不同光聚焦状态的光击穿声辐射特性

强激光通过光学系统聚焦于水下时,击穿水介质辐射强声波信号.光学聚焦特性的不同,使得激光击穿区域形状的差异,导致辐射的声信号在强度、频谱特征上具有较大的差异性.为研究不同光聚焦状态对激光击穿形成的空泡辐射声波的影响,推导了不同聚焦状态下激光空泡壁的运动方程及声辐射模型,构建了激光声实验测量系统,实验研究了不同聚焦状态下光击穿导致的空泡脉动参数、辐射声信号特征的差异性.结论:激光声信号强度与激光能量成线性关系,在激光能量高于一定参数情况下,声信号强度变化不大;在低激光功率下,扩束聚焦与非扩束聚焦,辐射的声信号特性差异不大;在高激光功率条件,扩束聚焦击穿形成的空泡半径大于非扩束聚焦击穿,辐射的声信号强度高于非扩束聚焦;为提高激光击穿辐射卢信号效率,应对激光束进行扩束聚焦.     

Compton散射对固体中激光能量与光声信号强度关系的影响

应用光声理论、相对论量子理论和电子与多光子集团非线性Compton散射模型,研究了Compton散射对掺杂固体中光声信号强度的影响。结果表明,当掺杂固体中发生电子与多光子集团之间的多光子非线性Compton散射时,在忽略光声转换效率的变化和基质晶格对介电函数贡献的情况下,耦合激光能量和磁感应强度是影响光声信号强度的两个主要因素,其中耦合磁感应强度起主导作用。Compton散射使光声信号强度随耦合光的磁感应强度和能量的增大而迅速增大。能量与光声信号声压之间存在对数线性关系,但它相对于Compton散射前的曲线下移。     

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应用光声理论、相对论量子理论和电子与多光子集团非线性Compton散射模型,研究了Compton散射对掺杂固体中光声信号强度的影响。结果表明,当掺杂固体中发生电子与多光子集团之间的多光子非线性Compton散射时,在忽略光声转换效率的变化和基质晶格对介电函数贡献的情况下,耦合激光能量和磁感应强度是影响光声信号强度的两个主要因素,其中耦合磁感应强度起主导作用。Compton散射使光声信号强度随耦合光的磁感应强度和能量的增大而迅速增大。能量与光声信号声压之间存在对数线性关系,但它相对于Compton散射前的曲线下移。     

同步双脉冲激光致声研究

利用高速摄像机拍摄了同步双脉冲激光聚焦击穿水介质产生激光空泡的运动过程。分析了该条件下产生的四种典型激光声信号特性,初步计算了光声能量转换效率。研究结果表明:通过调节脉冲激光的能量差,可以控制激光声信号峰值间隔,提高激光声信号脉宽;两空泡之间无量纲距离为1.5时,溃灭过程中出现融合现象,融合过程中空泡热力学能转化为机械能,融合后形成的单个空泡能量变大,其溃灭产生的激光声信号主频降低;同步双脉冲激光聚焦击穿情况下的光声能量转换效率为6%~10%。     

光声光谱分析中溶剂效应的研究Ⅰ——与水混溶的有机溶剂的光声信号增强作用

对液体光声光谱分析中的溶剂效应进行了研究,探讨了溶剂特性对于光声信号强度的影响。结果表明,有机溶剂的存在显著地增强了光声信号强度。这对于液体光声光谱分析,选择适当的溶剂介质,提高光声分析灵敏度具有十分重要的意义。     

光击穿机制下的光声能量转换效率

建立了激光声实验测量系统,利用脉冲激光聚焦击穿水介质产生声信号。由水听器将声信号转换成电信号并送入数字存储示波器,计算出声信号能量。对不同激光能量、激光聚焦位置和水体盐度下的光声能量转换效率进行了理论和实验研究。研究结果表明:随着激光能量的提高,能量转换效率逐渐减小;激光垂直水面入射时,随着激光聚焦深度的增加,能量转换效率会出现一次阶跃式变大,随后逐渐降低;等离子体对激光能量的吸收越充分,能量转换效率越高;水体盐度的变化对能量转换效率影响很小。     

透明液体中激光致声特性

研究透明液体的特性阻抗、激光声的传播距离和激光焦斑半径对激光声信号的影响.通过实验和数值计算发现,在一定范围内,随着特性阻抗的增加,声波的峰值声压、主频和声能也相应增加;随着激光声传播距离的增加,激光声的主频从急速下降变为相对稳定;在保证光击穿的条件下,选择较大的激光焦斑半径,可提高光声能量转换效率.     

