【作者】 杨子宁；

【导师】 陆启生； 王红岩；

【作者基本信息】 国防科学技术大学 ， 光学工程， 2012， 博士

【摘要】 半导体泵浦碱金属蒸汽激光器（DPAL）兼具固体和气体激光的优势，具有量子效率高、气体介质可循环流动散热、近红外原子谱线大气透过性好、全电操作、结构紧凑等特点，有望发展成为新一代高能激光光源。目前，人们对DPAL的功率定标放大能力开展了系统深入的研究，并于近期成功实现了高效的千瓦级连续输出，正处于功率提升的关键发展时期。鉴于此，本文从理论和实验两个方面对铷DPAL开展了研究，主要包括以下几个方面的内容：1、分析了铷原子的光学特性及其与缓冲气体的相互作用情况，在此基础上建立了基于纵向泵浦结构的速率方程模型，并提出具有快速收敛特性和高计算精度的数值算法，对激光器的动力学特性进行了全面的理论分析。结果表明，DPAL的激射过程会显著促进泵浦吸收效率的提高，激光器不是工作于小信号增益状态，而是依靠碱金属原子引擎式的快速循环工作完成泵浦能量的吸收、转移和激光输出的；饱和效应的本质来源于精细结构弛豫速率的不足，可以通过调节缓冲气体种类及分压或者增加碱金属原子密度的方案解决，使激光器工作于接近准二能级状态的线性输出区；作为三能级系统，泵浦强度应远大于阈值强度以有效抑制自发辐射的影响；激光器的操作存在最优温度，该温度下泵浦吸收和荧光损耗之间达到最佳平衡而获得最大的光光转换效率；泵浦强度一定时，采用线宽较窄的泵浦光源和较低缓冲气压的增益介质易于实现高光光转换效率，反之则需要提高泵浦强度才能达到相同的效率，但后者对光谱漂移等因素造成的影响具有更好的容忍度；最后指出纵向泵浦结构中增益长度和温度之间具有等价性，并分析了腔内损耗以及耦合输出率的影响。上述结论对激光器的优化设计具有指导意义。2、对大功率窄线宽半导体泵浦源开展了实验研究。采用Littman外腔结构对单宽面元半导体激光器进行线宽压窄，实现了线宽小于0.06nm功率10W的激光输出，调谐范围5nm，外腔效率60%；对线阵半导体激光器采用复合外腔方案，有效抑制了笑脸效应的不利影响，实现了线宽小于0.1nm功率41W的激光输出，外腔效率53%；最后对线阵半导体激光器采用体光栅方案，获得了线宽0.1nm，功率74W的激光输出，外腔效率达到95%。鉴于体光栅方案具有高外腔效率和结构紧凑等优势，采用该方案作为DPAL的泵浦源。3、对铷DPAL开展了实验研究。首先研究了铷-乙烷-氦气混合增益介质的吸收光谱特性、泵浦吸收特性以及荧光特性，在此基础上采用端面泵浦结构在准连续泵浦模式下进行了出光实验，获得了峰值功率1.4W的795nm铷激光输出，光光转换效率4.8%，斜率效率7.5%，分析认为泵浦吸收不足、低模式匹配因子以及高谐振腔损耗是造成光光效率较低的主要原因，并提出了进一步的解决方案；通过观察升温过程中出射光斑形貌变化研究了激光器的阈值行为；在56W连续泵浦模式下激光器工作了4.4s后窗口损坏，通过拉曼谱分析并结合文献报道，认为铷原子与乙烷发生了化学反应，且窗口材料也可能参与了反应，进一步的分析认为这一现象与局部温升过高有关，良好的热管理可以避免化学反应的发生。4、建立了横向泵浦流动介质DPAL理论模型，通过与文献中实验结果的对比验证了模型的有效性；在此基础上对泵浦、激光和气流方向三者垂直这一结构的高功率定标放大方案进行了研究，结果表明：在保持一定的合理泵浦强度条件下，通过增加介质长度及相应的泵浦功率能够在与热管理解耦的前提下实现按比例功率放大，是未来激光器功率提升的主要方案，宽度和高度方向的设计需要根据热管理、操作温度、输出光斑形状以及泵浦聚焦准直等实际工程因素综合考虑；对单侧、单侧双程以及对称双侧泵浦三种结构进行了对比分析；对兆瓦级DPAL进行了概念设计，在参数基本合理或是短期未来有望实现的条件下预测了输出功率1.7MW光光效率大于85%的结果，进一步理论证实了DPAL未来的发展潜力。5、建立了MOPA结构DPAL理论模型，提出新的用于计算体块状增益介质ASE效应的方法，综合考虑了纵向和横向ASE效应，将其耦合进速率方程进行计算，能够求解出ASE损耗的范围；在此基础上研究了种子光强、泵浦光强及操作温度等重要参量的影响，结果表明采用强注入种子光（~kW/cm2）进行饱和放大能够有效抑制ASE效应，确保其不会成为功率提升的瓶颈性因素；对高功率定标放大方案进行了研究，指出在固定泵浦强度下延展介质长度及相应泵浦功率将是最优选择方案。更多还原

