Transient and steady-state electrostrictive laser beam trapping

Electrostriction is a cause of laser beam self-trapping and damage in transparent glass. It predominates over other self-focusing mechanisms, such as Kerr effect or thermal self-focusing, under certain conditions. This paper shows that self-focusing can occur even if the laser pulse is so short that no steady state is achieved, provided the pulse power is large enough. In the steady state, the threshold power for self-focusing is independent of beam size. In the transient region, the threshold power increases and becomes proportional to the square of the initial beam radius, in the limit of large radius beams or short pulses. The theoretical trapping threshold derived here is less than typical experimental thresholds for track formation in glass.     

6A2--The influence of self-focusing on the stimulated Brillouin, Raman, and Rayleigh effects

In liquids with optically anisotropic molecules the largest nonlinear polarization, cubic in the field amplitudes, corresponds to the intensity dependent index of refraction. The resulting self-focusing of the laser beam is in turn responsible for the anomalous gain of the stimulated Raman emission. The stimulated Brillouin effect in these self-focusing fluids has the same threshold condition as the stimulated Raman effect. In both cases the condition is that self-focusing occurs within the cell length. The anomalous frequency broadening in these same liquids depends on the parameter(omega_{L} - omega'_{L})tau_{c}, where ωLandomega'_{L}are two frequencies occurring in the laser beam and τcis a correlation time for molecular reorientation. In agreement with this theory, more broadening was observed at higher temperatures, although the Raman gain has the opposite trend.     

Self-focusing as a pulse sharpening mechanism

The simplest aspect of a time-dependent analysis of self-focusing is a pronounced sharpening of the on-axis intensity pulse at distances less than the self-focusing length. We present a simple mathematical analysis of this effect and indicate its magnitude for a typical input pulse.     

Theory and simulation on the threshold of water breakdown inducedby focused ultrashort laser pulses

A comprehensive model is developed for focused pulse propagation in water. The model incorporates self-focusing, group velocity dispersion, and laser-induced breakdown in which an electron plasma is generated via cascade and multiphoton ionization processes. The laser-induced breakdown is studied first without considering self-focusing to give a breakdown threshold of the light intensity, which compares favorably with existing experimental results. The simple study also yields the threshold dependence on pulse duration and input spot size, thus providing a framework to view the results of numerical simulations of the full model. The simulations establish the breakdown threshold in input power and reveal qualitatively different behavior for picoand femto-second pulses. For longer pulses, the cascade process provides the breakdown mechanism, while for shorter pulses the cooperation between the self-focusing and the multiphoton plasma generation dominates the breakdown threshold     

Self-focusing during femtosecond micromachining of silicate glasses

Many recent investigations of micromachining with lasers, in vacuum and in ambient air environments, have demonstrated the improvements possible when using femtosecond-duration laser pulses compared with long laser pulses. There are obvious practical advantages for rapid micromachining in ambient air conditions. However, the maximum laser intensity and repetition rate are then eventually limited by the avalanche breakdown and nonlinear effects in the air through which the focused laser beam must propagate both outside the work piece and within the structure that is being machined. This paper investigates these limits in femtosecond deep hole drilling at high laser intensities in silicate glasses. In particular, it shows how nonlinear optical effects, particularly self-focusing, can dramatically affect hole shape and the rate of penetration during deep hole drilling. The experiments described here demonstrate how nonlinear Kerr focusing of femtosecond laser pulses occurs during propagation of intense femtosecond laser pulses through the atmosphere within the machined channel at powers levels significantly below the critical power for self-focusing in ambient air.     

Noise characteristics of stripe-geometry double-heterostructure junction lasers operating continuously--I. Intensity noise at room temperature

Measurements of the noise fluctuations present in the output intensity from stripe-geometry double-heterostructure junction lasers operating continuously at room temperature are reported. In some but not all the lasers studied, the low-frequency (50-MHz) fluctuations exhibit the quieting expected of an amplitude-stabilized oscillator operating above threshold. The intensity noise in these lasers becomes shot noise limited at currents about 10 percent above threshold, even when many longitudinal modes are oscillating. Additional measurements demonstrate explicitly the effective elimination of the wave-interaction, or excess, noise during the transit of the threshold region. However, in other lasers which are nominally similar, only a partial reduction of the excess noise occurs above threshold, resulting in a noise level which can be more than ten times the shot-noise limit. In addition, we find in all lasers studied thus far no quieting at high frequencies (4 GHz), in apparent contradiction to the behavior expected of a well-stabilized oscillator.     

