【作者】 王晓峰；

【导师】 谭吉春；

【作者基本信息】 国防科学技术大学 ， 光学工程， 2008， 博士

【摘要】 用波长较长的泵浦光在材料中激发出波长较短的荧光,利用该反斯托克斯荧光（Anti-Stokes Fluorescence）使材料降温,这一过程称为荧光致冷（FluorescenceCooling）。本文分为两部分,第一部分研究掺镱固体荧光致冷原理与实验方法,第二部分研究掺镱激光增益介质辐射冷却理论。主要内容如下:一、掺镱介质荧光致冷原理与实验研究分别从光谱和熵的角度对荧光致冷的机理进行了阐述,推导了荧光致冷效率的表达式及其热力学上限。对掺镱介质荧光致冷性能进行分析,对泵浦光波长进行优化。测量了Yb³⁺∶KGW和Yb³⁺∶KLuW晶体的偏振吸收光谱和偏振发射光谱,以及掺镱磷酸盐玻璃的吸收光谱和发射光谱。利用倒易法和F-L（Fuchtbauer-Ladenburg）公式相互补充、校验,分别计算吸收截面和发射截面。结果表明:两种掺镱晶体的吸收截面及发射截面比掺镱磷酸盐玻璃的截面约高一个量级。利用测得的吸收和发射光谱数据,以绝对致冷效率（Absolute Cooling Efficiency）为判据,对泵浦光波长进行了优化,计算结果为:两种国产掺镱晶体的最佳泵浦波长在1022nm附近,最佳偏振方向为E//m（致冷效率计算值为1.68%）,在1W泵浦功率下,两种国产掺镱晶体的致冷功率计算值约为24～26mW（E//m偏振）,掺镱磷酸盐玻璃的致冷功率计算值约为1.9mW。对荧光自吸收效应进行分析与计算。介质中的荧光自吸收（荧光捕获）效应使出射的平均荧光波长变长、外量子效率降低,导致致冷效率降低。本文利用蒙特卡罗方法分析了掺镱磷酸盐玻璃的荧光自吸收效应。利用Fluorolog Tan 3-21型荧光光谱仪测量了厚1mm和3mm的样品（直径10mm）的侧向荧光和后向荧光,结果表明:蒙特卡罗计算得到的荧光谱线轮廓与实验曲线基本一致。设计掺镱晶体的荧光致冷验证性实验。以10ns级脉冲可调谐OPO激光为泵浦源,采用傅立叶变换光谱步进扫描（Step-scan）技术测量了厚度为1～2mm样品的热辐射光谱,得到了国产Yb³⁺∶KGW和Yb³⁺∶KLuW晶体的半定量荧光致冷测量结果。利用上述光谱测量方法,得到了固体荧光冷却过程的时域演变曲线。而文献报道的荧光致冷测量方法（光热偏转法、荧光法等）受原理上的限制,得不到热辐射光谱随时间演变的测量结果。二、掺镱激光介质辐射冷却理论泵浦源向固体激光增益介质提供产生激光所需能量的同时,在介质中产生无用热,限制激光器的输出平均功率,同时导致激光光束质量下降。为了持续发光,并改善光束质量,必须及时从激光增益介质中去除无用热。以荧光辐射的方式直接从增益介质内部移出热量,是激光器热管理的一条新的技术途径。提出了一种激光发光与荧光冷却交替运行的调Q激光器模型。该激光器的基本原理是:利用Q开关关闭时的荧光致冷效应,抵消产生Q开关激光脉冲时所沉积的热量,以间歇式冷却方式使增益介质中的热沉积趋近于零。利用准二能级速率方程,对Yb³⁺∶KGW调Q激光器进行了模拟计算,得出了实现间歇式辐射冷却所需的介质参数和泵浦条件。这种冷却方式迄今未见国内外有相关报道。对辐射平衡激光器（Radiation-Balanced Laser）的原理与单元技术进行研究。推导了实现辐射平衡运转时泵浦光、激光强度与增益介质以及谐振腔参数之间满足的关系式。利用速率方程理论及材料的吸收、发射截面数据,以小信号增益系数和转换效率为目标函数,对辐射平衡激光器的泵浦光、激光波长及偏振态进行了优化计算。结果表明:对于5%掺杂Yb³⁺∶KGW晶体,辐射平衡运转时的最佳泵浦光波长为1001nm、偏振态为E//m,最佳激光波长为1039nm、偏振态为E//p（b）,该晶体发射的反斯托克斯荧光（平均波长995nm）起辐射致冷作用。根据辐射平衡激光器低热泵浦的需要,建立了二极管巴条—空心导管—增益介质一体化耦合计算模型,计算了增益介质内部泵浦光功率密度的三维分布。以传输效率为目标函数,对空心导管进行设计和优化。角谱传播法和速率方程结合,计算了Yb³⁺∶KGW激光器中自再现模的形成过程。分析了激光起振时的弛豫振荡现象,得出了激光光斑分布的时变过程。本文关于荧光致冷机理和掺镱激光介质辐射冷却技术的研究结果,对研制低内热固体激光器具有指导作用。更多还原

