超短脉冲激光对生物软组织的等离子体诱导蚀除

详细研究了超短脉冲激光与生物组织相互作用的机理 ,建立了生物软组织中激光诱导光学击穿模型 ;结果表明 ,对于纳秒或亚纳秒脉冲激光 ,强吸收介质的热电子发射对电子雪崩电离过程有很大影响 ,等离子体光学击穿阈值随生物组织吸收的增加而降低 ;在激光脉宽为亚皮秒量级时 ,多光子电离成为光学击穿的主要机制 ,介质的击穿阈值几乎与线性吸收系数无关     

超短脉冲激光与生物软组织相互作用机理研究

详细研究了超短脉冲激光与生物组织相互作用的机理,建立了生物软组织中激光诱导光学击穿模型;结果表明,对于纳秒或亚纳秒脉冲激光,强吸收介质的热电子发射对电子雪崩电离过程有很大影响,等离子体光学击穿阈值随生物组织吸收的增加而降低;在激光脉宽为亚皮秒量级时,多光子电离成为光学击穿的主要机制,介质的击穿阈值几乎与线性吸收系数无关.     

飞秒激光致双液滴光学击穿和等离子体分布研究

为了研究超短激光脉冲与双液滴相互作用过程中的光学击穿和等离子体分布,基于麦克斯韦方程组和电离速率方程,构建了飞秒激光与双液滴的瞬态耦合模型,使用有限元分析方法,对飞秒激光辐照微米量级双液滴的自由电子密度和光场分布进行了计算,得到了双液滴结构对液滴光学击穿和等离子体变化的影响。结果表明,第2个液滴的击穿阈值约为同等条件下单液滴击穿阈值的35%;等离子体的形态和击穿点的位置随双液滴间距发生变化,且在聚焦区域产生纳米等离子体射流;第2个液滴对激光能量的吸收随着双液滴间距的增加而减少;当分别使用满足击穿阈值的光强入射,双液滴吸收的能量约为单液滴的3%;第2个液滴对激光能量的吸收随光强增大而增大,能量吸收比例最终趋于0.01,仅为单液滴的1.5%。该研究为激光诱导水击穿和激光在大气中的传输提供了一定的参考。     

脉冲激光导致水光学击穿阈值计算的简化模型

通过对自由电子密度速率方程的简化,得到了计算光学击穿阂值的解析式。将计算结果与波长分别在可见光和近红外波段,脉宽分别为纳秒、皮秒和飞秒的激光脉冲在纯净水和含有杂质的水中实验测量的击穿阈值做了比较,吻合较好。对于纳秒激光脉冲。在纯净水中多光子电离提供初始电子,随后雪崩电离很快在电离的过程中占主导地位。对于短脉冲,多光子电离作用显得尤为重要,并且在击穿的过程中复合损失和碰撞损失对击穿阈值的影响逐渐消失。     

强脉冲CO_2激光对红外材料的破坏现象

指出了强脉冲激光对红外材料的破坏作用,用实验证明强激光对红外材料的破坏分为烧蚀破坏和冲击破坏。当光功率密度低于大气的光学击穿阈值时,是烧蚀破坏。当光功率密度超过光学击穿阈值时,主要是冲击破坏。在我们的实验条件下,激光对红外脆性材料的相互作用结果表明,红外光学材料极易产生微裂纹或粉碎性破坏。其破坏效果与激光的单脉冲能量、功     

355 nm脉冲激光诱导等离子体开关削波

利用激光诱导等离子体开关技术,对355 nm脉冲激光自削波进行了实验和理论研究。分别采用5种不同焦距的透镜,集中讨论了透镜焦距及激光器输出单脉冲能量对脉宽压缩的影响,发现采用焦距为200 mm的透镜能够获得最佳的脉冲压缩效果。在聚焦透镜焦距200 mm,单脉冲能量160 mJ时,获得最短脉宽3.47 ns;在激光电离Cu小孔内壁表面及空气击穿共同作用下,获得了脉宽最短达2.11 ns的脉冲激光输出。此外,根据实验结果得到了355 nm激光空气击穿阈值,并与理论估算值进行比较,两者结果较为一致。     

