Analysis of second-harmonic generation by unamplified,high-repetition-rate, ultrashort laser pulses at Si(001) interfaces

State-of-the-art femtosecond lasers have the potential to dramatically improve the effectiveness of interface nonlinear optics for diagnosing Si(001) interface characteristics that are relevant to Si microelectronics manufacturing. We present an analysis of signal acquisition rate and parasitic surface heating for SHG by ultrashort laser pulses at Si(001) interfaces, emphasizing their dependence on the pulse duration, energy, repetition rate, wavelength and focal geometry of the pulses. The results of the analysis are illustrated by several experimental examples of SHG by a Ti: sapphire femtosecond laser from a buried Si(001)-SiO₂ interface or a Si(001) surface during chemical vapor deposition     

Improved parallelism measurement using a laser interferometer

The authors discuss an improved method for obtaining parallelism measurements of two straight bars in a plane, considering the specific problem of making two vertical surfaces parallel to each other to within 0.00005 of an inch over a 50 inch length. The authors compare the existing method of using dial indicators and a slider with that of using a laser interferometer, cable and pulley system, and a microcomputer. Possible errors this system may generate and solutions to help minimize them are presented. The procedure presented will help to minimize errors due to the operator such as `reading between the lines' on an indicator and hand applied pressure. These methods allow improved parallelism resolution over that obtainable with the indicator     

Phase-controlled amplification of few-cycle laser pulses

Intense ultrashort waveforms of light that can be produced with an exactly predetermined electromagnetic field are essential in a number of applications of extreme nonlinear optics, most prominently in laser-driven sources of high-energy attosecond radiation. Field reproducibility in each laser shot requires stabilization of the carrier-envelope phase. The authors analyze different schemes of phase-stable pulse amplification and identify constraints limiting the precision with which the phase can be maintained. Next, they describe a phase-stabilized laser system based on a 20-fs multipass Ti:sapphire amplifier supplemented with a fiber compression stage for producing pulses in the few-cycle regime. It is shown that the amplifier introduces only a slow millihertz phase drift and, therefore, can be seeded by a standard phase-stabilized oscillator. This residual phase drift is assigned primarily to the beam pointing instability and can be precompensated in the phase-control loop of the seed oscillator using a feedback signal from a phase detector placed in the amplifier output. The phase stability of the resultant 5-fs 400-/spl mu/J pulses at a 1-kHz repetition rate is subsequently independently verified by higher order harmonic generation, in which different carrier-envelope phase settings are shown, both theoretically and experimentally, to produce distinctly different spectral shapes of the XUV radiation. From a series of such spectral patterns, the authors succeed in calibrating the value of the carrier envelope phase (with a /spl plusmn//spl pi/ ambiguity), which in turn allows them to fully characterize the temporal structure of the electric field of the laser pulses. The estimated precision of the phase control on the XUV target is better than /spl pi//5, which reduces the timing jitter between the driving laser pulse and the XUV bursts to /spl sim/ 250 as and opens the way to generate stable isolated attosecond pulses.     

Measurement of 10-fs laser pulses

We report full characterization of the intensity and phase of ~10-fs optical pulses using second-harmonic-generation frequency-resolved-optical-gating (SHG FROG). We summarize the subtleties in such measurements, compare these measurements with predicted pulse shapes, and describe the implications of these measurements for the creation of even shorter pulses. We also discuss the problem of validating these measurements. Previous measurements of such short pulses using techniques such as autocorrelation have been difficult to validate because at best incomplete information is obtained and internal self-consistency checks are lacking. FROG measurements of these pulses, in contrast, can be validated, for several reasons. First, the complete pulse-shape information provided by FROG allows significantly better comparison of experimental data with theoretical models than do measurements of the autocorrelation trace of a pulse. Second, there exist internal self-consistency checks in FROG that are not present in other pulse-measurement techniques. Indeed, we show how to correct a FROG trace with systematic error using one of these checks     

Generation of attosecond X-ray laser pulses

We propose to generate as-X-ray laser pulses by beating of two or more X-ray laser lines with a frequency separation in the range of 10/sup 15/ Hz. We focus on nickel-like X-ray lasers, some of which have a few almost equidistant laser gain lines with an appropriate difference frequency. It is shown that in the case of three or more lines, these can be phase-locked by means of a Langmuir wave generated in the gain medium at a suitable electron density.     

仪表放大器在激光外差玻璃测厚系统中的应用

为了放大激光外差玻璃厚度测量系统中的微弱信号,采用由分立元件构成仪表放大器作为光电探测器前置放大器,解决了放大器与光电接收器之间阻抗匹配,并运用Multsim对电路进行仿真和安装调试.结果证明,放大电路的输出信号符合系统设计的预期要求.     

