【作者】 张琛；

【导师】 张行愚；

【作者基本信息】 山东大学 ， 光学工程， 2009， 博士

【摘要】 本论文分两个部分,一部分是Nd:YAG陶瓷激光器的理论和实验研究;另一部分是TiO₂随机介质中的光扩散特性研究。在固体激光器领域,Nd:YAG单晶材料以其高增益、低阈值、热特性和机械性能优良而迅速成为应用最为广泛的固体激光增益介质。但是由于YAG单晶是用提拉法生长的,成本高,生长周期长,掺杂浓度低,使它的应用受到一定的限制。激光陶瓷是一种由细小、紧密填充的晶粒构成的马赛克结构,该材料除具有陶瓷的多晶体性质外,采用真空烧结方法制备的激光陶瓷还展现出非常接近单晶的透明性和热机械特性。Nd:YAG激光陶瓷不仅具有与Nd:YAG单晶几乎相同的热导率、吸收光谱、荧光光谱和荧光寿命,而且相对于单晶材料还有制作简单、成本低、可以生长大尺寸材料、机械特性良好、掺杂浓度高、多功能性以及可大批量生产等优点。由于光学陶瓷材料弥补了单晶的很多不足,Nd:YAG陶瓷材料很有希望成为同单晶Nd:YAG一样广泛应用的固体激光增益介质。目前有很多科研人员致力于陶瓷材料和陶瓷激光器的研究,已经取得了很多成果。日本的研究人员在开发陶瓷增益介质和陶瓷激光器方面作出了杰出的贡献,居世界领先地位。俄罗斯、美国、德国、罗马尼亚、英国、新加坡、印度和西班牙等国家的研究者都在积极地参与陶瓷激光器的研究。在中国大陆,山东大学、上海光学精密机械研究所、清华大学、四川大学也在积极地参与陶瓷激光器的研究工作,在理论和实验方面已经取得了一些研究成果。速率方程是研究激光器特性的有效理论工具。考虑腔内激光及反转粒子数密度空间分布的四能级调Q激光器的速率方程组已有详细分析并被广泛引用。目前还没有考虑激光下能级再吸收以及腔内激光和反转粒子数密度空间分布的准三能级主动调Q激光器速率方程组。光子局域化是电子局域化的光子学类比,是指散射介质中多重散射波的干涉效应使得光在其中的传播受到抑制。通过测量超短激光脉冲在随机样品中扩散过程的时间进化,可以得到光在该样品中的光子局域化程度,因而随机介质的光扩散时域研究成为研究光子局域化的一个重要方法。本论文中,对Nd:YAG陶瓷激光器的进行了理论和实验研究,对超短脉冲在TiO₂随机介质的光扩散特性进行了研究。研究内容主要有:1.进行了LD端面泵浦输出波长为1064 nm的Nd:YAG陶瓷激光器的实验研究,详细研究了在连续运转、主动调Q和被动调Q下的激光输出特性。并与相同实验条件下的1064 nm单晶Nd:YAG激光器进行了比较。Nd:YAG陶瓷激光器获得了比Nd:YAG单晶激光器略微好的实验结果。2.进行了LD端面泵浦输出波长为946 nm的主动调Q Nd:YAG陶瓷激光器的实验研究。当泵浦源为纤芯直径为600μm的光纤耦合LD,输入泵浦功率为17.4 W,脉冲重复率为50 kHz时,获得了平均功率为2.6 W的946 nm激光输出。3.在速率方程组中考虑激光下能级再吸收以及腔内激光和反转粒子数密度的空间分布,得到了准三能级主动调Q激光器的速率方程组。引入了一组综合参量,得到准三能级主动调Q激光器的归一化速率方程。通过数值求解该速率方程,得到了一组普适曲线,这些曲线不仅表明了归一化的主动调Q脉冲参量与这些综合参量之间关系,还可以用来估计任何一个LD泵浦或闪光灯泵浦的的准三能级主动调Q激光器的输出脉冲参量。用该理论对946nm的主动调Q Nd:YAG陶瓷激光器进行理论计算,得到的理论结果与实验结果大致符合。4.在考虑激光下能级再吸收以及腔内激光和反转粒子数密度空间分布的准三能级被动调Q激光器速率方程组中,引入一组综合参量,获得了准三能级被动调Q激光器的归一化速率方程。通过数值求解该归一化速率方程,得到一组普适曲线,这些曲线不仅表明了归一化的被动调Q脉冲参量与这些综合参量之间关系,还可以用来估计许多LD泵浦或闪光灯泵浦的准三能级被动调Q激光器的输出脉冲参量。用该理论对946 nm的被动调Q Nd:YAG陶瓷激光器进行理论计算,得到的理论结果与实验结果大致符合。5.进行了以TiO₂纳米颗粒的悬浮液为随机介质的532 nm超短激光脉冲的光扩散研究,激光脉冲宽度为60 ps,脉冲重复率为10 Hz,由条纹相机测量了透过随机样品后光脉冲的时间特性曲线,通过理论拟合得到光扩散常数,研究了光扩散常数随样品浓度的变化关系,扩散常数随着TiO₂纳米颗粒在悬浮液中浓度的增加而减小。6.进行了以TiO₂纳米颗粒为随机介质的800 nm超短激光脉冲的光扩散研究,激光脉冲宽度为10 ps,脉冲重复率为100 Hz,由数字存储示波器测量得到透过随机样品后光脉冲的时间特性曲线,当TiO₂纳米颗粒在随机样品中的浓度达到1.36×10¹⁵ cm^-3时,实验结果与光扩散理论结果出现偏离,说明开始出现弱光子局域化现象。本论文主要创新点如下:1.首次实现了LD端面泵浦946 nm主动调Q Nd:YAG陶瓷激光器的高效运转。当泵浦源为纤芯直径为600μm的光纤耦合LD,输入泵浦功率为17.4 W,脉冲重复率为50 kHz时,获得了平均功率为2.6W的946 nm激光输出。2.在速率方程中考虑激光下能级的再吸收以及腔内激光和反转粒子数密度的空间分布,首次得到了准三能级主动调Q激光器的速率方程组。引入了一组综合参量,得到准三能级主动调Q激光器的归一化速率方程。通过数值求解该速率方程,得到了一组普适曲线,这些曲线不仅表明了归一化的主动调Q脉冲参量与这些综合参量之间关系,还可以用来估计任何一个LD泵浦或闪光灯泵浦的的准三能级主动调Q激光器的输出脉冲参量。用该理论对946 nm的主动调Q Nd:YAG陶瓷激光器进行理论计算,得到的理论结果与实验结果大致符合。3.在考虑激光下能级的再吸收以及腔内激光和反转粒子数密度空间分布的准三能级被动调Q激光器速率方程组中,引入一组综合参量,首次获得了准三能级被动调Q激光器的归一化速率方程组。通过数值求解该归一化速率方程,得到一组普适曲线,这些曲线不仅表明了归一化的被动调Q脉冲参量与这些综合参量之间关系,还可以用来估计许多LD泵浦或闪光灯泵浦的被动调Q激光器的输出脉冲参量。用该理论对946 nm的被动调Q Nd:YAG陶瓷激光器进行理论计算,得到的理论结果与实验结果大致符合。4.进行了以TiO₂纳米颗粒为随机介质的超短激光脉冲的光扩散研究,首次测量了TiO₂随机介质光扩散中扩散常数与样品浓度的变化关系。当TiO₂纳米颗粒在随机样品中的浓度达到1.36×10¹⁵ cm^-3时,实验结果与光扩散理论结果出现偏离,说明开始出现弱光子局域化现象。更多还原

【Abstract】 There are two parts in this dissertation.One is the theoretical and experimental studies on Nd:YAG ceramic lasers.The other is the investigations on the light diffusion in TiO₂ random media.Neodymium doped yttrium aluminum garnet（Nd:YAG） single crystal has been the most widely used solid-state laser material.But its fabrication requires expensive equipment and crucible material.Neodymium doped yttrium aluminum garnet（YAG） transparent ceramics are nowadays attracting a great interest as a laser host material because of its advantages over the traditionally used Nd:YAG crystals.These advantages are the excellent thermal properties,the lower manufacturing costs,the possibility of high neodymium concentrations without any decrease in the optical quality of the gain medium,no growth limitation of shape and size,ease of power scaling,and the possibility of direct composite fabrication.Because of these advantages,excellent