高功率YAG激光在生理盐水中产生微等离子体的研究

利用光学阴影探测方法研究了调Ｑ－ＹＡＧ脉冲激光在生理盐水光学击穿阈值附近诱导微等离子体和冲击波产生的过程，得到了微等离子体和冲击波随人射激光能量和延迟时间而变的系列光学阴影图，并且指出了冲击波的机械作用在脉冲激光眼科医疗中的影响。     

不同盐度海水的光击穿声辐射特性研究

强激光聚焦于水下时,通过光击穿机制辐射强声波信号。水体介质的盐度不同,在同等的激光参数、光学系统条件下,光击穿辐射的声波在强度、脉冲波形、频谱特征上具有一定的差异性。为研究水介质盐度对激光击穿形成的空泡辐射声波的影响,构建了激光声测量系统,实验研究了不同水体盐度参数对光击穿辐射声信号的影响。结论：激光击穿水介质伴随空泡脉动、声信号辐射效应;激光声信号脉冲宽度与水体盐度无关;空泡尺寸和激光声信号强度与激光脉冲能量成正比关系;水体盐度与激光声信号的强度早非线性变化关系。研究结果有助于激光声在海洋中的应用。     

激光在液体中的声效应研究

以研究液体中激光声波的特点与应用为目的,从理论上分析脉冲激光在液体中产生的声波的波阵面、光声脉冲波形的特点,并用实验的方法观察脉冲CO2 激光光声脉冲的波形、频谱特性及光击穿现象。结果表明光声波的波阵面与吸收激光能量的媒质的形状有关;光声波的波形随激发机制而变化; CO2 脉冲激光在水中通过热膨胀机制激发的声波能量主要分布在100kHz以下,光击穿时水面可以观察到微爆炸和亮光现象。可以通过液体中光声效应现象和 光声信号波形的变化判断其激发机制,并确定液体的汽化与光击穿阈值,控制激发过程以获得确定特性的光声信号。     

激光模拟单粒子效应中单光子与双光子吸收诱导电荷量对比

通过理论推导和模拟试验,研究了单光子和双光子吸收分别占主导时,脉冲激光模拟试验中电荷收集效率之间的定量关系。分析不同光学参数对模拟试验中电离电荷浓度影响,确定具体激光波长、脉宽、能量及束斑等参数。根据脉冲激光在硅中单光子线性吸收和双光子非线性吸收的特点,推导单光子与双光子吸收产生电荷量比值的定量公式。通过1064 nm和1200 nm波长激光的验证试验,发现了响应脉冲及产生电荷量与脉冲激光能量或能量平方具有良好的线性关系,且在单光子吸收和双光子吸收各自占主导时,单光子吸收产生电荷率明显高于双光子吸收,证明了两种波长激光产生电荷量的比值近似等于公式计算结果。结果表明,1200 nm脉冲激光 1 nJ2 诱导电荷量等同于1064 nm脉冲激光 0.039 nJ诱导电荷量。     

Modeling of skin tissue ablation by nanosecond pulses from ultraviolet to near-infrared and comparison with experimental results

Comprehension of biological tissue ablation by short laser pulses in a broad optical spectrum is of fundamental importance to the understanding of laser-tissue interaction and advancing surgical applications of lasers. We report a new plasma ablation model in which the chromophore ionization pathway is incorporated to explain the skin tissue ablation by nanosecond laser pulses from ultraviolet to near-infrared. A rate equation is solved to examine the effects of chromophore, cascade, and multiphoton ionization on the optical breakdown of the tissue. The wavelength and spot size dependence of the breakdown thresholds have been measured and agreements have been found between the calculated and measured results.     

