激光辐照下皮肤组织光热响应有限元分析

为了更好地选择临床激光医疗曝光参量,采用有限元数值计算方法,模拟了脉冲激光与连续激光对人皮肤组织的光热作用及导致的温度变化效应,比较了两者的不同,得到了热响应时间及热弛豫时间与组织深度的关系,即组织越深(0～60 μm),其热响应时间(0～4 ms)与热弛豫时间(0.4～12.1 ms)越长;分析了激光脉宽长短对组织升温的影响;建立了评价脉冲间热损失的评价函数δ,并以此对脉冲间隔的选取作了探讨.     

脉宽对中红外激光带内损伤HgCdTe材料的影响

研究了脉宽对于中红外脉冲激光带内损伤碲镉汞（HgCdTe）材料阈值的影响,使用一维自洽模型对激光辐照HgCdTe材料程中的载流子数密度,载流子对数目流,载流子对能流,载流子温度和材料晶格温度等相关参数进行仿真计算。仿真结果表明,波长2.85 μm,脉宽30 ps～10 ns单脉冲激光带内辐照HgCdTe材料的损伤阈值为200～500 mJ/cm2。其中,300 ps～3 ns脉冲激光的损伤阈值相近,均为200 mJ/cm2且低于其他脉宽激光的损伤阈值。搭建实验光路并进行相关实验验证仿真模型的正确性。实验发现,波长2.85 μm、脉宽300 ps的单脉冲激光带内辐照HgCdTe材料的损伤阈值在200 mJ/cm2左右。相同条件下,10 ns单脉冲激光带内辐照HgCdTe材料的损伤阈值约474 mJ/cm2。百皮秒脉冲激光对HgCdTe材料的损伤过程结合了热击穿和光学击穿效应,其独特的毁伤机理加剧了材料的损伤。     

Measurement of gas temperature in a CO2-N2-He TEA amplifier using laser interferometry

Using a laser interferometric technique changes in the refractive index at 633 nm of the pulsed discharge of a TEA CO₂-N₂-He laser amplifier have been observed. Heating of the medium by the discharge reduces the density and hence the refractive index of the gas. The peak temperature was found to be reached between 0.15 and 0.25 msec after the current pulse. For values of voltage and pressure typical for 10.6 μm laser operation a temperature of ～430 K was measured. The system returned to thermal equilibrium after 23 and 27 msec. Thermal diffusion time calculations indicate that the heating is confined to within 3 to 4 mm of the central axis.     

60°C lead salt laser emission near 5-μm wavelength

A IV–VI double heterostructure laser made from molecular beam epitaxy grown PbSrSe/PbSe/PbSrSe has been operated in pulsed mode up to 333 K (60°C) at a wavelength of 4.78 μm. This is the highest mid-infrared diode laser operation temperature observed to date.     

12 W Q-switched Er:ZBLAN fiber laser at 2.8 μm

A diode-pumped, actively Q-switched 2.8 μm fiber laser oscillator with an average output power of more than 12 W has been realized through the use of a 35 μm core erbium-doped ZBLAN fiber and an acousto-optic modulator; to our knowledge, this is the first 3 μm pulsed fiber laser in the 10 W class. Pulse energy up to 100 μJ and pulse duration down to 90 ns, corresponding to a peak power of 0.9 kW, were achieved at a repetition rate of 120 kHz.     

Infrared dye laser excited by a diode laser

A waveguide dye laser with an inner diameter of 30 μm and a length of 1.0 nm has been excited with a pulsed GaAlAs diode laser. The infrared-absorbing dye 5,5′-dichloro-11-diphenylamino-3,3′-diethyl-10,12-ethylenethiatri- carbocyanine perchlorate was used in tetramethylene sulfoxide solution. By changing dye concentration, laser wavelengths from 950 to 1005 nm were obtained.     

Influence of scanning velocity on bovine shank bone ablation with pulsed CO2 laser

The influence of scanning speed on hard bone tissue ablation is studied with a 10.6-μm laser. The groove morphology and the thermal damage created in bovine shank bone by pulsed CO2 laser are examined as a function of incident fluence by optical microscope following standard histological processing. The results show that ablation groove width, depth and ablation volume, as well as the zone of thermal injury, increase gradually with incident fluence. As compared to the result for high scanning speed, the lower scanning speed always produces larger ablation volume but thicker zone of thermal injury. It is evident that scanning speed plays an important role in the ablation process. In clinical applications, it is important to select appropriate scanning speed to obtain both high ablation rates and minimal thermal injury.     

Realisation d'un laser monomode a four heat-pipe sur la raie a λ = 1.5 μm du baryum

We describe here the properties of a laser effect observed in a vapour provided in a heat pipe oven. The oscillation is a pulsed one. It occurs on the baryum line at 1.5 μm. The pulse duration is 20 ns. We have obtained laser effect in single mode operation.     