quality Nd:YAG ceramic laser materials are good alternative to the widely used Nd:YAG single crystals.Many scientists are engaged in the investigations of ceramic laser materials and ceramic lasers in recent years,and have made many achievemants.Scientists from Japan act as the leading role in the regime of ceramic growth and research on the ceramic lasers.Scientists from Russia,America,Germany,Ramania,Spain etc.are taking part actively in the field of the ceramic lasers.In China,the research groups from Shandong University,Shanghai Institute of Optics and Fine Mechanics,and Tsinghua University work at the research of the ceramic lasers.Rate equations are efficient tools describing the operation of lasers.The rate equations considering the spatial distributions of the intracavity photon density and the population inversion density for the four-level Q-switched lasers have been analyzed and widely cited.So far,there are no rate equations for a quasi-three level actively Q-switched lasers considering the influence of the reabsorption of the lower level and the spatial distributions of the intracavity photon density and the population inversion density.Photon localization is proposed as the analog of electron localization.It refers to an inhibition of wave transport in scattering media due to the interference of multiple scattered waves.The time-resolved measurement of light diffusion in random media is an effective method to investigate photon localization.In this dissertation,we have studied Nd:YAG ceramic lasers theoretically and experimentally.By using ultrashort pulses,we have studied the time-resolved transmission properties of TiO₂ random media.The main content of this dissertation is as follows:1.Diode-pumped continuous wave,actively Q-switched and passively Q-switched ceramic lasers operating at 1064 nm are investigated.Comparative studies of Nd:YAG ceramic lasers and Nd:YAG single crystal lasers are performed.The laser performance of the Nd:YAG ceramic is found to be a little better than that of Nd:YAG single crystal.2.The characteristics of the 946 nm actively Q-switched ceramic lasers are investigated.Using a fiber-coupled laser diode with a fiber diameter of 600μm as the pump source,an average output power of 2.6 W at a repetition rate of 50 kHz is obtained when the incident pump power is 17.4 W.3.The rate equations for the quasi-three-level actively Q-switched lasers are obtained by considering the reabsorption of the lower level and the spatial distributions of the intracavity photon density and the population inversion density.These rate equations are normalized by introducing some synthetic parameters.By solving the normalized rate equation numerically,a group of general curves are generated.These curves can give a good understanding of the dependences of the laser pulse characteristics on the synthetic parameters.They can also be used to estimate the laser pulse characteristics of any quasi-three-level actively Q-switched laser.The laser pulse characteristics of the 946 nm actively Q-switched Nd:YAG ceramic lasers are calculated,and the theoretical results are in fair agreement with the experimental results.4.By introducing some synthetic parameters into the rate equations considering the reabsorption of the lower level and the spatial distributions of the intracavity photon density and the population inversion density,the normalized rate equation for the quasi-three-level passively Q-switched lasers is obtained.By solving the normalized rate equation numerically,a group of general curves are generated. These curves can give a good understanding of the dependences of the laser pulse characteristics on the