Laser-induced plasma formation in water at nanosecond tofemtosecond time scales: calculation of thresholds, absorptioncoefficients, and energy density

The generation of plasmas in water by high-power laser pulses was investigated for pulse durations between 100 ns and 100 fs on the basis of a rate equation for the free electron density. The rate equation was numerically solved to calculate the evolution of the electron density during the laser pulse and to determine the absorption coefficient and energy density of the plasma. For nanosecond laser pulses, the generation of free electrons in distilled water is initiated by multiphoton ionization but then dominated by cascade ionization. For shorter laser pulses, multiphoton ionization gains ever more importance, and collision and recombination losses during breakdown diminish. The corresponding changes in the evolution of the free carrier density explain the reduction of the energy threshold for breakdown and of the plasma energy density observed with decreasing laser pulse duration. By solving the rate equation, we could also explain the complex pulse duration dependence of plasma transmission found in previous experiments. Good quantitative agreement was found between calculated and measured values for the breakdown threshold, plasma absorption coefficient, and plasma energy density     

1064nm激光诱导等离子体开关控制355nm脉宽可调输出

为得到脉宽可控的355nm紫外脉冲激光输出,采用1064nm脉冲激光诱导等离子体开关技术,控制355nm激光脉冲宽度,在激光电离Cu小孔内壁表面及空气击穿共同作用下,获得了2.8ns～10ns的脉宽可调输出。讨论了1064nm单脉冲输出能量对脉宽压缩的影响,在无延时情况下得到了脉宽最短达2.8ns的脉冲激光输出。在此基础上,保持1064nm单脉冲输出能量不变,采用延时装置改变两光路间的光程差,以控制等离子体开关相对于355nm激光脉冲的形成时间,最终得到脉宽可调的脉冲激光输出。结果表明,等离子体开关结构简单、操作方便、适用范围广,是一种较好的脉冲整形手段。     

空气击穿过程中电子损耗对击穿阈值的影响

通过对高功率激光空气击穿的物理机理、电离机制的讨论,得到空气击穿过程中激光焦区内自由电子密度公式。考虑电子弹性碰撞损耗、电子复合损耗影响,给出了新的击穿阈值公式。该公式的计算结果同实验数据一致。     

1064nm激光诱导等离子体开关控制355nm脉宽可调输出

为得到脉宽可控的355nm紫外脉冲激光输出,采用1064nm脉冲激光诱导等离子体开关技术,控制355nm激光脉冲宽度,在激光电离Cu小孔内壁表面及空气击穿共同作用下,获得了2.8ns～10ns的脉宽可调输出.讨论了1064nm单脉冲输出能量对脉宽压缩的影响,在无延时情况下得到了脉宽最短达2.8ns的脉冲激光输出.在此基础上,保持1064nm单脉冲输出能量不变,采用延时装置改变两光路间的光程差,以控制等离子体开关相对于355nm激光脉冲的形成时间,最终得到脉宽可调的脉冲激光输出.结果表明,等离子体开关结构简单、操作方便、适用范围广,是一种较好的脉冲整形手段.     

Penetration and precision of subsurface photodisruption in porcine skin tissue with infrared femtosecond laser pulses

The surgical precision of photodisruption with ultrafast optical pulses depends on the accurate delivery of optical energy to sites of interest. As light penetration is limited in turbid tissues, localization and precision of subsurface breakdown highly depend on the interacting effects of increased power requirements and external focusing conditions. Infrared femtosecond breakdown extent in excised porcine skin tissue was investigated using a high-frequency ultrasonic technique which sensitively detected laser-induced bubbles. Using a focused laser source, optical parameters including laser fluence and focusing numerical aperture (NA) were controlled. Decreasing NA improved penetration, while increasing NA improved precision. At lower NA, penetrations of up to 1 mm could be achieved with a single laser excitation at the cost of an expanded breakdown region which reduced precision. Even at higher NA, however, maximum penetration reduced to 400 microm and precision was still limited with extended breakdown regions for focusing depths greater than 100 microm. Glycerol was used as an index matching material, which helped reduce scattering and improved penetration by 150 to 400 microm for all NAs. Nonetheless, multiple breakdown sites and the corresponding reduction in precision were observed even with glycerol treatment.     

Laser-induced electric breakdown in solids

A review is given of recent experimental results on laser-induced electric breakdown in transparent optical solid materials. A fundamental breakdown threshold exists characteristic for each material. The threshold is determined by the same physical process as dc breakdown, namely, avalanche ionization. The dependence of the threshold on laser pulse duration and frequency is consistent with this process. The implication of this breakdown mechanism for laser bulk and surface damage to optical components is discussed. It also determines physical properties of self-focused filaments.