激光在医学中的应用

激光和普通光源相比,具有单色性、方向性和相干性好及能量集中等优越性能,所以激光一问世,就引起医学界的高度重视．激光技术近年来被广泛地应用于临床医学,目前临床上利用激光对一些疾病进行诊断和治疗的范围日益扩大,效果显著．医学临床应用激光主要用于以下几方面：１　激光手术１．１　切割术由于聚集后的激光束,在光点处可以产生高温和高压．照射到机体组织后,在很短的时间内就可使组织凝结、碳化、汽化和烧灼．因此,当光束以一定的速度移动时,就可以连续的切开组织,医生利用激光束作为“手术刀”,可以进行各种手术．由于激…     

激光与固体靶面烧蚀等离子体的能量耦合计算

 强激光辐照下固体靶表面迅速汽化产生靶蒸气等离子体,激光穿过等离子体区到达固体靶表面的过程就是激光束与等离子体的能量耦合与交换过程。采用具有五阶精度的WENO差分格式和简易等离子体状态方程模型对激光与等离子体相互作用的复杂物理过程进行了数值计算,分析了激光束能量在等离子体区中的吸收、屏蔽效应等动态耦合规律以及激光支持等离子体前驱冲击波传播。数值模拟结果表明：激光能量是支持靶面等离子体运动的唯一原因,能量屏蔽效应对激光与等离子体能量耦合有很大影响,通过控制激光脉冲宽度,可以合理调节屏蔽效应的影响。     

激光与靶相互作用体汽化模型的数值计算方法

利用克劳修斯-克拉珀龙(Clausius Clapeyron)方程描述相变过程,利用von Neumann差分方法构造辐射流体动力学方程组的计算方法,数值求解了激光辐照金属材料的热效应、汽化过程和激光维持爆震波,并同作者前期的工作作了比较.结果说明,该计算方法对激光与靶材料相互作用体汽化模型的数值计算是行之有效的,且该汽化模型为相关问题的二维、三维问题的数值计算提供了简便有效的途径.     

盲反卷积方法在水下激光图像复原中的应用

由于水体对激光存在着吸收和散射效应,距离选通水下激光成像系统所获得的图像存在不同程度的劣化问题,具有信噪比低、边缘模糊等特点。为提高图像质量,在分析水下激光成像劣化过程的基础上,研究了水下激光图像的基本噪声特征,并结合点扩展函数和调制传递函数,利用威尔斯小角度近似理论,将盲反卷积方法应用到水下激光图像复原中。在进行盲反卷积图像复原时,比较和讨论了将原始图像和经过降噪处理后的图像分别作为初始输入的处理结果;并对当人为改变调制传递函数和点扩展函数时所得到的图像复原结果进行了研究和讨论。处理结果表明该方法能达到抑制背景噪声、突出目标细节、提高对比度的效果,对水下激光图像增强十分有效。     

激光辐照下金属靶表面汽化现象研究

用高速纹影诊断技术研究激光辐照下金属表面发生汽化的特征及蒸气发展运动过程,结合汽化模型给出了凝聚相表面温度与激光入射功率密度的关系。由实验得到了不同入射激光功率密度下汽化表面(凝聚相)温度,蒸气羽阵面速度,前驱空气冲击波速度。     

核燃料棒反光表面条纹自适应中心提取方法

针对水下检测中激光条纹中心难以准确提取的问题,提出了一种适用于水下核燃料棒反光表面的条纹自适应中心提取方法.根据检测环境中存在水体散射及物体表面高反光的特点,去除水下噪点、反光噪点,实现激光条纹分割提取;充分利用BP神经网络曲线拟合及由光条几何信息生成的自适应卷积模板,实现反光区域的轮廓与灰度分布修正,使光条截面灰度分...     

激光等离子体特征参量和辐射耦合效应研究

采用一维单流体双温度辐射流体力学模型，在激光产生的等离子体做等温膨胀的假定下，研究了脉冲强激光辐照固态靶材时，计及再辐射后汽化等离子体特征参量与靶材物质和激光参数之间的定标关系，计算了等离子体再辐射对激光－靶材耦合效应的影响。     

约束层对激光驱动冲击波压力影响机理的理论研究

针对刚性约束层、柔性约束层以及液体约束层，从激光诱导冲击波阵面状态、汽化物(包括气体和等离子体)扩散以及冲击波的反射进行分析，发现对于脉宽小于冲击波通过汽化物层的时间间隔的短脉冲激光，约束层并不能直接提高冲击波的冲量，而对于脉宽大于冲击波通过汽化层时间间隔的激光，其增强冲击效果是通过约束汽化物的扩散，提高压力幅值和由于冲击波在约束层与工件表面的多次反射而延长对工件的作用时间来实现的.刚性约束层能最大地增加冲击冲量，而柔性约束层和液体约束层的主要优点是其形状可与非平面形工件表面符合. 关键词： 激光 约束层 扩散 反射波     