synthetic parameters.They can also be used to estimate the laser pulse characteristics of many quasi-three-level passively Q-switched lasers.The laser pulse characteristics of the 946 nm passively Q-switched Nd:YAG ceramic lasers are calculated,and the theoretical results are in fair agreement with the experimental results.5.Time-resolved transmission properties of 532 nm ultrashort pulse（60 ps pulse width,10 Hz repetition rate） in TiO₂ suspended solutions are investigated.A streak camera is used to record the transmitted signals.The diffusion constant is obtained by theoretical fitting,and the relation between the diffusion constant and the particle density of TiO₂ in the suspended solution is studied.The diffusion constant decreases with increasing particle density.6.Time-resolved transmission properties of 800 nm ultrashort pulse（10 ps pulse width,100 Hz repetition rate） in TiO₂ powders are investigated.A digital storage oscilloscope is used to record the transmitted signal.When the particle density is 1.36×10¹⁵ cm^-3,the experimental result shows a little deviation from the diffusion theory,which may be the signature of weak localization in the random media.The main innovations of this dissertation are as follows:1.The diode-pumped actively Q-switched Nd:YAG ceramic laser operating at 946 nm is obtained for the first time.Using a fiber-coupled laser diode with a fiber diameter of 600μm as the pump source,an average output power of 2.6 W at a repetition rate of 50 kHz is obtained when the incident pump power is 17.4 W.2.The rate equations for the quasi-three-level actively Q-switched lasers are obtained for the first time by considering the reabsorption of the lower level and the spatial distributions of the intracavity photon density and the population inversion density.These rate equations are normalized by introducing some synthetic parameters.By solving the normalized rate equation numerically,a group of general curves are generated.These curves can give a good understanding of the dependences of the laser pulse characteristics on the synthetic parameters.They can also be used to estimate the laser pulse characteristics of any quasi- three-level actively Q-switched laser.The laser pulse characteristics of the 946 nm actively Q-switched Nd:YAG ceramic lasers are calculated,and the theoretical results are in fair agreement with the experimental results.3.By introducing some synthetic parameters into the rate equations considering the reabsorption of the lower level and the spatial distributions of the intracavity photon density and the population inversion density,the normalized rate equation for the quasi-three-level passively Q-switched lasers is obtained for the first time. By solving the normalized rate equation numerically,a group of general curves are generated,These curves can give a good understanding of the dependences of the laser pulse characteristics on the synthetic parameters.They can also be used to estimate the laser pulse characteristics of many quasi-three-level passively Q-switched lasers.The laser pulse characteristics of the 946 nm passively Q-switched Nd:YAG ceramic lasers are calculated,and the theoretical results are in fair agreement with the experimental results.4.Time-resolved transmission properties of ultrashort pulse in TiO₂ random media are studied.The relation between the diffusion constant and partical density in TiO₂ random media is investigated for the first time.When using the TiO₂ powders as the sample with a particle density of 1.36×10¹⁵ cm^-3,the experimental result shows a little deviation from the diffusion theory,which may be the signature of weak localization in the random media.更多还原