节点文献
超快脉冲激光干涉技术及其在冲击动力学过程诊断中的应用
【作者】 翁继东；
【作者基本信息】 中国工程物理研究院 ， 光学， 2010， 博士
【摘要】 脉冲激光频域干涉技术是近10年以来国外发展起来的一种新型非接触式激光干涉测试技术,具有飞秒量级的时间分辨力和亚纳米量级的位移分辨力,结合泵浦-探测实验方法,可实现超快过程中物理参数的测量,是获得材料在飞秒或皮秒脉冲激光加载下动力学参数随时间变化历史的重要方法,目前美国、英国、法国和日本等国家的人已经建立了脉冲激光频域干涉技术。研制该技术的重要意义在于发展材料在超快脉冲激光加载下响应特性的测量手段,促进超快动力学物理机制的研究,对高能密度物理和武器物理研究具有重要的现实意义和应用前景。为了推动脉冲激光频域干涉技术在国内的发展和完善,使我国也具备测量飞秒脉冲激光与材料相互作用过程中冲击动力学参数的能力,本论文不但深入研究了脉冲激光频域干涉技术,可用于测量材料在平面分布冲击波或中心对称分布冲击波作用下的冲击波速度和波后粒子速度剖面,而且还研究了可测量材料在非中心对称分布冲击波作用下波后粒子速度剖面的超快显微干涉技术,对这两种技术的工作原理、设计方法、数据处理方法、调试方法和实验方法均进行了详细的分析。主要工作内容及创新点归纳如下：1、根据日本东京大学E.Tokunaga等人对频域干涉现象的诠释,本文对频域干涉技术的工作原理进行了深入细致的研究,得到了频域干涉的基本条件为：式中,T为参与频域干涉的两脉冲传输时间差,d为光谱仪的光栅常数,c为真空中光速,θ为光谱仪中特征谱线的衍射角。基于时域干涉原理和频域干涉条件,对频域干涉信号特征进行了详细分析,得到了频域干涉仪输出信号的数学表达式为：式中,f为光波频率,T为参与频域干涉的两脉冲传输时间差,φ为参与频域干涉的两脉冲传输相位差,R为参与频域干涉的两脉冲光强比值,I0(f)为脉冲激光器的光谱分布函数,I(f)为频域干涉信号的光谱分布函数。2、根据Michelson干涉仪的数据处理方法和频域干涉信号的数学表达式,对频域干涉技术的数据分析方法进行了深入研究,研究发现可以采用变量代换—等间隔插值—反傅立叶变换—数字滤波的计算步骤得到频域干涉仪测量的传输相位差,另外可采用变量代换—等间隔插值—数据扩展—反傅立叶变换—数字滤波的计算步骤得到频域干涉仪测量的传输时间差,如果不考虑被测物体表面光学折射率的变化,频域干涉仪测量的传输时间差或传输相位差与被测物体运动位移的关系为：式中,s为被测物体的运动位移,T为参与频域干涉的两脉冲传输时间差,φ为参与频域干涉的两脉冲传输相位差,λ为脉冲激光器的工作波长。分析结论表明通过计算频域干涉仪测量的传输时间差或传输相位差均可得到待测物体的运动位移历史,其区别在于传输相位差的计算精度较高,但要根据边界条件判断传输相位差是否存在π相位突变,采用传输相位差的计算方法并结合频域干涉仪的设计方法,本文研制的频域干涉仪时间测量精度优于0.001fs,位移分辨力高于0.1nm。3、通过对飞秒脉冲激光与物质相互作用机理的分析,提出了采用探测脉冲垂直入射被测物体表面的方式设计频域干涉仪的泵浦—探测实验光路,改进了国外探测脉冲斜入射被测物体表面的方法,并设计了新型飞秒脉冲激光频域干涉仪,与英国R.Evans等人设计的干涉仪相比,新型脉冲激光频域干涉仪具有结构简单、调试方便等优点,不但可测量金属膜在飞秒脉冲激光作用下的运动速度剖面,而且还可直接测量冲击波速度。4、对频域干涉仪的输出信号进行了数值模拟,研究发现：参与频域干涉的两脉冲之间的传输时间差T越小,则频域干涉条纹越稀；而时间延迟T越大,则频域干涉条纹越密,即频域干涉仪输出的条纹数目与两脉冲之间的传输时间差紧密相关,根据这种结论提出了泵浦—探测实验中泵浦脉冲和探测脉冲的同步方法,理论同步精度高于脉冲激光器脉宽,并设计了光路延迟器单步长延迟时间的标定方法。5、设计了光谱模式频域干涉仪和影像模式频域干涉仪,并提出了具体的调试方法,采用光谱模式频域干涉仪测量了200nm厚度铝膜在功率密度为1011W/cm2量级的飞秒脉冲激光作用下的纵波声速,实验测量数据表明具有高时空分辨力的频域干涉仪可用于探测脉冲激光作用材料诱导的超声波的波速；采用光谱模式频域干涉仪和影像模式频域干涉仪测量了200nm厚度铝膜在功率密度为1014W/cm2量级的飞秒脉冲激光作用下的冲击波速度、波后粒子速度和铝膜表面形貌变化历史,测量结果表明实测的冲击波速度和波后粒子速度满足铝材料的Hugoniot关系式。6、基于美国K.T.Gahagan等人对超快显微干涉技术的报道和球面波的干涉场理论,对超快显微干涉技术的工作原理、设计方法和数据处理方法进行了深入研究,提出了采用等倾干涉条纹数据分析方法处理超快显微干涉仪实验数据的思路。通过对飞秒脉冲激光超快显微干涉仪实验信号的理论分析和数值模拟,揭示了导致飞秒超快显微干涉场条纹对比度衰减的重要原因是飞秒脉冲激光器的线宽较宽。7、设计了一种紧凑性全光纤位移干涉仪,该干涉仪可采用Φ2.5或Φ1.25光纤探头测量金属膜在皮秒脉冲激光作用下的运动位移/速度历史,其测量结果不仅为超快显微干涉仪的实验参数设置提供依据,而且为超快显微干涉仪测量结果的可信度提供判据。
【Abstract】 The pulse frequency-domain interferometry, developed ten years ago, is an advanced noncontact laser interferometric instrument with fs time resolution and nm displacement resolution.By adopting pump-probe technology,the interferometry can capture the physics parameters changed to time during the ultrafast dynamic process. As a important instrument,the frequency-domain interferometry is used widely to measure the dynamic paraments of experimental sample loaded by fs laser or ps laser.Nowadays,the frequency-domain interferometric technology has been developed in America、Britain、France and Japan.The significance to develop frequency-domain interferometry is to provide a method for investigating dynamic response character of material loaded by ultrafast pulse laser and accelerating the development of ultrafast dynamics physics.In order to promote the development of pulse frequency-domain interferometry in china, the author has carried out a systematic investigation for the pulse frequency-domain interferometry during the past years which can be used to measure the dynamic parameters during these experiments in which the sample is loaded by plane-distributed or centrosymmetric-distributed shock wave.In addition,the interferometric microscopy has also been investigated to measure the dynamic parameters during those experiments in which the sample is loaded by non-centrosymmetric-distributed shock wave.The dissertation introduces the work principle、design method、data processing method and experiment method for the above two interferometrys in detail.The primary content and innovation is as follow.1、On the basis of the illumination proved by E.Tokunaga in university of Tokyo,the author has studied the work principle of frequency-domain interferometry carefully.It is found that the condition for two laser pulses interfering in frequency domain is as follows Where c is the velocity of light.