Far-infrared two-photon intraband transitions in n-InSb

Two-photon cyclotron resonance and two-photon shallow donor transitions have been excited by a high power pulsed D₂O laser emitting λ = 119 μm and 66 μm laser lines at the same time. Transitions involving the absorption of two 119 μm photons or simultaneously one 119 μm and one 66 μm photon were observed. Two-photon selection rules are discussed by a rigorous treatment of the symmetry of the free electron Hamiltonian.     

0.53 μm毫秒脉冲激光对GaAs热分解损伤特性

针对波长0.53 m的毫秒脉冲激光辐照GaAs的表面热分解损伤问题,建立了二维轴对称热传导模型,在考虑材料的热物性参数随温度变化的基础上,采用有限元法模拟了材料的瞬态温度场,得到了温度场分布特征及其随时间的变化规律,给出了材料表面发生热分解损伤阈值曲线。数值结果表明:毫秒脉冲激光对GaAs作用时,热传导影响着激光作用全过程,对应的损伤机理主要为热损伤;在激光作用下,被作用表面中心处温度最高,并且首先发生热分解损伤;随着作用激光能量密度的增加,GaAs表面发生热分解损伤的时刻不断提前。     

Theoretical considerations regarding pulsed CO2 laser annealing of silicon

We present a calculation of the surface temperature and investigate the “thermal runaway” phenomenon during pulsed CO₂ laser (λ = 10.6 μm) annealing of silicon. In calculating the temperature variation of free carrier absorption in n-Si, we have taken into account acoustic deformation potential scattering, optical deformation potential scattering, and ionized impurity scattering. The deformation potentials are adjusted to fit the experimentally observed values at 300°K. Also, we discuss the contribution of free carrier absorption during annealing with a Nd:glass laser (λ = 1.06 μm).     

High power repetitive TEA CO2 pulsed laser

A high power repetitive spark-pin UV-preionized TEA CO₂ laser system is presented. The discharge for generating laser pulses is controlled by a rotary spark switch and a high voltage pulsed trigger. Uniform glow discharge between two symmetrical Chang-electrodes is realized by using an auto-inversion circuit. A couple of high power axial-flow fans with the maximum wind speed of 80 m/s are used for gas exchange between the electrodes. At a repetitive operation, the maximum average output laser power of 10.4 kW 10.6 ??m laser is obtained at 300 Hz, with an electro-optical conversion efficiency of 15.6%. At single pulsed operation, more pumping energy and higher gases pressures can be injected, and the maximum output laser energy of 53 J is achieved.     

Continuous-wave room-temperature operation of a 2.8 μm GaSb-based semiconductor disk laser

We demonstrate an optically pumped semiconductor disk laser based on the (AlGaIn)(AsSb) material system, which operates at an emission wavelength of 2.8 μm. Up to 120 mW of output power were obtained in cw operation and more than 500 mW in pulsed mode. The performance of the present laser is discussed in comparison to shorter-wavelength semiconductor disk lasers based on the same materials system.     

30-W Yb<Superscript>3+</Superscript>-pulsed fiber laser with wavelength tuning

We have investigated various pulsed operation regimes of a diode-pumped Yb³⁺-doped fiber laser with both an acoustooptic filter and a shutter inside the resonator. To imbed the polarization-sensitive acoustooptic-tunable spectral filter into the polarization-nonmaintaining resonator, based on an “isotropic” single-mode fiber without “polarization’ losses, we have used a CaCO₃ single-crystal nondispersive thermostable polarization splitter. Stable smooth bell-shaped laser pulses were obtained in the Q-switch generation regime across the entire wavelength tuning band. Their duration depended on the resonator travel time and their repetition rate was determined exclusively by the outer high-frequency generator controlling the acoustooptic shutter. A pulsed laser radiation tuning bandwidth of more than 20-nm at a repetition rate band of 10–100 kHz was observed in the amplification band of the Yb³⁺-doped fiber. A stable average power of 30 W of the pulsed 70-ns 100-kHz laser radiation in a near Gaussian beam was reached by means of the two-stage amplifier based on Yb³⁺-doped fibers with an enlarged mode field diameter (14 μm). The amplifier was pumped by λ = 975 nm CW multimode laser diodes with a maximum average power of 42 W.     

强激光反射镜基体材料的热畸变特性有限元分析

 利用有限元方法数值模拟了单晶硅（Si）、铜（Cu）、钨(W)、氧化铍(BeO)等镜体材料的热畸变特性。结果表明：当入射激光功率为2 kW、镜面反射率为93%、光斑直径为17 mm、激光照射时间为10 s时，四种材料基板的中心最大热变形分别为0.984，3.32，1.55，1.88 μm。相比之下，Si镜的热变形最小，是比较理想的镜体材料；Cu镜的热变形最大；W和BeO两种材料的镜热变形介于Si镜和Cu镜之间，具有较高的强度和硬度，抗破坏能力较强。     

