【作者】 范书振；

【导师】 张行愚；

【作者基本信息】 山东大学 ， 光学工程， 2009， 博士

【摘要】 随机激光和光子局域化是近十几年来新发现的一种物理现象。随机激光器作为一种没有反射镜的激光器,在具有着类似传统激光器的单色性好的特点的同时,还有着多种有别于传统激光器的特点,如稳定性好,不怕振动,体积可以做的非常小,辐射的随机激光没有方向性和具有较低的空间相干特性等。因此随机激光在激光显示、照明工程、水文探测、医疗检测、地质研究、文档或材料的编码标记等多种领域有着众多的应用前景。同时,由于随机激光的相干反馈机制来源于光子的局域化,使得随机激光在基础物理研究领域也有着重要的应用。光子局域化来源于光的波动特性。当光在散射介质中受到强烈的散射时,光场的强烈相干会影响光的传播特性,导致光的传播受到抑制,并最终限制在一个局部区域内。光子局域化的研究在理解光的本质、研究光与物质在微观体系内的作用等方面有着重要的意义,对随机激光、光存储、激光防护等方面的研究有着重要的指导意义。基于以上原因,本文结合传统激光理论和随机激光的特性研究了光子的局域化特性。现有工作主要采用速率方程理论与光的扩散理论相结合来研究随机激光器。由于随机激光器的研究需要较大功率和较好的光束质量的激光光源,我们首先研究了两种泵浦方式的端面泵浦的固体激光器中的热透镜焦距的精确表达方式,以有利于高功率高光束质量的激光器的优化设计,并进行了实验研究。在第三章,我们研究了光子的预局域化的一种形式即光的相干背散射效应在弱信号情况下的简便测量方法,并根据研究提出了用于简化实验的方法。在第四章到第六章,我们分别从随机激光与放大自发辐射的光谱和时域特性比较方面,随机激光的光谱特性和时域特性随染料浓度及散射体浓度变化的方面对随机激光的特性和介质的局域化特性进行了研究。本文具体的研究内容为:1.研究了端泵固体激光器的热分布特性,结合失真的衍射理论（The Diffraction Theoryof Aberrations）,首次给出了经典激光器中热透镜焦距的更加精确的计算方法。首次提出了热透镜焦距不仅和激光器泵浦半径有关,还和激光的光束半径有关。理论研究了平顶光束和高斯光束两种不同泵浦模式下的热透镜效应。最后分别给出简化的拟合公式以方便计算应用。最后我们给出了平顶光束泵浦下两种不同泵浦半径下的实验结果。研究的结果对高功率固体激光器的优化有着重要的意义。2.研究了使用CCD器件对弱相干背散射信号的测量方法。首次给出了快速测量弱信号强度的相干背散射的改进方法,通过使用CCD对数据进行记录,然后通过数据处理,可以有效提取记录的信息。给出了拟合公式的处理,以及积分范围的优化言瘛Ｊ褂檬捣抡娴姆绞蕉苑椒ǖ目煽啃院徒峁奈ㄒ恍越辛搜橹?并对不同噪声程度引起的测量误差进行了估计。使用CCD测量了TiO₂纳米颗粒甲醇悬浊液的相干背散射特性,并使用此方法得到了有效的平均自由程信息。根据研究的结果,首次提出通过将传统的方法中的普通透镜换为柱透镜的方法,改变相干背散射信息的分布方式,以简化测量结果的数据处理,同时不损失相干背散射峰处的细节信息。3.对比研究了若丹明6G染料溶液在皮秒高能脉冲泵浦下的荧光辐射、软阈值以上的横向放大自发辐射、软阈值以上的纵向放大自发辐射、阈值以上的随机激光辐射以及我们称之为带有部分和较强相干反馈的纵向放大自发辐射的光谱特性和脉冲特性。通过对比,首次发现了不同相干反馈的程度对辐射光谱宽度和脉冲形式造成的不同影响。并根据实验的结果,首次提出了随机激光在一定的散射体浓度和染料浓度下,其辐射受到了一定程度的相干反馈影响。本发现打破了此种没有尖峰的随机激光辐射没有受到相干反馈影响的传统认识,也有利于加深对激光物理的认知。4.研究了皮秒高能激光脉冲泵浦的掺杂有纳米TiO₂散射体的染料溶液在不同散射体浓度和不同染料浓度下随机激光辐射的光谱特性,发现了不同染料浓度和不同散射体浓度下的随机激光辐射具有着不同的光谱峰值位置。首次发现了在特定染料浓度下,增加散射体的浓度引起的随机激光光谱峰值从蓝移向红移转变的拐点,并且发现在不同的染料浓度下,这个拐点位置的不同。通过理论研究,首次提出了基于染料的自吸收特性,从随机激光的辐射光谱峰值提取平均光程的方法,并通过扩散理论,首次验证了此光谱拐点代表了系统从扩散系统向弱局域化系统的转化。5.研究了高能皮秒激光脉冲泵浦的掺杂有纳米TiO₂散射体的染料溶液在不同散射体浓度和不同染料浓度的随机激光辐射的时域演化特性。实验中首次使用了双头光纤测量随机激光的时域特性,首次测量了随机激光的脉冲建立时间随染料浓度和散射体浓度变化的规律。结合经典激光理论的速率方程和扩散理论,对随机激光的动力学特性进行了初步研究,并结合实际实验情况对理论进行了改良。理论同实验结果进行比较发现,理论能够初步说明实验结果的趋势。本文的主要创新点:1.通过研究端泵固体激光器的热分布特性,结合失真的衍射理论（The DiffractionTheory of Aberrations）,首次给出了端泵固体激光器中平顶光束和高斯光束两种泵浦模式下热透镜焦距的更加精确的计算方法。首次提出热透镜焦距不仅和激光器泵浦半径有关,还和激光的光束半径有关。分别给出简化的拟合公式以方便计算应用,研究的结果对高功率固体激光器的优化有着重要的意义。2.通过研究使用CCD记录的样品的相干背散射图像的数据处理,首次给出了快速测量弱信号强度的相干背散射的简便方法,通过使用CCD对数据进行记录,然后通过数据处理,可以有效提取记录的信息。首次给出了这种方法中最佳积分范围的选择条件。首次给出了不同噪声引发的拟合结果误差估计。3.通过研究使用CCD记录的样品的相干背散射图像的数据处理,首次提出通过将传统的方法中的普通透镜换为柱透镜的方法,改变相干背散射信息的分布方式,有利于测量结果的进一步数据处理,同时不损失相干背散射的峰值细节信息。4.通过对比研究若丹明6G染料溶液的横向放大自发辐射、纵向放大自发辐射、随机激光辐射以及我们称之为带有部分和较强相干反馈的纵向放大自发辐射的光谱特性,脉冲特性,首次发现了相干反馈的不同程度对辐射光谱宽度和脉冲形式的不同影响,首次提出了随机激光在一定的散射体浓度和染料浓度下,其辐射受到了一定程度的相干反馈影响。本发现打破了此种没有尖峰的随机激光辐射中没有收到相干反馈影响的传统认识。5.通过研究掺杂有纳米TiO₂散射体的染料溶液的随机激光辐射的光谱特性,发现了不同染料浓度和不同散射体浓度下的随机激光辐射具有着不同的光谱峰值位置。首次发现了在特定染料浓度下,增加散射体的浓度引起的随机激光光谱峰值从蓝移向红移转变的拐点。6.通过研究掺杂有纳米TiO₂散射体的染料溶液的随机激光辐射的光谱特性,首次提出了基于染料的自吸收特性,从随机激光的辐射光谱峰值提取平均光程的方法。7.通过研究掺杂有纳米TiO₂散射体的染料溶液的随机激光辐射的光谱特性,利用扩散理论,首次提出了随机激光辐射的光谱拐点代表了系统从扩散系统向弱局域化系统的转化的观点。8.通过研究掺杂有纳米TiO₂散射体的染料溶液的随机激光辐射的时域特性,首次测量了随机激光的脉冲建立时间随染料浓度和散射体浓度变化的规律。结合经典激光理论的速率方程和扩散理论,对随机激光的动力学特性进行了初步研究,并结合实际实验情况对理论进行了改良。理论同实验结果进行比较发现,理论能够初步说明实验结果的趋势。更多还原

【Abstract】 Random laser and photon localization are both newly discovered less than two decades ago. Random laser is a special laser without mirrors. Its spectral width is narrow just like traditional lasers. And many properties of the random laser are different with the traditional laser. The random laser is robust because of the mirrorless structure, the scale can be very small, and the emission is not as directional and coherent as the traditional laser. These properties make the random laser an excellent candidate in the potential applications in laser displaying, lighting technology, monitoring of the liquid flow, medical biometrics, geological study, encoded marking of documents or material and so on. And because that the feedback of the random laser is believed to be