激光声信号的频谱特性

构建了激光声实验测量系统,利用脉冲激光聚焦击穿水介质产生声信号,由水听器将声信号转换成电信号并送入数字存储示波器。分析了激光声信号的时频域数学模型,实验研究了激光声信号的频域能量分布,以及激光器重复频率和激光声信号频谱特性的关系。结果表明:激光声信号能量主要集中在200 kHz内,其中100~200 kHz内的能量所占比例约50%。激光声信号的幅频响应极大值点可以受到激光器重复频率的控制。     

影响激光诱导等离子体稳定性的因素研究

采用1064nm的激光对炉渣样品进行烧蚀,分析了影响炉渣中信号稳定性的一些因素。激光能量在小范围波动(±2%)对信号的稳定性影响很小,参考激光能量和光谱信号强度无明显关系。当激光频率降低至4Hz以下时,信号的相对标准偏差下降至10%左右。当激光聚焦位置位于样品表面下3mm时信号强度和稳定性最优。熔渣样品信号的稳定性高于粉渣样品,不同渣系的炉渣的稳定性也不相同。对于原子线,不同点烧蚀的稳定性远远优于单点烧蚀稳定性,而对于离子谱线,两种不同的烧蚀稳定性差别不大。     

脉冲激光在液体中激发的声波特性研究

理论上分析了脉冲激光在液体中产生的声波波阵面随光声源形状的变化,以及不同激发机制下光声脉冲波形的差别,并从实验中得出了脉冲CO2激光光声脉冲频谱特性.结果表明光声波波阵面为球面或柱面,热弹机制激发双极性的光声脉冲,汽化机制激发单极性的光声脉冲,CO2脉冲激光在水中激发的声波频谱峰值主要在100 kHz以下.通过选择光声源的形状和激发机制可以获得所需的光声信号.     

Reconstruction of Optical Energy Deposition for Backward Optoacoustic Imaging

Visualizing optical properties, such as the optical absorption coefficient, helps us to obtain structural information of biological tissues. In this paper, we present an efficient reconstruction algorithm for optical energy deposition in backward optoacoustic imaging. Note that econstruction of optical energy deposition is the first step to imaging the optical absorption coefficient distribution. This algorithm is derived from the optoacoustic wave equations with line focusing, in which the focusing techniques were utilized to reduce the reconstruction problem from three dimensions (3-D) to one dimension (1-D). Simulations and experiments were conducted to verify efficacy of this algorithm. In the simulations, optoacoustic signals were generated based on the solution of the optoacoustic wave equations. In the experiments, a 3-D backward mode optoacoustic imaging system was built. The system consisted of a Nd YAG laser for optical irradiation and an acoustic detection system with a broadband hydrophone. A phantom was used to illustrate validity of the proposed algorithm. The results show that optical energy deposition can be efficiently reconstructed in both simulations and experiments.     

Real-time optodynamic monitoring of pulsed laser decoating rate

This paper deals with a laser-cleaning process for removing a layer of paint. The effects of each individual laser pulse were monitored by a three-probe system. Ultrasonic signals from the rear surface of the substrate were obtained using an arm compensated Michelson interferometer, the optoacoustic waves in the surrounding air were detected using a laser beam deflection probe and the shape of the growing crater was measured using a laser anamorph profilometer. Two different cleaning lasers were used: an Excimer and a Nd:YAG laser. Typical parameters of the optodynamic signals versus the laser-pulse number are presented. A linear correlation between the quantity of ablated material and the integral of the ultrasonic signals was found together with an exponential relationship between the time-of-flight of the optoacoustic signals and the decoating rate.     