【Abstract】 As a hybrid gas phase/solid state laser, diode pumped alkali vapor lasers (DPALs)have great potential in the future high power laser field due to its many advantages, forexample the high quantum efficiency, convenient thermal management by flowing thegaseous medium, high transmittance for near infrared laser spectrum, and electricallydriven compact system. In the past decade, the concept and power scaling ability ofDPALs have been demonstrated, and a high efficient CW kilowatt-class DPAL has beensuccessfully realized. Due to the potential and importance of DPALs, we have madestudies on the diode pumped rubidium vapor lasers. The main contents are presented asfollows:1. For rubidium atom, the optical properties and its interaction with buffer gasesare analyzed, the rate equation based model for longitudinally pumped DPALs isproposed as well as the fast convergent and high accurate numerical algorithm, based onthe model the kinetics of DPALs are studied. The results show that, for alkali lasers, thelasing process will dramatically enhance the pump absorption, these lasers are workingin an “atomic engine” mode rather than extracting energy from the small signal gain.The saturation effect is due to the insufficient fine-structure mixing rate, which can besolved by adjusting the component and pressure of buffer gases or by increasing thealkali concentration, and the ideal functional mode for lasers should be in a quasi-twoenergy state. As a three-level laser, the pump intensity should exceed far beyond thethreshold for effective fluorescence suppression. To balance the pump absorption andfluorescence loss, an optimal operation temperature exists for a highest opticalconversion efficiency. At constant pump intensity, the match of narrowed pumplinewidth and low pressure buffer gases will benefit high optical conversion efficiency,as a contrast, the situation that by using pump sources with broader linewidth and buffergases with higher pressure need more intense pump to obtain the same efficiency, butthe latter case shows better tolerance to the shift of wavelength. In longitudinallypumped configuration, the length of gain medium and operation temperature areequivalent. At last, the influence of inner cavity loss and output coupler are analyzed.The conclusions above will be important for a practical alkali laser system design.2. The linewidth narrowing on high power diode lasers are experimentally studied.By use of Littman configuration external cavity on a broad area single emitter laserdiode (BAL), we realize10W output with linewidth below0.06nm, the tuning range is5nm and efficiency is60%. By use of a compound external cavity on a laser diode array(LDA), we successfully suppress the smile effect and obtain41W output power withlinewidth below0.1nm and efficiency of53%. By coupling the LDA into the volumeBragg grating (VBG) based external cavity, we obtained74W output power with0.1nm linewidth and95%efficiency. Due to the high efficiency and compactness, we decide touse the VBG scheme as pumping source for rubidium laser study.3. The characteristics of a rubidium DPAL are experimentally studied. First, westudy the absorption spectrum, pump absorption and fluorescence characteristics of arubidium gain medium with buffer gases of helium and methane, based on this, we usethe VBG coupled LDA to do the pumping experiment with longitudinal configuration atQCW operation mode. As a result, we obtain1.4W peak power with795nm output, theoptical conversion and slope efficiencies are4.8%and7.5%. The analyses show themain reason for low efficiency are the low pump absorption, low mode overlap factorand high inner cavity losses. The threshold behavior is studied by observing the changeof beam pattern when raising the operation temperature. At a total CW pump power of56W, the lasing process last4.4s and terminate due to the damage of cell windows. Theanalyses show that the reason is the reaction between alkali atoms and ethane gas,maybe also the Pyrex window material, that induced by high local temperature rise, andan efficient thermal management could solve the problem.4. The model for transversely pumped DPALs with flowing medium is set up andvalidated by comparing with other researchers’ published experimental results. Basedon this, the power scaling schemes for DPAL with configuration of mutually orthogonalpump, laser and flow directions are studied. The results show that, at a constant andreasonable pump intensity, the increase of the gain length as well as the pump powercan realize linear power scaling and decouple with thermal management, which will bethe main scheme to realize high power DPALs. The design of width and height shouldcomprehensively consider many factors, such as thermal management, operationtemperature, output beam shape, and pump focusing etc. The comparison of single-side,single-side double-pass, and double-side configurations are made and analyzed. Amega-watt class DPAL is conceptually designed, the result shows that the laser couldrealize optical conversion efficiency over85%with all the other parameters reasonableor could be realized in the near future, which demonstrate the great potential of DPALs.5. The model for DPALs in MOPA configuration is set up, a new method tocalculate the ASE effect in a bulk gain material is proposed, which considers both thelongitudinal and transverse ASE effects and couple them into rate equations, and therange of ASE can be calculated. Based on this, the important influencing factors arestudied, such as operation temperature, seed and pump intensities. The results show thatby use of high seed laser intensity to realize saturated amplification, the ASE effect canbe effectively suppressed and will not become a bottleneck in power scaling of DPALs.The study of power scaled scheme show that the increase of gain length and thecorresponding pump power will be the first choice.更多还原