Enhanced hot-carrier-degradation in LDD MOSFET's under pulsedstress

Hot-carrier degradation of lightly doped drain (LDD) MOSFET's under ac stress is investigated. Enhanced ac degradation occurs in LDD MOSFET's as well as in single-drain MOSFET's. However, there is a peculiar degradation mechanism in LDD MOSFET's. For single-drain MOSFET's, enhanced ac degradation appears in both threshold voltage and transconductance at stress drain voltages larger than a critical value. On the other hand, for LDD MOSFET's, although the enhanced degradation in threshold voltage and transconductance appears at stress drain voltages larger than a critical value, the enhanced degradation in transconductance appears even under stress drain voltages lower than the critical value. The difference in the ac-enhanced degradation between LDD and single-drain structures can be explained by a hot hole generated neutral-electron-trap model and the change in hot-hole-injected oxide region according to stress bias conditions     

高能激光大气传输中热晕与自聚焦的比较

通过分析激光在大气中传输所产生热晕效应和自聚焦效应的临界功率PTB和PK表明,在同样大气条件下,产生自聚焦的临界功率PK比产生热晕的临界功率PTB大约高5个量级。另外,分析了在同样激光功率下,不同宽度的脉冲激光在大气中传输时的热晕效应与自聚焦效应,一般情况下,微秒量级的脉冲激光在大气传输中的自聚焦效应可忽略,而对于超短飞秒脉冲激光,必须考虑自聚焦效应对传输的影响。     

Electrostrictive self-focusing of picosecond laser pulse trains

A nanosecond-long train of picosecond laser pulses theoretically can produce self-focusing by electrostriction, even though the trapping threshold for a single picosecond pulse is very high. The relative roles of electrostriction and various types of Kerr effect in picosecond pulse train self-focusing should be assessed.     

Nonlinear mechanisms of filamentation in broad-area semiconductorlasers

There are three nonlinear mechanisms that can lead to filamentation in broad-area semiconductor lasers: gain-saturation-induced changes in the refractive index through the linewidth-enhancement factor, self-focusing due to heat-induced index changes, and self-defocusing through intensity-dependent index changes in the cladding layer. We present a theoretical model to analyze these mechanisms and their relative roles in destabilizing the laser output. We find that there exists a critical value for the linewidth-enhancement factor below which broad area lasers are stable for wide stripe widths (as wide as 250 μm) and high pumping levels (as high as 20 times threshold). We also find that broad-area lasers are less susceptible to filamentation through self-defocusing and show how an intensity-dependent index in the cladding layer may be used to suppress filamentation caused by the linewidth-enhancement factor     

Current dependence of pulsations in semiconductor lasers due to photon induced modulation of optical loss

Pulsations in semiconductor lasers are frequently observed to occur within a range of injection currents above threshold. A model is proposed to account for the existence of the current range and attributes it to a critical optical power level at which photon induced modulation of optical loss takes place, such as due to the effect of self-focusing or lateral mode deformation. This value is estimated to be0.36 times 10^{15}photons- cm^-3for double heterostructure oxide-striped lasers.     

Effects of nonlinear propagation on laser focusing properties

In a high-power laser system both the nonlinear growth of small-scale spatial irregularities on a pulse (beam breakup) and the accumulation of self-induced phase-front distortion on the spatial envelope of the pulse (whole-beam self-focusing) alter the focusing properties of the system output. We present experimental results which show simultaneously and in detail the influence of these two effects on the spatial and temporal profiles of the focused output of a simple system. in an extreme case the focused on-axis intensity is observed to be reduced by beam breakup to 20 percent of the value obtained in the absence of breakup. Diffraction code calculations of whole-beam self-focusing, which in these experiments tends to enhance the focused on-axis intensity, are found to agree closely with the measured results and demonstrate that whole-beam self-focusing effects depend strongly on the plane being observed.     

Analysis of lateral-mode behavior in broad-area InGaN quantum-welllasers

A wave-optical model that is coupled to a microscopic gain theory is used to investigate lateral mode behavior in group-III nitride quantum-well lasers. Beam filamentation due to self-focusing in the gain medium is found to limit fundamental-mode output to narrow stripe lasers or to operation close to lasing threshold. Differences between nitride and conventional near-infrared semiconductor lasers arise because of band structure differences, in particular, the presence of a strong quantum-confined Stark effect in the former. Increasing mirror reflectivities in plane-plane resonators to reduce lasing threshold current tends to exacerbate the filamentation problem. On the other hand, a negative-branch unstable resonator is found to mitigate filament effects, enabling fundamental-mode operation far above threshold in broad-area lasers     

非均匀辛格相关蓝绿激光波束通过海洋湍流的传输特性

根据广义惠更斯-菲涅尔原理,建立了非均匀辛格相关蓝绿激光波束在海洋湍流中的传输模型。基于交叉谱密度函数,讨论了不同传播距离下波束光强变化。数值计算了波束光强和光强最大值横向偏移受海洋湍流参数的影响。结果表明,传播距离和海洋湍流参数对非均匀辛格相关蓝绿激光波束的光强自聚焦现象有一定影响。当传播距离一定时,温度均方耗散率对光强自聚焦的影响大于湍流动能耗散率和温度盐度波动相对强度。     