基于激光偏振特性的水下声信号探测方法研究

水下航行体辐射的声波传播到水-空气分界面上时,会在水表面形成微扰,使水表面元产生微倾角.提出一种利用激光偏振特性对水下声信号进行探测的方法,不直接利用散射光强信息,而是利用激光在布儒斯特角附近入射水表面时,反射光偏振特性随入射角剧烈变化的特点对水表面微扰进行高精度探测.建立了该方法的理论模型,得到了入射波、出射波垂直分...     

激光水中衰减系数的比较测量法研究

研究了激光水下衰减系数的比较测量法,通过在激光传输路径上进行多点测量激光能量或功率,经过数据拟合获得准确的激光水中衰减系数,此方法可以消除一般测量方法容器侧壁的反射及吸收的影响,并可在较长距离测量激光传输路径上不同位置的衰减系数。在实验室中测量了脉冲绿光、连续绿光、连续蓝光在自来水中衰减系数并进行了比较,结果表明蓝光在水中的衰减系数大于绿光。对水下光通信及水下目标探测具有一定参考价值。     

Erbium:YAG (2.94 mum) laser effects on dental tissues

Past trials with soft and calcified tissues have demonstrated that long pulse train (2.5 mus) Er:YAG (2.94 mum) laser may be used to ablate tooth structure of human teeth. Determination of physical and thermal damage to surrounding tissue during removal of enamel and dentin is a primary objective of this study. Extracted human teeth with thermal probes imbedded in the pulp chambers were submitted to cavity preparation using an Erbium YAG laser with water mist. Wavelength selection as well as use of a water mist during the procedure resulted in efficient tissue removal without significant surrounding damage. Ground sections and SEM sections of teeth showed little or no melting or ash formation in adjacent dentin and enamel and no visible change in the pulp chamber. The surfaces produced by laser ablation were rough and irregular with craters and grooves. Average temperature change in the pulp chamber monitored during tooth preparation was 2.2 degrees Centigrade. These findings suggest that constantly available water aids vaporization and microexplosions, increasing the efficiency of tooth structure removal, and aids in cooling of the tooth structures. The long pulse Er:YAG (2.94 mum) laser may be an effective method for tooth reduction applications when used with a water mist.     

Ultrafast solid–liquid–vapor phase change of a gold film induced by pico- to femtosecond lasers

Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature, deeper melting and ablation depths.     

Driving mechanism of keyhole evolution during multi-pulse drilling with a millisecond laser

To better understand the physical processes of multi-pulse laser drilling, this study investigates the keyhole evolution and its driving mechanism in a time-resolved observation system. The evolution characteristics suggested a two-phase process of rapid penetration followed by moderate penetration. As revealed in the ejection and vaporization behavior, the keyhole evolution was dominated by ejection and vaporization during the rapid and moderate penetration stages, respectively. In a single laser-pulsed drilling experiment, the driving mechanism itself was found to be affected by the dimensionless laser power density. The effect of dimensionless laser power density on depth increment was then discussed by comparing the experimental observations with numerical simulation results. The results further confirmed the driving mechanism of the keyhole evolution. The results in this paper are useful for understanding the driving mechanism of the keyhole evolution during multi-pulse laser drilling.     

Single-walled carbon nanotube formation with double laser vaporization technique

Single-walled carbon nanotubes (SWNTs) were prepared with double laser vaporization of a graphite target and a metal/alloy target inside an electric furnace at 1200 ^°C ambient temperature with 500 torr Ar gas atmosphere. Each target was vaporized simultaneously with a different Nd:YAG laser. Several kinds of metal/alloy target (Ni, Co, Fe, and permalloy) were tested in order to see the difference in the resulting SWNT yield and the diameter distribution of them. The Raman spectra of SWNT-containing soot prepared by use of this technique with permalloy/carbon system indicated that permalloy gives almost the same yield as compared with Ni/Co carbon composite rod with single laser vaporization technique, though the diameter distribution of them is slightly different. Also, time-resolved images of the plume by carbon and permalloy nanoparticles after laser vaporization were collected using a high-speed video camera. These images suggest that the hot plumes due to carbon and permalloy nanoparticles do not mix together so extensively, at least in a few hundred microseconds after laser vaporization. The effect of time delay between two laser pulses on the yield and the diameter distribution of SWNTs was also presented and discussed.     

