板条固体激光器的三维瞬态热分布

本文建立了有限几何尺寸板条激光介质的三维温度分布模型.在普遍情况下,推导出温度分布的解析式.由此便于分析板条温度分布和泵浦参数、激光介质等的依赖关系.借助计算机进行的数值计算,证实了理论结果.     

A theoretical study on the melting of a finite slab with a pulsed laser

Calculations of the spatial and temporal temperature distributions in the molten layer thickness, the still solid part and in the backward surface of the finite slab were carried out during the irradiation with a pulsed laser. The two-dimensional Laplace integral transform technique has been applied to obtain the mathematical expressions for these temperature distributions and the molten layer thickness as a function of the melting time. The derivations have taken into account the temperature-dependent absorption coefficient of the irradiated surface and the cooling. As an illustrative example, computations were carried out on a finite aluminum (Al) target.     

板条固体激光器热稳特性的研究

本文报道板条激光器的热稳运行,在150W至10000W的泵浦功率下输出发散角基本保持不变,最大输出功率大于150W;并绘出了腔内插入柱面镜,以压缩输出激光发散角的初步结果.     

Pulsed microwave damage trend of bipolar transistor as a function of pulse parameters

In the present paper we conduct a theoretical study of the thermal accumulation effect of a typical bipolar transistor caused by high power pulsed microwave (HPM), and investigate the thermal accumulation effect as a function of pulse repetition frequency (PRF) and duty cycle. A study of the damage mechanism of the device is carried out from the variation analysis of the distribution of the electric field and the current density. The result shows that the accumulation temperature increases with PRF increasing and the threshold for the transistor is about 2 kHz. The response of the peak temperature induced by the injected single pulses indicates that the falling time is much longer than the rising time. Adopting the fitting method, the relationship between the peak temperature and the time during the rising edge and that between the peak temperature and the time during the falling edge are obtained. Moreover, the accumulation temperature decreases with duty cycle increasing for a certain mean power.     

A theoretical study of the evaporation induced by a pulsed laser in a finite slab

The two-dimensional integral Laplace transform technique has been applied to get the temperature distributions in the molten layer, the solid part and in the backward surface of a finite target irradiated with a pulsed laser. Formulas for the time dependence of the evaporated part and the molten layer thicknesses of the target were found. This is accomplished by considering the temperature dependence of the absorption coefficient of the irradiated surface as well as the chemical reaction. As an illustrative example computations were carried out on an aluminum (Al) target.     

The pulsed microwave damage trend of a bipolar transistor as a function of pulse parameters

In the present paper we conduct a theoretical study of the thermal accumulation effect of a typical bipolar transistor caused by high power pulsed microwaves(HPMs),and investigate the thermal accumulation effect as a function of pulse repetition frequency(PRF) and duty cycle.A study of the damage mechanism of the device is carried out from the variation analysis of the distribution of the electric field and the current density.The result shows that the accumulation temperature increases with PRF increasing and the threshold for the transistor is about 2 kHz.The response of the peak temperature induced by the injected single pulses indicates that the falling time is much longer than the rising time.Adopting the fitting method,the relationship between the peak temperature and the time during the rising edge and that between the peak temperature and the time during the falling edge are obtained.Moreover,the accumulation temperature decreases with duty cycle increasing for a certain mean power.     

Laser pulse optimization for practical laser drilling

Laser drilling is a common commercially developed technique for material processing. From the application viewpoint, it is the end product for a laser system, for instance a drilled hole, that matters. Laser pulse profile is the most important parameter controlling the laser hole drilling process. An efficient and practical method is therefore needed to develop a relationship between the pulse parameters and the depth of hole produced in a known material. In the present study, dimensionless groups are developed to optimize laser pulse parameters to give information on workpiece materials. Consequently, an optimal laser pulse for drilling an aluminum workpiece is predicted.     

超短脉冲染料放大器中的参数研究

对超短脉冲染料放大器(DLA)中放大的自发辐射(ASE)随泵浦强度变化的关系进行了实验研究,为了求得具有高储能、低自发辐射背景噪声的染料激光放大器的泵浦强度,我们用速率方程对实验结果进行了解析分析,获得了染料激光放大器的泵浦强度与染料参数(浓度、增益长度等)的近似关系式,以及相应条件下染料激光放大器储能和小信号增益的表达式,并与实验进行了比较.二者符合较好,从而为染料激光放大器的设计提供了参考.     

The thermal effect in a grazing-incidence slab laser with the novel composite cooling method

The thermal properties of the grazing-incidence slab laser are investigated when cooling with a novel composite method, both conducted cooling and air cooling. The 3D temperature distribution and the correspondent thermal lensing effect were analytically modeled and numerically calculated. The absorption saturation and stress-optic effect were taken into account in the modeling. The influence of the parameters of the laser on the thermal lensing effect were investigated in detail. The results show a lower temperature and weaker thermal lensing effect for the composite cooling than for the traditional contaction cooling.     

Control of laser pulse parameters in a cavity with a thermal-nonlinearity shutter

The light pulse generation by a laser controlled by shutter from a finite-thickness absorbing layer with thermal nonlinearity has been studied theoretically. The layer placed in the cavity is oriented with respect to the mirror so that conditions for nonlinear total internal reflection of generated radiation are created. In the plane-wave approximation, a self-consistent system of nonlinear truncated equations for the field amplitudes together with constitutive equations has been obtained. Numerical simulation with the use of the obtained system has shown that it is possible to control the form and duration of the generated pulse by varying the absorption coefficient of the nonlinear medium and the angle of incidence on the absorbing layer, as well as the layer thickness.Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 71, No. 6, pp. 771–777, November–December, 2004.This revised version was published online in April 2005 with a corrected cover date.     

