固体红外激光器理论与实验研究

【作者】 龙井宇；

【导师】 白晋涛；

【作者基本信息】 西北大学 ， 光学， 2009， 博士

【摘要】 激光二极管泵浦的高功率固体激光器具有效率高、光束质量好,结构紧凑、寿命长等优势,在工业生产、科学研究和军事领域都有着广泛的应用。发射1.06μm近红外激光的掺Nd³⁺固体激光器已经发展了棒状、盘片和光纤等多种工作介质形式,其中棒状激光晶体适应多样的泵浦耦合结构,拥有良好的谐振腔模式与泵浦模式交叠,以及实用性和紧凑性好等优势,依然是使用最广泛的激光介质。约2μm波长的中红外激光辐射由于具有对人眼安全,水的吸收系数高,适合于用光纤传输等诸多优点而被广泛应用于相干激光雷达,大气遥感探测,高精度外科手术等领域。其中氙灯泵浦室温工作的Cr:Tm:Ho:YAG激光器工作在2.1μm波长,可用于某些外科手术（包括切骨术、硬组织烧融和结石碎裂等）。目前,高功率固体激光器研究中最重要的问题之一是热管理问题,它包括三个方面:首先,提高泵浦源的辐射效率和激光的辐射转换效率以降低介质的上的热耗散总量;其次,实施有效的冷却,及时排除激光介质上的热沉积;第三,对冷却导致的热效应进行补偿和缓解。本论文围绕高功率固体激光器的热管理问题展开了较为深入的理论和实验研究。理论方面:给出了连续泵浦主动O开关运转Nd:YAG激光器热产生率的解析表达式。建立了热产生率与量子效率、斯托克斯效率、辐射量子效率、交叠效率和储能提取效率等因子的函数关系。考虑了Q开关重复频率及泵浦速率对热产生率的影响。研究结果表明,在其它条件相同时,主动Q开关运转固体激光器的热产生率高于连续运转情形。基于高斯光束的传输矩阵理论,构建了包含球面端面激光棒的谐振腔稳定性分析方法。并对特定的激光棒参数和谐振腔参数进行了数值计算。计算结果表明,对于几类典型的谐振腔如平平腔、平凹腔,球面端面激光棒可以显著的扩展强热聚焦作用下的稳定工作范围;球面端面激光棒在提高约束稳定性和热稳定性的同时,通常是以牺牲有效模式体积为代价的。实验方面:进行了LD侧面泵浦高功率Nd:YAG激光器的实验研究,采用了球面端面激光棒,LD阵列7向侧面泵浦。在最高泵浦电功率6820W时,获得最高的连续运转激光功率1414W,实现电光转换效率21%。实验上证明了球面端面激光棒的引入作为热透镜补偿的一种方式具有显著的可行性。设计出一种测量闪光灯泵浦固体激光器热透镜焦距的实验方法。该方法通过测定谐振腔由稳定区进入非稳定区的临界区域的泵浦能量,结合激光谐振腔稳定性条件,可以计算激光棒的热透镜效应系数。实验测量了氙灯泵浦Cr:Tm:Ho:YAG激光器不同泵浦条件下的热焦距值,结果表明,热焦距随泵浦脉冲宽度、冷却水温度等参数变化,在泵浦脉冲宽度分别为400μs,600μs,800μs时测得热透镜系数分别为5.59dpt/kW、4.84dpt/kW、4.38 dpt/kW。冷却水温分别为25℃,20℃,15℃时测得热透镜系数分别为5.76 dpt/kW、5.59 dpt/kW、5.36dpt/kw。利用算得的热焦距值,理论计算了激光棒的温度分布。对单脉冲和重复率泵浦Cr:Tm:Ho:YAG激光器的输出特性进行了实验研究。研究了谐振腔长度、输出镜透过率、冷却水温度、泵浦脉冲宽度、泵浦重复频率等参数对激光输出性能的影响。结果表明,激光输出能量随谐振腔长度的增加大幅度下降。输出能量随温度下降而上升的现象部分归结为Ho激光能级的准三能级本质,部分归结为高阶的热透镜效应。后两种参数对激光输出能量的影响归结为更复杂的多种因素,如上激光能级的有效储能,热透镜效应等。实验结果:单脉冲泵浦Cr:Tm:Ho:YAG激光器的输出能量最高达到3.3J,斜率效率4.9%。重复率10Hz泵浦时,获得最高平均功率25.8W,斜率效率4.3%。更多还原