Optical parametric amplification of femtosecond ultraviolet laser pulses

Broad-band amplification of femtosecond laser pulses using the scheme of noncollinear optical chirped pulse parametric amplification is modeled. The effect of two-photon absorption at the pump wavelength was also taken into account. The signal pulses range from 220 to 410 nm with pump pulses at 267, 248, and 213 nm. The best four crystals chosen among 12 possible ones are BBO, KDP, CLBO, and LB4. In an experiment, 30-fs laser pulses at 400 nm were amplified in a BBO crystal pumped by 267 nm pulses, exhibiting a single pass gain of 3550. The gain was found spectrally flat within the available 17-nm bandwidth of the signal pulse.     

Changes in electrical impedance of biological matter due to the application of ultrashort high voltage pulses

Time domain-based impedance measurements were used to study the changes in electrical parameters of biological samples following the application of ultrashort high voltage (HV) pulses. Pulses with very short duration (300 ns) caused a significant drop in post pulse resistance of the plasma membrane only at high field strengths in excess of 20 kV/cm. The conductivity of the plasma membrane returned to almost pre-pulse values within less than 10 ms after the field was applied. Further steps of recovery, attributed to pore shrinking and resealing with an exponential decay of the conductivity, as is expected in electroporation, were not observed. An increase in medium conductance, as recorded minutes after the pulse, arises mainly from cell damage. Although pore formation is a possible effect of the high electric field, our results suggest further disturbance of the membrane-like micelle formation or even the creation of large defects, forced by mechanical tension within the cell membrane.     

VUV laser in the Lyman band of molecular hydrogen pumped by fstitanium-sapphire laser pulses

We report lasing at 160 nm in the Lyman band of molecular hydrogen. The laser is pumped by 200 mJ/150 fs pulses from the ATLAS titanium-sapphire laser at our institute. The pump pulses are focused at an angle of incidence of 60° onto a 9-cm-long gold target to a line focus, generating traveling-wave excitation. With 80 mbar of hydrogen in the target chamber we measure an average gain of 1.1 cm^-1 and achieve a total gain-length product of 10. The evaluation of the far-field pattern shows that the beam originates from a region with an electron density of 5×10¹⁵ cm^-3. A simple model of the H₂ laser is presented which explains the main part of our observations and supports a pump mechanism of photoelectron pumping     

Optical guiding of intense laser pulses by fast Z-pinch discharge

We have proposed the optical guiding of intense laser pulses by fast Z-pinch for channel-guided laser wakefield acceleration (LWFA). The method has been developed based on capillary discharge-pumped X-ray laser technique. A discharge through preionized helium gas driven by a current of 4.8 kA with a rise time of 15 ns proved able to produce a uniform guiding channel with good reproducibility, less than the time jitter of 1.8 ns. The observed guiding channel formation process was corroborated by 1D-MHD simulation. With this new guiding method, an intense Ti-sapphire laser pulse (λ = 790 nm, 2.2 TW, 90 fs, 1 × 10¹⁷ W/cm²) was transported through the channel over a distance of 2 cm, corresponding to 12.5 times the Rayleigh length. © 2001 Scripta Technica, Electr Eng Jpn, 136(3): 19–27, 2001     

Self-starting 6.5-fs pulses from a Ti:sapphire laser using asemiconductor saturable absorber and double chirped mirrors

We demonstrate self-starting 6.5-fs pulses from a Kerr-lens-mode-locked Ti:sapphire laser with an average output power of 200 mW at a pulse repetition rate of 86 MHz. We have achieved a mode-locking buildup time of only 60 μs, using a broad-band semiconductor saturable absorber mirror to initiate the pulse formation. The dispersion has been compensated with a prism pair in combination with improved double-chirped mirrors. The prism pair allows for the flexible adjustment of both the duration and the center wavelength of the pulse. The double-chirped mirrors show a high reflectivity better than 99.8% over the full bandwidth of 300 nm and a controlled group delay over more than 250 nm. The choice of a proper output coupler turns out to be critical for ultrashort pulse generation directly from the laser     

Generation and spatiotemporal evolution of optical vortices in femtosecond laser pulses

Topology has become one of the key concepts allowing one to understand the intrinsic, qualitative properties of phenomena throughout various scientific fields. To date, this concept has been extended to the field of material science and technology. On the other hand, we can now utilize the spatially controlled light defined by the topology (so‐called “optical vortices”) in order to characterize the topological properties of materials. In particular, optical vortices in femtosecond pulses will be invaluable for advanced topological spectroscopy. In this work, the authors created femtosecond optical vortices using a spatial light modulator. Their spatiotemporal properties were evaluated using interferogram and correlation measurements. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(4): 39–46, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20791     

激光平面测量系统的研究

介绍了一种可用于精密平面测量的新方法,以柱面透镜产生的激光平面作为基准平面,凭借激光平面的稳定性进行系统的精密测量。先利用激光平面产生系统将点光源变成线光源,即把线状光束变成光平面,位置敏感元件PSD安装在被测平面的测量靶上。测量靶一边移动,PSD一边拾取激光平面信号,经放大滤波、A/D转换、单片机（MCU）数据处理后,将结果输送到显示屏（LED）。     