Laser effect on the tissue culture of rice

We treated the callus of high-quality Yunan rice DR453 by He-Ne laser, then found the seedling of these callus emerging to an earlier date and the green seedlings differentiation rate is much higher than that of the callus not treated with laser (CK). It is useful to research the breeding on the tissue culture.     

Intensified and controllable vaporization of phase-changeable nanodroplets induced by simultaneous exposure of laser and ultrasound

Phase-changeable contrast agents have been proposed as a next-generation ultrasound contrast agent over conventional microbubbles given its stability, longer circulation time and ability to extravasate. Safe vaporization of nanodroplets (NDs) plays an essential role in the practical translation of ND applications in industry and medical therapy. In particular, the exposure parameters for initializing phase change as well as the site of phase change are concerned to be controlled. Compared to the traditional optical vaporization or acoustic droplet vaporization, this study exhibited the potential of using simultaneous, single burst laser and ultrasound incidence as a means of activating phase change of NDs to generate cavitation nuclei with reduced fluence and sound pressure. A theoretical model considering the laser heating, vapor cavity nucleation and growth was established, where qualitative agreement with experiment findings were found in terms of the trend of combined exposure parameters in order to achieve the same level of vaporization outcome. The results indicate that using single burst laser pulse and 10-cycle ultrasound might be sufficient to lower the exposure levels under FDA limit for laser skin exposure and ultrasound imaging. The combination of laser and ultrasound also provides temporal and spatial control of ND vaporization and cavitation nucleation without altering the sound field, which is beneficial for further safe and effective applications of phase-changeable NDs in medical, environmental, food processing and other industrial areas.     

Investigation of pulsed laser ablation process of Hg0.8Cd0.2Te

The vaporization threshold was measured under the irradiation of 1.064-μm, 10-ns pulsed laser. Then we calculated the vaporization temperature based on the conservation law of energy and analyzed the vaporization time based on our established model. These results coincided well with the information from the micrograph of scanning electron microscope (SEM) and the spectra of the plasma. Besides, the laser ablation rate was also computed and discussed theoretically.     

Numerical model and experimental demonstration of high precision ablation of pulse CO_2 laser

To reveal the physical mechanism of laser ablation and establish the prediction model for figuring the surface of fused silica, a multi-physical transient numerical model coupled with heat transfer and fluid flow was developed under pulsed CO_2 laser irradiation. The model employed various heat transfer and hydrodynamic boundary and thermomechanical properties for assisting the understanding of the contributions of Marangoni convention,gravitational force, vaporization recoil pressure, and capillary force in the process of laser ablation and better prediction of laser processing. Simulation results indicated that the vaporization recoil pressure dominated the formation of the final ablation profile. The ablation depth increased exponentially with pulse duration and linearly with laser energy after homogenous evaporation. The model was validated by experimental data of pulse CO_2 laser ablation of fused silica. To further investigate laser beam figuring, local ablation by varying the overlap rate and laser energy was conducted, achieving down to 4 nm homogenous ablation depth.     

Material ejection in nanosecond Er:YAG laser ablation of water, liver, and skin

We investigated the mechanisms of material ejection in Q-switched Er:YAG laser tissue ablation (70-ns pulse duration) where moderate and large radiant exposures are associated with large volumetric energy densities in the target material. For water, an initial phase of non-equilibrium surface vaporization is followed by an explosive vaporization of the superficial liquid volume from a supercritical state. The ablation of deeper layers with lower peak temperatures proceeds as phase explosion. For mechanically strong tissues, non-equilibrium surface vaporization is followed by a vapour explosion coupled with thermal dissociation of the biomolecules into volatile products. In deeper layers, ablation proceeds as confined boiling with mechanical tearing of the tissue matrix by the vapour pressure. The recoil stress induced at a radiant exposure of 5.4 J/cm² is in the order of 500–900 MPa. For water and soft tissues such as liver, the recoil causes a powerful secondary material expulsion. For stronger tissues such as skin, no secondary expulsion was observed even though the recoil stress largely exceeds the static tensile strength of the tissue. Recoil-induced material expulsion results in an increase of both ablation efficiency and mechanical side effects of ablation. Theoretical modelling of the succession of phase transitions in nanosecond-laser tissue ablation and of recoil-induced material expulsion remain a major challenge for future work. PACS  42.62.Be; 79.20.Ds