in active random media. Light scattering, which had been regarded as detrimental to lasing action for a long time, actually provided coherent feedback for lasing. The fundamental difference and transition between a random laser with coherent feedback and a random laser with incoherent feedback were illustrated. We also trapped laser light in micrometer-sized random media. The trapping was caused by disorder-induced scattering and interference. This nontraditional way of light confinement has important applications to microlasers.     

Transient excitation boosting at Grand Coulee Third Power plant:power system application and field tests

Transient excitation boosting (TEB) has been installed on the Grand Coulee Third Power Plant hydrogenerators (three 600 MVA units and three 700 MVA units). The authors describe the application and implementation of transient excitation boosting based on direct detection of a critical disturbance. Using telemetry, a signal is sent to inject a decaying pulse into the excitation systems of large generators with very powerful static exciters. This open-loop control improves interarea transient stability. TEB is initiated for outages of the 3100 MW Pacific HVDC intertie, and results in a decaying pulse input to the generator voltage regulators. TEB temporarily raises Pacific Northwest transmission voltages which increases voltage-sensitive loads. The increased load brakes Northwest generators which are accelerating because of the loss of HVDC intertie power. Transient stability of the parallel Pacific AC intertie is thus improved. Power system-wide commissioning tests were conducted on May 7, 1991. The tests are described and test results compared with simulation results     

Multipole synchronous machines with fractional q < 1 tooth windings and excitation with permanent magnets

Specific features of multipole synchronous machines with fractional tooth windings and excitation with permanent magnets are considered. A comparison of such machines with motors with standard windings is given. Technical characteristics of manufactured motors are presented. Prospects for application of such motors are shown.     

Manufacturing with novel high-power diode lasers

Direct applications of high-power diode lasers (HPDLs) like hardening, heat conduction welding of metals, and joining of polymers have already been demonstrated also in the industrial environment. Relatively low intensities in the range of 10/sup 3/ W cm/sup -2/ are sufficient for these applications. While the commercial HPDL systems are built on the basis of diode laser bars with 40 W output power, in the meantime a record continuous-wave-output power of 267 W per bar has been demonstrated. The achievable higher output power per bar will lead to enhanced applicability of HPDLs and thus to a further steep increase of their industrial use. Improved packaging technology, multiplexing the emission of single bars and coherent coupling as well as promising new diode laser structures like Z-shaped broad area emitters, is discussed. In this paper, emphasis is laid on the potential applicability of commercial HPDLs for metal working with elevated intensities up to 10/sup 5/ W cm/sup -2/, like oxygen cutting and the worldwide first deep-penetration HPDL-weld up to a sheet thickness of 6 mm in stainless steel. These results have been predicted by proper theoretical modeling. Strong reduction of phase space dimension takes place in convective-diffusive-type free boundary problems typical for thermal processing. This property makes it possible to construct approximate finite-dimensional dynamical systems being solvable with controlled error. Numerical solutions of the full problem are used to investigate the quality of the approximate model. Observable quantities of the technical processes like signals from monitoring devices are part of the solution and solutions to the inverse problem are given.     

Corneal refractive surgery with femtosecond lasers

We investigated the use of ultrashort pulsed (femtosecond) laser technology in corneal refractive surgery. When compared to longer pulsewidth nanosecond or picosecond laser pulses, femtosecond laser-tissue interactions are characterized by significantly smaller and more deterministic photodisruptive energy thresholds, as well as reduced shock waves and smaller cavitation bubbles. We utilized a highly reliable all-solid-state femtosecond laser system for all studies to demonstrate practicality in real-world operating conditions. Contiguous tissue effects were achieved by scanning a 5-μm focused laser spot below the corneal surface at pulse energies of approximately 2-4 μJ. A variety of scanning patterns was used to perform three prototype procedures in animal eyes; corneal flap cutting, keratomileusis, and intrastromal vision correction. Superior dissection and surface quality results were obtained for lamellar procedures (corneal flap cutting and keratornileusis). Preliminary in vivo studies of intrastromal vision correction suggest that consistent refractive changes can also be achieved with this method. We conclude that femtosecond laser technology may be able to perform a variety of corneal refractive procedures with high precision, offering advantages over current mechanical and laser devices and techniques     

Utility experience with digital excitation systems

Ontario Hydro, a Canadian utility with an installed capacity of over 30000 MW, has utilized static excitation systems on a large number of generators, ranging from 3 MVA to 1100 MVA in size, since the early 1960's. Ontario Hydro has always attempted to take advantage of excitation system control as a means of eliminating or reducing stability constraints on their operation. The advent of digital excitation systems offered us the opportunity to work with manufacturers to incorporate special features or controls which had previously required complex additional hardware. This paper discusses the authors' experience with these systems, from 1988 to date, as well as providing a list of items which merit consideration by utilities and manufacturers as they move toward the second generation of digitally-implemented systems     

Review of biosensing with whispering-gallery mode lasers

Lasers are the pillars of modern optics and sensing.Microlasers based on whispering-gallery modes(WGMs)are miniature in size and have excellent lasing characteristics suitable for biosensing.WGM lasers have been used for labelfree detection of single virus particles,detection of molecular electrostatic changes at biointerfaces,and barcode-type live-cell tagging and tracking.The most recent advances in biosensing with WGM microlasers are described in this review.We cover the basic concepts of WGM resonators,the integration of gain media into various active WGM sensors and devices,and the cutting-edge advances in photonic devices for micro-and nanoprobing of biological samples that can be integrated with WGM lasers.     

Mode-locking ultrafast solid-state lasers with saturable Braggreflectors

We discuss mode-locking of low-gain solid-state lasers using a semiconductor saturable Bragg reflector structure. This recently developed low-loss mode-locking device consists of a single quantum well which acts as a saturable absorber incorporated into a high-reflectivity Bragg mirror. Highly stable mode-locking in solid-state lasers results from an ultrafast transient reflectivity in the device that is caused by saturation of the excitonic absorption in near-resonant conditions     

Short-wavelength capillary lasers with tunable spatial coherence

We present a method to continuously tune the degree of spatial coherence of a short-wavelength laser. The method is based on the optical properties of the hollow-waveguide Fabry-Perot resonators with external mirrors. The potential of the technique is demonstrated experimentally on a capillary nitrogen laser (λ=337.1 nm). The results of the work can be used for the construction of short-wavelength lasers with tunable spatial coherence     

瞬态电压抑制器在快上升沿电磁脉冲作用下的瞬态响应

为了研究瞬态电压抑制器(TVS)对快上升沿电磁脉冲的响应,基于传输线脉冲(TLP)测试理论,建立了由高频脉冲发生器、同轴电缆、专用测试夹具、30dB脉冲衰减器和高性能的数字存储示波器等组成的测试系统。参考时域传输(TDT)TLP测试方法,利用该测试系统,对典型双极TVS器件在快上升沿电磁脉冲作用下的电流特性和电压特性进行了研究,并对试验数据进行了拟合分析。结果表明,TVS器件的箝位电压、峰值电流等参数均随着测试电压的增大而增大,且与测试电压呈线性关系;而由箝位电压决定的箝位响应时间受测试电压的影响较小。经对比分析实验结果得出,对快沿电磁脉冲,采用固定响应时间下的箝位电压来衡量TVS器件的防护性能是较为合理的。