【Abstract】 Some materials emitted light at shorter wavelengths than that with which the material was illuminated due to thermal （phonon） interactions with the excited atoms. This process is termed anti-Stokes fluorescence cooling.This thesis consists of two parts: the first part focuses on the mechanism and experiments of solid fluorescence cooling, and the second parts focuses on the radiation cooling of Yb-doped laser mediums. The main contents are as follows:Ⅰ. Principles and experiment of fluorescence cooling of Yb-doped mediumThe mechanism of fluorescence cooling is discussed from the points of view of spectroscopy and entropy, and the expression and thermodynamics limitation of cooling efficiency are achieved.Fluorescence cooling capabilities of Yb-doped medium are analyzed and pump wavelength are optimized. The absorption and emission spectrums of Yb³⁺:KGW crystal, Yb³⁺:KLuW crystal, and Yb-doped phosphate glass are measured, and the absorption and emission cross sections are calculated with reciprocity methods and F-L equation. The results show that both crystals’ absorption cross sections are about ten times over those of the Yb-doped phosphate glass. By use of the absorption and emission spectrums, the pump wavelengths are optimized with the optimization function of absolute cooling efficiency. The results show that the optimal pump wavelength for Yb-doped phosphate glass is near 1025nm （cooling efficiency is 0.18%）, and the optimal pump wavelentgh for both crystal is near 1022nm, and the polarizaiton is E//m （cooling efficiency is 1.68%）.The influences of fluorescence reaborption are calculated and analyzed. Because of a spectral overlap between absorption and fluorescence, reaborption-reemission events cause a redshift in the observed fluorescence spectrums, so as to decrease the cooling efficiency. The effects on fluorescence cooling of fluorescence reaborption are analyzed with Monte Carlo method. The side and back emission spectrums of the samples with 1mm and 2mm thickness （diameter 10mm） are measured with Fluorolog Tan 3-21 fluorescence spectrometer. The fluorescence shape calculated by Monte Carlo method is agreed with the measured data.Validation experiments of fluorescence cooling of Yb-doped crystals are carried out. The thermal radiation spectrums of 1～2mm thickness samples are measured by use of the step-scan technology of Fourier transform spectrometer with the pump source of tunable OPO laser whose pulse duration is about 10 ns. The fluorescence cooling of the Yb³⁺:KGW and Yb³⁺:KLuW crystals are observed. The time resolved process of the solid fluorescence cooling is acquired, which is very difficult to get by the reportorial methods in literature （such as photothermal deflection method, fluorescence method, and etc） because of their principle limitations.Ⅱ. Radiation cooling of Yb-doped laser mediumThe processes of excitation and stimulated emission always results in heat generation within the lasing medium. This produces increased temperatures and stresses in the lasing medium which limit beam quality and average power. The fluorescence cooling removes heat from the medium directly, which can be used in solid-state laser cooling.A new approach to the design of Q-switched solid-state lasers is proposed which offsets heating loads by anti-Stokes fluorescence. In this ideal system, the pump will cool the gain media when the Q switch is off and removes the thermal loads that generated by the laser pulse when the Q switch is on. The operation of this model with Yb³⁺ :KGW crystal is simulated and the parameters and requirements is educed.Principles and cell technology of radiation-balanced lasers are researched. The relations among pump intensity, laser intensity, gain medium parameters and resonance cavity parameters are derived, and the output performances are discussed for radiation-balanced laser.The output laser wavelength and polarization are derived by use of rate equations. Pump wavelength, laser wavelength and polarization optimized based on small signal gain coefficiency and optics - optics efficiency function. The results show that the optimal pump and laser wavelenthes are 1001nm and 1039 nm, and the optimal pump and laser polarizations are E//m and E//p（b） for Yb³⁺:KGW crystal radiation-balanced laser.The LD bar-hollow duct-gain medium coupling calcalation model is proposed for the requirements of radiation-balanced laser. The 3D pump intensity is calculated and the initial gain distribution is achieved in the gain medium. The duct parameters are optimized based on the transmission efficiency.The forming process of self-reproductive mode is calculated with combining the rate equations and angular spectrum propagation theory. Thespatio-temporal dynamics of the mode formation are analyzed, and output laser characteristics are achieved for the Yb³⁺:KGW gain medium.The mechanism and application in Yb-doped laser mediums cooling by anti-Stokes fluorescence in this thesis have references to low-heat solid-state lasers.更多还原