【Abstract】 Laser diode pumped high power solid state lasers have many applications in the manufactures,scientific researchs and military technologies,owing to the advantages of high efficiency,high beam quality,compact structure and being last.Solid state lasers doped Nd³⁺ operating at near-infrared 1.06μm have been designed into kinds of geometry including rod, disc and fiber.The laser rods adaptive to various pumping structure,with good overlap between resonator mode and pumping mode,compaction and convenience,are most extensively applied.Infrared 2μm laser has the advantages including eyes -safety,the strong absorption in water and low absorption in atmosphere,adaptive to fiber transmission,which are fit for the coherent laser radar,remote sensing and the precise surgery.Xenon Flashlamp-pumped Cr:Tm:Ho:YAG laser operating at 2.1μm is applicable in the osteotomy, the lithotripsy,the ablation of soft tissue and so on.Nowadays one of the most important problems in research of high power solid state lasers is the thermal management which consists of the three aspects:firstly,improving the radiation efficiency of the pump source and the enengy conversion efficiency of laser to depress the total heat dissipation in laser material. Secondly,actualizing the effective cooling to dispel the heat dissipation.Thirdly, compensating or lowering the thermal effects.This dissertation expatiated on the theoretical and experimental studies on the thermal management of high power solid state lasers.Theoretical studies:The theoretical model for the heat generation coefficient of continuous pumped actively Q- switched Nd:YAG laser was developed.The functional relation between the heat generation coefficient and quantum efficiency,Stokes efficiency, radiation efficiency,overlap efficiency and energy storage efficiency was founded.The model took into account the effects on the heat generation coefficient from repetition rate of Q-switch and pumping rate.It was found that the actively Q- switched operating solid satet laser has more heat dissipation than the free operating laser does.The model for calculating the stability of the resonator containing a laser rod with curvature ends was developed,which was based on the transfer matrix theory of Gauss beam. The results from the numerical simulations on some typical resonators indicate that the laser rod with curvature ends can expand the stable region of plano-plano,plano-concave and plano-convex cavity notability;despite the laser rod with curvature ends can improve the stability of above mentioned cavities,it costs the effective mode volume.Experimental studies:An experimental study on LD side pumped high power Nd:YAG laser was detailed,which employed a laser rod with curvature ends and was pumped by seven LD arrays around the rod.A output laser power of 1414W was achieved at the maximum pump electrical power of 6820W,realizing the electrical-optical conversion efficiency of 21%. The experimental results confirmed that employing the laser rod with curvature ends to compensating thermal lens effect is available.An experimental method measuring the thermal focal length of solid state laser was designed basing on the analysis of the dynamic thermal stable resonator.By measuring the pump energy where a stable resonator becomes unstable,taking into account the resonator stability conditions,the thermal lens coefficient of the laser rod can be calculated.The thermal focal lengths of flashlamp-pumped Cr:Tm:Ho:YAG laser in various pumping conditions was calculated and the effects of the pumping pulse width and the cooling water temperature was analyzed.The measured thermal lens coefficient is 5.59dpt/kW,4.84 dpt/kW,4.38 dpt/kW at the pumped pulse width of 400μs,600μs,800μs,respectively.The measured thermal lens coefficient is 5.76 dpt/kW,5.59 dpt/kW,5.36dpt/kW at the cooling water temperature of 25℃,20℃,15℃,respectively.Utilizing the above results,the temperature distribution in laser rod cross section was estimated.A Xenon Flashlamp-pumped Cr:Tm:Ho:YAG laser operating at 2.1μm operating at both single pulse and repetiotion-rate pump has been demonstrated.The resonator length,output couple transition,cooling water temperature,pumped pulse width and pumped repetition-rate dependents of laser output energy was investigated experimentally.It was founded that laser output energy decline with the resonator length increasing.The quasi-three level nature of Ho laser at room temperature and high order term of thermal lens effect take responsibility for the phenomenon which laser output energy increase with cooling water temperature reduced.The laser energy varies with pumped pulse width and pumped repetition-rate,which is by reason of some factors,such as effective storage energy and thermal lens effect.The maximum laser average power of 25.8W at 10Hz repetition frequency with slope efficiency of 4.3%and the maximum laser pulse energy of 3.3J with a maximum slope efficiency of 4.9%at single pulse pump are obtained.更多还原