Nonlinear optoacoustic transformation in the system of dielectric substrate/submicron metal coating/liquid

The optoacoustic method has been shown to be an accurate technique for the measurement of the properties of submicron metal coatings deposited on a dielectric substrate, i.e., mirrors. The method has been previously theoretically described in terms of a linear model of optoacoustic transformation in a system substrate/coating/liquid. The goal of the present work was to determine the limits at which the linear model is still applicable. The modification of the laser induced acoustic signal profiles and transfer functions of optoacoustic transformation versus the laser fluence was studied for two liquids: ethanol and water.     

Methods of optoacoustic diagnostics of biological tissues

Acoustical Physics - Laser optoacoustic diagnostics is based on thermoelastic excitation of ultrasonic signals in a medium due to absorption of pulsed laser radiation. The pressure profile of such...     

Femto-, Picosecond,and Terahertz Optoacoustics

The possibilities offered by the optoacoustic monitoring of the sea shelf are considered, based on the development of reliable laser sources of femto- and picosecond pulse duration and new ways of detecting and processing high-frequency signals. The basis of these new possibilities is a volume source of sound produced as a result of the breakdown of the medium under the action of ultrashort laser pulses. An overview of new worldwide achievements is presented. The results from a number of model experiments on the optoacoustic monitoring of low concentrations of gas impurities, and on the precise monitoring of the sea shelf, are interpreted.     

Optoacoustic conversion in suspensions: The competition of mechanisms and statistical characteristics

Optoacoustic conversion in diluted suspensions under the effect of nanosecond laser pulses is considered. The mode of operation with moderate values of the laser fluence is studied theoretically and experimentally. In this mode, a competition of the thermooptical and cavitation mechanisms of sound conversion is observed, which leads to considerable fluctuations of the acoustic response from one laser pulse to another. Analytical expressions for the basic characteristics of the acoustic signal are obtained. A simulation of the statistical characteristics of the cavitation contribution to the signal is performed using the Monte Carlo method. The experiment is based on the use of second harmonic pulses of a YAG laser and test suspensions. The histograms of the amplitudes of acoustic signals can be used to discriminate between the mechanisms of optoacoustic conversion and also can serve as the basis for diagnosing a low content of an insoluble phase in a liquid.     

Method of Modeling Optoacoustic Signals in Composites Transparent Matrix – Metal Nanoparticles

Russian Physics Journal - Method of modeling optoacoustic signals initiated by a laser pulse in composites transparent matrix – metal nanoparticles taking into account melting processes has...     

Doppler-free optoacoustic spectroscopy

The first observation of Doppler-free optoacoustic spectroscopy is reported. As a first example the P (193) line of the 11-0 band of the B←X transition of ¹²⁷I₂ is used. The output of cw single mode dye laser is split into two equal intensity beams chopped at frequencies ω₁ and ω₂. The nonlinear compoment of the optoacoustic signal at the frequency (ω₁ + ω₂) is detected and Doppler-free resolution is obtained. Comparing the Doppler-free optoacoustic and fluorescence spectra of iodine measured under similar conditions, good agreement is found. Since optoacoustic and fluorescence methods complement each other, this opens up new possibilities for weakly or nonfluorescing molecules.     

Frequency Stabilization of Waveguide CO2 Laser by a Digital Technique

In this work we present a simple technique for laser frequency stabilization, based on Digital signal processing. The technique is used to stabilize a waveguide CO₂ laser of wide tunability by using three kinds of reference signals: the CO₂ laser ouptut power, an infrared absorption optoacoustic signal and the output power of a Far-Infrared optically pumped molecular laser.     

Control of tissue mechanics upon the repetitive-pulse laser heating of cartilage

The viscoelastic properties of laser-irradiated cartilages are studied with optoacoustic methods upon the thermal excitation of mechanical oscillations by repetitive-pulse laser radiation. The effect of laser power, pulse duration, repetition rate, and irradiation time on the shape of the optoacoustic signal is analyzed. It is demonstrated that the optoacoustic response of the cartilage to the repetitive-pulse radiation of a fiber laser depends on the softening of the tissue upon the variation in its shape. Under repetitive-pulse laser irradiation, the optoacoustic response of the cartilage depends on the mechanical characteristics of the biotissue (elastic modulus, hydraulic permeability, and thickness). A simple model that makes it possible to estimate the contribution of the viscoelastic properties to the formation of the optoacoustic response at various laser repetition rates is proposed.