激光二极管双端直接抽运混合腔板条激光器

为减少热透镜效应的影响, 获得高光束质量的激光, 本文利用激光二极管阵列双端直接抽运Nd:YVO₄, 采用负支共焦折叠混合腔结构, 获得最高功率为416 W 1064 nm的激光输出, 光–光转换效率为54.3%, 斜效率为61.6%. 在输出功率为370 W时, 光束质量因子M²在非稳腔和稳腔方向上分别为3.9和4.7.     

Inverse estimation of the pulse parameters of a time-varying laser pulse to obtain desired temperature at the material surface

A proper selection of the pulse parameters is essential to achieve desired temperature at the material surface. This leads to obtain the required metallurgical changes in the surface vicinity when a time-varying laser pulse is used in a heating process such as surface modification. In this paper, the conjugate gradient method (CGM) for parameter estimation is successfully applied to estimate the unknown laser pulse parameters for those purposes during laser heating process. The determination of the pulse parameters is treated as a one-dimensional, transient, non-linear inverse heat conduction problem (IHCP). Based on a sensitivity analysis, the inverse problem is solved as an optimization problem comparing a desired temperature at the surface and a calculated one where the objective function is minimized by CGM. The method has been applied to a test case of a heating process on steel, appropriate pulse parameters and desired temperature distribution can also be returned.     

Theoretical investigation of laser pulse width dependence in a thermal confinement regime

Previous molecular dynamics (MD) simulations of ultraviolet (UV) laser ablation demonstrate the distinct dependence of material ejection on laser fluence and laser pulse duration. In this paper, we examine the pulse width dependence when the laser pulse widths are appropriate for the thermal confinement regime. We perform MD simulations of laser ablation with a laser pulse duration of 1 ns and compare with a pulse width of 150 ps as in previous simulations. The simulations confirm that the pulse width in thermal confinement regime does not dramatically influence the molecular ejection mechanism. The simulations reveal differentiations, however, in plume composition and the ablation threshold value. PACS 02.70.Ns; 61.80.Az; 79.20.Ap     

Laser diode partially end-pumped electro-optically Q-switched Yb:YAG slab laser

A laser diode partially end-pumped, electro-optically Q-switched, Yb:Y_3Al_5O_(12)(Yb:YAG) slab laser was reported. We obtained output energy of 14.6 mJ/pulse with a pulse width of 30 ns at a repetition frequency of 2 k Hz, and the corresponding peak power was 480 kW. The beam quality factors M~2 in the unstable direction and the stable direction was 1.32 and 1.25, respectively.     

Transfer function of thermal FM,FSK step response and the dip in the FM response of laser diodes

A transfer function of the thermal FM response of laser diodes is theoretically derived that is in excellent agreement with the experimentally measured FM response and agrees in general with an empirical expression reported earlier in the literature. The characteristic dip in the overall sinewave FM response and the time-domain stepresponse of the laser diode are analysed theoretically as a function of a thermal cutoff frequency and the ratio of thermal FM at d.c. to the carrier FM at d.c. From the step response analysis, the lower bound on the bit rate for penalty-free Alternate Mark Inversion FSK (AMI-FSK) transmission is derived which tallies well with the experimental observations. A first-order equalization network to correct the drooping nature of the FSK output pulse response is analysed theoretically.     

Time-resolved current response of a nanosecond laser pulse illuminated STM tip

Time-resolved dependence of the transient current through a ns laser pulse illuminated scanning tunneling microscope (STM) tip/sample gap in tunneling mode and out of tunneling range is presented. A self-designed fast STM-preamplifier (bandwidth 35 MHz) allows one to resolve the fine structure of the transient signal as well as the observation of some effects that are undetectable by using conventional low-band preamplifiers. The dependence of the threshold laser pulse intensity, which corresponds to the beginning of electron emission from tip (in non-tunneling mode), as a function of the tip/sample distance was investigated. At tip/sample distances from tunnel contact up to approximately 1 μm a linear dependence is found. This behavior is in good agreement with the theory for field enhancement in a STM tip/sample system. In tunneling mode a ns (fast component) as well as a μs (slow component) current response was found as a result of the laser pulse illumination. These data suggest the tip bending to be an important factor in clarifying the thermal/mechanical mechanism of laser-assisted surface nanomodification. Received: 4 May 1998 / Accepted: 29 January 1999 / Published online: 28 April 1999     

Entropy generation rate during laser pulse heating: Effect of laser pulse parameters on entropy generation rate

Laser pulse heating of solid surface and entropy generation during the heating process are considered. Time exponentially decaying pulse is accommodated in the analysis and the laser pulse parameter (β₁/β₂) resulting in minimum entropy generation rate is computed. Analytical solutions for temperature rise are presented and volumetric entropy generation rate is formulated. Two laser pulses resulting in low volumetric entropy generation rate are examined in detail and volumetric entropy generation rate is associated with the laser pulse parameter (β₁/β₂). It is found that volumetric entropy generation rate attains high values in the early heating period due to large (1/T²). Moreover, the laser pulse with high-peak intensity results in lower volumetric entropy generation rate than that corresponding to the low-intensity laser pulse with the same energy content.     

Laser irradiance of a homogeneous metal slab

Numerical calculations of the effect of laser irradiance was performed for three homogeneous slabs. The temperature profile and the temperature of the front surface for three metals, silver, steel, and aluminium was calculated. The formulation of the temperature distribution of the slab was carried out using Fourier series expansions for the case of time dependent laser irradiance. The exact expressions for the thermal penetration depth and the critical time required to initiate melting (i.e. damage) were found.