the photon localization, it is also important in the study of the photon localization. The photon localization comes from the interference effect of the light wave. When the light is strongly scattered in the medium, the interference may lead to the slow-down of the light diffusion. And the light is expressed as localized when the diffusion coefficient is approaching zero. The studying of the photon localization is important in exploring the underlying physics of the light and in studying the interaction of the light with the matter microscopically. And also it is important in the engineering of the random laser, the optical storage, the laser protection and so on.So in this thesis, we study the properties of the photon localization by using the random laser and the traditional laser theory. The rate equations combined with diffusion theory are employed in the studying of the random laser. As a powerful laser with good beam quality is needed in the studying of the random laser as the pumping source, we first study the precise determination of the thermal focal length in the end-pumped solid state laser in the second chapter, which is useful in designing the laser with high power and good beam quality. In the third chapter, we study the pre-localization: coherent backscattering. We show a simple method of measuring weak coherent backscattering signal. In chapter four, we compared the random laser and various emissions from the dye solution. In chapter five and six, we show the studies of the random laser on the spectral and the temporal properties. The studies in chapter four to six show the localization in certain samples.The main works in the thesis are:1. The thermal distribution in the end-pumped solid state laser is studied. Using the diffraction theory of aberrations, we give the more precise determination of the thermal focal length. We show that the thermal focal length is determined not only by the pumping beam radius, but also the laser beam radius. Two pumping distributions are considered theoretically. The simplified equations are also presented for convenience. Experimental results are also given. This study is useful in the optimization of the laser with high power and good beam quality.2. A simple method of measuring the weak coherent backscattering signal using CCD is studied. The CCD is used to record the weak signal of the coherent backscattering. And the obtained data image is integrated to increase the signal to noise ratio. The optimized integration interval is suggested. The equation for fitting is studied to obtain the useful information. Simulation is presented to validate the method and to make sure the results are exclusive. The dependence of the fitting error on the noise is presented. Experiment results are given. And we give two new experimental configurations based on this method. The new configuration uses a cylindrical lens instead of the normal lens, which make the numerical process much easier and the detail of the coherent backscattering peak is maintained.3. The fluorescence, the transverse amplified spontaneous emission, the longitudinal amplified spontaneous emission, the random laser and the emissions which we call the longitudinal amplified spontaneous emission with partial coherent feedback and with stronger coherent feedback from the Rhodamine 6G dye solution under picosecond pulse pumping are compared. We show the effect on the spectra and the pulse width of the degree of the coherent feedback. And we show that in certain dye concentration and scatterer density, the random laser show partial coherent feedback. This study changed the idea that in such random laser there is no coherent feedback, and may help to increase the knowledge on the laser physics.4. The spectral properties of the random laser from the dye solution with nanoscatterers under picosecond