Mechanism for self-synchronization of femtosecond pulses in atwo-color Ti:sapphire laser

An experimental and theoretical analysis of the nonlinear coupling mechanism between the two solitary pulses circulating in a two-color femtosecond laser is presented. Two operation regimes; synchronized; and nonsynchronized; and a hysteresis of the transition between the two regimes are clearly observed; while independent modelocking and tunability of the output pulse trains is found in both regimes. Pulses in the range from 15 to 100 fs are synchronized with a timing jitter below 2 fs. The combined effects of cross-phase modulation and negative group velocity dispersion are shown to be responsible for the strong pulse correlation in the synchronized regime. Our experimental observations are in agreement with numerical simulations, thus confirming the theoretical model     

Development of a laser radar using LDS dye and sum frequency generation for NO2 measurement

A laser radar for measuring vertical concentration profiles of atmospheric NO₂ was developed. The laser transmitter uses an LDS765 dye laser pumped by a Nd:YAG laser. The dye laser output is converted to wavelengths 446.8 and 448.1 nm, which are used for NO₂ measurement, by sum frequency generation (SFG) using a pair of KD^*P crystals. The dependence of the LDS765 dye laser output on wavelength and dye concentration was investigated, and the performance of the laser transmitter was evaluated. Compared to the Coumarin 445 dye laser conventionally used for NO₂ measurement, this laser transmitter represents a significant improvement in output stability, as well as in maximum output energy and overall conversion efficiency. Using the newly developed laser radar, NO₂ concentrations of 20 to 40 ppb were measured for vertical range 500 to 1000 m, which were in rough agreement with values monitored on the ground. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 144(2): 26–33, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10231     

Real-time measurement of mean frequency in two-Machine system during power swings

This paper presents an algorithm for measuring mean frequency in two-machine system during power swings, which applies to sampled current signals in transmission lines with two sinusoidal components. The algorithm is based on rigorous mathematical deducing and recombines itself with Newton iterative technique and Taylor series expansion. The response of the algorithm is provided and the effects of key parameters, that affect the performance of the algorithm, are discussed. Computation requirements are modest and the technique has been implemented on a modern digital signal processor. The proposed technique has also been extensively tested using current signals obtained from dynamic frequency sources. Some test results are presented in the paper.     

Real-time control of power systems using nodal prices

This article presents a novel control scheme for achieving optimal power balancing and congestion management in electrical power systems via nodal prices. We develop a dynamic controller that guarantees economically optimal steady-state operation while respecting all line flow constraints in steady-state. A benchmark example illustrates the effectiveness of the proposed control scheme.     

Time-development of transient-carrier temperature, density, andgain spectrum in ultrashort optical pulse excited InGaAsmultiquantum-well laser structure

We present a novel transient gain-spectra measurement method based on the traditional variable pump-stripe technique. Using the pump-stripe technique with ultra-short optical pulse excitation, time-resolved amplified spontaneous emission spectroscopy of an InGaAs-InGaAsP multiquantum-well (MQW) laser structure was measured, and time-development of the transient optical net gain spectra was obtained accordingly. By fitting the measured gain spectra with a model for band-to-band transitions including momentum conservation and an energy- and density-dependent lifetime broadening, dynamics of band filling, carrier density, carrier temperature and bandgap renormalization have been obtained. This opens the possibility to study simultaneously the influence of transient-carrier density and, in particular, transient-carrier temperature on the transient optical gain. Strong gain compression in the whole gain-spectra region due to transient high carrier temperature after ultra-short pulse injection is clearly demonstrated for the first time     

用于主动减振的振动信号实时高精度检测

为了满足主动减振对振动信号实时高精度检测的需求,提出了一种自适应的振动信号频率、相位和幅值的在线检测方法。首先采用自适应陷波器实时估计振动信号频率,为降低噪声对频率估计精度的影响,对振动信号进行低通滤波预处理;接着利用检测出的频率生成参考正弦信号,通过最小均方算法（1east mean—square algorithm,LMS）提取出经滤波处理后的振动信号;最后,利用检测出的信号频率和已知的低通滤波器信息,还原出无噪声干扰的原始振动信号。根据主动减振应用中振动信号的特点,设计了一组检测仿真实验,实验结果显示本文方法可以实现对频率和相位的无偏估计,幅值的检测精度优于99％,可以满足主动减振应用的要求。     

Contrast-modified gradient echo imaging using rotary echo preparatory pulses

The use of on-resonance binomial composite pulses in two- or three-dimensional magnetization-prepared gradient-recalled echo magnetic resonance imaging experiments generates rotary echoes, leading to an increase in contrast range that is, in part, determined by the ratio ofT ₂ toT ₁. In comparison with other fast gradient-recalled echo imaging techniques designed for enhancedT ₂ contrast, this method is more robust with respect to radiofrequency field inhomogeneity and less sensitive with respect to motion artifacts. Three-dimensional parametric images may be calculated using least-squares fitting based on a simple model for steady-state longitudinal magnetization during the imaging sequences.