基于多程增益工作机理的高功率2μm激光器性能研究

The 2μm solid-state laser has broad prospects and application requirements in research fields such as surgery and pain nerve stimulation. With the rapid development of clinical medicine and diagnosis and treatment technology, more and higher requirements have been put forward for the 2μm laser source, which is not limited to output power. The reliable light source with high beam quality, high power and high efficiency will be the key to further enhance its application value. The line-pumped multi-pass gain slab laser with multi-fold resonator theory proposed in this project. A novel steady-state operation model of multi-pass gain laser system is established. While ensuring the mode matching, high coupling efficiency and good beam quality of end-pumped solid laser, the linear array end pumped technology is used to improve the pump power greatly under the limitation of the limited pump power density. The theory of fundamental mode astigmatism correction is proposed to realize the transverse mode control of Tm laser system with large mode volume linear array end pumped structure. By establishing horizontal and vertical transverse mode control model, the asymmetry of output transverse mode in two directions of the linear array end pumped slab Tm laser is improved. Finally, high power, high efficiency, high symmetry and high beam quality 2μm Tm laser output is realized. It provides theoretical basis and technical means for the extensive and deep application of compact high power 2μm Tm laser.

2μm波段激光在外科手术临床、疼痛神经刺激研究领域都有着广阔前景和应用需求。但随之对手术的激光源提出了更多更高的要求，所以对于激光的要求不仅仅限于输出功率，迫切需要进一步拓展到高光束质量、高功率、高效率三个重要因素兼备的可靠光源，是进一步提升其应用价值的关键。本项目提出的线阵泵浦多折叠腔多程增益板条激光理论，建立全新机制的多程增益激光系统稳态工作模型，采用线阵端泵技术，在端泵固体激光的模式匹配、耦合效率高、光束质量好的同时，实现在有限泵浦功率密度的限制下大幅提高泵浦功率；提出基模像散矫正理论，实现大模式体积线阵端泵结构下Tm激光系统的横模调控问题，通过建立水平和竖直两个方向横模控制模型，改进现有线阵端泵板条Tm激光输出横模在两方向上的不对称性，最终实现高功率高效率高度对称高光束质量2μm Tm激光输出。为紧凑型高功率2μm Tm激光的广泛深入应用提供理论基础和技术手段。

目前2μm激光在工业加工以及外科手术临床领域的应用不断拓展，尤其随着新型透明复合材料的精细加工要求在不断提升，临床手术刀的创口伤害要求在不断缩小，因此需要进一步提升2μm激光器性能，在保证结构紧凑化高功率输出的前提下，进一步优化输出光束质量。本课题针对激光加工、外科手术临床领域的应用需求，开展了高功率多程折叠腔型结构2μm激光器研究工作。首先针对Tm:YLF激光介质高功率运转严重热效应问题，构建阵列结构混合泵浦板条增益介质热应力模型，仿真分析混合泵浦大功率下的晶体损伤阈值极限，获得最佳泵浦光半径参数。最终获得端面和侧面最佳光斑尺寸分别为5×1mm2和 10×1mm2，在注入最大总功率为400W时，晶体内最高温度为333K，最大应力为27.4MPa，为激光器的整体设计提供理论支持。在此基础上，基于泵浦光场多横模分布构型，建立准三能级多横模增益竞争Tm:YLF激光速率方程模型，掌握不同模式下粒子数变化规律，得出了随着泵浦功率的提高不同模式粒子数竞争逐渐加剧，基模粒子数逐渐下降，光束质量出现恶化。为此在激光系统构型设计中采用双程折叠腔和三程折叠腔构型，增长增益长度，抑制多横模竞争。并进一步构建多程增益过程的等效谐振腔矩阵传递模型，对双程折叠腔以及三程折叠腔内基模光斑半径进行模拟，实现良好的腔模匹配。最终实现了双程折叠腔激光器在泵浦功率为59.8w下，获得了5.23W的激光输出，其斜率效率为15.6%；三程折叠腔激光器获得了2.35W的激光输出，其斜率效率为11.7%。同时测得双程折叠腔下激光输出光束质量为M2y=1.49，M2x=2.06，三程折叠腔下激光输出光束质量为M2y=1.23，M2x= 1.15。验证了多程增益构型下实现高功率高光束质量2μm激光输出的可行性，为获得高功率下高光束质量的2μm激光提供了一种新的技术途径。