脉冲式2 μm KTiOAsO4光参变振荡器

为了验证KTiOAsO4(KTA)晶体用于光参变振荡产生2μm激光的可行性,设计了脉冲式2μm KTA光参变振荡器,采用θ=49°切割的KTA晶体作为光参变振荡晶体,实现了2μm激光的输出;计算了该系统在双谐振情况下的起振阈值,在实验中测得输出的信号光、闲频光波长在2.16μm和2.09μm附近,与理论计算基本吻合,并测量了脉冲宽度。同时选用了对信闲光透过率分别为60%、70%、90%的镜片作为光参变振荡器(OPO)输出镜,测得了三种情况下的输出能量及电光效率的数值;测得了在380 V电压下,输出镜透过率一定时输出能量与光参变振荡器腔长之间的关系曲线,从而验证了KTA晶体用于光参变振荡产生2μm激光的可行性,为下一步的研究工作打下了基础。     

Projection-patterned etching of silicon in chlorine atmosphere with a KrF excimer laser

The formation of relief features in silicon by a one-step process that avoids resist patterning has been achieved by laser-projection-patterned etching in a chlorine atmosphere. Etching is performed with a pulsed KrF excimer laser (λ=248 nm, τ=15 ns) and deep UV projection optics having an optical resolution of 2 μm. Etching takes place in two steps. Between laser pulses, the silicon surface is covered with a monolayer of chemisorbed chlorine atoms (one Cl per Si). During the laser pulse, surface transient heating at temperatures in excess of 1250 K results in the desorption of the reaction products (mainly SiCl₂). At laser energy densities that induce surface melting, this desorption results in a saturated etch. rate of 0.06 nm per pulse, corresponding to the removal of about 0.5 Si monolayer per pulse. At densities below the melting threshold, reduced thermal and possibly a small amount of photochemical etching result in lower etch rates. Projection of a resolution test photomask onto the silicon surface shows that the size of etched features differs from the size of the projected features and strongly depends on the laser energy density. As a result of the heat spread in silicon and of the highly nonlinear character of the etching reaction, etched features smaller than the irradiated area are obtained at all fluences in the range 350–700 mJ/cm². Etched lines having a width down to about 1.3 μm were produced. Proximity effects due to heat spread were also evidenced for small projected features (<4 μm). The characteristics of the etched patterns are compared with those obtained for GaAs etching in chlorinated gases with the same experimental set-up. Significant differences in pattern resolution for Si and GaAs etching are observed. This variation in resolution is believed to result from the fact that Si has a greater thermal diffusivity than GaAs.     

激光泵浦碲化物产生中红外超连续谱数值模拟

对2 m波段脉冲激光泵浦碲化物光子晶体光纤产生中红外超连续谱进行了数值研究。通过材料的拉曼增益谱间接求得了对应的拉曼响应函数;由光子晶体光纤的材料折射率和波导结构,通过COMSOL软件获得了碲化物光子晶体光纤中基模等效折射率,计算了相应的色散曲线和限制损耗 ;利用自适应的分步傅里叶算法,模拟了中心波长为1.96m、峰值功率为20 kW的50 fs脉冲光泵浦碲化物光子晶体光纤时超连续谱的产生,当光纤长度为6 cm时,产生的中红外超连续谱波长范围为1.0~4.5 m。     

聚甲基丙烯酸甲酯掺杂Ce3+的飞秒激光三维信息存储

用飞秒激光(200 fs,1 kHz,800 nm)脉冲在掺杂稀土离子Ce3 的聚甲基丙烯酸甲酯(PMMA)膜中进行了光存储实验研究,包括对样品的吸收光谱、激光照射前后的电子旋转共振(Electron spin resonance,ESR)光谱的测量和讨论。结果表明掺杂稀土离子Ce3 的聚甲基丙烯酸甲酯膜具有较低的写入阈值,有利于高速、并行的三维光存储。实验结果采用传统光学显微镜并行读出。给出了四层存储结果(点间距和层间距分别是4μm和16μm),并讨论了脉冲能量的大小对空腔尺寸的影响,进行高密度存储时,在保证读出信号灰度值足够大的情况下,应选择尽量小的激光脉冲写入能量。实验结果表明这种材料可以应用于三维光信息存储。     

脉冲激光辐照硅材料引起表面波纹的特性研究

叙述了激光与材料相互作用过程中引起相干受激光散射的机制,以及形成材料表面波纹的特性。在激光波长1．06μm、能量15mJ、光斑直径2mm、脉冲半峰全宽约10ns和入射方向为布儒斯特角的条件下,进行了脉冲激光辐照硅材料形成表面波纹的实验研究。在脉冲激光辐照硅材料表面功率密度略大于材料损伤阈值的条件下,发现了硅材料表面形成的平行等间距直线条纹结构。用光学显微镜和原子力显微镜分别测量了被辐照硅材料表面的波纹形貌特征。在假设硅材料表面波纹的产生与声波在材料中的传播速度有关的条件下,由声波传播速度和激光辐照硅材料的脉冲宽度较好地解释了材料表面形成条纹的宽度．并认为在形成表面波纹的过程中,热应力起主要作用。