pulse pumping are studied. The spectral peaks are shown to be at different wavelength for various dye solutions and scatterer densities. The inflection points of the spectral peak from blue shift to red shift are found for various dye concentration with increasing scatterer density. The inflection points are shown at different scatterer density for different dye concentration. Using the diffusion theory, the average optical path length can be estimated and the inflection points are believed to be the points when the system changing from the diffusive to weakly localized.5. The temporal properties of the random laser from the dye solution with nanoscatterers under picosecond pulse pumping are studied. A special fiber is used to measure the temporal properties. The build-up time of the random laser from samples with various dye solutions and scatterer densities. We studied the dynamic of the random laser using the traditional laser theory and the diffusion theory. The results from the theory and the experiments are agreed in the trend.The main innovations in the thesis:1. For the first time, we give the more precise determination of the thermal focal length of the end-pumped solid state lasers. We show that the thermal focal length depends on the laser beam radius for the first time. We give the simple equations for convenience.2. For the first time we present a simple method for recording the coherent backscattering using CCD. We present the optimized selection of the integration interval. We calculated the dependence of the error of the results on the noise.3. For the first time we show a new experimental configuration based on the method we present. A cylindrical lens is used instead of normal lens to make the process of the CBS image much easier. The detail of the CBS peak is maintained using this configuration.4. By comparing the transverse amplified spontaneous emission, the longitudinal amplified spontaneous emission, the random laser and the emissions which we call the longitudinal amplified spontaneous emission with partial coherent feedback and with stronger coherent feedback, for the first time we show the effect on the spectra and the pulse width of the degree of the coherent feedback. And for the first time we show that in certain dye concentration and scatterer density, the random laser show partial coherent feedback. This study changed the idea that in such random laser there is no coherent feedback, and may help to increase the knowledge on the laser physics5. By studying the spectral properties of the random laser from the dye solution with nanoscatterers under picosecond pulse pumping, for the first time we find the inflection points of the spectral peak from blue shift to red shift for various dye concentration with increasing scatterer density. The inflection points are shown at different scatterer density for different dye concentration.6. By studying the spectral properties of the random laser from the dye solution with nanoscatterers under picosecond pulse pumping, for the first time we give the method of estimating the average optical path length of the random laser by using the spectral peak and the diffusion theory.7. By using the diffusion theory and the dependence of the obtained average optical path lengths on the scatterer density, for the first time we show that the inflection points of the spectral peak is showing that the system is changing from the diffusion system to a weakly localized system.8. For the first time, the build-up time of the random laser from the dye solutions with nanoscatterers under picosecond pulse pumping with various dye solutions and scatterer densities are measured. We studied the dynamic of the random laser using the traditional laser theory and the diffusion theory. The results from the theory and the experiments are agreed in the trend.更多还原