Breakdown and self-focusing effects in gases produced by means of a single-mode ruby laser

Using a passivelyQ-switched ruby laser, operating in a single axial and transverse mode, sparks produced in a number of gases at pressures ranging from 760 to 9000 mmHg have been studied. Breakdown threshold measurements for both single and multimode laser radiation have been made and in addition, the characteristics of the sparks produced with single-mode radiation have been investigated. Photography at 90° of the scattered laser light showed the existence of scattering regions with transverse dimensions not exceeding the 5-μ resolution of the optical system. Furthermore, a large amount of laser light is scattered in the forward direction. The intensity, angular distribution, and spectral characteristics of this scattered radiation have been determined. Various possible mechanisms that could account for these phenomena are discussed, and it is concluded that self-focusing of the laser beam after the initiation of the breakdown process may be occurring.     

Capacity and mutual information of wideband multipath fadingchannels

We investigate the capacity and mutual information of a broadband fading channel consisting of a finite number of time-varying paths. We show that the capacity of the channel in the wideband limit is the same as that of a wideband Gaussian channel with the same average received power. However, the input signals needed to achieve the capacity must be “peaky” in time or frequency. In particular, we show that if white-like signals are used instead (as is common in spread-spectrum systems), the mutual information is inversely proportional to the number of resolvable paths L˜ with energy spread out, and in fact approaches 0 as the number of paths gets large. This is true even when the paths are assumed to be tracked perfectly at the receiver. A critical parameter L˜_crit is defined in terms of system parameters to delineate the threshold on L over which such overspreading phenomenon occurs     

飞秒强激光的拉曼延迟克尔非线性效应研究

飞秒强脉冲激光具有高功率、窄脉宽和宽光谱等特点,其在大气传输过程中涉及很多线性和非线性的物理机制,高阶效应对传输将产生一系列重要的影响.采用数值方法求解包含拉曼延迟响应的非线性传输方程,研究了与拉曼延迟响应时间相当的、具有几十飞秒脉宽的飞秒强激光大气传输中拉曼延迟克尔非线性效应引起的非线性折射率变化、自聚焦和光谱展宽特...     

高清晰度粒子全息再现像的获取方法

为了掌握同轴粒子场全息诊断中再现系统参量,如放大率、光强、图像采集设备及背景光等因素对粒子再现像质量的影响,开展了系列的实验研究.由实验可知,调节再现光的强度,使背景光强尽量接近CCD阈值下限,可以得到背景噪声均匀、再现清晰度高的图像;而对于粒径在20靘以下的较小粒子及具有一定放大率的再现系统,则需要通过适当增加再现光强度来得到高清晰度的图像;保持实验室的暗背景环境对图像采集是非常有利的.结果表明,此方法可以在粒子场全息再现过程中获得高清晰度粒子再现像.     

Instability of semiconductor lasers due to optical feedback from distant reflectors

We explain an istability occurring in continuously operating lasers due to moderate feedback from distant reflectors. This instability occurs despite the fact that the laser is stable with respect to small deviations from steady-state operation. It is the result of finite phase and carrier number changes caused by fluctuations in spontaneous emission. We predict several properties that agree with recent experimental observations: 1) the instability only occurs when the laser reaches a steady state that maximizes coherent feedback and laser light intensity; 2) the instability vanishes at strong feedback levels; and 3) at moderate feedback levels, the laser will be nearly stable at threshold, but unstable when operated well above threshold. The latter behavior results in a nonlinear "kinked" shape in the light versus current relation.     

8A2--The self-focusing of light of different polarizations

In an effort to verify the hypothesis that molecular reorientation (ac Kerr effect) is responsible for the self-focusing of light beams in certain liquids, we have measured and compared the thresholds and other characteristics of self-focusing for circularly and linearly polarized beams incident on these liquids. We show that for plane waves propagating in a homogeneous, isotropic, ensemble of molecules having anisotropic polarizability tensors, the nonlinear index should be four times as great for linearly as for circularly polarized waves. The hope that this difference in indexes would be reflected in a four-fold increase in the threshold power for self-trapping when circularly instead of linearly polarized light is incident was not realized. In practice, the increase was always found to be much less. However, in every case studied, the trapped light from a beam, circularly polarized to better than 1 part in 200, was markedly, if not completely, depolarized as soon as self-trapping could be detected. We show qualitatively that this polarization instability should exist for all but linearly polarized light and for a variety of nonlinear mechanisms. However, in the absence of even an approximate quantitative theory of the self-trapping of light that is not linearly polarized, the comparative measurements of thresholds cannot be said either to verify or disprove the hypothesis of molecular reorientation.