θis the diffraction angle of characteristic spectrum.d is the grating and T is the delay time between the two pulses came from one laser.By analyzing the character of frequency-domain interference,it is found that the light intensity equation I(f) of frequency-domain interference can be expressed as follows Where f is the frequency of light.T is the delay time between the two laser pulses.Φis the phase between the two laser pulses. R is the intensity ratio of the two laser pulses. I0(f) is the spectrum function of the laser.2、After analyzing the data processing method of Michelson interferometer,the author has brought forward a unique data processing method for frequency-domain interferometry. By adopted the Fourier transform arithmetic,it is easy to calculate the transfers-phase difference or transfers-time difference between the two laser pulses.In addition,the author has deduced the relation between the transfers-time difference or transfers-phase difference measured by fequency-domain interferometry,which is expressed as follows Where s is the dynamic displacement of moving surface. T is the transfers-time difference between the two laser pulses.Φis the transfers-phase difference between the two laser pulses.λis the work wavelength of the pulse laser.From the above formula,it is easy to be found that the movement displacement can be got by calculating the transfers-time difference or transfers-phase difference measured by frequency-domain interferometry.By Adopting the data processing method for transfers-phase difference,we can get 0.001fs time resolution and 0.1nm displacement resolution which is more high than by the one for transfers-time difference.3、By irradiated the probe light to the target surface slantways, a new frequency-domain interferometry, different with foreign ones,has been developed to conduct the parameters measurement for ultrafast physics process taking place within tens of picoseconds.Compared with the frequency-domain interferometry designed by R.Evans, the unique interferometry designed by the author is of more advanges such as easy to debugging and can be used to measure the shock wave velocity and free-surface velocity profile at the same time.4、After investigating the output character of frequency-domain interferometry in detail, it came to a conclusion that the stripe number will become more and more with the transfers-time difference increasing.On the other hand, the stripe number will become less and less with the transfers-time difference decreasing. By means of the phenomenon, we can make the pump pulse and probe pulse arrive at the measured target synchronously when only one stripe is appear in frequency-domain interferometry.5、On the basis of elaborate investigation to the work principle,the author has improved the frequency-domain interferometry which can work on spectrum mode and image mode.The viability of frequency-domain interferometry was demonstrated by measuring the shock wave velocity、variety of surface shape and free-surface moving velocity of 200nm-thickness aluminium foil load by pulse lase. 6、To measure the three-dimensional shape,the author has constructed a interferometric microscopy and given a detailed illustration for it’s work principle、data processing method and design method.After elaborate analyzing for femtosecond-pulse interferometric microscopy in theory,it came to a conclusion that very wide spectrum is the result in the decreace of interference fringe visibility.7、By adopting theΦ2.5 orΦ1.25 measurement head and using the currently available conventional telecommunications elements,the authors have constructed a compact velocimeter called all-fiber displacement interferometry(AFDI). The velocimeter can measure the movement displacement profile of target loaded by picosecond pulse laser.Depend on the measurement result of AFDI,it is not only easy for us to set the parameters in those experiments using interferometric microscopy,but also help us to verify the viability of the interferometric microscopy.